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[SOURCE: https://en.wikipedia.org/wiki/Indian_subcontinent] | [TOKENS: 1165] |
Contents Indian subcontinent The Indian subcontinent is a physiographic region of Asia below the Himalayas which projects into the Indian Ocean between the Bay of Bengal to the east and the Arabian Sea to the west. It is now divided between Bangladesh, India, and Pakistan. Although the terms "Indian subcontinent" and "South Asia" are often also used interchangeably to denote a wider region which includes, in addition, Bhutan, the Maldives, Nepal and Sri Lanka, the "Indian subcontinent" is more of a geophysical term, whereas "South Asia" is more geopolitical. "South Asia" frequently also includes Afghanistan, which is not considered part of the subcontinent even in extended usage. Name Historically, the region surrounding and southeast of the Indus River was often simply referred to as India in many historical sources. Even today, historians use this term to denote the entire Indian subcontinent when discussing history up until the era of the British Raj. Over time, however, "India" evolved to refer to a distinct political entity that eventually became a nation-state (today the Republic of India). According to the Oxford English Dictionary, the term subcontinent signifies a "subdivision of a continent which has a distinct geographical, political, or cultural identity" and also a "large land mass somewhat smaller than a continent". Its use to signify the Indian subcontinent is evidenced from the early twentieth century when most of the territory was either part of the British Empire or allied with them. It was a convenient term to refer to the region comprising both British India and the princely states. The term has been particularly common in the British Empire and its successors, while the term South Asia is the more common usage in Europe and North America as well as in most countries in South Asia itself sometimes. According to historians Sugata Bose and Ayesha Jalal, the Indian subcontinent has come to be known as South Asia "in more recent and neutral parlance". Indologist Ronald B. Inden argues that the usage of the term South Asia is becoming more widespread since it clearly distinguishes the region from East Asia. While South Asia, a more accurate term that reflects the region's contemporary political demarcations, is replacing the Indian subcontinent, a term closely linked to the region's colonial heritage, as a cover term, the latter is still widely used in typological studies. Since the Partition of India, citizens of Pakistan (which became independent of British India in 1947) and Bangladesh (which became independent of Pakistan in 1971) often perceive the use of the Indian subcontinent as offensive and suspicious because of the dominant placement of India in the term. As such it is being increasingly less used in those countries.[note 7] Meanwhile, many Indian analysts prefer to use the term because of the socio-cultural commonalities of the region. The region has also been called the "Asian subcontinent", the "South Asian subcontinent", as well as "India" or "Greater India" in the classical and pre-modern sense. The sport of cricket, introduced to the region by the British, is notably popular in India, Pakistan, Sri Lanka, Nepal, Bangladesh and Afghanistan. Within a cricket context, these countries are sometimes referred to simply as the subcontinent e.g. "Australia's tour of the subcontinent". The term is also sometimes used adjectivally in cricket e.g. "subcontinental conditions". Geology Before the Indian plate rifted from Gondwana and drifted northward toward Eurasia, two other landmasses, the Qiangtang terrane and Lhasa terrane,[note 8] had accreted to Eurasia. The Qiantang and Lhasa terranes were part of the string of microcontinents Cimmeria, today constituting parts of Turkey, Iran, Pakistan (including the Karakoram), China, Myanmar, Thailand and Malaysia, which closed the Paleo-Tethys Ocean above them and opening the Neo-Tethys Ocean between them and Gondwana, eventually colliding with Eurasia, and creating the Cimmerian Orogeny. After the Lhasa terrane had adjoined Eurasia, an active continental margin opened along its southern flank, below which the Neo-Tethys oceanic plate had begun to subduct. Magmatic activity along this flank produced the Gangdese batholith in what is today the Tibetan trans-Himalaya. Another subduction zone opened to the west, in the ocean basin above the Kohistan-Ladakh island arc. This island arc—formed by one oceanic plate subducting beneath another, its magma rising and creating continental crust—drifted north, closed its ocean basin and collided with Eurasia. Ladakh is today in the Indian-administered region of Kashmir and Kohistan in the Khyber-Pakhtunkhwa province of Pakistan, both on the Indian subcontinent. The collision of India with Eurasia closed the Neo-Tethys Ocean. The suture zone (in this instance, the remnants of the Neo-Tethys subduction zone pinched between the two continental crusts), which marks India's welding to Eurasia, is called the Indus-Yarlung suture zone. It lies north of the Himalayas. The headwaters of the Indus River and the Yarlung Tsangpo (later in its course, the Brahmaputra) flow along this suture zone. These two Eurasian rivers, whose courses were continually diverted by the rising Himalayas, define the western and eastern limits, respectively, of the Himalayan mountain range. See also Notes References Bibliography External links Africa Antarctica Asia Australia Europe North America South America Afro-Eurasia Americas Eurasia Oceania |
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[SOURCE: https://en.wikipedia.org/wiki/Arabian_Peninsula] | [TOKENS: 5150] |
Contents Arabian Peninsula The Arabian Peninsula (Arabic: شبه الجزيرة العربية, romanized: shibh al-jazīra al-ʿarabiyya, or جزيرة العرب, jazīrat al-ʿarab, 'the Island of Arabs'), or simply Arabia, is a peninsula in West Asia, situated north-east of Africa on the Arabian plate. At 3,237,500 km2 (1.25 million mi2), comparable in size to India, the Arabian Peninsula is the largest peninsula in the world. Geographically, the Arabian Peninsula comprises Bahrain,[a] Kuwait, Oman, Qatar, Saudi Arabia, the United Arab Emirates (UAE) and Yemen, as well as southern Iraq and Jordan. The largest of these is Saudi Arabia. In ancient antiquity, particularly from the 9th century BC to the 7th century AD, the Sinai Peninsula was also considered a part of Arabia. The Arabian Peninsula formed as a result of the rifting of the Red Sea between 56 and 23 million years ago, and is bordered by the Red Sea to the west and south-west, the Persian Gulf and the Gulf of Oman to the north-east, the Levant and Mesopotamia to the north and the Arabian Sea and the Indian Ocean to the south-east. The peninsula plays a critical geopolitical role in the Arab world and globally due to its vast reserves of oil and natural gas. The era of human settlement in the Arabian Peninsula predating any systematic written records is known as Prehistoric Arabia. The period of Arabian history beginning with the appearance of systematic records, until the rise of Islam, is known as Pre-Islamic Arabia. In the medieval Islamic period, geographers divided the Peninsula into four main regions: the Central Plateau (Najd and Al-Yamama), South Arabia (Yemen, Hadhramaut and south-western Oman), Al-Bahrain (Eastern Arabia or Al-Hassa), and the Hejaz (Tihamah for the western coast). Etymology In antiquity, the term "Arabia" encompassed a larger area than the current term "Arabian Peninsula" and included the Arabian Desert and large parts of the Syrian–Arabian desert. During the Hellenistic period, the area was known as Arabia (Ancient Greek: Ἀραβία). The Romans named three regions "Arabia": One of the nomes of Ptolemaic Egypt was named Arabia. Arabians used a north–south division of Arabia: ash-Sham vs. al-Yaman, or Arabia Deserta vs. Arabia Felix. Arabia Felix had originally been used for the whole peninsula, and at other times only for the southern region. Because its use became limited to the south, the whole peninsula was simply called Arabia. Arabia Deserta was the entire desert region extending north from Arabia Felix to Palmyra and the Euphrates, including all the area between Pelusium on the Nile and Babylon. This area was also called Arabia and not sharply distinguished from the peninsula. The Arabs and the Ottoman Empire considered the west of the Arabian Peninsula region where the Arabs lived 'the land of the Arabs'—billad al-'Arab (Arabia), and its major divisions were the bilad al-Sham (Levant), bilad al-Yaman (Yemen), and bilad al-'Iraq (Iraq). The Ottomans used the term Arabistan in a broad sense for the region starting from Cilicia, where the Euphrates river makes its descent into Syria, through Palestine, and on through the remainder of the Sinai and Arabian peninsulas. The provinces of Arabia were: al-Tih, the Sinai peninsula, Hejaz, Asir, Yemen, Hadramaut, Mahra and Shilu, Oman, Hasa, Bahrain, Dahna, Nufud, the Hammad, which included the deserts of Syria, Mesopotamia and Babylonia. Geography The Arabian Peninsula is located in the continent of Asia and is bounded by (clockwise) the Persian Gulf on the north-east, the Strait of Hormuz and the Gulf of Oman on the east, the Arabian Sea on the south-east, the Gulf of Aden, and the Guardafui Channel on the south, and the Bab-el-Mandeb strait on the south-west and the Red Sea, which is located on the south-west and west. The northern portion of the peninsula merges with the Syrian Desert with no clear borderline, although the northern boundary of the peninsula is generally considered to be the northern borders of Saudi Arabia and Kuwait, also southern regions of Iraq and Jordan. The most prominent feature of the peninsula is desert, but in the south-west, there are mountain ranges, which receive greater rainfall than the rest of the peninsula. Harrat ash Shaam is a large volcanic field that extends from north-western Arabia into Jordan and southern Syria. The Peninsula's constituent countries are (clockwise from north to south) Kuwait, Qatar, and the United Arab Emirates (UAE) on the east, Oman on the south-east, Yemen on the south, and Saudi Arabia at the center. The island country of Bahrain lies just off the east coast of the Peninsula. Due to Yemen's jurisdiction over the Socotra Archipelago, the Peninsula's geopolitical outline faces the Guardafui Channel and the Somali Sea to the south. The six countries of Bahrain, Kuwait, Oman, Qatar, Saudi Arabia, and the UAE form the Gulf Cooperation Council (GCC). The Kingdom of Saudi Arabia covers the greater part of the Peninsula. The Peninsula contains the world's largest reserves of oil. Saudi Arabia and the UAE are economically the wealthiest in the region. Qatar, the only peninsular country in the Persian Gulf on the larger peninsula, is home to the Arabic television station Al Jazeera and its English-language subsidiary Al Jazeera English. Kuwait, on the border with Iraq, is an important country strategically, forming one of the main staging grounds for coalition forces mounting the United States–led 2003 invasion of Iraq. Despite its historically sparse population, political Arabia stands out for its rapid population growth, driven by both significant inflows of migrant labor and persistently high birth rates. The population is characterized by its relative youth and a heavily skewed gender ratio favoring males. In several states, the number of South Asians surpasses that of the native population. The four smallest states (by area), with coastlines entirely bordering the Persian Gulf, showcase the world's most extreme population growth, nearly tripling every two decades. In 2014, the estimated population of the Arabian Peninsula was 77,983,936 (including expatriates). The Arabian Peninsula is known for having one of the most uneven adult sex ratios in the world, with females in some regions (especially the east) constituting only a quarter of people aged between 20 and 40. The ten most populous cities on the Arabian Peninsula are: The rocks exposed vary systematically across Arabia, with the oldest rocks exposed in the Arabian-Nubian Shield near the Red Sea, overlain by earlier sediments that become younger towards the Persian Gulf. Perhaps the best-preserved ophiolite on Earth, the Semail Ophiolite, lies exposed in the mountains of the UAE and northern Oman. The peninsula consists of: Arabia has few lakes or permanent rivers. Most areas are drained by ephemeral watercourses called wadis, which are dry except during the rainy season. Plentiful ancient aquifers exist beneath much of the peninsula, however, and where this water surfaces, oases form (e.g. Al-Hasa and Qatif, two of the world's largest oases) and permit agriculture, especially palm trees, which allowed the peninsula to produce more dates than any other region in the world. In general, the climate is extremely hot and arid, although there are exceptions. Higher elevations are made temperate by their altitude, and the Arabian Sea coastline can receive cool, humid breezes in summer due to cold upwelling offshore. The peninsula has no thick forests. Desert-adapted wildlife is present throughout the region. A plateau more than 2,500 feet (760 m) high extends across much of the Arabian Peninsula. The plateau slopes eastwards from the massive, rifted escarpment along the coast of the Red Sea, to the shallow waters of the Persian Gulf. The interior is characterized by cuestas and valleys, drained by a system of wadis. A crescent of sand and gravel deserts lies to the east. There are mountains at the eastern, southern and north-western borders of the peninsula. Broadly, the ranges can be grouped as follows: From the Hijaz southwards, the mountains show a steady increase in altitude westward as they get nearer to Yemen, and the highest peaks and ranges are all located in Yemen. The highest, Jabal An-Nabi Shu'ayb or Jabal Hadhur of the Haraz subrange of the Sarawat range, is 3,666 metres (12,028 ft) high. By comparison, the Tuwayr, Shammar and Dhofar generally do not exceed 1,000 m (3,300 ft) in height. Not all mountains in the peninsula are visibly within ranges. Jebel Hafeet in particular, on the border of the UAE and Oman, measuring between 1,100 and 1,300 m (3,600 and 4,300 ft), is not within the Hajar range, but may be considered an outlier of that range. Most of the Arabian Peninsula is unsuited to agriculture, making irrigation and land reclamation projects essential. The narrow coastal plain and isolated oases, amounting to less than 1% of the land area, are used to cultivate grains, coffee and tropical fruits. Goat, sheep, and camel husbandry is widespread elsewhere throughout the rest of the Peninsula. Some areas have a summer humid tropical monsoon climate, in particular the Dhofar and Al Mahrah areas of Oman and Yemen. These areas allow for large scale coconut plantations. Much of Yemen has a tropical monsoon rain influenced mountain climate. The plains usually have either a tropical or subtropical arid desert climate or arid steppe climate. The sea surrounding the Arabian Peninsula is generally tropical with a very rich sea life and some of the world's largest and most pristine coral reefs. In addition, the protozoa and zooxanthellae living in symbiosis with Red Sea corals have a unique hot weather adaptation to sudden rise (and fall) in sea water temperature. Hence, these coral reefs are not affected by coral bleaching caused by rise in temperatures, as Indo-Pacific coral reefs are. The reefs are also unaffected by mass tourism and diving or other large scale human interference. The Persian gulf has suffered significant loss and degradation of coral reefs with the biggest ongoing threat believed to be coastal construction activity altering the marine environment. The fertile soils of Yemen have encouraged settlement of almost all of the land from sea level up to the mountains at 10,000 feet (3,000 m). In the higher elevations, elaborate terraces have been constructed to facilitate grain, fruit, coffee, ginger and khat cultivation. The Arabian peninsula is known for its rich oil, i.e. petroleum production due to its geographical location. According to NASA's Gravity Recovery and Climate Experiment (GRACE) satellite data (2003–2013) analysed in a University of California, Irvine (UCI)-led study published in Water Resources Research on 16 June 2015, the most over-stressed aquifer system in the world is the Arabian Aquifer System, upon which more than 60 million people depend for water. Twenty-one of the 37 largest aquifers "have exceeded sustainability tipping points and are being depleted" and thirteen of them are "considered significantly distressed". History Prehistoric Arabia is the period of the Arabian Peninsula before any written records are known, going back to when humans first began to settle in the region, until around 1000 BC, when systematic written documentation begins to appear in the archaeological record. Stone tools from the Middle Paleolithic era along with fossils of other animals discovered at Ti's al Ghadah, in north-western Saudi Arabia, might imply that hominins migrated through a "Green Arabia" between 300,000 and 500,000 years ago. Two-hundred-thousand-year-old stone tools were discovered at Shuaib Al-Adgham in the eastern Al-Qassim Province, which would indicate that many prehistoric sites, located along a network of rivers, had once existed in the area. Acheulean tools found in Sadaqah, Riyadh Region reveal that hominids lived in the Arabian Peninsula around 188,000 years ago. Human habitation in Arabia may have occurred as early as 130,000 years ago. A fossilized Homo sapiens finger bone found at Al Wusta in the Nefud Desert dates to approximately 90,000 years ago and is the oldest human fossil discovered outside of Africa and the Levant. This indicates human migrations from Africa to Arabia occurred around this time. The Arabian Peninsula may have been the homeland of a 'Basal Eurasian' population, which diverged from other Eurasians soon after the Out-of-Africa migration, and subsequently became isolated, until it started to mix with other populations in the Middle East around 25,000 years ago. These different Middle Eastern populations would later spread Basal Eurasian ancestry via the Neolithic Revolution to all of Western Eurasia. Archaeology has revealed the existence of many civilizations in pre-Islamic Arabia (such as the Thamud), especially in South Arabia. South Arabian civilizations include the Kingdom of Saba, Awsan, Ma'in, and Himyar. From 106 AD to 630 AD north-western Arabia was under the control of the Roman Empire, which renamed it Arabia Petraea. Central Arabia was the location of the Kingdom of Kinda in the 4th, 5th and early 6th centuries, as well as the Ma'add tribes. Eastern Arabia was home to the Dilmun civilization. The earliest known events in Arabian history are migrations from the peninsula into neighbouring areas. The Arabian Peninsula has long been accepted as the original Urheimat of the Semitic languages by most scholars. The seventh century saw the rise of Islam as the peninsula's dominant religion. The Islamic prophet Muhammad was born in Mecca in about 570 and first began preaching in the city in 610, but migrated to Medina in 622. From there he and his companions united the tribes of Arabia under the banner of Islam and created the first Islamic state—a single Arab Muslim religious polity in the Arabian Peninsula. Under the subsequent Rashidun and Umayyad Caliphates, rapid expansion of Arab power well beyond the Arabian Peninsula formed a vast Muslim Arab Empire with an area of influence that stretched from the north-west Indian subcontinent, across Central Asia, the Middle East, North Africa, southern Italy, and the Iberian Peninsula, to the Pyrenees. With Muhammad's death in 632, disagreement broke out over who would succeed him as leader of the Muslim community. Umar ibn al-Khattab, a prominent companion of Muhammad, nominated Abu Bakr, who was Muhammad's intimate friend and collaborator. Others added their support and Abu Bakr was made the first caliph. This choice was disputed by some of Muhammad's companions, who held that Ali ibn Abi Talib, his cousin and son-in-law, had been designated his successor. Abu Bakr's immediate task was to avenge a recent defeat by Byzantine (or Eastern Roman Empire) forces, although he first had to put down a rebellion by Arab tribes in an episode known as the Ridda wars, or "Wars of Apostasy". On his death in 634, he was succeeded by Umar as caliph, followed by Uthman ibn al-Affan and Ali ibn Abi Talib. The period of these first four caliphs is known as al-khulafā' ar-rāshidūn: the Rashidun or "rightly guided" Caliphate. Under the Rashidun Caliphs, and, from 661, their Umayyad successors, the Arabs rapidly expanded the territory under Muslim control outside of Arabia. In a matter of decades Muslim armies decisively defeated the Byzantine army and destroyed the Persian Empire, conquering huge swathes of territory from the Iberian Peninsula to India. The political focus of the Muslim world then shifted to the newly conquered territories. Nevertheless, Mecca and Medina remained the spiritually most important places in the Muslim world. The Qur'an requires every able-bodied Muslim who can afford it, as one of the five pillars of Islam, to make a pilgrimage, or Hajj, to Mecca during the Islamic month of Dhu al-Hijjah at least once in his or her lifetime. The Masjid al-Haram (the Grand Mosque) in Mecca is the location of the Kaaba, Islam's holiest site, and the Masjid al-Nabawi (the Prophet's Mosque) in Medina is the location of Muhammad's grave; as a result, from the 7th century, Mecca and Medina became the pilgrimage destinations for large numbers of Muslims from across the Islamic world. Despite its spiritual importance, in political terms Arabia soon became a peripheral region of the Islamic world, in which the most important medieval Islamic states were based at various times in such far away cities as Damascus, Baghdad, and Cairo. However, from the 10th century (and, in fact, until the 20th century) the Hashemite Sharifs of Mecca maintained a state in the most developed part of the region, the Hejaz. Their domain originally comprised only the holy cities of Mecca and Medina but in the 13th century it was extended to include the rest of the Hejaz. Although, the Sharifs exercised at most times independent authority in the Hejaz, they were usually subject to the suzerainty of one of the major Islamic empires of the time. In the Middle Ages, these included the Abbasids of Baghdad, and the Fatimids, Ayyubids, and Mamluks of Egypt. The provincial Ottoman Army for Arabia (Arabistan Ordusu) was headquartered in Syria, which included Palestine, the Transjordan region in addition to Lebanon (Mount Lebanon was, however, a semi-autonomous mutasarrifate). It was put in charge of Syria, Cilicia, Iraq, and the remainder of the Arabian Peninsula. The Ottomans never had any control over central Arabia, also known as the Najd region.[citation needed] The emergence of what was to become the Saudi royal family, known as the Al Saud, began in Najd in central Arabia in 1744, when Muhammad bin Saud, founder of the dynasty, joined forces with the religious leader Muhammad ibn Abd al-Wahhab, founder of the Wahhabi movement, a strict puritanical form of Sunni Islam. The Emirate of Diriyah established in the area around Riyadh rapidly expanded and briefly controlled most of the present-day territory of Saudi Arabia, sacking Karbala in 1802, and capturing Mecca in 1803. The Damascus Protocol of 1914 provides an illustration of the regional relationships. Arabs living in one of the existing districts of the Arabian peninsula, the Emirate of Hejaz, asked for a British guarantee of independence. Their proposal included all Arab lands south of a line roughly corresponding to the northern frontiers of present-day Syria and Iraq. They envisioned a new Arab state, or confederation of states, adjoining the southern Arabian Peninsula. It would have comprised Cilicia—İskenderun and Mersin, Iraq with Kuwait, Syria, Mount Lebanon Mutasarrifate, Jordan, and Palestine. In the modern era, the term bilad al-Yaman came to refer specifically to the south-western parts of the peninsula. Arab geographers started to refer to the whole peninsula as 'jazirat al-Arab', or the peninsula of the Arabs. The railway was started in 1900 at the behest of the Ottoman Sultan Abdul Hamid II and was built largely by the Turks, with German advice and support. A public subscription was opened throughout the Islamic world to fund the construction. The railway was to be a waqf, an inalienable religious endowment or charitable trust. The major developments of the early 20th century were the Arab Revolt during World War I and the subsequent collapse and partitioning of the Ottoman Empire. The Arab Revolt (1916–1918) was initiated by the Sherif Hussein ibn Ali with the aim of securing independence from the ruling Ottoman Empire and creating a single unified Arab state spanning from Aleppo in Syria to Aden in Yemen. During World War I, the Sharif Hussein entered into an alliance with the United Kingdom and France against the Ottomans in June 1916.[citation needed] These events were followed by the foundation of Saudi Arabia under King Abdulaziz Ibn Saud. After the collapse of the Emirate of Diriyah, the House of Saud regrouped and in 1824 founded the Second Saudi State, which would control most of Arabia for the next two-thirds of a century. Ibn Saud, after his family lost power in 1891, would establish the Third Saudi State, capturing Riyadh in 1902, and, successively subduing Al-Hasa, Jabal Shammar and Hejaz between 1913 and 1926. The Saudis then absorbed the Emirate of Asir, with their expansion only ending in 1934 after a war with Yemen.[citation needed] The second major development has been the discovery of vast reserves of oil in the 1930s. Its production brought great wealth to all countries of the region, with the exception of Yemen. The North Yemen Civil War was fought in North Yemen between royalists of the Mutawakkilite Kingdom of Yemen and factions of the Yemen Arab Republic from 1962 to 1970. The war began with a coup d'état carried out by the republican leader, Abdullah as-Sallal, which dethroned the newly crowned Muhammad al-Badr and declared Yemen a republic under his presidency. The Imam escaped to the Saudi Arabian border and rallied popular support. The royalist side received support from Saudi Arabia, while the republicans were supported by Egypt and the Soviet Union. Both foreign irregular and conventional forces were also involved. The Egyptian President, Gamal Abdel Nasser, supported the republicans with as many as 70,000 troops. Despite several military moves and peace conferences, the war sank into a stalemate. Egypt's commitment to the war is considered to have been detrimental to its performance in the Six-Day War of June 1967, after which Nasser found it increasingly difficult to maintain his army's involvement and began to pull his forces out of Yemen. By 1970, King Faisal of Saudi Arabia recognized the republic and a truce was signed. Egyptian military historians refer to Egypt's role in the war in Yemen as analogous to the United States' role in the Vietnam War. In 1990, Iraq invaded Kuwait. The invasion of Kuwait by Iraqi forces led to the 1990–91 Gulf War. Egypt, Qatar, Syria, and Saudi Arabia joined a multinational coalition that opposed Iraq. Displays of support for Iraq by Jordan and Palestine resulted in strained relations between many of the Arab states. After the war, a so-called "Damascus Declaration" formalized an alliance for future joint Arab defensive actions between Egypt, Syria, and the GCC member states. The Arab Spring reached Yemen in January 2011. People of Yemen took to the street demonstrating against three decades of rule by President Ali Abdullah Saleh. The demonstration led to cracks in the ruling General People's Congress (GPC) and Saleh's Sanhani clan. Saleh used tactics of concession and violence to save his presidency. After numerous attempts, Saleh accepted the Gulf Cooperation Council's mediation. He eventually handed power to Vice President Hadi, who was sworn in as President of Yemen on 25 February 2012. Hadi launched a national dialogue to address new constitutional, political and social issues. The Houthi movement, dissatisfied with the outcomes of the national dialogue, launched an offensive and stormed the Yemeni capital Sanaa on 21 September 2014. In response, Saudi Arabia launched a military intervention in Yemen in March 2015. The civil war and subsequent military intervention and blockade caused a famine in Yemen. Gallery See also Explanatory notes References Further reading External links Africa Antarctica Asia Australia Europe North America South America Afro-Eurasia Americas Eurasia Oceania Other notes: 23°N 46°E / 23°N 46°E / 23; 46 |
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[SOURCE: https://en.wikipedia.org/wiki/Troposphere] | [TOKENS: 3575] |
Contents Troposphere The troposphere is the lowest layer of the atmosphere of Earth. Pronounced /ˈtrɒpəsfɪərˌ-poʊ-/, the name comes from Ancient Greek τρόπος (trópos) 'turning, change' and -sphere. It contains 80% of the total mass of the planetary atmosphere and 99% of the total mass of water vapor and aerosols, and is where most weather phenomena occur. From the planetary surface of the Earth, the average height of the troposphere is 18 km (11 mi; 59,000 ft) in the tropics; 11 km (6.8 mi; 36,000 ft) in the middle latitudes; and 6 km (3.7 mi; 20,000 ft) in the high latitudes of the polar regions in winter; thus the average height of the troposphere is 13 km (8.1 mi; 43,000 ft). The term troposphere derives from the Greek words tropos (rotating) and sphaira (sphere) indicating that rotational turbulence mixes the layers of air and so determines the structure and the phenomena of the troposphere. The rotational friction of the troposphere against the planetary surface affects the flow of the air, and so forms the planetary boundary layer (PBL) that varies in height from hundreds of meters up to 2 km (1.2 mi; 6,600 ft). The measures of the PBL vary according to the latitude, the landform, and the time of day when the meteorological measurement is realized. Atop the troposphere is the tropopause, which is the functional atmospheric border that demarcates the troposphere from the stratosphere. As such, because the tropopause is an inversion layer in which air-temperature increases with altitude, the temperature of the tropopause remains constant. The layer has the largest concentration of nitrogen. Structure The Earth's planetary atmosphere is water vapour which is carbonic acid rain water, which therefore has an approximate natural pH of 5.0 to 5.5 (slightly acidic). (Water other than atmospheric water vapour fallen as fresh rain, such as fresh/sweet/potable/river water, will usually be affected by the physical environment and may not be in this pH range.) Atmospheric water vapour holds suspended gases in it (not by mass), 78.08% nitrogen as N2, 20.95% oxygen as O2, 0.93% argon, trace gases, and variable amounts of condensing water (from saturated water vapor). Any carbon dioxide released into the atmosphere from a pressurised source combines with the carbonic acid water vapour and momentarily reduces the atmospheric pH by negligible amounts. Respiration from animals releases out of equilibrium carbonic acid and low levels of other ions. Combustion of hydrocarbons releases to atmosphere carbonic acid water as; saturates, condensates, vapour or gas (invisible steam). Combustion can releases particulates (carbon/soot and ash) as well as molecules forming nitrites and sulphites which will reduce the atmospheric pH of the water slightly or harmfully in highly industrialised areas where this is classed as air pollution and can create the phenomena of acid rain, a pH lower than the natural pH5.56. The negative effects of the by-products of combustion released into the atmospheric vapour can be removed by the use of scrubber towers and other physical means, the captured pollutants can be processed into a valuable by-product. The sources of atmospheric water vapor are the bodies of water (oceans, seas, lakes, rivers, swamps), and vegetation on the planetary surface, which humidify the troposphere through the processes of evaporation and transpiration respectively, and which influences the occurrence of weather phenomena; the greatest proportion of water vapor is in the atmosphere nearest the surface of the Earth. The temperature of the troposphere decreases at high altitude by way of the inversion layers that occur in the tropopause, which is the atmospheric boundary that demarcates the troposphere from the stratosphere. At higher altitudes, the low air-temperature consequently decreases the saturation vapor pressure, the amount of atmospheric water vapor in the upper troposphere. The maximum air pressure (weight of the atmosphere) is at sea level and decreases at high altitude because the atmosphere is in hydrostatic equilibrium, wherein the air pressure is equal to the weight of the air above a given point on the planetary surface. The relation between decreased air pressure and high altitude can be equated to the density of a fluid, by way of the following hydrostatic equation: where: The planetary surface of the Earth heats the troposphere by means of latent heat, thermal radiation, and sensible heat. The gas layers of the troposphere are less dense at the geographic poles and denser at the equator, where the average height of the tropical troposphere is 13 km, approximately 7.0 km greater than the 6.0 km average height of the polar troposphere at the geographic poles; therefore, surplus heating and vertical expansion of the troposphere occur in the tropical latitudes. At the middle latitudes, tropospheric temperatures decrease from an average temperature of 15 °C (59 °F) at sea level to approximately −55 °C (−67 °F) at the tropopause. At the equator, the tropospheric temperatures decrease from an average temperature of 20 °C (68 °F) at sea level to approximately −70 to −75 °C (−94 to −103 °F) at the tropopause. At the geographical poles, the Arctic and the Antarctic regions, the tropospheric temperature decreases from an average temperature of 0 °C (32 °F) at sea level to approximately −45 °C (−49 °F) at the tropopause. The temperature of the troposphere decreases with increased altitude, and the rate of decrease in air temperature is measured with the environmental lapse rate ( − d T / d z {\displaystyle -dT/dz} ), which is the numeric difference between the temperature of the planetary surface and the temperature of the tropopause divided by the altitude. Functionally, the ELR equation presumes that the planetary atmosphere is static and that there is no mixing of the layers of air by either vertical atmospheric convection or winds that could create turbulence. The difference in temperature derives from the planetary surface absorbing most of the energy from the sun, which then radiates outwards and heats the troposphere (the first layer of the atmosphere of Earth) while the radiation of surface heat to the upper atmosphere results in the cooling of that layer of the atmosphere. The ELR equation also assumes that the atmosphere is static, but heated air becomes buoyant, expands, and rises. The dry adiabatic lapse rate (DALR) accounts for the effect of the expansion of dry air as it rises in the atmosphere, and the wet adiabatic lapse rate (WALR) includes the effect of the condensation-rate of water vapor upon the environmental lapse rate. A parcel of air rises and expands because of the lower atmospheric pressure at high altitudes. The expansion of the air parcel pushes outwards against the surrounding air, and transfers energy (as work) from the parcel of air to the atmosphere. Transferring energy to a parcel of air by way of heat is a slow and inefficient exchange of energy with the environment, which is an adiabatic process (no energy transfer by way of heat). As the rising parcel of air loses energy while it acts upon the surrounding atmosphere, no heat energy is transferred from the atmosphere to the air parcel to compensate for the heat loss. The parcel of air loses energy as it reaches greater altitude, which is manifested as a decrease in the temperature of the air mass. Analogously, the reverse process occurs within a cold parcel of air that is being compressed and is sinking to the planetary surface. The compression and the expansion of an air parcel are reversible phenomena in which energy is not transferred into or out of the air parcel; atmospheric compression and expansion are measured as an isentropic process ( d S = 0 {\displaystyle dS=0} ) wherein there occurs no change in entropy as the air parcel rises or falls within the atmosphere. Because the heat exchanged ( d Q = 0 {\displaystyle dQ=0} ) is related to the change in entropy ( d S {\displaystyle dS} by d Q = T d S {\displaystyle dQ=TdS} ) the equation governing the air temperature as a function of altitude for a mixed atmosphere is: d S d z = 0 {\displaystyle {\frac {\,dS\,}{dz}}=0} where S is the entropy. The isentropic equation states that atmospheric entropy does not change with altitude; the adiabatic lapse rate measures the rate at which temperature decreases with altitude under such conditions. If the air contains water vapor, then cooling of the air can cause the water to condense, and the air no longer functions as an ideal gas. If the air is at the saturation vapor pressure, then the rate at which temperature decreases with altitude is called the saturated adiabatic lapse rate. The actual rate at which the temperature decreases with altitude is the environmental lapse rate. In the troposphere, the average environmental lapse rate is a decrease of about 6.5 °C for every 1.0 km (1,000m) of increased altitude. For dry air, an approximately ideal gas, the adiabatic equation is: p ( z ) [ T ( z ) ] − γ γ − 1 = constant {\displaystyle p(z){\Bigl [}T(z){\Bigr ]}^{-{\frac {\gamma }{\,\gamma \,-\,1\,}}}={\text{constant}}} wherein γ {\displaystyle \gamma } is the heat capacity ratio ( γ ≈ {\displaystyle \gamma \approx \,} 7⁄5) for air. The combination of the equation for the air pressure yields the dry adiabatic lapse rate: d T d z = − m g R γ − 1 γ = − 9.8 ∘ C / k m {\displaystyle {\frac {\,dT\,}{dz}}=-{\frac {\;mg\;}{R}}{\frac {\;\gamma \,-\,1\;}{\gamma }}=-9.8^{\circ }\mathrm {C/km} } . The environmental lapse rate ( d T / d z {\displaystyle dT/dz} ), at which temperature decreases with altitude, usually is unequal to the adiabatic lapse rate ( d S / d z ≠ 0 {\displaystyle dS/dz\neq 0} ). If the upper air is warmer than predicted by the adiabatic lapse rate ( d S / d z > 0 {\displaystyle dS/dz>0} ), then a rising and expanding parcel of air will arrive at the new altitude at a lower temperature than the surrounding air. In which case, the air parcel is denser than the surrounding air, and so falls back to its original altitude as an air mass that is stable against being lifted. If the upper air is cooler than predicted by the adiabatic lapse rate, then, when the air parcel rises to a new altitude, the air mass will have a higher temperature and a lower density than the surrounding air and will continue to accelerate and rise. The tropopause is the atmospheric boundary layer between the troposphere and the stratosphere, and is located by measuring the changes in temperature relative to increased altitude in the troposphere and in the stratosphere. In the troposphere, the temperature of the air decreases at high altitude, however, in the stratosphere the air temperature initially is constant, and then increases with altitude. The increase of air temperature at stratospheric altitudes results from the ozone layer's absorption and retention of the ultraviolet (UV) radiation that Earth receives from the Sun. The coldest layer of the atmosphere, where the temperature lapse rate changes from a positive rate (in the troposphere) to a negative rate (in the stratosphere) locates and identifies the tropopause as an inversion layer in which limited mixing of air layers occurs between the troposphere and the stratosphere. Atmospheric flow The general flow of the atmosphere is from west to east, which, however, can be interrupted by polar flows, either north-to-south flow or a south-to-north flow, which meteorology describes as a zonal flow and as a meridional flow. The terms are used to describe localized areas of the atmosphere at a synoptic scale; the three-cell model more fully explains the zonal and meridional flows of the planetary atmosphere of the Earth. The three-cell model of the atmosphere of the Earth describes the actual flow of the atmosphere with the tropical-latitude Hadley cell, the mid-latitude Ferrel cell, and the polar cell to describe the flow of energy and the circulation of the planetary atmosphere. Balance is the fundamental principle of the model – that the solar energy absorbed by the Earth in a year is equal to the energy radiated (lost) into outer space. The Earth's energy balance does not equally apply to each latitude because of the varying strength of the sunlight that strikes each of the three atmospheric cells, consequent to the inclination of the axis of planet Earth within its orbit of the Sun. The resultant atmospheric circulation transports warm tropical air to the geographic poles and cold polar air to the tropics. The effect of the three cells is the tendency to the equilibrium of heat and moisture in the planetary atmosphere of Earth. A zonal flow regime is the meteorological term meaning that the general flow pattern is west to east along the Earth's latitude lines, with weak shortwaves embedded in the flow. The use of the word "zone" refers to the flow being along the Earth's latitudinal "zones". This pattern can buckle and thus become a meridional flow. When the zonal flow buckles, the atmosphere can flow in a more longitudinal (or meridional) direction, and thus the term "meridional flow" arises. Meridional flow patterns feature strong, amplified troughs of low pressure and ridges of high pressure, with more north–south flow in the general pattern than west-to-east flow. Solar System Within the Solar System, other planetary bodies with a substantial atmosphere have a troposphere. These include Venus, Mars, and the Saturnian moon Titan. Jupiter does not have a solid surface, and the lowest atmospheric layer, the troposphere, smoothly transitions into the planet's fluid interior. The troposphere of Venus is the densest part of the atmosphere, starting at the surface and extendind upwards to 65 km. The winds are slow near the surface, but at the top of the troposphere the temperature and pressure reaches Earth-like levels and clouds pick up speed to 100 m/s (360 km/h). The large amount of CO2 in the atmosphere together with water vapour and sulfur dioxide create a strong greenhouse effect, trapping solar energy and raising the surface temperature to around 740 K (467 °C). The thick troposphere makes the difference in temperature between the day and night side small, even though the slow retrograde rotation of the planet causes a single solar day to last 116.5 Earth days. On the night side of Venus clouds can still be found at 80 km (50 mi) above the surface. The troposphere of Mars contains most of the planet's weather phenomena, including convection and dust storms. Its dynamics are heavily driven by the daytime surface heating and the amount of suspended dust. Mars has a higher scale height of 11.1 km than Earth because of its weaker gravity. The theoretical dry adiabatic lapse rate of Mars is 4.3 °C km−1, but the measured average lapse rate is about 2.5 °C km−1 because the suspended dust particles absorb solar radiation and heat the air. The planetary boundary layer can extend to over 10 km thick during the daytime. The near-surface diurnal temperature range is huge (60 °C) due to the low thermal inertia. Under dusty conditions, the suspended dust particles can reduce the surface diurnal temperature range to only 5 °C. The temperature above 15 km is controlled by radiative processes instead of convection. Mars is a rare exception to the "0.1-bar tropopause" rule found in the other atmospheres in our solar system. Titan is the only planetary satellite with a substantial atmosphere, and it is the only atmosphere besides Earth's composed primarily of nitrogen. Titan's lower surface gravity creates a more extended atmosphere than Earth, with scale heights of 15–50 km (9.3–31.1 mi). The troposphere of Titan is well defined, extending to a tropopause at an altitude of around 40 km, where the temperature is 70 K. Methane condenses out of Titan's atmosphere at high altitudes, with its abundance increasing below the tropopause, leveling off at a value of 4.9% between 8 km (5.0 mi) and the surface. Methane rain, haze rainout, and varying cloud layers are found in the troposphere. See also References External links |
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[SOURCE: https://en.wikipedia.org/wiki/Stratosphere] | [TOKENS: 2241] |
Contents Stratosphere The stratosphere is the second-lowest layer of the atmosphere of Earth, located above the troposphere and below the mesosphere. Pronounced /ˈstrætəˌsfɪər, -toʊ-/, the name originates from Ancient Greek στρωτός (strōtós) 'layer, stratum' and -sphere. The stratosphere is composed of stratified temperature zones, with the warmer layers of air located higher (closer to outer space) and the cooler layers lower (closer to the planetary surface of the Earth). The increase of temperature with altitude is a result of the absorption of the Sun's ultraviolet (UV) radiation by the ozone layer, where ozone is exothermically photolyzed into oxygen in a cyclical fashion. This temperature inversion is in contrast to the troposphere, where temperature decreases with altitude, and between the troposphere and stratosphere is the tropopause border that demarcates the beginning of the temperature inversion. Near the equator, the lower edge of the stratosphere is as high as 20 km (66,000 ft; 12 mi), at mid-latitudes around 10 km (33,000 ft; 6.2 mi), and at the poles about 7 km (23,000 ft; 4.3 mi). Temperatures range from an average of −51 °C (−60 °F; 220 K) near the tropopause to an average of −15 °C (5.0 °F; 260 K) near the mesosphere. Stratospheric temperatures also vary within the stratosphere as the seasons change, reaching particularly low temperatures in the polar night (winter). Winds in the stratosphere can far exceed those in the troposphere, reaching near 60 m/s (220 km/h; 130 mph) in the Southern polar vortex. Discovery In 1902, Léon Teisserenc de Bort from France and Richard Assmann from Germany, in separate but coordinated publications and following years of observations, published the discovery of an isothermal layer at around 11–14 km (6.8-8.7 mi), which is the base of the lower stratosphere. This was based on temperature profiles from mostly unmanned and a few manned instrumented balloons. Ozone layer The mechanism describing the formation of the ozone layer was described by British mathematician and geophysicist Sydney Chapman in 1930, and is known as the Chapman cycle or ozone–oxygen cycle. Molecular oxygen absorbs high energy sunlight in the UV-C region, at wavelengths shorter than about 240 nm. Radicals produced from the homolytically split oxygen molecules combine with molecular oxygen to form ozone. Ozone in turn is photolyzed much more rapidly than molecular oxygen as it has a stronger absorption that occurs at longer wavelengths, where the solar emission is more intense. Ozone (O3) photolysis produces O and O2. The oxygen atom product combines with atmospheric molecular oxygen to reform O3, releasing heat. The rapid photolysis and reformation of ozone heat the stratosphere, resulting in a temperature inversion. This increase of temperature with altitude is characteristic of the stratosphere; its resistance to vertical mixing means that it is stratified. Within the stratosphere temperatures increase with altitude (see temperature inversion); the top of the stratosphere has a temperature of about 270 K (−3°C or 26.6°F). This vertical stratification, with warmer layers above and cooler layers below, makes the stratosphere dynamically stable: there is no regular convection and associated turbulence in this part of the atmosphere. However, exceptionally energetic convection processes, such as volcanic eruption columns and overshooting tops in severe supercell thunderstorms, may carry convection into the stratosphere on a very local and temporary basis. Overall, the attenuation of solar UV at wavelengths that damage DNA by the ozone layer allows life to exist on the planet's surface outside of the ocean. All air entering the stratosphere must pass through the tropopause, the temperature minimum that divides the troposphere and stratosphere. The rising air is literally freeze-dried; the stratosphere is a very dry place. The top of the stratosphere is called the stratopause, above which the temperature decreases with height. Sydney Chapman gave a correct description of the source of stratospheric ozone and its ability to generate heat within the stratosphere;[citation needed] he also wrote that ozone may be destroyed by reacting with atomic oxygen, making two molecules of molecular oxygen. We now know that there are additional ozone loss mechanisms and that these mechanisms are catalytic, meaning that a small amount of the catalyst can destroy a great number of ozone molecules. The first is due to the reaction of hydroxyl radicals (•OH) with ozone. •OH is formed by the reaction of electrically excited oxygen atoms produced by ozone photolysis, with water vapor. While the stratosphere is dry, additional water vapor is produced in situ by the photochemical oxidation of methane (CH4). The HO2 radical produced by the reaction of OH with O3 is recycled to OH by reaction with oxygen atoms or ozone. In addition, solar proton events can significantly affect ozone levels via radiolysis with the subsequent formation of OH. Nitrous oxide (N2O) is produced by biological activity at the surface and is oxidized to NO in the stratosphere; the so-called NOx radical cycles also deplete stratospheric ozone. Finally, chlorofluorocarbon molecules are photolyzed in the stratosphere releasing chlorine atoms that react with ozone giving ClO and O2. The chlorine atoms are recycled when ClO reacts with O in the upper stratosphere, or when ClO reacts with itself in the chemistry of the Antarctic ozone hole. Paul J. Crutzen, Mario J. Molina and F. Sherwood Rowland were awarded the Nobel Prize in Chemistry in 1995 for their work describing the formation and decomposition of stratospheric ozone. Aircraft flight Commercial airliners typically cruise at altitudes of 9–12 km (30,000–39,000 ft) which is in the lower reaches of the stratosphere in temperate latitudes. This optimizes fuel efficiency, mostly due to the low temperatures encountered near the tropopause and low air density, reducing parasitic drag on the airframe. Stated another way, it allows the airliner to fly faster while maintaining lift equal to the weight of the plane. (The fuel consumption depends on the drag, which is related to the lift by the lift-to-drag ratio.) It also allows the airplane to stay above the turbulent weather of the troposphere. The Concorde aircraft cruised at Mach 2 at about 60,000 ft (18 km), and the SR-71 cruised at Mach 3 at 85,000 ft (26 km), all within the stratosphere. Because the temperature in the tropopause and lower stratosphere is largely constant with increasing altitude, very little convection and its resultant turbulence occurs there. Most turbulence at this altitude is caused by variations in the jet stream and other local wind shears, although areas of significant convective activity (thunderstorms) in the troposphere below may produce turbulence as a result of convective overshoot. On October 24, 2014, Alan Eustace became the record holder for reaching the altitude record for a manned balloon at 135,890 ft (41,419 m). Eustace also broke the world records for vertical speed skydiving, reached with a peak velocity of 1,321 km/h (822 mph) and total freefall distance of 123,414 ft (37,617 m) – lasting four minutes and 27 seconds. Circulation and mixing The stratosphere is a region of intense interactions among radiative, dynamical, and chemical processes, in which the horizontal mixing of gaseous components proceeds much more rapidly than does vertical mixing. The overall circulation of the stratosphere is termed as Brewer-Dobson circulation, which is a single-celled circulation, spanning from the tropics up to the poles, consisting of the tropical upwelling of air from the tropical troposphere and the extra-tropical downwelling of air. Stratospheric circulation is a predominantly wave-driven circulation in that the tropical upwelling is induced by the wave force by the westward propagating Rossby waves, in a phenomenon called Rossby-wave pumping. An interesting feature of stratospheric circulation is the quasi-biennial oscillation (QBO) in the tropical latitudes, which is driven by gravity waves that are convectively generated in the troposphere. The QBO induces a secondary circulation that is important for the global stratospheric transport of tracers, such as ozone or water vapor. Another large-scale feature that significantly influences stratospheric circulation is the breaking planetary waves resulting in intense quasi-horizontal mixing in the midlatitudes. This breaking is much more pronounced in the winter hemisphere where this region is called the surf zone. This breaking is caused due to a highly non-linear interaction between the vertically propagating planetary waves and the isolated high potential vorticity region known as the polar vortex. The resultant breaking causes large-scale mixing of air and other trace gases throughout the midlatitude surf zone. The timescale of this rapid mixing is much smaller than the much slower timescales of upwelling in the tropics and downwelling in the extratropics. During northern hemispheric winters, sudden stratospheric warmings, caused by the absorption of Rossby waves in the stratosphere, can be observed in approximately half of the winters when easterly winds develop in the stratosphere. These events often precede unusual winter weather and may even be responsible for the cold European winters of the 1960s. Stratospheric warming of the polar vortex results in its weakening. When the vortex is strong, it keeps the cold, high-pressure air masses contained in the Arctic; when the vortex weakens, air masses move equatorward, and results in rapid changes of weather in the mid latitudes. Upper-atmospheric lightning Upper-atmospheric lightning is a family of short-lived electrical breakdown phenomena that occur well above the altitudes of normal lightning and storm clouds. Upper-atmospheric lightning is believed to be electrically induced forms of luminous plasma. Lightning extending above the troposphere into the stratosphere is referred to as blue jet, and that reaching into the mesosphere as red sprite. Life Bacterial life survives in the stratosphere, making it a part of the biosphere. In 2001, dust was collected at a height of 41 kilometres in a high-altitude balloon experiment and was found to contain bacterial material when examined later in the laboratory. See also References External links |
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[SOURCE: https://en.wikipedia.org/wiki/Atmosphere_of_Earth] | [TOKENS: 7176] |
Contents Atmosphere of Earth The atmosphere of Earth consists of a layer of mixed gas (commonly referred to as air) that is retained by gravity, surrounding the Earth's surface. It contains variable quantities of suspended aerosols and particulates that create weather features such as clouds and hazes. The atmosphere serves as a protective buffer between the Earth's surface and outer space. It shields the surface from most meteoroids and ultraviolet solar radiation, reduces diurnal temperature variation – the temperature extremes between day and night, and keeps it warm through heat retention via the greenhouse effect. The atmosphere redistributes heat and moisture among different regions via air currents, and provides the chemical and climate conditions that allow life to exist and evolve on Earth. By mole fraction (i.e., by quantity of molecules), dry air contains 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, and small amounts of other trace gases (see Composition below for more detail). Air also contains a variable amount of water vapor, on average around 1% at sea level, and 0.4% over the entire atmosphere. Earth's primordial atmosphere consisted of gases accreted from the solar nebula, but the composition changed significantly over time, affected by many factors such as volcanism, outgassing, impact events, weathering and the evolution of life (particularly the photoautotrophs). In the present day, human activity has contributed to atmospheric changes, such as climate change (mainly through deforestation and fossil-fuel–related global warming), ozone depletion and acid deposition. The atmosphere has a mass of about 5.15×1018 kg, three quarters of which is within about 11 km (6.8 mi; 36,000 ft) of the surface. The atmosphere becomes thinner with increasing altitude, with no definite boundary between the atmosphere and outer space. The Kármán line at 100 km (62 mi) is often used as a conventional definition of the edge of space. Several layers can be distinguished in the atmosphere based on characteristics such as temperature and composition, namely the troposphere, stratosphere, mesosphere, thermosphere (formally the ionosphere), and exosphere. Air composition, temperature and atmospheric pressure vary with altitude. Air suitable for use in photosynthesis by terrestrial plants and respiration of terrestrial animals is found within the troposphere. The study of Earth's atmosphere and its processes is called atmospheric science (aerology), and includes multiple subfields, such as climatology and atmospheric physics. Early pioneers in the field include Léon Teisserenc de Bort and Richard Assmann. The study of the historic atmosphere is called paleoclimatology. Composition The three major constituents of Earth's atmosphere are nitrogen, oxygen, and argon. Water vapor accounts for roughly 0.25% of the atmosphere by mass. In the lower atmosphere, the concentration of water vapor (a greenhouse gas) varies significantly from around 10 ppm by mole fraction in the coldest portions of the atmosphere to as much as 5% by mole fraction in hot, humid air masses, and concentrations of other atmospheric gases are typically quoted in terms of dry air (without water vapor).: 8 The remaining gases are often referred to as trace gases, among which are other greenhouse gases, principally carbon dioxide, methane, nitrous oxide, and ozone. Besides argon, other noble gases, neon, helium, krypton, and xenon are also present. Filtered air includes trace amounts of many other chemical compounds. Many substances of natural origin may be present in locally and seasonally variable small amounts as aerosols in an unfiltered air sample, including dust of mineral and organic composition, pollen and spores, sea spray, and volcanic ash. Various industrial pollutants also may be present as gases or aerosols, such as chlorine (elemental or in compounds), fluorine compounds, and elemental mercury vapor. Sulfur compounds such as hydrogen sulfide and sulfur dioxide (SO2) may be derived from natural sources or from industrial air pollution. The total ppm above adds up to more than 1 million (currently 83.43 above it) due to experimental error. Notes (A) In the atmosphere the pressure is low enough for the ideal gas laws to be correct within 1%. Therefore, the mole fraction is very close to the volume fraction.: 4 (B) ppm: parts per million by molecular count (C) The concentration of CO2 has been increasing in recent decades, as has that of CH4. (D) Water vapor is about 0.25% by mass over full atmosphere (E) Water vapor varies significantly locally The average molecular weight of dry air, which can be used to calculate densities or to convert between mole fraction and mass fraction, is about 28.946 or 28.964: 257 g/mol. This is decreased when the air is humid. Up to an altitude of around 100 km (62 mi), atmospheric turbulence mixes the component gases so that their relative concentrations remain the same. There exists a transition zone from roughly 80 to 120 km (50 to 75 mi) where this turbulent mixing gradually yields to molecular diffusion. The latter process forms the heterosphere where the relative concentration of lighter gases increase with altitude. Stratification In general, air pressure and density decrease with altitude in the atmosphere. However, temperature has a more complicated profile with altitude and may remain relatively constant or even increase with altitude in some regions (see the temperature section). Because the general pattern of the temperature/altitude profile, or lapse rate, is constant and measurable by means of instrumented balloon soundings, the temperature behavior provides a useful metric to distinguish atmospheric layers. This atmospheric stratification divides the Earth's atmosphere into five main layers with these typical altitude ranges: The exosphere is the outermost layer of Earth's atmosphere (though it is so tenuous that some scientists consider it to be part of interplanetary space rather than part of the atmosphere). It extends from the thermopause (also known as the "exobase") at the top of the thermosphere to a poorly defined boundary with the solar wind and interplanetary medium. The altitude of the exobase varies from about 500 kilometres (310 mi; 1,600,000 ft) to about 1,000 kilometres (620 mi) in times of higher incoming solar radiation. The upper limit varies depending on the definition. Various authorities consider it to end at about 10,000 kilometres (6,200 mi) or about 190,000 kilometres (120,000 mi)—about halfway to the moon, where the influence of Earth's gravity is about the same as radiation pressure from sunlight. The geocorona visible in the far ultraviolet (caused by neutral hydrogen) extends to at least 100,000 kilometres (62,000 mi). This layer is mainly composed of extremely low densities of hydrogen, with limited amounts of helium, carbon dioxide, and nascent oxygen closer to the exobase. The atoms and molecules are so far apart that they can travel hundreds of kilometres without colliding with one another.: 14–4 Thus, the exosphere no longer behaves like a gas, and the particles constantly escape into space. These free-moving particles follow ballistic trajectories and may migrate in and out of the magnetosphere or the solar wind. Every second, the Earth loses about 3 kg of hydrogen, 50 g of helium, and much smaller amounts of other constituents. The exosphere is too far above Earth for meteorological phenomena to be possible. The exosphere contains many of the artificial satellites that orbit Earth. The thermosphere is the second-highest layer of Earth's atmosphere. It extends from the mesopause (which separates it from the mesosphere) at an altitude of about 80 km (50 mi) up to the thermopause at an altitude range of 500–1000 km (310–620 mi). The height of the thermopause varies considerably due to changes in solar activity. The passage of the dusk and dawn solar terminator creates background density perturbations up to a factor of two through this layer, forming a dominant feature in this region. Because the thermopause lies at the lower boundary of the exosphere, it is also referred to as the exobase. Overlapping the thermosphere, from 50 to 600 kilometres (31 to 373 mi) above Earth's surface, is the ionosphere – a region of enhanced plasma density. The temperature of the thermosphere gradually increases with height and can rise as high as 1500 °C (2700 °F), though the gas molecules are so far apart that its temperature in the usual sense is not very meaningful. This temperature increase is caused by absorption of ionizing UV and X-ray emission from the Sun. The air is so rarefied that an individual molecule (of oxygen, for example) travels an average of 1 kilometre (0.62 mi; 3300 ft) between collisions with other molecules. Although the thermosphere has a high proportion of molecules with high energy, it would not feel hot to a human in direct contact, because its density is too low to conduct a significant amount of energy to or from the skin. This layer is completely cloudless and free of water vapor. However, non-hydrometeorological phenomena such as the aurora borealis and aurora australis are occasionally seen in the thermosphere at an altitude of around 100 km (62 mi). The colors of the aurora are linked to the properties of the atmosphere at the altitude they occur. The most common is the green aurora, which comes from atomic oxygen in the 1S state, and occurs at altitudes from 120 to 400 km (75 to 250 mi). The International Space Station orbits in the thermosphere, between 370 and 460 km (230 and 290 mi). It is this layer where many of the satellites orbiting the Earth are present. The mesosphere is the third highest layer of Earth's atmosphere, occupying the region above the stratosphere and below the thermosphere. It extends from the stratopause at an altitude of about 50 km (31 mi) to the mesopause at 80–85 km (50–53 mi) above sea level. Temperatures drop with increasing altitude to the mesopause that marks the top of this middle layer of the atmosphere. It is the coldest place on Earth and has an average temperature around −85 °C (−120 °F; 190 K). Because the atmosphere absorbs sound waves at a rate that is proportional to the square of the frequency, audible sounds from the ground do not reach the mesosphere. Infrasonic waves can reach this altitude, but they are difficult to emit at a high power level. Just below the mesopause, the air is so cold that even the very scarce water vapor at this altitude can condense into polar-mesospheric noctilucent clouds of ice particles. These are the highest clouds in the atmosphere and may be visible to the naked eye if sunlight reflects off them about an hour or two after sunset or similarly before sunrise. They are most readily visible when the Sun is around 4 to 16 degrees below the horizon. Lightning-induced discharges known as transient luminous events (TLEs) occasionally form in the mesosphere above tropospheric thunderclouds. The mesosphere is also the layer where most meteors and satellites burn up upon atmospheric entrance. It is too high above Earth to be accessible to jet-powered aircraft and balloons, and too low to permit orbital spacecraft. The mesosphere is mainly accessed by sounding rockets and rocket-powered aircraft. The stratosphere is the second-lowest layer of Earth's atmosphere. It lies above the troposphere and is separated from it by the tropopause. This layer extends from the top of the troposphere at roughly 12 km (7.5 mi) above Earth's surface to the stratopause at an altitude of about 50 to 55 km (31 to 34 mi). 99% of the total mass of the atmosphere lies below 30 km (19 mi), and the atmospheric pressure at the top of the stratosphere is roughly 1/1000 the pressure at sea level. It contains the ozone layer, which is the part of Earth's atmosphere that contains relatively high concentrations of that gas. The stratosphere defines a layer in which temperatures rise with increasing altitude. This rise in temperature is caused by the absorption of ultraviolet radiation (UV) from the Sun by the ozone layer, which restricts turbulence and mixing. Although the temperature may be −80 °C (−110 °F; 190 K) at the tropopause, the top of the stratosphere is much warmer, and may be just below 0 °C. This layer is unique to the Earth; neither Mars nor Venus have a stratosphere because of low abundances of oxygen in their atmospheres. The stratospheric temperature profile creates very stable atmospheric conditions, so the stratosphere lacks the weather-producing air turbulence that is so prevalent in the troposphere. Consequently, the stratosphere is almost completely free of clouds and other forms of weather. However, polar stratospheric or nacreous clouds are occasionally seen in the lower part of this layer of the atmosphere where the air is coldest. The stratosphere is the highest layer that can be accessed by jet-powered aircraft. The troposphere is the lowest layer of Earth's atmosphere. It extends from Earth's surface to an average height of about 12 km (7.5 mi), although this altitude varies from about 9 km (5.6 mi) at the geographic poles to 17 km (11 mi) at the Equator, with some variation due to weather. The troposphere is bounded above by the tropopause, a boundary marked in most places by a temperature inversion (i.e. a layer of relatively warm air above a colder one), and in others by a zone that is isothermal with height. Although variations do occur, the temperature usually declines with increasing altitude in the troposphere because the troposphere is mostly heated through energy transfer from the surface. Thus, the lowest part of the troposphere (i.e. Earth's surface) is typically the warmest section of the troposphere. This promotes vertical mixing (hence, the origin of its name in the Greek word τρόπος, tropos, meaning "turn"). The troposphere contains roughly 80% of the mass of Earth's atmosphere. The troposphere is denser than all its overlying layers because a larger atmospheric weight sits on top of the troposphere and causes it to be more severely compressed. Fifty percent of the total mass of the atmosphere is located in the lower 5.5 km (3.4 mi) of the troposphere. Nearly all atmospheric water vapor or moisture is found in the troposphere, so it is the layer where most of Earth's weather takes place. The ability of the atmosphere to retain water decreases as the temperature declines, so 90% of the water vapor is held in the lower part of the troposphere. It has basically all the weather-associated cloud genus types generated by active wind circulation, although very tall cumulonimbus thunder clouds can penetrate the tropopause from below and rise into the lower part of the stratosphere. Most conventional aviation activity takes place in the troposphere, and it is the only layer accessible by propeller-driven aircraft. Contrails are formed from jet engine water emission at altitudes where the atmospheric temperature is about −53 °C (−63 °F); typically around 7.7 km (4.8 mi) for modern engines. Within the five principal layers above, which are largely determined by temperature, several secondary layers may be distinguished by other properties: The average temperature of the atmosphere at Earth's surface is 14 °C (57 °F; 287 K) or 15 °C (59 °F; 288 K), depending on the reference. Physical properties The average atmospheric pressure at sea level is defined by the International Standard Atmosphere as 101325 pascals (760.00 Torr; 14.6959 psi; 760.00 mmHg).: 257 This is sometimes referred to as a unit of standard atmospheres (atm). Total atmospheric mass is 5.1480×1018 kg (1.13494×1019 lb), about 2.5% less than would be inferred from the average sea-level pressure and Earth's area of 51007.2 megahectares,: 240 this portion being displaced by Earth's mountainous terrain. Atmospheric pressure is the total weight of the air above unit area at the point where the pressure is measured. Thus air pressure varies with location and weather. Air pressure decreases exponentially with altitude at a rate that depends on the air temperature. The rate of decrease is determined by a temperature-dependent parameter called the scale height: for each increase in altitude by this height, the pressure decreases by a factor of e (the base of natural logarithms, approximately 2.718). For Earth, this value is typically 5.5 to 6 km for altitudes up to around 80 km (50 mi). However, the atmosphere is more accurately modeled with a customized equation for each layer that takes gradients of temperature, molecular composition, solar radiation and gravity into account. At heights over 100 km, the atmosphere is not well mixed, so each chemical species has its own scale height. At altitudes of 200 to 300 km, the combined scale height is 20 to 30 km. The mass of Earth's atmosphere is distributed approximately as follows: By comparison, the summit of Mount Everest is at 8,848 m (29,029 ft); commercial airliners typically cruise between 9 and 12 km (30,000 and 38,000 ft), where the lower density and temperature of the air improve fuel economy; weather balloons reach about 35 km (115,000 ft); and the highest X-15 flight in 1963 reached 108.0 km (354,300 ft). Even above the Kármán line, significant atmospheric effects such as auroras still occur. Meteors begin to glow in this region, though the larger ones may not burn up until they penetrate more deeply. The various layers of Earth's ionosphere, important to HF radio propagation, begin below 100 km and extend beyond 500 km. By comparison, the International Space Station typically orbit at 370–460 km, within the F-layer of the ionosphere,: 271 where they encounter enough atmospheric drag to require reboosts every few months, otherwise orbital decay will occur, resulting in a return to Earth. Depending on solar activity, satellites can experience noticeable atmospheric drag at altitudes as high as 600–800 km. Starting at sea level, the temperature decreases with altitude until reaching the stratosphere at around 11 km. Above, the temperature stabilizes over a large vertical distance. Starting above about 20 km, the temperature increases with height, due to heating within the ozone layer caused by the capture of significant ultraviolet radiation from the Sun by the molecular oxygen and ozone gas in this region. A second region of increasing temperature with altitude occurs at very high altitudes, in the aptly-named thermosphere above 90 km. During the night, the ground radiates more energy than it gains from the atmosphere. As energy is conducted from the nearby atmosphere to the cooler ground, it creates a temperature inversion where the local temperature increases with altitude up to around 1,000 m. Because in an ideal gas of constant composition the speed of sound depends only on temperature and not on pressure or density, the speed of sound in the atmosphere with altitude takes on the form of the complicated temperature profile (see illustration to the right), and does not mirror altitudinal changes in density or pressure. For example, at sea level the speed of sound is 340 m/s. At the average temperature of the stratosphere, −60 °C, the speed of sound decreases to 290 m/s. The density of air at sea level is about 1.29 kg/m3 (1.29 g/L, 0.00129 g/cm3).: 257 Density is not measured directly but is calculated from measurements of temperature, pressure and humidity using the equation of state for air (a form of the ideal gas law). Atmospheric density decreases as the altitude increases. This variation can be approximately modeled using the barometric formula. More sophisticated models are used to predict the orbital decay of satellites. The average mass of the atmosphere is about 5 quadrillion (5×1015) tonnes or 1/1,200,000 the mass of Earth. According to the American National Center for Atmospheric Research, "The total mean mass of the atmosphere is 5.1480×1018 kg with an annual range due to water vapor of 1.2 or 1.5×1015 kg, depending on whether surface pressure or water vapor data are used; somewhat smaller than the previous estimate. The mean mass of water vapor is estimated as 1.27×1016 kg and the dry air mass as 5.1352 ±0.0003×1018 kg." Optical properties Solar radiation (or sunlight) is the energy Earth receives from the Sun. Earth also emits radiation back into space, but at longer wavelengths that humans cannot see. As energy propagates through the atmosphere, it is impacted by the process of radiative transfer. That is, some of the incoming and emitted radiation is subject to absorption, emission, and scattering by the atmosphere. Another portion of the incident energy is reflected, with the two most important atmospheric reflectors being dust and clouds. Depending on the properties of the aerosol, clouds can reflect up to 70% of the incident radiation. Globally, clouds reflect 20% of the incoming energy, contributing two thirds of the planet's total albedo. In May 2017, glints of light, seen as twinkling from an orbiting satellite a million miles away, were found to be reflected light from ice crystals in the troposphere. When light passes through Earth's atmosphere, photons interact with it through scattering. If the light does not interact with the atmosphere, it is called direct radiation and is what you see if you were to look directly at the Sun. Indirect radiation is light that has been scattered in the atmosphere. For example, on an overcast day when you cannot see your shadow, there is no direct radiation reaching you, it has all been scattered. As another example, due to a phenomenon called Rayleigh scattering, shorter (blue) wavelengths scatter more easily than longer (red) wavelengths. This is why the sky looks blue; you are seeing scattered blue light. This is also why sunsets are red. Because the Sun is close to the horizon, the Sun's rays pass through more atmosphere than normal before reaching your eye. Much of the blue light has been scattered out, leaving the red light in a sunset. Different molecules absorb different wavelengths of radiation. For example, O2 and O3 absorb almost all radiation with wavelengths shorter than 300 nanometres. Water (H2O) absorbs at many wavelengths above 700 nm. When a molecule absorbs a photon, it increases the energy of the molecule. This heats the atmosphere, but the atmosphere also cools by emitting radiation, as discussed below. In astronomical spectroscopy, the absorption of specific frequencies by the atmosphere is referred to as telluric contamination. The combined absorption spectra of the gases in the atmosphere leave "windows" of low opacity, allowing the transmission of only certain bands of light. The optical window runs from around 300 nm (ultraviolet-C) up into the range humans can see, the visible spectrum (commonly called light), at roughly 400–700 nm and continues to the infrared to around 1100 nm. There are also infrared and radio windows that transmit some infrared and radio waves at longer wavelengths. For example, the radio window runs from about one centimetre to about eleven-metre waves. Emission is the opposite of absorption, it is when an object emits radiation. Objects tend to emit amounts and wavelengths of radiation depending on their "black body" emission curves, therefore hotter objects tend to emit more radiation, with shorter wavelengths. Colder objects emit less radiation, with longer wavelengths. For example, the Sun is approximately 6,000 K (5,730 °C; 10,340 °F), its radiation peaks near 500 nm, and is visible to the human eye. Earth is approximately 290 K (17 °C; 62 °F), so its radiation peaks near 10,000 nm, and is much too long to be visible to humans. Because of its temperature, the atmosphere emits infrared radiation. For example, on clear nights Earth's surface cools down faster than on cloudy nights. This is because clouds (H2O) are strong absorbers and emitters of infrared radiation. This is also why it becomes colder at night at higher elevations. The greenhouse effect is directly related to this absorption and emission effect. Some gases in the atmosphere absorb and emit infrared radiation, but do not interact in this manner with sunlight in the visible spectrum. Common examples of these are CO2 and H2O. Without greenhouse gases in the atmosphere, the average temperature of Earth's surface would be a frozen −18 °C (0 °F), rather than the present comfortable average of 15 °C (59 °F). The refractive index of air is close to, but just greater than, 1. Systematic variations in the refractive index can lead to the bending of light rays over long optical paths. One example is that, under some circumstances, observers on board ships can see other vessels just over the horizon because light is refracted in the same direction as the curvature of Earth's surface. The refractive index of air depends on temperature, giving rise to refraction effects when the temperature gradient is large. An example of such effects is the mirage. Circulation Atmospheric circulation is the large-scale movement of air through the troposphere, and the means (with ocean circulation) by which heat is distributed around Earth. The large-scale structure of the atmospheric circulation varies from year to year, but the basic structure remains fairly constant because it is determined by Earth's rotation rate and the difference in solar radiation between the equator and poles. The axial tilt of the planet means the location of maximum heat is continually changing, resulting in seasonal variations. The uneven distribution of land and water further breaks up the flow of air. The flow of air around the planet is divided into three main convection cells by latitude. Around the equator, the Hadley cell is driven by the rising flow of air along the equator. In the upper atmosphere, this air flows toward the poles. At mid latitudes, this circulation is reversed, with ground air flowing toward the poles with the Ferrel cell. Finally, in the high latitudes is the Polar cell, where air again rises and flows toward the poles. The interface between these cells is responsible for jet streams. These are narrow, fast moving bands that flow from west to east and typically form at an elevation of around 9,100 m (30,000 ft). Jet streams can shift around depending on conditions. They are strongest in winter, when the boundaries between hot and cold air are the most pronounced. In the middle latitudes, it is instabilities in the jet streams that are responsible for moving weather systems. As with the oceans, the Earth's atmosphere is subject to waves and tidal forces. These are triggered by non-uniform heating by the Sun, and by the daily solar cycle, respectively. Wave-like behavior can occur on a variety of scales, from smaller gravity waves that transfer momentum into the higher atmospheric layers, to much larger planetary waves, or Rossby waves. Atmospheric tides are periodic oscillations of the troposphere and stratosphere that transport energy to the upper atmosphere. Evolution of Earth's atmosphere The first atmosphere, during the Early Earth's Hadean eon, consisted of gases in the solar nebula, primarily hydrogen, and probably simple hydrides such as those now found in the gas giants (Jupiter and Saturn), notably water vapor, methane and ammonia. During this earliest era, the Moon-forming collision and numerous impacts with large meteorites heated the atmosphere, driving off the most volatile gases. The collision with Theia, in particular, melted and ejected large portions of Earth's mantle and crust and outgassed significant amounts of steam which eventually cooled and condensed to contribute to ocean water at the end of the Hadean.: 10 The increasing solidification of Earth's crust at the end of the Hadean closed off most of the advective heat transfer to the surface, causing the atmosphere to cool, which condensed most of the water vapor out of the air precipitating into a superocean. Further outgassing from volcanism, supplemented by gases introduced by huge asteroids during the Late Heavy Bombardment, created the subsequent Archean atmosphere, which consisted largely of nitrogen plus carbon dioxide, methane and inert gases. A major part of carbon dioxide emissions dissolved in water and reacted with metals such as calcium and magnesium during weathering of crustal rocks to form carbonates that were deposited as sediments. Water-related sediments have been found that date from as early as 3.8 billion years ago. About 3.4 billion years ago, nitrogen formed the major component of the then-stable "second atmosphere". The influence of the evolution of life has to be taken into account rather soon in the history of the atmosphere because hints of earliest life forms appeared as early as 3.5 billion years ago. How Earth at that time maintained a climate warm enough for liquid water and life, if the early Sun put out 30% lower solar radiance than today, is a puzzle known as the "faint young Sun paradox". The geological record however shows a continuous relatively warm surface during the complete early temperature record of Earth – with the exception of one cold glacial phase about 2.4 billion years ago. In the late Neoarchean, an oxygen-containing atmosphere began to develop, apparently due to a billion years of cyanobacterial photosynthesis (known as the Great Oxygenation Event), which have been found as stromatolite fossils from 2.7 billion years ago. The early basic carbon isotopy (isotope ratio proportions) strongly suggests conditions similar to the current, and that the fundamental features of the carbon cycle became established as early as 4 billion years ago. Ancient sediments in the Gabon dating from between about 2.15 and 2.08 billion years ago provide a record of Earth's dynamic oxygenation evolution. These fluctuations in oxygenation were likely driven by the Lomagundi-Jatuli Carbon Isotope Excursion. The constant re-arrangement of continents by plate tectonics influences the long-term evolution of the atmosphere by transferring carbon dioxide to and from large continental carbonate stores. Free oxygen did not exist in the atmosphere until about 2.4 billion years ago during the Great Oxygenation Event and its appearance is indicated by the end of banded iron formations (which signals the depletion of substrates that can react with oxygen to produce ferric deposits) during the early Proterozoic eon. Before this time, any oxygen produced by cyanobacterial photosynthesis would be readily removed by the oxidation of reducing substances on the Earth's surface, notably ferrous iron, sulfur and atmospheric methane. Free oxygen molecules did not start to accumulate in the atmosphere until the rate of production of oxygen began to exceed the availability of reductant materials that removed oxygen. This point signifies a shift from a reducing atmosphere to an oxidizing atmosphere. O2 showed major variations during the Proterozoic, including a billion-year period of euxinia, until reaching a steady state of more than 15% by the end of the Precambrian. The rise of the more robust eukaryotic photoautotrophs (green and red algae) injected further oxygenation into the air, especially after the end of the Cryogenian global glaciation, which was followed by an evolutionary radiation event during the Ediacaran period known as the Avalon explosion, where complex metazoan life forms (including the earliest cnidarians, placozoans and bilaterians) first proliferated. The following time span from 539 million years ago to the present day is the Phanerozoic eon, during the earliest period of which, the Cambrian, more actively moving metazoan life began to appear and rapidly diversify in another radiation event called the Cambrian explosion, whose locomotive metabolism was fuelled by the rising oxygen level. The amount of oxygen in the atmosphere has fluctuated over the last 600 million years, reaching a peak of about 35% around 280 million years ago during the Carboniferous period, significantly higher than today's 21%. Two main processes govern changes in the atmosphere: the evolution of plants and their increasing role in carbon fixation, and the consumption of oxygen by rapidly diversifying animal faunae and also by plants for photorespiration and their own metabolic needs at night. Breakdown of pyrite and volcanic eruptions release sulfur into the atmosphere, which reacts and hence reduces oxygen in the atmosphere. However, volcanic eruptions also release carbon dioxide, which can fuel oxygenic photosynthesis by terrestrial and aquatic plants. The cause of the variation of the amount of oxygen in the atmosphere is not precisely understood. Periods with more oxygen in the atmosphere were often associated with more rapid development of animals. Air pollution Air pollution is the introduction of airborne chemicals, particulate matter or biological materials that cause harm or discomfort to organisms. The population growth, industrialization and motorization of human societies have significantly increased the amount of airborne pollutants in the Earth's atmosphere, causing noticeable problems such as smogs, acid rains and pollution-related diseases. The depletion of the stratospheric ozone layer, which shields the surface from harmful ionizing ultraviolet radiations, is also caused by air pollution, chiefly from chlorofluorocarbons and other ozone-depleting substances. Since 1750, human activity, especially after the Industrial Revolution, has increased the concentrations of various greenhouse gases, most importantly carbon dioxide, methane and nitrous oxide. Greenhouse gas emissions, coupled with deforestation and destruction of wetlands via logging and land developments, have caused an observed rise in global temperatures, with the global average surface temperatures being 1.1 °C higher in the 2011–2020 decade than they were in 1850. It has raised concerns of man-made climate change, which can have significant environmental impacts such as sea level rise, ocean acidification, glacial retreat (which threatens water security), increasing extreme weather events and wildfires, ecological collapse and mass dying of wildlife. See also References External links |
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[SOURCE: https://en.wikipedia.org/wiki/Prebiotic_atmosphere] | [TOKENS: 3519] |
Contents Prebiotic atmosphere The prebiotic atmosphere is the second atmosphere present on Earth before today's biotic, oxygen-rich third atmosphere, and after the first atmosphere (which was mainly water vapor and simple hydrides) of Earth's formation. The formation of the Earth, roughly 4.5 billion years ago, involved multiple collisions and coalescence of planetary embryos. This was followed by an over 100 million year period on Earth where a magma ocean was present, the atmosphere was mainly steam, and surface temperatures reached up to 8,000 K (14,000 °F). Earth's surface then cooled and the atmosphere stabilized, establishing the prebiotic atmosphere. The environmental conditions during this time period were quite different from today: the Sun was about 30% dimmer overall yet brighter at ultraviolet and x-ray wavelengths; there was a liquid ocean; it is unknown if there were continents but oceanic islands were likely; Earth's interior chemistry (and thus, volcanic activity) was different; there was a larger flux of impactors (e.g. comets and asteroids) hitting Earth's surface. Studies have attempted to constrain the composition and nature of the prebiotic atmosphere by analyzing geochemical data and using theoretical models that include our knowledge of the early Earth environment. These studies indicate that the prebiotic atmosphere likely contained more CO2 than the modern Earth, had N2 within a factor of 2 of the modern levels, and had vanishingly low amounts of O2. The atmospheric chemistry is believed to have been "weakly reducing", where reduced gases like CH4, NH3, and H2 were present in small quantities. The composition of the prebiotic atmosphere was likely periodically altered by impactors, which may have temporarily caused the atmosphere to have been "strongly reduced". Constraining the composition of the prebiotic atmosphere is key to understanding the origin of life, as it may facilitate or inhibit certain chemical reactions on Earth's surface believed to be important for the formation of the first living organism. Life on Earth originated and began modifying the atmosphere at least 3.5 billion years ago and possibly much earlier, which marks the end of the prebiotic atmosphere. Environmental context Earth is believed to have formed over 4.5 billion years ago by accreting material from the solar nebula. Earth's Moon formed in a collision, the Moon-forming impact, believed to have occurred 30-50 million years after the Earth formed. In this collision, a Mars-sized object named Theia collided with the primitive Earth and the remnants of the collision formed the Moon. The collision likely supplied enough energy to melt most of Earth's mantle and vaporize roughly 20% of it, heating Earth's surface to as high as 8,000 K (~14,000 °F). Earth's surface in the aftermath of the Moon-forming impact was characterized by high temperatures (~2,500 K), an atmosphere made of rock vapor and steam, and a magma ocean. As the Earth cooled by radiating away the excess energy from the impact, the magma ocean solidified and volatiles were partitioned between the mantle and atmosphere until a stable state was reached. It is estimated that Earth transitioned from the hot, post-impact environment into a potentially habitable environment with crustal recycling, albeit different from modern plate tectonics, roughy 10-20 million years after the Moon-forming impact, around 4.4 billion years ago. The atmosphere present from this point in Earth's history until the origin of life is referred to as the prebiotic atmosphere. It is unknown when exactly life originated. The oldest direct evidence for life on Earth is around 3.5 billion years old, such as fossil stromatolites from North Pole, Western Australia. Putative evidence of life on Earth from older times (e.g. 3.8 and 4.1 billion years ago) lacks additional context necessary to claim it is truly of biotic origin, so it is still debated. Thus, the prebiotic atmosphere concluded 3.5 billion years ago or earlier, placing it in the early Archean Eon or mid-to-late Hadean Eon. Knowledge of the environmental factors at play on early Earth is required to investigate the prebiotic atmosphere. Much of what we know about the prebiotic environment comes from zircons - crystals of zirconium silicate (ZrSiO4). Zircons are useful because they record the physical and chemical processes occurring on the prebiotic Earth during their formation and they are especially durable. Most zircons that are dated to the prebiotic time period are found at the Jack Hills formation of Western Australia, but they also occur elsewhere. Geochemical data from several prebiotic zircons show isotopic evidence for chemical change induced by liquid water, indicating that the prebiotic environment had a liquid ocean and a surface temperature that did not cause it to freeze or boil. It is unknown when exactly the continents emerged above this liquid ocean. This adds uncertainty to the interaction between Earth's prebiotic surface and atmosphere, as the presence of exposed land determines the rate of weathering processes and provides local environments that may be necessary for life to form. However, oceanic islands were likely. Additionally, the oxidation state of Earth's mantle was likely different at early times, which changes the fluxes of chemical species delivered to the atmosphere from volcanic outgassing. Environmental factors from elsewhere in the Solar System also affected prebiotic Earth. The Sun was ~30% dimmer overall around the time the Earth formed. This means greenhouse gases may have been required in higher levels than present day to keep Earth from freezing over. Despite the overall reduction in energy coming from the Sun, the early Sun emitted more radiation in the ultraviolet and x-ray regimes than it currently does. This indicates that different photochemical reactions may have dominated early Earth's atmosphere, which has implications for global atmospheric chemistry and the formation of important compounds that could lead to the origin of life. Finally, there was a significantly higher flux of objects that impacted Earth - such as comets and asteroids - in the early Solar System. These impactors may have been important in the prebiotic atmosphere because they can deliver material to the atmosphere, eject material from the atmosphere, and change the chemical nature of the atmosphere after their arrival. Atmospheric composition The exact composition of the prebiotic atmosphere is unknown due to the lack of geochemical data from the time period. Current studies generally indicate that the prebiotic atmosphere was "weakly reduced", with elevated levels of CO2, N2 within a factor of 2 of the modern level, negligible amounts of O2, and more hydrogen-bearing gases than the modern Earth (see below). Noble gases and photochemical products of the dominant species were also present in small quantities. Carbon dioxide (CO2) is an important component of the prebiotic atmosphere because, as a greenhouse gas, it strongly affects the surface temperature; also, it dissolves in water and can change the ocean pH. The abundance of carbon dioxide in the prebiotic atmosphere is not directly constrained by geochemical data and must be inferred. Evidence suggests that the carbonate-silicate cycle regulates Earth's atmospheric carbon dioxide abundance on timescales of about 1 million years. The carbonate-silicate cycle is a negative feedback loop that modulates Earth's surface temperature by partitioning carbon between the atmosphere and the mantle via several surface processes. It has been proposed that the processes of the carbonate-silicate cycle would result in high CO2 levels in the prebiotic atmosphere to offset the lower energy input from the faint young Sun. This mechanism can be used to estimate the prebiotic CO2 abundance, but it is debated and uncertain. Uncertainty is primarily driven by a lack of knowledge about the area of exposed land, early Earth's interior chemistry and structure, the rate of reverse weathering and seafloor weathering, and the increased impactor flux. One extensive modeling study suggests that CO2 was roughly 20 times higher in the prebiotic atmosphere than the preindustrial modern value (280 ppm), which would result in a global average surface temperature around 259 K (6.5 °F) and an ocean pH around 7.9. This is in agreement with other studies, which generally conclude that the prebiotic atmospheric CO2 abundance was higher than the modern one, although the global surface temperature may still be significantly colder due to the faint young Sun. Nitrogen in the form of N2 is 78% of Earth's modern atmosphere by volume, making it the most abundant gas. N2 is generally considered a background gas in the Earth's atmosphere because it is relatively unreactive due to the strength of its triple bond. Despite this, atmospheric N2 was at least moderately important to the prebiotic environment because it impacts the climate via Rayleigh scattering and it may have been more photochemically active under the enhanced x-ray and ultraviolet radiation from the young Sun. N2 was also likely important for the synthesis of compounds believed to be critical for the origin of life, such as hydrogen cyanide (HCN) and amino acids derived from HCN. Studies have attempted to constrain the prebiotic atmosphere N2 abundance with theoretical estimates, models, and geologic data. These studies have resulted in a range of possible constraints on the prebiotic N2 abundance. For example, a recent modeling study that incorporates atmospheric escape, magma ocean chemistry, and the evolution of Earth's interior chemistry suggests that the atmospheric N2 abundance was probably less than half of the present day value. However, this study fits into a larger body of work that generally constrains the prebiotic N2 abundance to be between half and double the present level. Oxygen in the form of O2 makes up 21% of Earth's modern atmosphere by volume. Earth's modern atmospheric O2 is due almost entirely to biology (e.g. it is produced during oxygenic photosynthesis), so it was not nearly as abundant in the prebiotic atmosphere. This is favorable for the origin of life, as O2 would oxidize organic compounds needed in the origin of life. The prebiotic atmosphere O2 abundance can be theoretically calculated with models of atmospheric chemistry. The primary source of O2 in these models is the breakdown and subsequent chemical reactions of other oxygen containing compounds. Incoming solar photons or lightning can break up CO2 and H2O molecules, freeing oxygen atoms and other radicals (i.e. highly reactive gases in the atmosphere). The free oxygen can then combine into O2 molecules via several chemical pathways. The rate at which O2 is created in this process is determined by the incoming solar flux, the rate of lightning, and the abundances of the other atmospheric gases that take part in the chemical reactions (e.g. CO2, H2O, OH), as well as their vertical distributions. O2 is removed from the atmosphere via photochemical reactions that mainly involve H2 and CO near the surface. The most important of these reactions starts when H2 is split into two H atoms by incoming solar photons. The free H then reacts with O2 and eventually forms H2O, resulting in a net removal of O2 and a net increase in H2O. Models that simulate all of these chemical reactions in a potential prebiotic atmosphere show that an extremely small atmospheric O2 abundance is likely. In one such model that assumed values for CO2 and H2 abundances and sources, the O2 volume mixing ratio is calculated to be between 10−18 and 10−11 near the surface and up to 10−4 in the upper atmosphere. The hydrogen abundance in the prebiotic atmosphere can be viewed from the perspective of reduction-oxidation (redox) chemistry. The modern atmosphere is oxidizing, due to the large volume of atmospheric O2. In an oxidizing atmosphere, the majority of atoms that form atmospheric compounds (e.g. C) will be in an oxidized form (e.g. CO2) instead of a reduced form (e.g. CH4). In a reducing atmosphere, more species will be in their reduced, generally hydrogen-bearing forms. Because there was very little O2 in the prebiotic atmosphere, it is generally believed that the prebiotic atmosphere was "weakly reduced" - although some argue that the atmosphere was "strongly reduced". In a weakly reduced atmosphere, reduced gases (e.g. CH4 and NH3) and oxidized gases (e.g CO2) are both present. The actual H2 abundance in the prebiotic atmosphere has been estimated by doing a calculation that takes into account the rate at which H2 is volcanically outgassed to the surface and the rate at which it escapes to space. One of these recent calculations indicates that the prebiotic atmosphere H2 abundance was around 400 parts per million, but could have been significantly higher if the source from volcanic outgassing was enhanced or atmospheric escape was less efficient than expected. The abundances of other reduced species in the atmosphere can then be calculated with models of atmospheric chemistry. Post-impact atmospheres It has been proposed that the large flux of impactors in the early solar system may have significantly changed the nature of the prebiotic atmosphere. During the time period of the prebiotic atmosphere, it is expected that a few asteroid impacts large enough to vaporize the oceans and melt Earth's surface could have occurred, with smaller impacts expected in even larger numbers. These impacts would have significantly changed the chemistry of the prebiotic atmosphere by heating it up, ejecting some of it to space, and delivering new chemical material. Studies of post-impact atmospheres indicate that they would have caused the prebiotic atmosphere to be strongly reduced for a period of time after a large impact. On average, impactors in the early solar system contained highly reduced minerals (e.g. metallic iron) and were enriched with reduced compounds that readily enter the atmosphere as a gas. In these strongly reduced post-impact atmospheres, there would be significantly higher abundances of reduced gases like CH4, HCN, and perhaps NH3. Reduced, post-impact atmospheres after the ocean condensed are predicted to last up to tens of millions of years before returning to the background state. Model studies haved refined this by dividing post-impact evolution into three phases: initial H2 production from iron-steam reactions, cooling with CH4 and NH3 formation (catalyzed by nickel surfaces), and long-term photochemical production of nitriles. When CH4 to CO2 ratio > 0.1, hazy atmospheres with HCN/HCCCN rainout up to 109 molecules per cm2 per second; smaller CH4 to CO2 ratios yield negligible HCCCN. Such production of nitriles would continue until H2 escape to space on the order of a few million years. Minimum masses for effective reduction are 4×1020–5×1021 kg, depending on iron efficiency and melt equilibration. In additional to the nitrile bombardment hypothesis, other studies find that serpentinization from deep mantle processes may have been sufficient on their own to produce HCN an order of magnitude less than the bombardment mechanism—though without HCCCN. Relationship to the origin of life The prebiotic atmosphere can supply chemical ingredients and facilitate environmental conditions that contribute to the synthesis of organic compounds involved in the origin of life. For example, compounds potentially involved in the origin of life were synthesized in the Miller-Urey experiment. In this experiment, assumptions must be made about what gases were present in the prebiotic atmosphere. Proposed important ingredients for the origin of life include (but are not limited to) methane (CH4), ammonia (NH3), phosphate (PO43-), hydrogen cyanide (HCN), cyanoacetylene (HCCCN), various organics, and various photochemical byproducts. The atmospheric composition will impact the stability and production of these compounds at Earth's surface. For example, the "weakly reduced" prebiotic atmosphere may produce some, but not all, of these ingredients via reactions with lightning. On the other hand, the production and stability of origin of life ingredients in a strongly reduced atmosphere are greatly enhanced, making post-impact atmospheres particularly relevant. It is also proposed that the conditions required for the origin of life could have emerged locally, in a system that is isolated from the atmosphere (e.g. a hydrothermal vent). Arguments against this hypothesis have emphasized that compounds such as cyanides used to make nucleobases of RNA would be too dilute in the ocean, unlike lakes on land which might readily store them as ferrocyanide salts. This may be overcome by imposing a boundary condition such as shallow water vents that experienced localized evaporative cycles. The vent mechanism might also produce HCCCN, but would require extremely high pressure and temperature for efficient stockpiling. Methods that readily produce HCCCN are important as it is a required constituent in the current best understanding of pyrimidine synthesis. Once life originated and began interacting with the atmosphere, the prebiotic atmosphere transitioned into the post-biotic atmosphere, by definition. References |
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[SOURCE: https://en.wikipedia.org/wiki/History_of_Earth] | [TOKENS: 12224] |
Contents History of Earth The natural history of Earth concerns the development of planet Earth from its formation to the present day. Nearly all branches of natural science have contributed to understanding of the main events of Earth's past, characterized by constant geological change and biological evolution. The geological time scale (GTS), as defined by international convention, depicts the large spans of time from the beginning of Earth to the present, and its divisions chronicle some definitive events of Earth history. Earth formed around 4.54 billion years ago, approximately one-third the age of the universe, by accretion from the solar nebula. Volcanic outgassing probably created the primordial atmosphere and then the ocean, but the early atmosphere contained almost no oxygen. Much of Earth was molten because of frequent collisions with other bodies which led to extreme volcanism. While Earth was in its earliest stage (Early Earth), a giant impact collision with a planet-sized body named Theia is thought to have formed the Moon. Over time, Earth cooled, causing the formation of a solid crust, and allowing liquid water on the surface. The Hadean eon represents the time before a reliable (fossil) record of life; it began with the formation of the planet and ended 4.0 billion years ago. The following Archean and Proterozoic eons produced the beginnings of life on Earth and its earliest evolution. The succeeding eon is the Phanerozoic, divided into three eras: the Palaeozoic, an era of arthropods, fishes, and the first life on land; the Mesozoic, which spanned the rise, reign, and climactic extinction of the non-avian dinosaurs; and the Cenozoic, which saw the rise of mammals. Recognizable humans emerged at most 2 million years ago, a vanishingly small period on the geological scale. The earliest undisputed evidence of life on Earth dates at least from 3.5 billion years ago, during the Eoarchean Era, after a geological crust started to solidify following the earlier molten Hadean eon. There are microbial mat fossils such as stromatolites found in 3.48 billion-year-old sandstone discovered in Western Australia. Other early physical evidence of a biogenic substance is graphite in 3.7 billion-year-old metasedimentary rocks discovered in southwestern Greenland as well as "remains of biotic life" found in 4.1 billion-year-old rocks in Western Australia. According to one of the researchers, "If life arose relatively quickly on Earth … then it could be common in the universe." Photosynthetic organisms appeared between 3.2 and 2.4 billion years ago and began enriching the atmosphere with oxygen. Life remained mostly small and microscopic until about 580 million years ago, when complex multicellular life arose, developed over time, and culminated in the Cambrian Explosion about 538.8 million years ago. This sudden diversification of life forms produced most of the major phyla known today, and divided the Proterozoic Eon from the Cambrian Period of the Paleozoic Era. It is estimated that 99 percent of all species that ever lived on Earth, over five billion, have gone extinct. Estimates on the number of Earth's current species range from 10 million to 14 million, of which about 1.2 million are documented, but over 86 percent have not been described. Earth's crust has constantly changed since its formation, as has life since its first appearance. Species continue to evolve, taking on new forms, splitting into daughter species, or going extinct in the face of ever-changing physical environments. The process of plate tectonics continues to shape Earth's continents and oceans and the life they harbor. Eons In geochronology, time is generally measured in mya (million years ago), each unit representing the period of approximately 1,000,000 years in the past. The history of Earth is divided into four great eons, starting 4,540 mya with the formation of the planet. Each eon saw the most significant changes in Earth's composition, climate and life. Each eon is subsequently divided into eras, which in turn are divided into periods, which are further divided into epochs. Geologic time scale The history of Earth can be organized chronologically according to the geologic time scale, which is split into intervals based on stratigraphic analysis. The following five timelines show the geologic time scale to scale. The first shows the entire time from the formation of Earth to the present, but this gives little space for the most recent eon. The second timeline shows an expanded view of the most recent eon. In a similar way, the most recent era is expanded in the third timeline, the most recent period is expanded in the fourth timeline, and the most recent epoch is expanded in the fifth timeline. (Horizontal scale is millions of years for the above timelines; thousands of years for the timeline below) Solar System formation The standard model for the formation of the Solar System (including Earth) is the solar nebula hypothesis. In this model, the Solar System formed from a large, rotating cloud of interstellar dust and gas called the solar nebula. It was composed of hydrogen and helium created shortly after the Big Bang 13.8 Ga (billion years ago) and heavier elements ejected by supernovae. About 4.5 Ga, the nebula began a contraction that may have been triggered by the shock wave from a nearby supernova. A shock wave would have also made the nebula rotate. As the cloud began to accelerate, its angular momentum, gravity, and inertia flattened it into a protoplanetary disk perpendicular to its axis of rotation. Small perturbations due to collisions and the angular momentum of other large debris created the means by which kilometer-sized protoplanets began to form, orbiting the nebular center. The center of the nebula, not having much angular momentum, collapsed rapidly, the compression heating it until nuclear fusion of hydrogen into helium began. After more contraction, a T Tauri star ignited and evolved into the Sun. Meanwhile, in the outer part of the nebula gravity caused matter to condense around density perturbations and dust particles, and the rest of the protoplanetary disk began separating into rings. In a process known as runaway accretion, successively larger fragments of dust and debris clumped together to form planets. Earth formed in this manner about 4.54 billion years ago (with an uncertainty of 1%) and was largely completed within 10–20 million years. In June 2023, scientists reported evidence that the planet Earth may have formed in just three million years, much faster than the 10−100 million years thought earlier. Nonetheless, the solar wind of the newly formed T Tauri star cleared out most of the material in the disk that had not already condensed into larger bodies. The same process is expected to produce accretion disks around virtually all newly forming stars in the universe, some of which yield planets. The proto-Earth grew by accretion until its interior was hot enough to melt the heavy, siderophile metals. Having higher densities than the silicates, these metals sank. This so-called iron catastrophe resulted in the separation of a primitive mantle and a (metallic) core only 10 million years after Earth began to form, producing the layered structure of Earth and setting up the formation of Earth's magnetic field. J.A. Jacobs was the first to suggest that Earth's inner core—a solid center distinct from the liquid outer core—is freezing and growing out of the liquid outer core due to the gradual cooling of Earth's interior (about 100 degrees Celsius per billion years). Hadean and Archean Eons The first eon in Earth's history, the Hadean, begins with Earth's formation and is followed by the Archean eon at 3.8 Ga.: 145 The oldest rocks found on Earth date to about 4.0 Ga, and the oldest detrital zircon crystals in rocks to about 4.4 Ga, soon after the formation of Earth's crust and Earth itself. The giant impact hypothesis for the Moon's formation states that shortly after formation of an initial crust, the proto-Earth was impacted by a smaller protoplanet, which ejected part of the mantle and crust into space and created the Moon. From crater counts on other celestial bodies, it is inferred that a period of intense meteorite impacts, called the Late Heavy Bombardment, began about 4.1 Ga, and concluded around 3.8 Ga, at the end of the Hadean. In addition, volcanism was severe due to the large heat flow and geothermal gradient. Nevertheless, detrital zircon crystals dated to 4.4 Ga show evidence of having undergone contact with liquid water, suggesting that Earth already had oceans or seas at that time. By the beginning of the Archean, Earth had cooled significantly. Present life forms could not have survived at Earth's surface, because the Archean atmosphere lacked oxygen hence had no ozone layer to block ultraviolet light. Nevertheless, it is believed that primordial life began to evolve by the early Archean, with candidate fossils dated to around 3.5 Ga. Some scientists even speculate that life could have begun during the early Hadean, as far back as 4.4 Ga, surviving the possible Late Heavy Bombardment period in hydrothermal vents below Earth's surface. Earth's only natural satellite, the Moon, is larger relative to its planet than any other satellite in the Solar System.[nb 1] During the Apollo program, rocks from the Moon's surface were brought to Earth. Radiometric dating of these rocks shows that the Moon is 4.53 ± 0.01 billion years old, formed at least 30 million years after the Solar System. New evidence suggests the Moon formed even later, 4.48 ± 0.02 Ga, or 70–110 million years after the start of the Solar System. Theories for the formation of the Moon must explain its late formation as well as the following facts. First, the Moon has a low density (3.3 times that of water, compared to 5.5 for Earth) and a small metallic core. Second, Earth and Moon have the same oxygen isotopic signature (relative abundance of the oxygen isotopes). Of the theories proposed to account for these phenomena, one is widely accepted: The giant impact hypothesis proposes that the Moon originated after a body the size of Mars (sometimes named Theia) struck the proto-Earth a glancing blow.: 256 The collision released about 100 million times more energy than the more recent Chicxulub impact that is believed to have caused the extinction of the non-avian dinosaurs. It was enough to vaporize some of Earth's outer layers and melt both bodies.: 256 A portion of the mantle material was ejected into orbit around Earth. The giant impact hypothesis predicts that the Moon was depleted of metallic material, explaining its abnormal composition. The ejecta in orbit around Earth could have condensed into a single body within a couple of weeks. Under the influence of its own gravity, the ejected material became a more spherical body: the Moon. Plate tectonics is driven by mantle convection, the flow of heated rock from Earth's interior to the surface.: 2 The rising mantle extruded at mid-oceanic ridges builds up rigid tectonic plates, which are eventually shifted to subduction zones where they sink back into the mantle. During the early Archean (about 3.0 Ga) the mantle was probably around 1,600 °C (2,910 °F) hotter than today,: 82 so convection in the mantle was faster, leading to a faster tectonic process during the Hadean and Archean. Subduction zones were more common and tectonic plates smaller.: 258 The initial crust, which formed when Earth's surface first solidified, totally disappeared from a combination of this fast Hadean plate tectonics and the intense impacts of the Late Heavy Bombardment. It is thought the original crust was basaltic like today's oceanic crust, because little crustal differentiation had yet taken place.: 258 The first larger pieces of continental crust, formed of lighter elements which float upward during partial melting in the lower crust, appeared at the end of the Hadean, about 4.0 Ga. What is left of these first small continents are called cratons; they form the cores around which today's continents grew. The oldest rocks on Earth are found in the North American craton of Canada. They are tonalites from about 4.0 Ga. They show traces of metamorphism by high temperature, but also sedimentary grains that have been rounded by erosion during transport by water, showing that rivers and seas existed then. Cratons consist primarily of two alternating types of terranes. The first are so-called greenstone belts, consisting of low-grade metamorphosed sedimentary rocks. These "greenstones" are similar to the sediments today found in oceanic trenches, above subduction zones, suggesting the start of subduction during the Archean. The second type is a complex of felsic magmatic rocks, mostly tonalite, trondhjemite or granodiorite, similar to granite: such terranes are called TTG-terranes. TTG-complexes are seen as the relicts of the first continental crust, formed by partial melting in basalt.: Chapter 5 Earth is often described as having had three atmospheres. The first, captured from the solar nebula, was composed of light (atmophile) elements from the solar nebula, mostly hydrogen and helium. A combination of the solar wind and Earth's heat would have driven off this atmosphere, and today's atmosphere is depleted of these elements compared to cosmic abundances. After the impact which created the Moon, the molten Earth released volatile gases; and later more gases were released by volcanoes, completing a second atmosphere rich in greenhouse gases but poor in oxygen.: 256 Finally, the third atmosphere, rich in oxygen, emerged when bacteria began to produce oxygen about 2.8 Ga.: 83–84, 116–117 In early models for the formation of the atmosphere and ocean, the second atmosphere was formed by outgassing of volatiles from Earth's interior. Now it is considered likely that many of the volatiles were delivered during accretion by a process known as impact degassing in which incoming bodies vaporize on impact. The ocean and atmosphere would, therefore, have started to form even as Earth formed. The new atmosphere probably contained water vapor, carbon dioxide, nitrogen, and smaller amounts of other gases. Planetesimals at a distance of 1 astronomical unit (AU), the distance of Earth from the Sun, probably did not contribute any water to Earth because the solar nebula was too hot for ice to form and the hydration of rocks by water vapor would have taken too long. The water must have been supplied by meteorites from the outer asteroid belt and some large planetary embryos from beyond 2.5 AU. Comets may also have contributed. Though most comets are today in orbits farther away from the Sun than Neptune, computer simulations show that they were originally far more common in the inner parts of the Solar System.: 130–132 As Earth cooled, clouds formed. Rain created the oceans. Recent evidence suggests the oceans may have begun forming as early as 4.4 Ga. By the start of the Archean eon, they already covered much of Earth. This early formation has been difficult to explain because of a problem known as the faint young Sun paradox. Stars are known to get brighter as they age, and the Sun has become 30% brighter since its formation 4.5 billion years ago. Many models indicate that the early Earth should have been covered in ice. A likely solution is that there was enough carbon dioxide and methane to produce a greenhouse effect. The carbon dioxide would have been produced by volcanoes and the methane by early microbes. It is hypothesized that there also existed an organic haze created from the products of methane photolysis that caused an anti-greenhouse effect as well. Another greenhouse gas, ammonia, would have been ejected by volcanos but quickly destroyed by ultraviolet radiation.: 83 One of the reasons for interest in the early atmosphere and ocean is that they form the conditions under which life first arose. There are many models, but little consensus, on how life emerged from non-living chemicals; chemical systems created in the laboratory fall well short of the minimum complexity for a living organism. The first step in the emergence of life may have been chemical reactions that produced many of the simpler organic compounds, including nucleobases and amino acids, that are the building blocks of life. An experiment in 1952 by Stanley Miller and Harold Urey showed that such molecules could form in an atmosphere of water, methane, ammonia and hydrogen with the aid of sparks to mimic the effect of lightning. Although atmospheric composition was probably different from that used by Miller and Urey, later experiments with more realistic compositions also managed to synthesize organic molecules. Computer simulations show that extraterrestrial organic molecules could have formed in the protoplanetary disk before the formation of Earth. Additional complexity could have been reached from at least three possible starting points: self-replication, an organism's ability to produce offspring that are similar to itself; metabolism, its ability to feed and repair itself; and external cell membranes, which allow food to enter and waste products to leave, but exclude unwanted substances. Even the simplest members of the three modern domains of life use DNA to record their "recipes" and a complex array of RNA and protein molecules to "read" these instructions and use them for growth, maintenance, and self-replication. The discovery that a kind of RNA molecule called a ribozyme can catalyze both its own replication and the construction of proteins led to the hypothesis that earlier life-forms were based entirely on RNA. They could have formed an RNA world in which there were individuals but no species, as mutations and horizontal gene transfers would have meant that the offspring in each generation were quite likely to have different genomes from those that their parents started with. RNA would later have been replaced by DNA, which is more stable and therefore can build longer genomes, expanding the range of capabilities a single organism can have. Ribozymes remain as the main components of ribosomes, the "protein factories" of modern cells. Although short, self-replicating RNA molecules have been artificially produced in laboratories, doubts have been raised about whether natural non-biological synthesis of RNA is possible. The earliest ribozymes may have been formed of simpler nucleic acids such as PNA, TNA or GNA, which would have been replaced later by RNA. Other pre-RNA replicators have been posited, including crystals: 150 and even quantum systems. In 2003 it was proposed that porous metal sulfide precipitates would assist RNA synthesis at about 100 °C (212 °F) and at ocean-bottom pressures near hydrothermal vents. In this hypothesis, the proto-cells would be confined in the pores of the metal substrate until the later development of lipid membranes. Another long-standing hypothesis is that the first life was composed of protein molecules. Amino acids, the building blocks of proteins, are easily synthesized in plausible prebiotic conditions, as are small peptides (polymers of amino acids) that make good catalysts.: 295–297 A series of experiments starting in 1997 showed that amino acids and peptides could form in the presence of carbon monoxide and hydrogen sulfide with iron sulfide and nickel sulfide as catalysts. Most of the steps in their assembly required temperatures of about 100 °C (212 °F) and moderate pressures, although one stage required 250 °C (482 °F) and a pressure equivalent to that found under 7 kilometers (4.3 mi) of rock. Hence, self-sustaining synthesis of proteins could have occurred near hydrothermal vents. A difficulty with the metabolism-first scenario is finding a way for organisms to evolve. Without the ability to replicate as individuals, aggregates of molecules would have "compositional genomes" (counts of molecular species in the aggregate) as the target of natural selection. However, a recent model shows that such a system is unable to evolve in response to natural selection. It has been suggested that double-walled "bubbles" of lipids like those that form the external membranes of cells may have been an essential first step. Experiments that simulated the conditions of the early Earth have reported the formation of lipids, and these can spontaneously form liposomes, double-walled "bubbles", and then reproduce themselves. Although they are not intrinsically information-carriers as nucleic acids are, they would be subject to natural selection for longevity and reproduction. Nucleic acids such as RNA might then have formed more easily within the liposomes than they would have outside. Some clays, notably montmorillonite, have properties that make them plausible accelerators for the emergence of an RNA world: they grow by self-replication of their crystalline pattern, are subject to an analog of natural selection (as the clay "species" that grows fastest in a particular environment rapidly becomes dominant), and can catalyze the formation of RNA molecules. Although this idea has not become the scientific consensus, it still has active supporters.: 150–158 Research in 2003 reported that montmorillonite could also accelerate the conversion of fatty acids into "bubbles", and that the bubbles could encapsulate RNA attached to the clay. Bubbles can then grow by absorbing additional lipids and dividing. The formation of the earliest cells may have been aided by similar processes. A similar hypothesis presents self-replicating iron-rich clays as the progenitors of nucleotides, lipids and amino acids. It is believed that of this multiplicity of protocells, only one line survived. Current phylogenetic evidence suggests that the last universal ancestor (LUA) lived during the early Archean eon, perhaps 3.5 Ga or earlier. This LUA cell is the ancestor of all life on Earth today. It was probably a prokaryote, possessing a cell membrane and probably ribosomes, but lacking a nucleus or membrane-bound organelles such as mitochondria or chloroplasts. Like modern cells, it used DNA as its genetic code, RNA for information transfer and protein synthesis, and enzymes to catalyze reactions. Some scientists believe that instead of a single organism being the last universal common ancestor, there were populations of organisms exchanging genes by lateral gene transfer. Proterozoic Eon The Proterozoic eon lasted from 2.5 Ga to 538.8 Ma (million years) ago. In this time span, cratons grew into continents with modern sizes. The change to an oxygen-rich atmosphere was a crucial development. Life developed from prokaryotes into eukaryotes and multicellular forms. The Proterozoic saw a couple of severe ice ages called Snowball Earths. After the last Snowball Earth about 600 Ma, the evolution of life on Earth accelerated. About 580 Ma, the Ediacaran biota formed the prelude for the Cambrian Explosion.[citation needed] The earliest cells absorbed energy and food from the surrounding environment. They used fermentation, the breakdown of more complex compounds into less complex compounds with less energy, and used the energy so liberated to grow and reproduce. Fermentation can only occur in an anaerobic (oxygen-free) environment. The evolution of photosynthesis made it possible for cells to derive energy from the Sun.: 377 Most of the life that covers the surface of Earth depends directly or indirectly on photosynthesis. The most common form, oxygenic photosynthesis, turns carbon dioxide, water, and sunlight into food. It captures the energy of sunlight in energy-rich molecules such as ATP, which then provide the energy to make sugars. To supply the electrons in the circuit, hydrogen is stripped from water, leaving oxygen as a waste product. Some organisms, including purple bacteria and green sulfur bacteria, use an anoxygenic form of photosynthesis that uses alternatives to hydrogen stripped from water as electron donors; examples are hydrogen sulfide, sulfur and iron. Such extremophile organisms are restricted to otherwise inhospitable environments such as hot springs and hydrothermal vents.: 379–382 The simpler anoxygenic form arose about 3.8 Ga, not long after the appearance of life. The timing of oxygenic photosynthesis is more controversial; it had certainly appeared by about 2.4 Ga, but some researchers put it back as far as 3.2 Ga. The latter "probably increased global productivity by at least two or three orders of magnitude". Among the oldest remnants of oxygen-producing lifeforms are fossil stromatolites. At first, the released oxygen was bound up with limestone, iron, and other minerals. The oxidized iron appears as red layers in geological strata called banded iron formations that formed in abundance during the Siderian period (between 2500 Ma and 2300 Ma).: 133 When most of the exposed readily reacting minerals were oxidized, oxygen finally began to accumulate in the atmosphere. Though each cell only produced a minute amount of oxygen, the combined metabolism of many cells over a vast time transformed Earth's atmosphere to its current state. This was Earth's third atmosphere.: 50–51 : 83–84, 116–117 Some oxygen was stimulated by solar ultraviolet radiation to form ozone, which collected in a layer near the upper part of the atmosphere. The ozone layer absorbed, and still absorbs, a significant amount of the ultraviolet radiation that once had passed through the atmosphere. It allowed cells to colonize the surface of the ocean and eventually the land: without the ozone layer, ultraviolet radiation bombarding land and sea would have caused unsustainable levels of mutation in exposed cells.: 219–220 Photosynthesis had another major impact. Oxygen was toxic; much life on Earth probably died out as its levels rose in what is known as the oxygen catastrophe. Resistant forms survived and thrived, and some developed the ability to use oxygen to increase their metabolism and obtain more energy from the same food. The natural evolution of the Sun made it progressively more luminous during the Archean and Proterozoic eons; the Sun's luminosity increases 6% every billion years.: 165 As a result, Earth began to receive more heat from the Sun in the Proterozoic eon. However, Earth did not get warmer. Instead, the geological record suggests it cooled dramatically during the early Proterozoic. Glacial deposits found in South Africa date back to 2.2 Ga, at which time, based on paleomagnetic evidence, they must have been located near the equator. Thus, this glaciation, known as the Huronian glaciation, may have been global. Some scientists suggest this was so severe that Earth was frozen over from the poles to the equator, a hypothesis called Snowball Earth. The Huronian ice age might have been caused by the increased oxygen concentration in the atmosphere, which caused the decrease of methane (CH4) in the atmosphere. Methane is a strong greenhouse gas, but with oxygen it reacts to form CO2, a less effective greenhouse gas.: 172 When free oxygen became available in the atmosphere, the concentration of methane could have decreased dramatically, enough to counter the effect of the increasing heat flow from the Sun. However, the term Snowball Earth is more commonly used to describe later extreme ice ages during the Cryogenian period. There were four periods, each lasting about 10 million years, between 750 and 580 million years ago, when Earth is thought to have been covered with ice apart from the highest mountains, and average temperatures were about −50 °C (−58 °F). The snowball may have been partly due to the location of the supercontinent Rodinia straddling the Equator. Carbon dioxide combines with rain to weather rocks to form carbonic acid, which is then washed out to sea, thus extracting the greenhouse gas from the atmosphere. When the continents are near the poles, the advance of ice covers the rocks, slowing the reduction in carbon dioxide, but in the Cryogenian the weathering of Rodinia was able to continue unchecked until the ice advanced to the tropics. The process may have finally been reversed by the emission of carbon dioxide from volcanoes or the destabilization of methane gas hydrates. According to the alternative Slushball Earth theory, even at the height of the ice ages there was still open water at the Equator. Modern taxonomy classifies life into three domains. The time of their origin is uncertain. The Bacteria domain probably first split off from the other forms of life (sometimes called Neomura), but this supposition is controversial. Soon after this, by 2 Ga, the Neomura split into the Archaea and the Eukaryota. Eukaryotic cells (Eukaryota) are larger and more complex than prokaryotic cells (Bacteria and Archaea), and the origin of that complexity is only now becoming known. The earliest fossils possessing features typical of fungi date to the Paleoproterozoic era, some 2.4 Ga ago; these multicellular benthic organisms had filamentous structures capable of anastomosis. Around this time, the first proto-mitochondrion was formed. A bacterial cell related to today's Rickettsia, which had evolved to metabolize oxygen, entered a larger prokaryotic cell, which lacked that capability. Perhaps the large cell attempted to digest the smaller one but failed (possibly due to the evolution of prey defenses). The smaller cell may have tried to parasitize the larger one. In any case, the smaller cell survived inside the larger cell. Using oxygen, it metabolized the larger cell's waste products and derived more energy. Part of this excess energy was returned to the host. The smaller cell replicated inside the larger one. Soon, a stable symbiosis developed between the large cell and the smaller cells inside it. Over time, the host cell acquired some genes from the smaller cells, and the two kinds became dependent on each other: the larger cell could not survive without the energy produced by the smaller ones, and these, in turn, could not survive without the raw materials provided by the larger cell. The whole cell is now considered a single organism, and the smaller cells are classified as organelles called mitochondria. A similar event occurred with photosynthetic cyanobacteria entering large heterotrophic cells and becoming chloroplasts.: 60–61 : 536–539 Probably as a result of these changes, a line of cells capable of photosynthesis split off from the other eukaryotes more than 1 billion years ago. There were probably several such inclusion events. Besides the well-established endosymbiotic theory of the cellular origin of mitochondria and chloroplasts, there are theories that cells led to peroxisomes, spirochetes led to cilia and flagella, and that perhaps a DNA virus led to the cell nucleus, though none of them are widely accepted. Archaeans, bacteria, and eukaryotes continued to diversify and to become more complex and better adapted to their environments. Each domain repeatedly split into multiple lineages. Around 1.1 Ga, the plant, animal, and fungi lines had split, though they still existed as solitary cells. Some of these lived in colonies, and gradually a division of labor began to take place; for instance, cells on the periphery might have started to assume different roles from those in the interior. Although the division between a colony with specialized cells and a multicellular organism is not always clear, around 1 billion years ago, the first multicellular plants emerged, probably green algae. Possibly by around 900 Ma: 488 true multicellularity had also evolved in animals. At first, it probably resembled today's sponges, which have totipotent cells that allow a disrupted organism to reassemble itself.: 483–487 As the division of labor was completed in the different lineages of multicellular organisms, cells became more specialized and more dependent on each other. Reconstructions of tectonic plate movement in the past 250 million years (the Cenozoic and Mesozoic eras) can be made reliably using fitting of continental margins, ocean floor magnetic anomalies and paleomagnetic poles. No ocean crust dates back further than that, so earlier reconstructions are more difficult. Paleomagnetic poles are supplemented by geologic evidence such as orogenic belts, which mark the edges of ancient plates, and past distributions of flora and fauna. The further back in time, the scarcer and harder to interpret the data get and the more uncertain the reconstructions.: 370 Throughout the history of Earth, there have been times when continents collided and formed a supercontinent, which later broke up into new continents. About 1000 to 830 Ma, most continental mass was united in the supercontinent Rodinia.: 370 Rodinia may have been preceded by Early-Middle Proterozoic continents called Nuna and Columbia.: 374 After the break-up of Rodinia about 800 Ma, the continents may have formed another short-lived supercontinent around 550 Ma. The hypothetical supercontinent is sometimes referred to as Pannotia or Vendia.: 321–322 The evidence for it is a phase of continental collision known as the Pan-African orogeny, which joined the continental masses of current-day Africa, South America, Antarctica and Australia. The existence of Pannotia depends on the timing of the rifting between Gondwana (which included most of the landmass now in the Southern Hemisphere, as well as the Arabian Peninsula and the Indian subcontinent) and Laurentia (roughly equivalent to current-day North America).: 374 It is at least certain that by the end of the Proterozoic eon, most of the continental mass lay united in a position around the south pole. The end of the Proterozoic saw at least two Snowball Earths, so severe that the surface of the oceans may have been completely frozen. This happened about 716.5 and 635 Ma, in the Cryogenian period. The intensity and mechanism of both glaciations are still under investigation and harder to explain than the early Proterozoic Snowball Earth. Most paleoclimatologists think the cold episodes were linked to the formation of the supercontinent Rodinia. Because Rodinia was centered on the equator, rates of chemical weathering increased and carbon dioxide (CO2) was taken from the atmosphere. Because CO2 is an important greenhouse gas, climates cooled globally. In the same way, during the Snowball Earths most of the continental surface was covered with permafrost, which decreased chemical weathering again, leading to the end of the glaciations. An alternative hypothesis is that enough carbon dioxide escaped through volcanic outgassing that the resulting greenhouse effect raised global temperatures. Increased volcanic activity resulted from the break-up of Rodinia at about the same time. The Cryogenian period was followed by the Ediacaran period, which was characterized by a rapid development of new multicellular lifeforms. Whether there is a connection between the end of the severe ice ages and the increase in diversity of life is not clear, but it does not seem coincidental. The new forms of life, called Ediacara biota, were larger and more diverse than ever. Though the taxonomy of most Ediacaran life forms is unclear, some were ancestors of groups of modern life. Important developments were the origin of muscular and neural cells. None of the Ediacaran fossils had hard body parts like skeletons. These first appear after the boundary between the Proterozoic and Phanerozoic eons or Ediacaran and Cambrian periods. Phanerozoic Eon The Phanerozoic is the current eon on Earth, which started approximately 538.8 million years ago. It consists of three eras: The Paleozoic, Mesozoic, and Cenozoic, and is the time when multi-cellular life greatly diversified into almost all the organisms known today. The Paleozoic ("old life") era was the first and longest era of the Phanerozoic eon, lasting from 538.8 to 251.9 Ma. During the Paleozoic, many modern groups of life came into existence. Life colonized the land, first plants, then animals. Two significant extinctions occurred. The continents formed at the break-up of Pannotia and Rodinia at the end of the Proterozoic slowly moved together again, forming the supercontinent Pangaea in the late Paleozoic. The Mesozoic ("middle life") era lasted from 251.9 Ma to 66 Ma. It is subdivided into the Triassic, Jurassic, and Cretaceous periods. The era began with the Permian–Triassic extinction event, the most severe extinction event in the fossil record whereby 95% of species on Earth died out, and ended with the Cretaceous–Paleogene extinction event that wiped out the dinosaurs. The Cenozoic ("new life") era began at 66 Ma, and is subdivided into the Paleogene, Neogene, and Quaternary periods. These three periods are further split into seven subdivisions, with the Paleogene composed of The Paleocene, Eocene, and Oligocene, the Neogene divided into the Miocene, Pliocene, and the Quaternary composed of the Pleistocene, and Holocene. Mammals, birds, amphibians, crocodilians, turtles, and lepidosaurs survived the Cretaceous–Paleogene extinction event that killed off the non-avian dinosaurs and many other forms of life, and this is the era during which they diversified into their modern forms. At the end of the Proterozoic, the supercontinent Pannotia had broken apart into the smaller continents Laurentia, Baltica, Siberia and Gondwana. During periods when continents move apart, more oceanic crust is formed by volcanic activity. Because the young volcanic crust is relatively hotter and less dense than the old oceanic crust, the ocean floors rise during such periods. This causes the sea level to rise. Therefore, in the first half of the Paleozoic, large areas of the continents were below sea level.[citation needed] Early Paleozoic climates were warmer than today, but the end of the Ordovician saw a short ice age during which glaciers covered the south pole, where the huge continent Gondwana was situated. Traces of glaciation from this period are only found on former Gondwana. During the Late Ordovician ice age, a few mass extinctions took place, in which many brachiopods, trilobites, Bryozoa and corals disappeared. These marine species could probably not contend with the decreasing temperature of the sea water. The continents Laurentia and Baltica collided between 450 and 400 Ma, during the Caledonian Orogeny, to form Laurussia (also known as Euramerica). Traces of the mountain belt this collision caused can be found in Scandinavia, Scotland, and the northern Appalachians. In the Devonian period (416–359 Ma) Gondwana and Siberia began to move towards Laurussia. The collision of Siberia with Laurussia caused the Uralian Orogeny, the collision of Gondwana with Laurussia is called the Variscan or Hercynian Orogeny in Europe or the Alleghenian Orogeny in North America. The latter phase took place during the Carboniferous period (359–299 Ma) and resulted in the formation of the last supercontinent, Pangaea. By 180 Ma, Pangaea broke up into Laurasia and Gondwana.[citation needed] The rate of the evolution of life as recorded by fossils accelerated in the Cambrian period (542–488 Ma). The sudden emergence of many new species, phyla, and forms in this period is called the Cambrian Explosion. It was a form of adaptive radiation, where vacant niches left by the extinct Ediacaran biota were filled up by the emergence of new phyla. The biological fomenting in the Cambrian Explosion was unprecedented before and since that time.: 229 Whereas the Ediacaran life forms appear yet primitive and not easy to put in any modern group, at the end of the Cambrian, most modern phyla were already present. The development of hard body parts such as shells, skeletons or exoskeletons in animals like molluscs, echinoderms, crinoids and arthropods (a well-known group of arthropods from the lower Paleozoic are the trilobites) made the preservation and fossilization of such life forms easier than those of their Proterozoic ancestors. For this reason, much more is known about life in and after the Cambrian period than about life in older periods. Some of these Cambrian groups appear complex but are seemingly quite different from modern life; examples are Anomalocaris and Haikouichthys. More recently, however, these seem to have found a place in modern classification. During the Cambrian, the first vertebrate animals, among them the first fishes, had appeared.: 357 A creature that could have been the ancestor of the fishes, or was probably closely related to it, was Pikaia. It had a primitive notochord, a structure that could have developed into a vertebral column later. The first fishes with jaws (Gnathostomata) appeared during the next geological period, the Ordovician. The colonisation of new niches resulted in massive body sizes. In this way, fishes with increasing sizes evolved during the early Paleozoic, such as the titanic placoderm Dunkleosteus, which could grow 7 meters (23 ft) long. The diversity of life forms did not increase significantly because of a series of mass extinctions that define widespread biostratigraphic units called biomeres. After each extinction pulse, the continental shelf regions were repopulated by similar life forms that may have been evolving slowly elsewhere. By the late Cambrian, the trilobites had reached their greatest diversity and dominated nearly all fossil assemblages.: 34 Oxygen accumulation from photosynthesis resulted in the formation of an ozone layer that absorbed much of the Sun's ultraviolet radiation, meaning unicellular organisms that reached land were less likely to die, and prokaryotes began to multiply and become better adapted to survival out of the water. Prokaryote lineages had probably colonized the land as early as 3 Ga even before the origin of the eukaryotes. For a long time, the land remained barren of multicellular organisms. The supercontinent Pannotia formed around 600 Ma and then broke apart a short 50 million years later. Fish, the earliest vertebrates, evolved in the oceans around 530 Ma.: 354 A major extinction event occurred near the end of the Cambrian period, which ended 488 Ma. Several hundred million years ago, plants (probably resembling algae) and fungi started growing at the edges of the water and then out of it.: 138–140 The oldest fossils of land fungi and plants date to 480–460 Ma, though molecular evidence suggests the fungi may have colonized the land as early as 1000 Ma and the plants 700 Ma. Initially remaining close to the water's edge, mutations and variations resulted in further colonization of this new environment. The timing of the first animals to leave the oceans is not precisely known: the oldest clear evidence is of arthropods on land around 450 Ma, perhaps thriving and becoming better adapted due to the vast food source provided by the terrestrial plants. There is also unconfirmed evidence that arthropods may have appeared on land as early as 530 Ma. At the end of the Ordovician period, 443 Ma, additional extinction events occurred, perhaps due to a concurrent ice age. Around 380 to 375 Ma, the first tetrapods evolved from fish. Fins evolved to become limbs that the first tetrapods used to lift their heads out of the water to breathe air. This would let them live in oxygen-poor water, or pursue small prey in shallow water. They may have later ventured on land for brief periods. Eventually, some of them became so well adapted to terrestrial life that they spent their adult lives on land, although they hatched in the water and returned to lay their eggs. This was the origin of the amphibians. About 365 Ma, another period of extinction occurred, perhaps as a result of global cooling. Plants evolved seeds, which dramatically accelerated their spread on land, around this time (by approximately 360 Ma). About 20 million years later (340 Ma: 293–296 ), the amniotic egg evolved, which could be laid on land, giving a survival advantage to tetrapod embryos. This resulted in the divergence of amniotes from amphibians. Another 30 million years (310 Ma: 254–256 ) saw the divergence of the synapsids (including mammals) from the sauropsids (including birds and reptiles). Other groups of organisms continued to evolve, and lines diverged—in fish, insects, bacteria, and so on—but less is known of the details.[citation needed] After yet another, the most severe extinction of the period (251~250 Ma), around 230 Ma, dinosaurs split off from their reptilian ancestors. The Triassic–Jurassic extinction event at 200 Ma spared many of the dinosaurs, and they soon became dominant among the vertebrates. Though some mammalian lines began to separate during this period, existing mammals were probably small animals resembling shrews.: 169 The boundary between avian and non-avian dinosaurs is unclear, but Archaeopteryx, traditionally considered one of the first birds, lived around 150 Ma. The earliest evidence for the angiosperms evolving flowers is during the Cretaceous period, some 20 million years later (132 Ma). The first of five great mass extinctions was the Ordovician-Silurian extinction. Its possible cause was the intense glaciation of Gondwana, which eventually led to a Snowball Earth. 60% of marine invertebrates became extinct, and 25% of all families.[citation needed] The second mass extinction was the Late Devonian extinction, probably caused by the evolution of trees, which could have led to the depletion of greenhouse gases (like CO2) or the eutrophication of water. 70% of all species became extinct. The third mass extinction was the Permian-Triassic, or the Great Dying, event. The event was possibly caused by some combination of the Siberian Traps volcanic event, an asteroid impact, methane hydrate gasification, sea level fluctuations, and a major anoxic event. Either the proposed Wilkes Land crater in Antarctica or Bedout structure off the northwest coast of Australia may indicate an impact connection with the Permian-Triassic extinction. But it remains uncertain whether these or other proposed Permian-Triassic boundary craters are real impact craters or even contemporary with the Permian-Triassic extinction event. This was by far the deadliest extinction ever, with about 57% of all families and 83% of all genera killed. The fourth mass extinction was the Triassic-Jurassic extinction event in which almost all synapsids and archosaurs became extinct, probably due to new competition from dinosaurs. The fifth and most recent mass extinction was the Cretaceous-Paleogene extinction event. In 66 Ma, a 10-kilometer (6.2 mi) asteroid struck Earth just off the Yucatán Peninsula—somewhere in the southwestern tip of then Laurasia—where the Chicxulub crater is today. This ejected vast quantities of particulate matter and vapor into the air that occluded sunlight, inhibiting photosynthesis. 75% of all life, including the non-avian dinosaurs, became extinct, marking the end of the Cretaceous period and Mesozoic era.[citation needed] The first true mammals evolved in the shadows of dinosaurs and other large archosaurs that filled the world by the late Triassic. The first mammals were very small, and were probably nocturnal to escape predation. Mammal diversification truly began only after the Cretaceous-Paleogene extinction event. By the early Paleocene Earth recovered from the extinction, and mammalian diversity increased. Creatures like Ambulocetus took to the oceans to eventually evolve into whales, whereas some creatures, like primates, took to the trees. This all changed during the mid to late Eocene when the circum-Antarctic current formed between Antarctica and Australia which disrupted weather patterns on a global scale. Grassless savanna began to predominate much of the landscape, and mammals such as Andrewsarchus rose up to become the largest known terrestrial predatory mammal ever, and early whales like Basilosaurus took control of the seas. [citation needed] The evolution of grasses brought a remarkable change to Earth's landscape, and the new open spaces pushed mammals to get bigger and bigger. Grass started to expand in the Miocene, and the Miocene is where many modern- day mammals first appeared. Giant ungulates like Paraceratherium and Deinotherium evolved to rule the grasslands. The evolution of grass also brought primates down from the trees, and started human evolution. The first big cats evolved during this time as well. The Tethys Sea was closed off by the collision of Africa and Europe. The formation of Panama was perhaps the most important geological event to occur in the last 60 million years. Atlantic and Pacific currents were closed off from each other, which caused the formation of the Gulf Stream, which made Europe warmer. The land bridge allowed the isolated creatures of South America to migrate over to North America and vice versa. Various species migrated south, leading to the presence in South America of llamas, the spectacled bear, kinkajous and jaguars.[citation needed] Three million years ago saw the start of the Pleistocene epoch, which featured dramatic climatic changes due to the ice ages. The ice ages led to the evolution and expansion of modern man in Saharan Africa. The mega-fauna that dominated fed on grasslands that, by now, had taken over much of the subtropical world. The large amounts of water held in the ice allowed various water bodies to shrink and sometimes disappear, such as the North Sea and the Bering Strait. It is believed by many that a huge migration took place along Beringia, which is why, today, there are camels (which evolved and became extinct in North America), horses (which evolved and became extinct in North America), and Native Americans. The end of the last ice age coincided with the expansion of man and a massive die out of ice age mega-fauna. A small African ape living around 6 Ma was the last animal whose descendants would include both modern humans and their closest relatives, the chimpanzees.: 100–101 Only two branches of its family tree have surviving descendants. Very soon after the split, for reasons that are still unclear, apes in one branch developed the ability to walk upright.: 95–99 Brain size increased rapidly, and by 2 Ma, the first animals classified in the genus Homo had appeared.: 300 Around the same time, the other branch split into the ancestors of the common chimpanzee and the ancestors of the bonobo as evolution continued simultaneously in all life forms.: 100–101 The ability to control fire probably began in Homo erectus (or Homo ergaster), probably at least 790,000 years ago but perhaps as early as 1.5 Ma.: 67 The use and discovery of controlled fire may even predate Homo erectus. Fire was possibly used by the early Lower Paleolithic (Oldowan) hominid Homo habilis or strong australopithecines such as Paranthropus. It is more difficult to establish the origin of language; it is unclear whether Homo erectus could speak or if that capability had not begun until Homo sapiens.: 67 As brain size increased, babies were born earlier, before their heads grew too large to pass through the pelvis. As a result, they exhibited more plasticity, thus possessing an increased capacity to learn and requiring a longer period of dependence. Social skills became more complex, language became more sophisticated, and tools became more elaborate. This contributed to further cooperation and intellectual development.: 7 Modern humans (Homo sapiens) are believed to have originated around 200,000 years ago or earlier in Africa; the oldest fossils date back to around 160,000 years ago. The first humans to show signs of spirituality are the Neanderthals (usually classified as a separate species with no surviving descendants); they buried their dead, often with no sign of food or tools.: 17 However, evidence of more sophisticated beliefs, such as the early Cro-Magnon cave paintings (probably with magical or religious significance): 17–19 did not appear until 32,000 years ago. Cro-Magnons also left behind stone figurines such as Venus of Willendorf, probably also signifying religious belief.: 17–19 By 11,000 years ago, Homo sapiens had reached the southern tip of South America, the last of the uninhabited continents (except for Antarctica, which remained undiscovered until 1820 AD). Tool use and communication continued to improve, and interpersonal relationships became more intricate.[citation needed] Throughout more than 90% of its history, Homo sapiens lived in small bands as nomadic hunter-gatherers.: 8 As language became more complex, the ability to remember and communicate information resulted in a new replicator: the meme. Ideas could be exchanged quickly and passed down the generations. Cultural evolution quickly outpaced biological evolution, and history proper began. Between 8500 and 7000 BC, humans in the Fertile Crescent in the Middle East began the systematic husbandry of plants and animals: agriculture. This spread to neighboring regions and developed independently elsewhere until most Homo sapiens lived sedentary lives in permanent settlements as farmers. Not all societies abandoned nomadism, especially those in isolated areas of the globe poor in domesticable plant species, such as Australia. However, among those civilizations that did adopt agriculture, the relative stability and increased productivity provided by farming allowed the population to expand.[citation needed] Agriculture had a major impact; humans began to affect the environment as never before. Surplus food allowed a priestly or governing class to arise, followed by increasing division of labor. This led to Earth's first civilization at Sumer in the Middle East, between 4000 and 3000 BC.: 15 Additional civilizations quickly arose in ancient Egypt, at the Indus River valley and in China. The invention of writing enabled complex societies to arise: record-keeping and libraries served as a storehouse of knowledge and increased the cultural transmission of information. Humans no longer had to spend all their time working for survival, enabling the first specialized occupations (e.g. craftsmen, merchants, priests, etc.). Curiosity and education drove the pursuit of knowledge and wisdom, and various disciplines, including science (in a primitive form), arose. This in turn led to the emergence of increasingly larger and more complex civilizations, such as the first empires, which at times traded with one another, or fought for territory and resources. By around 500 BC, there were advanced civilizations in the Middle East, Iran, India, China, and Greece, at times expanding, at times entering into decline.: 3 In 221 BC, China became a single polity that would grow to spread its culture throughout East Asia, and it has remained the most populous nation in the world. During this period, famous Hindu texts known as vedas came in existence in Indus Valley Civilization. This civilization developed in warfare, arts, science, mathematics and architecture.[citation needed] The fundamentals of Western civilization were largely shaped in Ancient Greece, with the world's first democratic government and major advances in philosophy and science, and in Ancient Rome with advances in law, government, and engineering. The Roman Empire was Christianized by Emperor Constantine in the early 4th century and declined by the end of the 5th. Beginning with the 7th century, Christianization of Europe began, and since at least the 4th century Christianity has played a prominent role in the shaping of Western civilization. In 610, Islam was founded and quickly became the dominant religion in Western Asia. The House of Wisdom was established in Abbasid-era Baghdad, Iraq. It is considered to have been a major intellectual center during the Islamic Golden Age, where Muslim scholars in Baghdad and Cairo flourished from the ninth to the thirteenth centuries until the Mongol sack of Baghdad in 1258 AD. In 1054 AD the Great Schism between the Roman Catholic Church and the Eastern Orthodox Church led to the prominent cultural differences between Western and Eastern Europe. In the 14th century, the Renaissance began in Italy with advances in religion, art, and science.: 317–319 At that time the Christian Church as a political entity lost much of its power. In 1492, Christopher Columbus reached the Americas, initiating great changes to the New World. European civilization began to change beginning in 1500, leading to the Scientific Revolution and Industrial Revolution. That continent began to exert political and cultural dominance over human societies around the world, a time known as the Colonial era (see also: Age of Discovery).: 295–299 In the 18th century a cultural movement known as the Age of Enlightenment further shaped the mentality of Europe and contributed to its secularization.[citation needed] In 1776, the United States of America declared independence from the British Empire in part because of the ideas popularized by The Enlightenment. The resulting war contributed to a chain of events that led to the French Revolution and the Napoleonic Wars. In the 20th century, imperialism and nationalism spread around the world setting the stage for World War I, World War II, the Cold War, the Space Race, nuclear proliferation, and decolonization. The 20th century also saw the rise of rapid technological advancements with humanity moving from the Wright brothers' first powered airplane flight in 1903, to the Apollo 11 Moon landing in 1969, to the development of the internet in the last few decades of the millenium. The human population also grew from 1 billion people in 1800 to over 8 billion today. In the 21st century, the planet continues to face the impact of human activities and technology. Today some scientists argue that because of climate change and other environmental impacts the Earth is now in the midst of a 6th mass extinction and/or a new geological epoch called the Anthropocene though these concepts are both disputed. See also Notes References Further reading External links |
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[SOURCE: https://en.wikipedia.org/wiki/Climatology] | [TOKENS: 2540] |
Contents Climatology Climatology (from Greek κλίμα, klima, "slope"; and -λογία, -logia) or climate science is the scientific study of Earth's climate, typically defined as weather conditions averaged over a period of at least 30 years. Climate concerns the atmospheric condition during an extended to indefinite period of time; weather is the condition of the atmosphere during a relative brief period of time. The main topics of research are the study of climate variability, mechanisms of climate changes and modern climate change. This topic of study is regarded as part of the atmospheric sciences and a subdivision of physical geography, which is one of the Earth sciences. Climatology includes some aspects of oceanography and biogeochemistry. The main methods employed by climatologists are the analysis of observations and modelling of the physical processes that determine climate. Short term weather forecasting can be interpreted in terms of knowledge of longer-term phenomena of climate, for instance climatic cycles such as the El Niño–Southern Oscillation (ENSO), the Madden–Julian oscillation (MJO), the North Atlantic oscillation (NAO), the Arctic oscillation (AO), the Pacific decadal oscillation (PDO), and the Interdecadal Pacific Oscillation (IPO). Climate models are used for a variety of purposes from studying the dynamics of the weather and climate system to predictions of future climate. History The Greeks began the formal study of climate; in fact, the word "climate" is derived from the Greek word klima, meaning "slope", referring to the slope or inclination of the Earth's axis. Arguably the most influential classic text concerning climate was On Airs, Water and Places written by Hippocrates about 400 BCE. This work commented on the effect of climate on human health and cultural differences between Asia and Europe. This idea that climate controls which populations excel depending on their climate, or climatic determinism, remained influential throughout history. Chinese scientist Shen Kuo (1031–1095) inferred that climates naturally shifted over an enormous span of time, after observing petrified bamboos found underground near Yanzhou (modern Yan'an, Shaanxi province), a dry-climate area unsuitable at that time for the growth of bamboo. The invention of thermometers and barometers during the Scientific Revolution allowed for systematic recordkeeping, that began as early as 1640–1642 in England. Early climate researchers include Edmund Halley, who published a map of the trade winds in 1686 after a voyage to the southern hemisphere. Benjamin Franklin (1706–1790) first mapped the course of the Gulf Stream for use in sending mail from North America to Europe. Francis Galton (1822–1911) invented the term anticyclone. Helmut Landsberg (1906–1985) fostered the use of statistical analysis in climatology. During the early 20th century, climatology mostly emphasized the description of regional climates. This descriptive climatology was mainly an applied science, giving farmers and other interested people statistics about what the normal weather was and how great chances were of extreme events. To do this, climatologists had to define a climate normal, or an average of weather and weather extremes over a period of typically 30 years. While scientists knew of past climate change such as the ice ages, the concept of climate as changing only very gradually was useful for descriptive climatology. This started to change during the decades that followed, and while the history of climate change science started earlier, climate change only became one of the main topics of study for climatologists during the 1970s and afterward. Subfields Various subtopics of climatology study different aspects of climate. There are different categorizations of the sub-topics of climatology. The American Meteorological Society for instance identifies descriptive climatology, scientific climatology and applied climatology as the three subcategories of climatology, a categorization based on the complexity and the purpose of the research. Applied climatologists apply their expertise to different industries such as manufacturing and agriculture. Paleoclimatology is the attempt to reconstruct and understand past climates by examining records such as ice cores and tree rings (dendroclimatology). Paleotempestology uses these same records to help determine hurricane frequency over millennia. Historical climatology is the study of climate as related to human history and is thus concerned mainly with the last few thousand years. Boundary-layer climatology concerns exchanges in water, energy and momentum near surfaces. Further identified subtopics are physical climatology, dynamic climatology, tornado climatology, regional climatology, bioclimatology, and synoptic climatology. The study of the hydrological cycle over long time scales is sometimes termed hydroclimatology, in particular when studying the effects of climate change on the water cycle. Methods The study of contemporary climates incorporates meteorological data accumulated over many years, such as records of rainfall, temperature and atmospheric composition. Knowledge of the atmosphere and its dynamics is also embodied in models, either statistical or mathematical, which help by integrating different observations and testing how well they match. Modeling is used for understanding past, present and potential future climates. Climate research is made difficult by the large scale, long time periods, and complex processes which govern climate. Climate is governed by physical principles which can be expressed as differential equations. These equations are coupled and nonlinear, so that approximate solutions are obtained by using numerical methods to create global climate models. Climate is sometimes modeled as a stochastic process but this is generally accepted as an approximation to processes that are otherwise too complicated to analyze. The collection of a long record of climate variables is essential for the study of climate. Climatology deals with the aggregate data that meteorologists have recorded. Scientists use both direct and indirect observations of the climate, from Earth observing satellites and scientific instrumentation such as a global network of thermometers, to prehistoric ice extracted from glaciers. As measuring technology changes over time, records of data often cannot be compared directly. As cities are generally warmer than the areas surrounding, urbanization has made it necessary to constantly correct data for this urban heat island effect. Climate models use quantitative methods to simulate the interactions of the atmosphere, oceans, land surface, and ice. They are used for a variety of purposes from study of the dynamics of the weather and climate system to projections of future climate. All climate models balance, or very nearly balance, incoming energy as short wave (including visible) electromagnetic radiation to the Earth with outgoing energy as long wave (infrared) electromagnetic radiation from the Earth. Any unbalance results in a change of the average temperature of the Earth. Most climate models include the radiative effects of greenhouse gases such as carbon dioxide. These models predict a trend of increase of surface temperatures, as well as a more rapid increase of temperature at higher latitudes. Models can range from relatively simple to complex: Additionally, they are available with different resolutions ranging from >100 km to 1 km. High resolutions in global climate models are computational very demanding and only few global datasets exists. Examples are ICON or mechanistically downscaled data such as CHELSA (Climatologies at high resolution for the Earth's land surface areas). Topics of research Topics that climatologists study comprise three main categories: climate variability, mechanisms of climatic change, and modern changes of climate. Various factors affect the average state of the atmosphere at a particular location. For instance, midlatitudes will have a pronounced seasonal cycle of temperature whereas tropical regions show little variation of temperature over a year. Another major variable of climate is continentality: the distance to major water bodies such as oceans. Oceans act as a moderating factor, so that land close to it has typically less difference of temperature between winter and summer than areas further from it. The atmosphere interacts with other parts of the climate system, with winds generating ocean currents that transport heat around the globe. Classification is an important method of simplifying complicated processes. Different climate classifications have been developed over the centuries, with the first ones in Ancient Greece. How climates are classified depends on what the application is. A wind energy producer will require different information (wind) in a classification than someone more interested in agriculture, for whom precipitation and temperature are more important. The most widely used classification, the Köppen climate classification, was developed during the late nineteenth century and is based on vegetation. It uses monthly data concerning temperature and precipitation. There are different types of variability: recurring patterns of temperature or other climate variables. They are quantified with different indices. Much in the way the Dow Jones Industrial Average, which is based on the stock prices of 30 companies, is used to represent the fluctuations of stock prices in general, climate indices are used to represent the essential elements of climate. Climate indices are generally devised with the twin objectives of simplicity and completeness, and each index typically represents the status and timing of the climate factor it represents. By their very nature, indices are simple, and combine many details into a generalized, overall description of the atmosphere or ocean which can be used to characterize the factors which effect the global climate system. El Niño–Southern Oscillation (ENSO) is a coupled ocean-atmosphere phenomenon in the Pacific Ocean responsible for much of the global variability of temperature, and has a cycle between two and seven years. The North Atlantic oscillation is a mode of variability that is mainly contained to the lower atmosphere, the troposphere. The layer of atmosphere above, the stratosphere is also capable of creating its own variability, most importantly the Madden–Julian oscillation (MJO), which has a cycle of approximately 30 to 60 days. The Interdecadal Pacific oscillation can create changes in the Pacific Ocean and lower atmosphere on decadal time scales. Climate change occurs when changes of Earth's climate system result in new weather patterns that remain for an extended period of time. This duration of time can be as brief as a few decades to as long as millions of years. The climate system receives nearly all of its energy from the sun. The climate system also gives off energy to outer space. The balance of incoming and outgoing energy, and the passage of the energy through the climate system, determines Earth's energy budget. When the incoming energy is greater than the outgoing energy, earth's energy budget is positive and the climate system is warming. If more energy goes out, the energy budget is negative and earth experiences cooling. Climate change also influences the average sea level. Modern climate change is caused largely by the human emissions of greenhouse gas from the burning of fossil fuel which increases global mean surface temperatures. Increasing temperature is only one aspect of modern climate change, which also includes observed changes of precipitation, storm tracks and cloudiness. Warmer temperatures are causing further changes of the climate system, such as the widespread melt of glaciers, sea level rise and shifts of flora and fauna. Differences with meteorology In contrast to meteorology, which emphasises short term weather systems lasting no more than a few weeks, climatology studies the frequency and trends of those systems. It studies the periodicity of weather events over years to millennia, as well as changes of long-term average weather patterns in relation to atmospheric conditions. Climatologists study both the nature of climates – local, regional or global – and the natural or human-induced factors that cause climates to change. Climatology considers the past and can help predict future climate change. Phenomena of climatological interest include the atmospheric boundary layer, circulation patterns, heat transfer (radiative, convective and latent), interactions between the atmosphere and the oceans and land surface (particularly vegetation, land use and topography), and the chemical and physical composition of the atmosphere. Use in weather forecasting A relative difficult method of forecast, the analog technique requires remembering a previous weather event which is expected to be mimicked by an upcoming event. What makes it a difficult technique is that there is rarely a perfect analog for an event of the future. Some refer to this type of forecasting as pattern recognition, which remains a useful method of estimating rainfall over data voids such as oceans using knowledge of how satellite imagery relates to precipitation rates over land, as well as the forecasting of precipitation amounts and distribution of the future. A variation of this theme, used for medium range forecasting, is known as teleconnections, when systems in other locations are used to help determine the location of a system within the regime surrounding. One method of using teleconnections are by using climate indices such as ENSO-related phenomena. See also References Further reading External links |
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[SOURCE: https://en.wikipedia.org/wiki/Earth_symbol] | [TOKENS: 560] |
Contents Earth symbol A variety of symbols or iconographic conventions are used to represent Earth, whether in the sense of planet Earth, or the inhabited world, or as a classical element. A circle representing the round world, with the rivers of Garden of Eden separating the four corners of the world, or rotated 45° to suggest the four continents, remains a common pictographic convention to express the notion of "worldwide". The current astronomical symbols for the planet are a circle with an intersecting cross, , and a globus cruciger, . Although the International Astronomical Union (IAU) now discourages the use of planetary symbols, this is an exception, being used in abbreviations such as M🜨 or M♁ for Earth mass. History The earliest type of symbols are allegories, personifications or deifications, mostly in the form of an Earth goddess (in the case of Egyptian mythology a god, Geb). Before the recognition of the spherical shape of the Earth in the Hellenistic period, the main attribute of the Earth was its being flat. The Egyptian hieroglyph for "earth, land" depicts a stretch of flat alluvial land with grains of sand (Gardiner N16: 𓇾). The Sumerian cuneiform sign for "earth, place" KI (𒆠) originates as a picture of a "threshing floor", and the Chinese character (土) originated as a lump of clay on a potting wheel. Earth, the classical element In Chinese mysticism, the classical element "Earth" is represented by the trigram of three broken lines in the I Ching (☷). The Western (early modern) alchemical symbol for earth is a downward-pointing triangle bisected by a horizontal line (🜃). Other symbols for the earth in alchemy or mysticism include the square and the serpent. The planet In the Roman period, the globe, a representation of the spherical Earth, became the main symbol representing the concept. The globe depicted the "universe" (pictured as the celestial sphere) as well as the Earth. The globus cruciger (♁) is the globe surmounted by a Christian cross, held by Byzantine Emperors on the one hand to represent the Christian ecumene, on the other hand the akakia represented the mortal nature of all men. In the medieval period, the known world was also represented by the T-and-O figure, representing an extremely simplified world map of the three classical continents of the Old World, viz. Asia, Europe and Africa (in various orientations: , , , ). Unicode encodes four characters representing the globe in the Miscellaneous Symbols and Pictographs block: See also References External links |
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[SOURCE: https://en.wikipedia.org/wiki/Biosphere] | [TOKENS: 2145] |
Contents Biosphere The biosphere (from Ancient Greek βίος (bíos) 'life' and σφαῖρα (sphaîra) 'sphere'), also called the ecosphere (from Ancient Greek οἶκος (oîkos) 'settlement, house' and σφαῖρα (sphaîra) 'sphere'), is the worldwide sum of all ecosystems. It can also be termed the zone of life on the Earth. The biosphere (which is technically a spherical shell) is virtually a closed system with regard to matter, with minimal inputs and outputs. Regarding energy, it is an open system, with photosynthesis capturing solar energy at a rate of around 100 terawatts. By the most general biophysiological definition, the biosphere is the global ecological system integrating all living beings and their relationships, including their interaction with the elements of the lithosphere, cryosphere, hydrosphere, and atmosphere. The biosphere is postulated to have evolved, beginning with a process of biopoiesis (life created naturally from non-living matter, such as simple organic compounds) or biogenesis (life created from living matter), at least some 3.5 billion years ago. In a general sense, biospheres are any closed, self-regulating systems containing ecosystems. This includes artificial biospheres such as Biosphere 2 and BIOS-3, and potentially ones on other planets or moons. Origin and use of the term The term "biosphere" was coined in 1875 by geologist Eduard Suess, who defined it as the place on Earth's surface where life dwells. While the concept has a geological origin, it is an indication of the effect of both Charles Darwin and Matthew F. Maury on the Earth sciences. The biosphere's ecological context comes from the 1920s (see Vladimir I. Vernadsky), preceding the 1935 introduction of the term "ecosystem" by Sir Arthur Tansley (see ecology history). Vernadsky defined ecology as the science of the biosphere. It is an interdisciplinary concept for integrating astronomy, geophysics, meteorology, biogeography, evolution, geology, geochemistry, hydrology and, generally speaking, all life and Earth sciences. Geochemists define the biosphere as being the total sum of living organisms (the "biomass" or "biota" as referred to by biologists and ecologists). In this sense, the biosphere is but one of four separate components of the geochemical model, the other three being geosphere, hydrosphere, and atmosphere. When these four component spheres are combined into one system, it is known as the ecosphere. This term was coined during the 1960s and encompasses both biological and physical components of the planet. The Second International Conference on Closed Life Systems defined biospherics as the science and technology of analogs and models of Earth's biosphere; i.e., artificial Earth-like biospheres. Others may include the creation of artificial non-Earth biospheres—for example, human-centered biospheres or a native Martian biosphere—as part of the topic of biospherics. Earth's biosphere Currently, the total number of living cells on the Earth is estimated to be 1030; the total number since the beginning of Earth, as 1040, and the total number for the entire time of a habitable planet Earth as 1041. This is much larger than the total number of estimated stars (and Earth-like planets) in the observable universe as 1024, a number which is more than all the grains of beach sand on planet Earth; but less than the total number of atoms estimated in the observable universe as 1082; and the estimated total number of stars in an inflationary universe (observed and unobserved), as 10100. The earliest evidence for life on Earth includes biogenic graphite found in 3.7 billion-year-old metasedimentary rocks from Western Greenland and microbial mat fossils found in 3.48 billion-year-old sandstone from Western Australia. More recently, in 2015, "remains of biotic life" were found in 4.1 billion-year-old rocks in Western Australia. In 2017, putative fossilized microorganisms (or microfossils) were announced to have been discovered in hydrothermal vent precipitates in the Nuvvuagittuq Belt of Quebec, Canada that were as old as 4.28 billion years, the oldest record of life on earth, suggesting "an almost instantaneous emergence of life" after ocean formation 4.4 billion years ago, and not long after the formation of the Earth 4.54 billion years ago. According to biologist Stephen Blair Hedges, "If life arose relatively quickly on Earth ... then it could be common in the universe." Every part of the planet, from the polar ice caps to the equator, features life of some kind. Recent advances in microbiology have demonstrated that microbes live deep beneath the Earth's terrestrial surface and that the total mass of microbial life in so-called "uninhabitable zones" may, in biomass, exceed all animal and plant life on the surface. The actual thickness of the biosphere on Earth is difficult to measure. Birds typically fly at altitudes as high as 1,800 m (5,900 ft; 1.1 mi) and fish live as much as 8,372 m (27,467 ft; 5.202 mi) underwater in the Puerto Rico Trench. There are more extreme examples for life on the planet: Rüppell's vulture has been found at altitudes of 11,300 metres (37,100 feet; 7.0 miles); bar-headed geese migrate at altitudes of at least 8,300 m (27,200 ft; 5.2 mi); yaks live at elevations as high as 5,400 m (17,700 ft; 3.4 mi) above sea level; mountain goats live up to 3,050 m (10,010 ft; 1.90 mi). Herbivorous animals at these elevations depend on lichens, grasses, and herbs. Life forms live in every part of the Earth's biosphere, including soil, hot springs, inside rocks at least 19 km (12 mi) deep underground, and at least 64 km (40 mi) high in the atmosphere. Marine life under many forms has been found in the deepest reaches of the world ocean while much of the deep sea remains to be explored. Under certain test conditions, microorganisms have been observed to survive the vacuum of outer space. The total amount of soil and subsurface bacterial carbon is estimated as 5 × 1017 g. The mass of prokaryote microorganisms—which includes bacteria and archaea, but not the nucleated eukaryote microorganisms—may be as much as 0.8 trillion tons of carbon (of the total biosphere mass, estimated at between 1 and 4 trillion tons). Barophilic marine microbes have been found at more than a depth of 10,000 m (33,000 ft; 6.2 mi) in the Mariana Trench, the deepest spot in the Earth's oceans. In fact, single-celled life forms have been found in the deepest part of the Mariana Trench, by the Challenger Deep, at depths of 11,034 m (36,201 ft; 6.856 mi). Other researchers reported related studies that microorganisms thrive inside rocks up to 580 m (1,900 ft; 0.36 mi) below the sea floor under 2,590 m (8,500 ft; 1.61 mi) of ocean off the coast of the northwestern United States, as well as 2,400 m (7,900 ft; 1.5 mi) beneath the seabed off Japan. Culturable thermophilic microbes have been extracted from cores drilled more than 5,000 m (16,000 ft; 3.1 mi) into the Earth's crust in Sweden, from rocks between 65–75 °C (149–167 °F). Temperature increases with increasing depth into the Earth's crust. The rate at which the temperature increases depends on many factors, including the type of crust (continental vs. oceanic), rock type, geographic location, etc. The greatest known temperature at which microbial life can exist is 122 °C (252 °F) (Methanopyrus kandleri Strain 116). It is likely that the limit of life in the "deep biosphere" is defined by temperature rather than absolute depth.[citation needed] On 20 August 2014, scientists confirmed the existence of microorganisms living 800 m (2,600 ft; 0.50 mi) below the ice of Antarctica. Earth's biosphere is divided into several biomes, inhabited by fairly similar flora and fauna. On land, biomes are separated primarily by latitude. Terrestrial biomes lying within the Arctic and Antarctic Circles are relatively barren of plant and animal life. In contrast, most of the more populous biomes lie near the equator. Artificial biospheres Experimental biospheres, also called closed ecological systems, have been created to study ecosystems and the potential for supporting life outside the Earth. These include spacecraft and the following terrestrial laboratories: Extraterrestrial biospheres No biospheres have been detected beyond the Earth; therefore, the existence of extraterrestrial biospheres remains hypothetical. The rare Earth hypothesis suggests they should be very rare, save ones composed of microbial life only. On the other hand, Earth analogs may be quite numerous, at least in the Milky Way galaxy, given the large number of planets. Three of the planets discovered orbiting TRAPPIST-1 could possibly contain biospheres. Given limited understanding of abiogenesis, it is currently unknown what percentage of these planets actually develop biospheres. Based on observations by the Kepler Space Telescope team, it has been calculated that provided the probability of abiogenesis is higher than 1 to 1000, the closest alien biosphere should be within 100 light-years from the Earth. It is also possible that artificial biospheres will be created in the future, for example with the terraforming of Mars. See also References Further reading External links |
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[SOURCE: https://en.wikipedia.org/wiki/List_of_sovereign_states] | [TOKENS: 1624] |
List of sovereign states The following is a list providing an overview of sovereign states around the world with information on their status and recognition of their sovereignty. The 205 listed states can be divided into three categories based on membership within the United Nations System: 193 UN member states, 2 UN General Assembly non-member observer states, and 10 other states. The sovereignty dispute column indicates states having undisputed sovereignty (189 states, of which there are 188 UN member states and one UN General Assembly non-member observer state), states having disputed sovereignty (14 states, of which there are 5 UN member states, 1 UN General Assembly non-member observer state, and 8 de facto states), and states having a special political status (two states, both in free association with New Zealand). Compiling a list such as this can be complicated and controversial, as there is no definition that is binding on all the members of the community of nations concerning the criteria for statehood. For more information on the criteria used to determine the contents of this list, please see the criteria for inclusion section below. The list is intended to include entities that have been recognised as having de facto status as sovereign states, and inclusion should not be seen as an endorsement of any specific claim to statehood in legal terms. Criteria for inclusion The dominant customary international law standard of statehood is the declarative theory of statehood, which was codified by the Montevideo Convention of 1933. The Convention defines the state as a person of international law if it "possess[es] the following qualifications: (a) a permanent population; (b) a defined territory; (c) government; and (d) a capacity to enter into relations with the other states" so long as it was not "obtained by force whether this consists in the employment of arms, in threatening diplomatic representations, or in any other effective coercive measure". Debate exists on the degree to which recognition should be included as a criterion of statehood. The declarative theory of statehood argues that statehood is purely objective and recognition of a state by other states is irrelevant. On the other end of the spectrum, the constitutive theory of statehood defines a state as a person under international law only if it is recognised as sovereign by other states. For the purposes of this list, included are all polities that consider themselves sovereign states (through a declaration of independence or some other means) and either: In some cases, there is a divergence of opinion over the interpretation of the first point, and whether an entity satisfies it is disputed. Unique political entities which fail to meet the classification of a sovereign state are considered proto-states. On the basis of the above criteria, this list includes the following 205 entities:[a][b] The table includes bullets in the right-hand column representing entities that are either not sovereign states or have a close association to another sovereign state. It also includes subnational areas where the sovereignty of the titular state is limited by an international agreement. Taken together, these include: List of states "Membership within the UN System" column legend "Sovereignty dispute" column legend and Brčko District, a self-governing administrative district.[j] China claims, but does not control, Taiwan, which is governed by a rival administration (the Republic of China) that claims all of China as its territory.[q] China is not recognised by 11 UN member states and Vatican City, which, with the exception of Bhutan, all recognise the Republic of China (Taiwan) instead.[r] Cyprus is not recognised by Turkey due to the Cyprus problem, with Turkey recognising Northern Cyprus. The metropolitan territory of Denmark, the Faroe Islands and Greenland form the three constituent countries of the Kingdom.[v] The Kingdom of Denmark as a whole is a member of the EU, but EU law (in most cases) does not apply to the Faroe Islands and Greenland. See Greenland and the European Union, and Faroe Islands and the European Union. Israel is not recognised as a state by 28 UN members and the Sahrawi Republic. The Palestine Liberation Organization, recognised by a majority of UN member states as the representative of the Palestinian people, recognised Israel in 1993. The Metropolitan Netherlands, Aruba, Curaçao and Sint Maarten form the four constituent countries of the Kingdom. Three overseas parts of the Netherlands (Bonaire, Saba and Sint Eustatius) are special municipalities of the metropolitan Netherlands.[ae] The Kingdom of the Netherlands as a whole is a member of the EU, but EU law only wholly applies to parts within Europe. The New Zealand Government acts for the entire Realm of New Zealand in all international contexts, which has responsibilities for (but no rights of control over) two freely associated states: The Cook Islands and Niue have diplomatic relations with 63 and 25 UN members respectively. They have full treaty-making capacity in the UN, and are members of some UN specialized agencies. Norway has one dependent territory and two claimed Antarctic dependent territories in the Southern Hemisphere: Azad Kashmir describes itself as a "self-governing state under Pakistani control", while Gilgit-Baltistan is described in its governance order as a group of "areas" with self-government. These territories are not usually regarded as sovereign, as they do not fulfil the criteria set out by the declarative theory of statehood (for example, their current laws do not allow them to engage independently in relations with other states). Several state functions of these territories (such as foreign affairs and defense) are performed by Pakistan. South Korea is not recognised by North Korea, which claims to be the sole legitimate government of Korea. The Abyei Area is a zone with "special administrative status" established by the Comprehensive Peace Agreement in 2005. It is de jure a condominium of South Sudan and Sudan, but de facto administered by two competing administrations and the United Nations. The Abyei Area is a zone with "special administrative status" established by the Comprehensive Peace Agreement in 2005. It is de jure a condominium of South Sudan and Sudan, but de facto administered by two competing administrations and the United Nations. Syria has one self-declared autonomous region, Rojava. The British monarch also has direct sovereignty over three self-governing Crown Dependencies: Additionally, the federal government of the United States has sovereignty over 13 unincorporated territories. Of these unincorporated territories, five are inhabited possessions: The United States also has sovereignty over eight uninhabited territories: The United States disputes sovereignty over two territories: Three sovereign states have become associated states of the United States under the Compact of Free Association: "Membership within the UN System" column legend "Sovereignty dispute" column legend The territories under its control, the so-called Free Zone, are claimed in whole by Morocco. In turn, the Sahrawi Republic claims the Moroccan-occupied part of Western Sahara to the west of the Moroccan sand wall. Its government resides in exile in the Sahrawi refugee camps of Tindouf, Algeria. In addition to these relations, the ROC also maintains unofficial relations with 58 UN member states, one self-declared state (Somaliland), three territories (Guam, Hong Kong, and Macau), and the European Union via its representative offices and consulates under the One China principle. Taiwan has the 31st-largest diplomatic network in the world with 110 offices. The territory of the ROC is claimed in whole by the PRC.[q] The ROC participates in international organizations under a variety of pseudonyms, most commonly "Chinese Taipei". In the WTO, the ROC has full membership under the designation of "Separate Customs Territory of Taiwan, Penghu, Kinmen and Matsu". The ROC was a founding member of the UN and enjoyed membership from 1945 to 1971, with veto power in the UN Security Council. See China and the United Nations. See also Notes References Bibliography |
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[SOURCE: https://en.wikipedia.org/wiki/Earth_in_culture] | [TOKENS: 2583] |
Contents Earth in culture The cultural perspective on Earth, or the world, varies by society and time period. Religious beliefs often include a creation belief as well as personification in the form of a deity. The exploration of the world has modified many of the perceptions of the planet, resulting in a viewpoint of a globally integrated ecosystem. Unlike the remainder of the planets in the Solar System, mankind did not perceive the Earth as a planet until the sixteenth century. Etymology Unlike the other planets in the Solar System, in English, Earth does not directly share a name with an ancient Roman deity. The name Earth derives from the eighth century Anglo-Saxon word erda, which means ground or soil, and ultimately descends from Proto-Germanic *erþō. From this it has cognates throughout the Germanic languages, including with Jörð, the name of the giantess of Norse myth. Earth was first used as the name of the sphere of the Earth in the early fifteenth century. The planet's name in Latin, used academically and scientifically in the West during the Renaissance, is the same as that of Terra Mater, the Roman goddess, which translates to English as Mother Earth. Planetary symbol The standard astronomical symbol of the Earth consists of a cross circumscribed by a circle. This symbol is known as the wheel cross, sun cross, Odin's cross or Woden's cross. Although it has been used in various cultures for different purposes, it came to represent the compass points, Earth and the land. Another version of the symbol is a cross on top of a circle; a stylized globus cruciger that was also used as an early astronomical symbol for the planet Earth. Religious beliefs Earth has often been personified as a deity, in particular a goddess. In many cultures the mother goddess is also portrayed as a fertility deity. To the Aztecs, Earth was called Tonantzin—"our mother"; to the Incas, Earth was called Pachamama—"mother earth". The Chinese Earth goddess Hou Tu is similar to Gaia, the Greek goddess personifying the Earth. Bhumi Devi is the goddess of Earth in Hinduism, influenced by Graha. The Tuluva people of Tulunadu in Southern India celebrate a Three Day "Earth Day" called Keddaso. This festival comes in usually on 10th,12th,13 February every Calendar year. In Norse mythology, the Earth giantess Jörð was the mother of Thor and the daughter of Annar. Ancient Egyptian mythology is different from that of other cultures because Earth (Geb) is male and the sky (Nut) is female. Creation myths in many religions recall a story involving the creation of the world by a supernatural deity or deities. A variety of religious groups, often associated with fundamentalist branches of Protestantism or Islam, assert that their interpretations of the accounts of creation in sacred texts are literal truth and should be considered alongside or replace conventional scientific accounts of the formation of the Earth and the origin and development of life. Such assertions are opposed by the scientific community as well as other religious groups. A prominent example is the creation–evolution controversy. Creation myths in different cultures and religions Tiāmat is a sea monster known as the monster of monsters. She is killed and her body is cut in half in order to create heaven and Earth. The upper part of Tiāmat is used to create heaven with her belly as the separation line. The lower part of her body was used to create Earth, but the way that specific body parts were used to create other things is not described. Odin and his two brothers killed the frost-giant Ymir and took his body with them. From Ymir's body Odin and his brothers created the Earth. As Ymir's blood drained from his body, Odin created oceans and lakes, from his teeth they formed broken bits of rocks and placed them on mountains, from his bones they made boulders, his skull fashioned the sky and respectively his brain formed clouds. After Odin's creation of Earth he sent four dwarves down to each corner of the Earth one being Austri meaning east, another called Vestri meaning west, another named Nordri meaning north, and the final one named Sudri meaning south. This is where directions came from. Odin and his brothers then set out to make the first people. Odin and his brothers gathered wood from the seashore and created the first people, Ask being the man and Embla being the woman. Light and dark was the final step that Odin had to create. He took Night who is the daughter of a giant that is dark in color. Odin gave Night a chariot pulled by a horse called Hrimfaxi. He instructed Night to ride around the Earth and with her she brought darkness, from her horse's saliva dew was created. He then took Day, the son of Night and Delling of the AEsir, who was bright and attractive and gave him a chariot pulled by a horse named Skinfaxi. He instructed Day to ride around the Earth and with him he brought light and from his horse's mane streamed light. In the myth of the god Tlaltecuhtli, her dismembered body was the basis for the world in the Aztec creation story of the fifth and final cosmos. Before the fifth sun was created, the "earth monster" dwelled in the ocean after the fourth Great Flood. The gods Quetzalcoatl and Tezcatlipoca descended from the heavens in the form of serpents and found the monstrous Tlaltecuhtli. The two gods decided that the fifth cosmos could not prosper with such a horrible creature roaming the world, and so they set out to destroy her. After a long struggle, Tezcatlipoca and Quetzalcoatl managed to rip her body in two — from the upper half came the sky, and from the lower came the Earth. The other gods were angered to hear of Tlaltecuhtli's treatment and decreed that the various parts of her dismembered body would become the features of the new world. Her skin became grasses and small flowers, her hair the trees and herbs, her eyes the springs and wells, her nose the hills and valleys, her shoulders the mountains, and her mouth the caves and rivers. In the Yoruba religion, there are many deities, but the Supreme One is called Ọlọrun and they are said to be perfect. Before Ayé (the Earth) was created there was only Ọrun (the sky or heavens) above and water, swamps, and mist below. One day, one of the deities named Ọbatala asked Ọlọrun if he could make a world out of what was below. Ọlọrun granted him permission to make a world from the things down below. Before taking action, Ọbatala consulted with Ọrunmila (the deity of divination) who told Ọbatala to get a golden chain and lower it from Ọrun to the waters below so that he could eventually return to the other deities. Ọrunmila also told him to take a snail shell with soil in it, a hen, a black cat, and a palm nut. Ọbatala heeded his words and descended down the golden chain with all of the things he was told to take. Once Ọbatala reached the waters below he poured all of the soil onto the water. He then set the hen down, who spread the soil out by pecking and scratching at it. After the soil was spread, he planted the palm nut which grew and produced more nuts which respectively grew more trees. Obatala thought that this new world needed more light, so he consulted with Ọlọrun through their chameleon servant and then Ọlọrun created the Sun and Moon and sent fire on a vulture's head for light when the Sun was gone. Ọbatala got lonely on this new world of his, so he fashioned human beings out of clay and asked Ọlọrun for help. Ọlọrun breathed life into the clay figures and humans became alive. Ọlọrun also gave life to animals, plants, rivers, and language for the people to utilize. When Ọbatala was pleased with his work he climbed back up the golden chain and lived with the other deities in the sky above. In fiction While in general, a planet can be considered "too large, and its lifetime too long, to be comfortably accommodated within fiction as a topic in its own right", this has not prevented some writers from engaging with the topic (for example, Camille Flammarion's Lumen (1887), David Brin's Earth (1990), or Terry Pratchett's, Ian Stewart's and Jack Cohen's The Science of Discworld (1999)). The iconic photo of Earth known as The Blue Marble, taken by the crew of Apollo 17 (1972), and similar images of Earth from space, might have popularized Earth as a theme in fiction. Additionally, it is undeniable that an overwhelming majority of fiction is set on or features the Earth. Earth as a planet has been subject to various works of literary treatments. Its climate itself is related to the entire genre known as climate fiction, and its future is a major aspect of the Dying Earth genre as well as the apocalyptic and post-apocalyptic fiction. Depiction of Earth In the ancient past there were varying levels of belief in a flat Earth, with the Mesopotamian culture portraying the world as a flat disk afloat in an ocean. The spherical form of the Earth was suggested by early Greek philosophers; a belief espoused by Pythagoras. By the Middle Ages—as evidenced by thinkers such as Thomas Aquinas—European belief in a spherical Earth was widespread. The technological developments of the latter half of the 20th century are widely considered to have altered the public's perception of the Earth. Before space flight, the popular image of Earth was of a green world. Science fiction artist Frank R. Paul provided perhaps the first image of a cloudless blue planet (with sharply defined land masses) on the back cover of the July 1940 issue of Amazing Stories, a common depiction for several decades thereafter. Earth was first photographed from a satellite by Explorer 6 in 1959. Yuri Gagarin became the first human to view Earth from space in 1961. The crew of the Apollo 8 was the first to view an Earth-rise from lunar orbit in 1968, and astronaut William Anders's photograph of it, Earthrise, became iconic. In 1972 the crew of the Apollo 17 produced The Blue Marble, another famous photograph of the planet Earth from cislunar space. These became iconic images of the planet as a marble of cloud-swirled blue ocean broken by green-brown continents. NASA archivist Mike Gentry has speculated that The Blue Marble is the most widely distributed image in human history. Inspired by The Blue Marble poet-diplomat Abhay K has penned an Earth Anthem describing the planet as a "Cosmic Blue Pearl". A photo taken of a distant Earth by Voyager 1 in 1990 inspired Carl Sagan to name it and describe the planet as a Pale Blue Dot. On Earth Day (22 April) 2023, a collection of images to date of Earth taken from various deep space distances in the Solar System was published. Since the 1960s, Earth has also been described as a massive "Spaceship Earth," with a life support system that requires maintenance, or, in the Gaia hypothesis, as having a biosphere that forms one large organism. Since 2010 the Cupola of the ISS has allowed for a wealth of intricate images of Earth from orbit. Over the past two centuries a growing environmental movement has emerged that is concerned about humankind's effects on the Earth. The key issues of this socio-political movement are the conservation of natural resources, elimination of pollution, and the usage of land. Although diverse in interests and goals, environmentalists as a group tend to advocate sustainable management of resources and stewardship of the environment through changes in public policy and individual behavior. Of particular concern is the large-scale exploitation of non-renewable resources. Changes sought by the environmental movements are sometimes in conflict with commercial interests due to the additional costs associated with managing the environmental impact of those interests. See also Notes References |
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[SOURCE: https://en.wikipedia.org/wiki/Dependent_territory] | [TOKENS: 2188] |
Contents Dependent territory A dependent territory, dependent area, or dependency (sometimes referred as an external territory) is a territory that does not possess full political independence or sovereignty as a sovereign state and remains politically outside the controlling state's integral area. A dependent territory is commonly distinguished from a country subdivision by being considered not to be a constituent part of a sovereign state. An administrative subdivision, instead, is understood to be a division of a state proper. A dependent territory, conversely, often maintains a great degree of autonomy from its controlling state. Historically, most colonies were considered to be dependent territories. Not all autonomous entities are considered to be dependent territories.[failed verification] Most inhabited, dependent territories have their own ISO 3166 country codes. Some political entities inhabit a special position guaranteed by an international treaty or another agreement, thereby creating a certain level of autonomy (e.g. a difference in immigration rules). Those entities are sometimes considered to be, or are at least grouped with, dependent territories, but are officially considered by their governing states to be an integral part of those states. Such an example is Åland, an autonomous region of Finland or Hong Kong, a special administrative region of China Summary The lists below include the following: Lists of dependent territories This list includes all territories that have not been legally incorporated into their governing state, including several territories that are not on the list of non-self-governing territories of the General Assembly of the United Nations. All claims in Antarctica are listed in italics. New Zealand has two self-governing associated states, one dependent territory, and a territorial claim in Antarctica.[better source needed] Norway has one dependent territory and two Antarctic claims. Norway also possesses the inhabited islands of Svalbard where Norwegian sovereignty is limited (see below). The United Kingdom has three "Crown Dependencies", thirteen "Overseas Territories" (ten autonomous, two used primarily as military bases, and one uninhabited), and one Antarctic claim. The United States has 13 "unincorporated" dependent territories under its administration and two claimed territories outside its control. The uninhabited Palmyra Atoll is administered similarly to some of these territories, and is usually included on lists of U.S. overseas territories, but it is excluded from this list because it is classified in U.S. law as an incorporated territory. The U.S. Constitution does not apply in full to the insular areas. Lists of similar entities The following entities are, according to the law of their state, integral parts of the state but exhibit many characteristics of dependent territories. This list is generally limited to entities that are either subject to an international treaty on their status, uninhabited, or have a unique level of autonomy and are largely self-governing in matters other than international affairs. It generally does not include entities with no unique autonomy, such as the five overseas departments and regions (French Guiana, Guadeloupe, Martinique, Mayotte, and Réunion) of France; the BES islands (Bonaire, Sint Eustatius, and Saba) of the Netherlands; Jan Mayen of Norway; and Palmyra Atoll of the United States. Entities with only limited unique autonomy—such as Barbuda of Antigua and Barbuda; Sabah and Sarawak of Malaysia; the two autonomous regions (the Azores and Madeira) of Portugal; Nevis of Saint Kitts and Nevis; the Canary Islands and the two autonomous cities (Ceuta and Melilla) of Spain; Northern Ireland of the United Kingdom; and Kurdistan of Iraq—and entities with non-recognized unique autonomy—such as Wa of Myanmar; Gaza of Palestine; Puntland of Somalia; Zapatista of Mexico; and The autonomous administration north and east Syria of Syria—are also not included. All claims in Antarctica are listed in italics. Australia has six external territories in its administration and one Antarctic claim. Debate remains as to whether the external territories are integral parts of Australia,[citation needed] due to their not being part of Australia in 1901, when its constituent states federated (with the exception of the Coral Sea Islands, which was a part of Queensland). Norfolk Island was self-governing from 1979 to 2016. The external territories are often grouped separately from Australia proper for statistical purposes.[citation needed] The People's Republic of China (PRC) has two special administrative regions (SARs) that are governed according to the constitution and respective basic laws. The SARs greatly differ from Mainland China in administrative, economic, legislative, and judicial terms including by currency, left-hand versus right-hand traffic, official languages, and immigration control. Although the PRC does claim sovereignty over Taiwan (governed by the Republic of China), it is not listed here as the PRC government does not have de facto control of the territory. The Kingdom of Denmark contains two autonomous territories with their own governments and legislatures, and input into foreign affairs. Finland has one autonomous region that is also subject to international treaties. France has overseas six autonomous collectivities and two uninhabited territories (one of which includes an Antarctic claim). This does not include its "standard" overseas regions (which are also overseas departments) of French Guiana, Guadeloupe, Martinique, Mayotte, and Réunion. Although also located overseas, they have the same status as the regions of metropolitan France. Nonetheless, all of France's overseas territory is considered an integral part of the French Republic. The Kingdom of the Netherlands comprises three autonomous "constituent countries" in the Caribbean (listed below) and one constituent country, the Netherlands, with most of its area in Europe but also encompassing three overseas Caribbean municipalities—Bonaire, Sint Eustatius, and Saba (these three Caribbean municipalities are excluded here because they are directly administered by the Government of the Netherlands). All citizens of the Dutch Kingdom share the same nationality and are thus citizens of the European Union, but only the European portion of the Kingdom is a part of the territory of the Union, the Customs Union, and the Eurozone while other areas have overseas countries and territory status. Norway has, in the Arctic, one inhabited archipelago with restrictions placed on Norwegian sovereignty — Svalbard. Unlike the country's dependent territory (Bouvet Island) and Antarctic claims (see above), Svalbard is a part of the Kingdom of Norway. Norway also has one uninhabited remote archipelago located in the Arctic, Jan Mayen, but it is excluded in this list as the island is directly administered by the Nordland County Municipality and none of the considerations established for Svalbard Treaty are attributed to it. Description Three Crown Dependencies are in a form of association with the United Kingdom. They are independently administrated jurisdictions, although the British Government is solely responsible for defence and international representation and has ultimate responsibility for ensuring good government. They do not have diplomatic recognition as independent states, but neither are they integrated into the UK. The UK Parliament retains the ability to legislate for the crown dependencies even without the agreement of their legislatures. No crown dependency has representation in the UK Parliament. Although they are British Overseas Territories, Bermuda and Gibraltar have similar relationships to the UK as do the Crown Dependencies. While the United Kingdom is officially responsible for their defence and international representation, these jurisdictions maintain their own militaries and have been granted limited diplomatic powers, in addition to having internal self-government. New Zealand and its dependencies share the same governor-general and constitute one monarchic realm. The Cook Islands and Niue are officially termed associated states. Puerto Rico (since 1952) and the Northern Mariana Islands (since 1986) are non-independent states freely associated with the United States. The mutually negotiated Covenant to Establish a Commonwealth of the Northern Mariana Islands (CNMI) in Political Union with the United States was approved in 1976. The covenant was fully implemented on November 3, 1986, under Presidential Proclamation no. 5564, which conferred U.S. citizenship on legally qualified CNMI residents. Under the Constitution of Puerto Rico, Puerto Rico is described as a Commonwealth and Puerto Ricans have a degree of administrative autonomy similar to that of a citizen of a U.S. state. Puerto Ricans "were collectively made U.S. citizens" in 1917, as a result of the Jones–Shafroth Act. The commonly used name in Spanish of the Commonwealth of Puerto Rico, Estado Libre Asociado de Puerto Rico, literally "Associated Free State of Puerto Rico", which sounds similar to "free association" particularly when loosely used in Spanish, is sometimes erroneously interpreted to mean that Puerto Rico's relationship with the United States is based on a Compact of Free Association and at other times is erroneously held to mean that Puerto Rico's relationship with the U.S. is based on an Interstate compact. This is a constant source of ambiguity and confusion when trying to define, understand, and explain Puerto Rico's political relationship with the United States. For various reasons Puerto Rico's political status differs from that of the Pacific Islands that entered into Compacts of Free Association with the United States. As sovereign states, these islands have the full right to conduct their foreign relations, while the Commonwealth of Puerto Rico has territorial status subject to U.S. congressional authority under the Constitution's Territory Clause, "to dispose of and make all needful Rules and Regulations respecting the Territory… belonging to the United States." Puerto Rico does not have the right to unilaterally declare independence, and at the last referendum (1998), the narrow majority voted for "none of the above", which was a formally undefined alternative used by commonwealth supporters to express their desire for an "enhanced commonwealth" option. This kind of relationship can also be found in the Kingdom of the Netherlands, which is termed a federacy. The European continental part is organised like a unitary state. However, the status of its "constituent countries" in the Caribbean (Aruba, Curaçao, and Sint Maarten) can be considered akin to dependencies or "associated non-independent states." The Kingdom of Denmark also operates similarly, akin to another federacy. The Faroe Islands and Greenland are two self-governing territories or regions within the Kingdom. The relationship between Denmark proper and these two territories is semi-officially termed the Rigsfællesskabet ("Unity of the Realm"). Overview of inhabited dependent territories See also Notes References Bibliography • British Overseas Territories • Overseas France (Overseas collectivity Overseas country of France) • Realm of New Zealand (Political status of the Cook Islands and Niue) • States and territories of Australia (Australian Indian Ocean Territories) |
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[SOURCE: https://en.wikipedia.org/wiki/Natural_environment] | [TOKENS: 3990] |
Contents Natural environment The natural environment or natural world encompasses all biotic and abiotic things occurring naturally, meaning in this case not artificial. The term is most often applied to Earth or some parts of Earth. This environment encompasses the interaction of all living species, climate, weather and natural resources that affect human survival and economic activity. The concept of the natural environment can be distinguished as components: In contrast to the natural environment is the built environment. Built environments are where humans have fundamentally transformed landscapes such as urban settings and agricultural land conversion, the natural environment is greatly changed into a simplified human environment. Even acts which seem less extreme, such as building a mud hut or a photovoltaic system in the desert, the modified environment becomes an artificial one. Though many animals build things to provide a better environment for themselves, they are not human, hence beaver dams and the works of mound-building termites are thought of as natural. There are no absolutely natural environments on Earth. Naturalness usually varies in a continuum, from 100% natural in one extreme to 0% natural in the other. The massive environmental changes of humanity in the Anthropocene have fundamentally affected all natural environments including: climate change, biodiversity loss and pollution from plastic and other chemicals in the air and water. More precisely, we can consider the different aspects or components of an environment, and see that their degree of naturalness is not uniform. If, for instance, we take an agricultural field, and consider the mineralogic composition and the structure of its soil, we will find that whereas the first is quite similar to that of an undisturbed forest soil, the structure is quite different. Composition Earth science generally recognizes four spheres, the lithosphere, the hydrosphere, the atmosphere and the biosphere as correspondent to rocks, water, air and life respectively. Some scientists include as part of the spheres of the Earth, the cryosphere (corresponding to ice) as a distinct portion of the hydrosphere, as well as the pedosphere (to soil) as an active and intermixed sphere. Earth science (also known as geoscience, the geographical sciences or the Earth Sciences), is an all-embracing term for the sciences related to the planet Earth. There are four major disciplines in earth sciences, namely geography, geology, geophysics and geodesy. These major disciplines use physics, chemistry, biology, chronology and mathematics to build a qualitative and quantitative understanding of the principal areas or spheres of Earth. Geological activity The Earth's crust or lithosphere, is the outermost solid surface of the planet and is chemically, physically and mechanically different from underlying mantle. It has been generated greatly by igneous processes in which magma cools and solidifies to form solid rock. Beneath the lithosphere lies the mantle which is heated by the decay of radioactive elements. The mantle though solid is in a state of rheic convection. This convection process causes the lithospheric plates to move, albeit slowly. The resulting process is known as plate tectonics. Volcanoes result primarily from the melting of subducted crust material or of rising mantle at mid-ocean ridges and mantle plumes. Water on Earth Most water is found in various kinds of natural body of water. An ocean is a major body of saline water and a component of the hydrosphere. Approximately 71% of the surface of the Earth (an area of some 362 million square kilometers) is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas. More than half of this area is over 3,000 meters (9,800 ft) deep. Average oceanic salinity is around 35 parts per thousand (ppt) (3.5%), and nearly all seawater has a salinity in the range of 30 to 38 ppt. Though generally recognized as several separate oceans, these waters comprise one global, interconnected body of salt water often referred to as the World Ocean or global ocean. The deep seabeds are more than half the Earth's surface, and are among the least-modified natural environments. The major oceanic divisions are defined in part by the continents, various archipelagos and other criteria, these divisions are, in descending order of size, the Pacific Ocean, the Atlantic Ocean, the Indian Ocean, the Southern Ocean and the Arctic Ocean. A river is a natural watercourse, usually freshwater, flowing toward an ocean, a lake, a sea or another river. A few rivers simply flow into the ground and dry up completely without reaching another body of water. The water in a river is usually in a channel, made up of a stream bed between banks. In larger rivers there is often also a wider floodplain shaped by waters over-topping the channel. Flood plains may be very wide in relation to the size of the river channel. Rivers are a part of the hydrological cycle. Water within a river is generally collected from precipitation through surface runoff, groundwater recharge, springs and the release of water stored in glaciers and snowpacks. Small rivers may also be called by several other names, including stream, creek and brook. Their current is confined within a bed and stream banks. Streams play an important corridor role in connecting fragmented habitats and thus in conserving biodiversity. The study of streams and waterways in general is known as surface hydrology. A lake (from Latin lacus) is a terrain feature, a body of water that is localized to the bottom of basin. A body of water is considered a lake when it is inland, is not part of an ocean and is larger and deeper than a pond. Natural lakes on Earth are generally found in mountainous areas, rift zones and areas with ongoing or recent glaciation. Other lakes are found in endorheic basins or along the courses of mature rivers. In some parts of the world, there are many lakes because of chaotic drainage patterns left over from the last ice age. All lakes are temporary over geologic time scales, as they will slowly fill in with sediments or spill out of the basin containing them. A pond is a body of standing water, either natural or human-made, that is usually smaller than a lake. A wide variety of human-made bodies of water are classified as ponds, including water gardens designed for aesthetic ornamentation, fish ponds designed for commercial fish breeding and solar ponds designed to store thermal energy. Ponds and lakes are distinguished from streams by their current speed. While currents in streams are easily observed, ponds and lakes possess thermally driven micro-currents and moderate wind-driven currents. These features distinguish a pond from many other aquatic terrain features, such as stream pools and tide pools. Humans impact the water in different ways such as modifying rivers (through dams and stream channelization), urbanization and deforestation. These impact lake levels, groundwater conditions, water pollution, thermal pollution, and marine pollution. Humans modify rivers by using direct channel manipulation. We build dams and reservoirs and manipulate the direction of the rivers and water path. Dams can usefully create reservoirs and hydroelectric power. However, reservoirs and dams may negatively impact the environment and wildlife. Dams stop fish migration and the movement of organisms downstream. Urbanization affects the environment because of deforestation and changing lake levels, groundwater conditions, etc. Deforestation and urbanization go hand in hand. Deforestation may cause flooding, declining stream flow and changes in riverside vegetation. The changing vegetation occurs because when trees cannot get adequate water they start to deteriorate, leading to a decreased food supply for the wildlife in an area. Atmosphere, climate and weather The atmosphere of the Earth serves as a key factor in sustaining the planetary ecosystem. The thin layer of gases that envelops the Earth is held in place by the planet's gravity. Dry air consists of 78% nitrogen, 21% oxygen, 1% argon, inert gases and carbon dioxide. The remaining gases are often referred to as trace gases. The atmosphere includes greenhouse gases such as carbon dioxide, methane, nitrous oxide and ozone. Filtered air includes trace amounts of many other chemical compounds. Air also contains a variable amount of water vapor and suspensions of water droplets and ice crystals seen as clouds. Many natural substances may be present in tiny amounts in an unfiltered air sample, including dust, pollen and spores, sea spray, volcanic ash and meteoroids. Various industrial pollutants also may be present, such as chlorine (elementary or in chlorine compounds), fluorine compounds, elemental mercury, and sulfur compounds such as sulfur dioxide (SO2). The ozone layer of the Earth's atmosphere plays an important role in reducing the amount of ultraviolet (UV) radiation that reaches the surface. As DNA is readily damaged by UV light, this serves to protect life at the surface. The atmosphere also retains heat during the night, thereby reducing the daily temperature extremes. Earth's atmosphere can be divided into five main layers. These layers are mainly determined by whether temperature increases or decreases with altitude. From highest to lowest, these layers are: Within the five principal layers determined by temperature there are several layers determined by other properties. The dangers of global warming are being increasingly studied by a wide global consortium of scientists. These scientists are increasingly concerned about the potential long-term effects of global warming on our natural environment and on the planet. Of particular concern is how climate change and global warming caused by anthropogenic, or human-made releases of greenhouse gases, most notably carbon dioxide, can act interactively and have adverse effects upon the planet, its natural environment and humans' existence. It is clear the planet is warming, and warming rapidly. This is due to the greenhouse effect, which is caused by greenhouse gases, which trap heat inside the Earth's atmosphere because of their more complex molecular structure which allows them to vibrate and in turn trap heat and release it back towards the Earth. This warming is also responsible for the extinction of natural habitats, which in turn leads to a reduction in wildlife population. The most recent report from the Intergovernmental Panel on Climate Change (the group of the leading climate scientists in the world) concluded that the earth will warm anywhere from 2.7 to almost 11 degrees Fahrenheit (1.5 to 6 degrees Celsius) between 1990 and 2100. Efforts have been increasingly focused on the mitigation of greenhouse gases that are causing climatic changes, on developing adaptative strategies to global warming, to assist humans, other animal, and plant species, ecosystems, regions and nations in adjusting to the effects of global warming. Some examples of recent collaboration to address climate change and global warming include: A significantly profound challenge is to identify the natural environmental dynamics in contrast to environmental changes not within natural variances. A common solution is to adapt a static view neglecting natural variances to exist. Methodologically, this view could be defended when looking at processes which change slowly and short time series, while the problem arrives when fast processes turns essential in the object of the study. Climate looks at the statistics of temperature, humidity, atmospheric pressure, wind, rainfall, atmospheric particle count and other meteorological elements in a given region over long periods of time. Weather, on the other hand, is the present condition of these same elements over periods up to two weeks. Climates can be classified according to the average and typical ranges of different variables, most commonly temperature and precipitation. The most commonly used classification scheme is the one originally developed by Wladimir Köppen. The Thornthwaite system, in use since 1948, uses evapotranspiration as well as temperature and precipitation information to study animal species diversity and the potential impacts of climate changes. Weather is a set of all the phenomena occurring in a given atmospheric area at a given time. Most weather phenomena occur in the troposphere, just below the stratosphere. Weather refers, generally, to day-to-day temperature and precipitation activity, whereas climate is the term for the average atmospheric conditions over longer periods of time. When used without qualification, weather is understood to be the weather of Earth. Weather occurs due to density (temperature and moisture) differences between one place and another. These differences can occur due to the sun angle at any particular spot, which varies by latitude from the tropics. The strong temperature contrast between polar and tropical air gives rise to the jet stream. Weather systems in the mid-latitudes, such as extratropical cyclones, are caused by instabilities of the jet stream flow. Because the Earth's axis is tilted relative to its orbital plane, sunlight is incident at different angles at different times of the year. On the Earth's surface, temperatures usually range ±40 °C (100 °F to −40 °F) annually. Over thousands of years, changes in the Earth's orbit have affected the amount and distribution of solar energy received by the Earth and influenced long-term climate. Surface temperature differences in turn cause pressure differences. Higher altitudes are cooler than lower altitudes due to differences in compressional heating. Weather forecasting is the application of science and technology to predict the state of the atmosphere for a future time and a given location. The atmosphere is a chaotic system, and small changes to one part of the system can grow to have large effects on the system as a whole. Human attempts to control the weather have occurred throughout human history, and there is evidence that civilized human activity such as agriculture and industry has inadvertently modified weather patterns. Life Evidence suggests that life on Earth has existed for about 3.7 billion years. All known life forms share fundamental molecular mechanisms, and based on these observations, theories on the origin of life attempt to find a mechanism explaining the formation of a primordial single cell organism from which all life originates. There are many different hypotheses regarding the path that might have been taken from simple organic molecules via pre-cellular life to protocells and metabolism. Although there is no universal agreement on the definition of life, scientists generally accept that the biological manifestation of life is characterized by organization, metabolism, growth, adaptation, response to stimuli and reproduction. Life may also be said to be simply the characteristic state of organisms. In biology, the science of living organisms, "life" is the condition which distinguishes active organisms from inorganic matter, including the capacity for growth, functional activity and the continual change preceding death. A diverse variety of living organisms (life forms) can be found in the biosphere on Earth, and properties common to these organisms—plants, animals, fungi, protists, archaea, and bacteria—are a carbon- and water-based cellular form with complex organization and heritable genetic information. Living organisms undergo metabolism, maintain homeostasis, possess a capacity to grow, respond to stimuli, reproduce and, through natural selection, adapt to their environment in successive generations. More complex living organisms can communicate through various means. Ecosystems An ecosystem (also called an environment) is a natural unit consisting of all plants, animals, and micro-organisms (biotic factors) in an area functioning together with all of the non-living physical (abiotic) factors of the environment. Central to the ecosystem concept is the idea that living organisms are continually engaged in a highly interrelated set of relationships with every other element constituting the environment in which they exist. Eugene Odum, one of the founders of the science of ecology, stated: "Any unit that includes all of the organisms (i.e.: the "community") in a given area interacting with the physical environment so that a flow of energy leads to clearly defined trophic structure, biotic diversity, and material cycles (i.e.: exchange of materials between living and nonliving parts) within the system is an ecosystem." The human ecosystem concept is then grounded in the deconstruction of the human/nature dichotomy, and the emergent premise that all species are ecologically integrated with each other, as well as with the abiotic constituents of their biotope. A more significant number or variety of species or biological diversity of an ecosystem may contribute to greater resilience of an ecosystem because there are more species present at a location to respond to change and thus "absorb" or reduce its effects. This reduces the effect before the ecosystem's structure changes to a different state. This is not universally the case and there is no proven relationship between the species diversity of an ecosystem and its ability to provide goods and services on a sustainable level. The term ecosystem can also pertain to human-made environments, such as human ecosystems and human-influenced ecosystems. It can describe any situation where there is relationship between living organisms and their environment. Fewer areas on the surface of the earth today exist free from human contact, although some genuine wilderness areas continue to exist without any forms of human intervention. Biogeochemical cycles Global biogeochemical cycles are critical to life, most notably those of water, oxygen, carbon, nitrogen and phosphorus. Wilderness Wilderness is generally defined as a natural environment on Earth that has not been significantly modified by human activity. The WILD Foundation goes into more detail, defining wilderness as: "The most intact, undisturbed wild natural areas left on our planet – those last truly wild places that humans do not control and have not developed with roads, pipelines or other industrial infrastructure." Wilderness areas and protected parks are considered important for the survival of certain species, ecological studies, conservation, solitude, and recreation. Wilderness is deeply valued for cultural, spiritual, moral, and aesthetic reasons. Some nature writers believe wilderness areas are vital for the human spirit and creativity. The word, "wilderness", derives from the notion of wildness; in other words that which is not controllable by humans. The word etymology is from the Old English wildeornes, which in turn derives from wildeor meaning wild beast (wild + deor = beast, deer). From this point of view, it is the wildness of a place that makes it a wilderness. The mere presence or activity of people does not disqualify an area from being "wilderness". Many ecosystems that are, or have been, inhabited or influenced by activities of people may still be considered "wild". This way of looking at wilderness includes areas within which natural processes operate without very noticeable human interference. Wildlife includes all non-domesticated plants, animals and other organisms. Domesticating wild plant and animal species for human benefit has occurred many times all over the planet, and has a major impact on the environment, both positive and negative. Wildlife can be found in all ecosystems. Deserts, rain forests, plains, and other areas—including the most developed urban sites—all have distinct forms of wildlife. While the term in popular culture usually refers to animals that are untouched by civilized human factors, most scientists agree that wildlife around the world is (now) impacted by human activities. Challenges It is the common understanding of natural environment that underlies environmentalism—a broad political, social and philosophical movement that advocates various actions and policies in the interest of protecting what nature remains in the natural environment, or restoring or expanding the role of nature in this environment. While true wilderness is increasingly rare, wild nature (e.g., unmanaged forests, uncultivated grasslands, wildlife, wildflowers) can be found in many locations previously inhabited by humans. Goals for the benefit of people and natural systems, commonly expressed by environmental scientists and environmentalists include: Criticism In some cultures the term environment is meaningless because there is no separation between people and what they view as the natural world, or their surroundings. Specifically in the United States and Arabian countries many native cultures do not recognize the "environment", or see themselves as environmentalists. See also References Further reading External links |
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[SOURCE: https://en.wikipedia.org/wiki/Nature] | [TOKENS: 7691] |
Contents Nature Nature is an inherent character or constitution, particularly of the ecosphere or the universe as a whole. In this general sense nature refers to the laws, elements and phenomena of the physical world, including life. Although humans are part of nature, human activity or humans as a whole are often described as at times at odds, or outright separate and even superior to nature. During the advent of modern scientific method in the last several centuries, nature became the passive reality, organized and moved by divine laws. With the Industrial Revolution, nature increasingly became seen as the part of reality deprived from intentional intervention: it was hence considered as sacred by some traditions (Rousseau, American transcendentalism) or a mere decorum for divine providence or human history (Hegel, Marx). However, a vitalist vision of nature, closer to the pre-Socratic one, got reborn at the same time, especially after Charles Darwin. Within the various uses of the word today, "nature" often refers to geology and wildlife. Nature can refer to the general realm of living beings, and in some cases to the processes associated with inanimate objects—the way that particular types of things exist and change of their own accord, such as the weather and geology of the Earth. It is often taken to mean the "natural environment" or wilderness—wild animals, rocks, forest, and in general those things that have not been substantially altered by human intervention, or which persist despite human intervention. For example, manufactured objects and human interaction generally are not considered part of nature, unless qualified as, for example, "human nature" or "the whole of nature". This more traditional concept of natural things that can still be found today implies a distinction between the natural and the artificial, with the artificial being understood as that which has been brought into being by a human consciousness or a human mind. Depending on the particular context, the term "natural" might also be distinguished from the unnatural or the supernatural. Etymology The word nature is borrowed from the Old French nature and is derived from the Latin word natura, or "essential qualities, innate disposition", and in ancient times, literally meant "birth". In ancient philosophy, natura is mostly used as the Latin translation of the Greek word physis (φύσις), which originally related to the intrinsic characteristics of plants, animals, and other features of the world to develop of their own accord. The concept of nature as a whole, the physical universe, is one of several expansions of the original notion; it began with certain core applications of the word φύσις by pre-Socratic philosophers (though this word had a dynamic dimension then, especially for Heraclitus), and has steadily gained currency ever since. Earth Earth is the only planet known to support life, and its natural features are the subject of many fields of scientific research. Within the Solar System, it is third closest to the Sun; it is the largest terrestrial (rocky) planet and the fifth largest overall. Its most prominent climatic features are its two large polar regions, two relatively narrow temperate zones, and a wide equatorial tropical to subtropical region. Precipitation varies widely with location, from several metres of water per year to less than a millimetre. 71 percent of the Earth's surface is covered by salt-water oceans. The remainder consists of continents and islands, with a majority of the inhabited land in the Northern Hemisphere. Earth has evolved through geological and biological processes that have left few traces of the original conditions. The outer surface is divided into several gradually migrating tectonic plates. The interior remains active, with a thick layer of plastic mantle and an iron-filled core that generates a magnetic field. This iron core is composed of a solid inner phase, and a fluid outer phase. Convective motion in the outer core generates electric currents through dynamo action, and these, in turn, generate the geomagnetic field. The atmospheric conditions have been significantly altered from the original conditions by the presence of life-forms, which create an ecological balance that stabilizes the surface conditions. Despite the wide regional variations in climate by latitude and other geographic factors, the long-term average global climate is quite stable during interglacial periods, and variations of a degree or two of average global temperature have historically had major effects on the ecological balance, and on the actual geography of the Earth. Geology is the science and study of the solid and liquid matter that constitutes the Earth. The field of geology encompasses the study of the composition, structure, physical properties, dynamics, and history of Earth materials, and the processes by which they are formed, moved, and changed. The field is a major academic discipline, and is also important for mineral and hydrocarbon extraction, knowledge about and mitigation of natural hazards, some Geotechnical engineering fields, and understanding past climates and environments. The geology of an area evolves through time as rock units are deposited and inserted and deformational processes change their shapes and locations. Rock units are first emplaced either by deposition onto the surface or intrude into the overlying rock. Deposition can occur when sediments settle onto the surface of the Earth and later lithify into sedimentary rock, or when as volcanic material such as volcanic ash or lava flows, blanket the surface. Igneous intrusions such as batholiths, laccoliths, dikes, and sills, push upwards into the overlying rock, and crystallize as they intrude. After the initial sequence of rocks has been deposited, the rock units can be deformed and/or metamorphosed. Deformation typically occurs as a result of horizontal shortening, horizontal extension, or side-to-side (strike-slip) motion. These structural regimes broadly relate to convergent boundaries, divergent boundaries, and transform boundaries, respectively, between tectonic plates. Earth is estimated to have formed 4.54 billion years ago from the solar nebula, along with the Sun and other planets. The Moon formed roughly 20 million years later. Initially molten, the outer layer of the Earth cooled, resulting in the solid crust. Outgassing and volcanic activity produced the primordial atmosphere. Condensing water vapor, most or all of which came from ice delivered by comets, produced the oceans and other water sources. The highly energetic chemistry is believed to have produced a self-replicating molecule around 4 billion years ago. Continents formed, then broke up and reformed as the surface of Earth reshaped over hundreds of millions of years, occasionally combining to make a supercontinent. Roughly 750 million years ago, the earliest known supercontinent Rodinia, began to break apart. The continents later recombined to form Pannotia which broke apart about 540 million years ago, then finally Pangaea, which broke apart about 180 million years ago. During the Neoproterozoic era, freezing temperatures covered much of the Earth in glaciers and ice sheets. This hypothesis has been termed the "Snowball Earth", and it is of particular interest as it precedes the Cambrian explosion in which multicellular life forms began to proliferate about 530–540 million years ago. Since the Cambrian explosion there have been five distinctly identifiable mass extinctions. The last mass extinction occurred some 66 million years ago, when a meteorite collision probably triggered the extinction of the non-avian dinosaurs and other large reptiles, but spared small animals such as mammals. Over the past 66 million years, mammalian life diversified. Several million years ago, a species of small African ape gained the ability to stand upright. The subsequent advent of human life, and the development of agriculture and further civilization allowed humans to affect the Earth more rapidly than any previous life form, impacting both the nature and quantity of other organisms as well as global climate. By comparison, the Great Oxygenation Event, produced by the proliferation of algae during the Siderian period, required about 400 million years to culminate. The present era is classified as part of a mass extinction event, the Holocene extinction event, the fastest ever to have occurred. Some, such as E. O. Wilson of Harvard University, predict that human destruction of the biosphere could cause the extinction of one-half of all species in the next 100 years. The extent of the current extinction event is still being researched, debated and calculated by biologists. Atmosphere, climate, and weather The Earth's atmosphere is a key factor in sustaining the ecosystem. The thin layer of gases that envelops the Earth is held in place by gravity. Air is mostly nitrogen, oxygen, water vapor, with much smaller amounts of carbon dioxide, argon, etc.: 258 The atmospheric pressure and density declines steadily with altitude. The ozone layer plays an important role in depleting the amount of ultraviolet (UV) radiation that reaches the surface. As DNA is readily damaged by UV light, this serves to protect life at the surface. The atmosphere also retains heat during the night, thereby reducing the daily temperature extremes. Terrestrial weather occurs almost exclusively in the lower part of the atmosphere, and serves as a convective system for redistributing heat. Weather is a chaotic system that is readily modified by small changes to the environment, so accurate weather forecasting is limited to only a few days. Weather is also influenced by the seasons, which result from the Earth's axis being tilted relative to its orbital plane. Thus, at any given time during the summer or winter, one part of the Earth is more directly exposed to the rays of the sun. This exposure alternates as the Earth revolves in its orbit. At any given time, regardless of season, the Northern and Southern Hemispheres experience opposite seasons. Weather can have both beneficial and harmful effects. Lightning strikes can cause wildfires, while heavy rain can cause flooding and mud slides. Extremes in weather, such as tornadoes or hurricanes and cyclones, can expend large amounts of energy along their paths, and produce devastation. Surface vegetation has evolved a dependence on the seasonal variation of the weather, and sudden changes lasting only a few years can have a stress effect on the plants. These pose a threat to the animals that depend on its growth for their food. Climate is a measure of the long-term trends in the weather. Various factors are known to influence the climate, including ocean currents, surface albedo, greenhouse gases, variations in the solar luminosity, and changes to the Earth's orbit. Based on historical and geological records, the Earth is known to have undergone drastic climate changes in the past, including ice ages. In the present day, two things are happening worldwide: (1) temperature is increasing on the average; and (2) regional climates have been undergoing noticeable changes. Ocean currents are an important factor in determining climate, particularly the major underwater thermohaline circulation which distributes heat energy from the equatorial oceans to the polar regions. These currents help to moderate the differences in temperature between winter and summer in the temperate zones. Also, without the redistributions of heat energy by the ocean currents and atmosphere, the tropics would be much hotter, and the polar regions much colder. The climate of a region depends on a number of factors, including topology, prevailing winds, proximity to a large body of water, and especially latitude. A latitudinal band of the surface with similar climatic attributes forms a climate region. There are a number of such regions, ranging from the tropical climate at the equator to the polar climate in the northern and southern extremes. The latter regions are typically below the freezing temperature of water for much of the year, which can allow frozen water to accumulate in ice caps and thereby changing the surface albedo. Water on Earth Water is a chemical substance that is composed of hydrogen and oxygen (H2O) and is vital for all known forms of life. In typical usage, "water" refers only to its liquid form, but it also has a solid state, ice, and a gaseous state, water vapor, or steam. Water covers 71% of the Earth's surface. On Earth, it is found mostly in oceans and other large bodies of water, with 1.6% of water below ground in aquifers and 0.001% in the air as vapor, clouds, and precipitation. Oceans hold 96.5% of surface water; glaciers and polar ice caps, 2.4%; and other land surface water such as rivers, lakes, ponds, underground aquifers, and groundwater, 1%. The smallest freshwater reserve is the 0.1% in the atmosphere. Through subduction processes in the Earth's crust, an equivalent mass of the planet's surface water has been interred in the upper mantle alone. An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface (an area of some 361 million square kilometers) is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas. More than half of this area is over 3,000 meters (9,800 feet) deep. Average oceanic salinity is around 35 parts per thousand (ppt) (3.5%), and nearly all seawater has a salinity in the range of 30 to 38 ppt. Though generally recognized as several 'separate' oceans, these waters comprise one global, interconnected body of salt water often referred to as the World Ocean or global ocean. This is a fundamental concept in oceanography: a global-spanning ocean that functions as a continuous body of water with relatively free interchange among its bodies. The major oceanic divisions are determined by the various continents, archipelagos, and other criteria. In descending order of size, they are the Pacific Ocean, the Atlantic Ocean, the Indian Ocean, the Southern Ocean, and the Arctic Ocean. Smaller regions of the oceans are called seas, gulfs, bays and other names. There are also salt lakes, which are smaller bodies of landlocked saltwater that are not interconnected with the World Ocean. Two notable examples of salt lakes are the Great Salt Lake and the Caspian Sea. No other planet in the Solar System has surface oceans, although there are 15 moons that are suspected of having ice-covered oceans. A lake (from Latin word lacus) is a terrain feature (or physical feature), a body of liquid on the surface of a world that is localized to the bottom of basin (another type of landform or terrain feature; that is, it is not global) and moves slowly if it moves at all. On Earth, a body of water is considered a lake when it is inland, not part of the ocean, is larger and deeper than a pond, and is fed by a river. The only world other than Earth known to harbor lakes is Titan, Saturn's largest moon, which has lakes of ethane, most likely mixed with methane. It is not known if Titan's lakes are fed by rivers, though Titan's surface is carved by numerous river beds. Natural lakes on Earth are generally found in mountainous areas, rift zones, and areas with ongoing or recent glaciation. Other lakes are found in endorheic basins, along the courses of mature rivers, or human-made reservoirs behind dams. In some parts of the world, there are many lakes because of chaotic drainage patterns left over from the last ice age. All lakes are temporary over geologic time scales, as they will slowly fill in with sediments or spill out of the basin containing them. Small bodies of standing water, typically less than 2 Hectare, are termed a pond or pool. They can be natural or human-made. A wide variety of human-made bodies of water are classified as ponds, including water gardens designed for aesthetic ornamentation, fish ponds designed for commercial fish breeding, and solar ponds designed to store thermal energy. Ponds and lakes are distinguished from streams via current speed. While currents in streams are easily observed, ponds possess thermally driven micro-currents and moderate wind driven currents. These features distinguish a pond from many other aquatic terrain features, such as stream pools and tide pools.[citation needed] A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. A river is part of the hydrological cycle. Water within a river is generally collected from precipitation through surface runoff, groundwater recharge, springs, and the release of stored water in natural ice and snowpacks (i.e., from glaciers). Where a river merges with a slow-moving body of water, the deposited sedimentation can build up to form a delta. There is no general rule that defines what can be called a river. Smaller scale water flows with a steady current are termed a stream, creek, brook, rivulet, or rill. These are confined within a stream bed and bank. Many names for small rivers are specific to geographic location; one example is Burn in Scotland and North-east England. In US naming, sometimes a river is said to be larger than a creek, but this is not always the case, due to vagueness in the language; consequently the US Geographic Names Information System calls all "linear flowing bodies of water" streams. Streams are important as conduits in the water cycle, instruments in groundwater recharge, and they serve as corridors for fish and wildlife migration. The biological habitat in the immediate vicinity of a stream is called a riparian zone. Given the status of the ongoing Holocene extinction, streams play an important corridor role in connecting fragmented habitats and thus in conserving biodiversity. The study of streams and waterways in general involves many branches of inter-disciplinary natural science and engineering, including hydrology, fluvial geomorphology, aquatic ecology, fish biology, riparian ecology, and others. Ecosystems Ecosystems are composed of a variety of biotic and abiotic components that function in an interrelated way. The structure and composition is determined by various environmental factors that are interrelated. Variations of these factors will initiate dynamic modifications to the ecosystem. Some of the more important components are soil, atmosphere, radiation from the sun, water, and living organisms. Central to the ecosystem concept is the idea that living organisms interact with every other element in their local environment. Eugene Odum, a founder of ecology, stated: "Any unit that includes all of the organisms (i.e.: the "community") in a given area interacting with the physical environment so that a flow of energy leads to clearly defined trophic structure, biotic diversity, and material cycles (i.e.: exchange of materials between living and nonliving parts) within the system is an ecosystem." Within the ecosystem, species are connected and dependent upon one another in the food chain, and exchange energy and matter between themselves as well as with their environment. The human ecosystem concept is based on the human/nature dichotomy and the idea that all species are ecologically dependent on each other, as well as with the abiotic constituents of their biotope. A smaller unit of size is called a microecosystem. For example, a microsystem can be a stone and all the life under it. A macroecosystem might involve a whole ecoregion, with its drainage basin. Wilderness is generally defined as areas that have not been significantly modified by human activity. Wilderness areas can be found in preserves, estates, farms, conservation preserves, ranches, national forests, national parks, and even in urban areas along rivers, gulches, or otherwise undeveloped areas. Wilderness areas and protected parks are considered important for the survival of certain species, ecological studies, conservation, and solitude. Some nature writers believe wilderness areas are vital for the human spirit and creativity, and some ecologists consider wilderness areas to be an integral part of the Earth's self-sustaining natural ecosystem (the biosphere). They may also preserve historic genetic traits and that they provide habitat for wild flora and fauna that may be difficult or impossible to recreate in zoos, arboretums, or laboratories. Life Although there is no universal agreement on the definition of life, scientists generally accept that the biological manifestation of life is characterized by organization, metabolism, growth, adaptation, response to stimuli, and reproduction. Life may also be said to be simply the characteristic state of organisms. The latter can then be defined in terms of biochemistry, genetics, or thermodynamics. Properties common to terrestrial organisms (plants, animals, fungi, protists, archaea, and bacteria) are that they are cellular and based on a complex chemical organization. However, not every definition of life considers these properties to be essential. Human-made analogs of life may also be considered to be life. Present day organisms from viruses to humans possess a self-replicating informational molecule (genome), either DNA or RNA (as in some viruses), and such an informational molecule is probably intrinsic to life. It is likely that the earliest forms of life were based on a self-replicating informational molecule (genome), perhaps RNA or a molecule more primitive than RNA or DNA. The specific nucleotide sequence in each organism contains information that functions to promotes survival, reproduction, and the capacity to acquire resources necessary for reproduction; such sequences probably emerged early in the evolution of life. Survival functions present early in the evolution of life likely also included genomic sequences that promote the avoidance of damage to the self-replicating molecule and also the capability to repair such damages that do occur. Repair of some genome damages may have involved using information from another similar molecule by a process of recombination (a primitive form of sexual interaction). The biosphere is the part of Earth's outer shell—including land, surface rocks, water, air and the atmosphere—within which life occurs, and which biotic processes in turn alter or transform. From the broadest geophysiological point of view, the biosphere is the global ecological system integrating all living beings and their relationships, including their interaction with the elements of the lithosphere (rocks), hydrosphere (water), and atmosphere (air). The entire Earth contains over 75 billion tons (150 trillion pounds or about 6.8×1013 kilograms) of biomass (life), which lives within various environments within the biosphere. Over nine-tenths of the total biomass on Earth is plant life, on which animal life depends very heavily for its existence. More than 2 million species of plant and animal life have been identified to date, and estimates of the actual number of existing species range from several million to well over 50 million. The number of individual species of life is constantly in some degree of flux, with new species appearing and others ceasing to exist on a continual basis. The total number of species is in rapid decline. The origin of life on Earth is not well understood, but it is known to have occurred at least 3.5 billion years ago, during the hadean or archean eons on a primordial Earth that had a substantially different environment than is found at present. These life forms possessed the basic traits of self-replication and inheritable traits. Once life had appeared, the process of evolution by natural selection resulted in the development of ever-more diverse life forms. Species that were unable to adapt to the changing environment and competition from other life forms became extinct. However, the fossil record retains evidence of many of these older species. Current fossil and DNA evidence shows that all existing species can trace a continual ancestry back to the first primitive life forms. When basic forms of plant life developed the process of photosynthesis the sun's energy could be harvested to create conditions which allowed for more complex life forms. The resultant oxygen accumulated in the atmosphere and gave rise to the ozone layer. The incorporation of smaller cells within larger ones resulted in the development of yet more complex cells called eukaryotes. Cells within colonies became increasingly specialized, resulting in true multicellular organisms. With the ozone layer absorbing harmful ultraviolet radiation, life colonized the land surface of Earth. The first form of life to develop on the Earth were unicellular, and they remained the only form of life until about a billion years ago when multi-cellular organisms began to appear. Microorganisms or microbes are microscopic, and smaller than the human eye can see. Microorganisms can be single-celled, such as Bacteria, Archaea, many Protista, and a minority of Fungi. These life forms are found in almost every location on the Earth where there is liquid water, including in the Earth's interior. Their reproduction is both rapid and profuse. The combination of a high mutation rate and a horizontal gene transfer ability makes them highly adaptable, and able to survive in new and sometimes very harsh environments, including outer space. They form an essential part of the planetary ecosystem. However, some microorganisms are pathogenic and can post health risk to other organisms. Viruses are infectious agents, but they are not autonomous life forms, as it is the case for viroids, satellites, DPIs and prions. Originally Aristotle divided all living things between plants, which generally do not move fast enough for humans to notice, and animals. In Linnaeus' system, these became the kingdoms Vegetabilia (later Plantae) and Animalia. Since then, it has become clear that the Plantae as originally defined included several unrelated groups, and the fungi and several groups of algae were removed to new kingdoms. However, these are still often considered plants in many contexts. Bacterial life is sometimes included in flora, and some classifications use the term bacterial flora separately from plant flora. Among the many ways of classifying plants are by regional floras, which, depending on the purpose of study, can also include fossil flora, remnants of plant life from a previous era, including pollen. People in many regions and countries take great pride in their individual arrays of characteristic flora, which can vary widely across the globe due to differences in climate and terrain. Regional floras commonly are divided into categories such as native flora or agricultural and garden flora. Some types of "native flora" actually have been introduced centuries ago by people migrating from one region or continent to another, and become an integral part of the native, or natural flora of the place to which they were introduced. These invasive species are examples of how human interaction with the ecosystem can blur the boundary of what is considered nature. Another category of plant has historically been carved out for weeds. Though the term has fallen into disfavor among botanists as a formal way to categorize "useless" plants, the informal use of the word "weeds" to describe those plants that are deemed worthy of elimination is illustrative of the general tendency of people and societies to seek to alter or shape the course of nature. Similarly, animals are often categorized in ways such as domestic, laboratory, farm animals, wild animals, pests, etc. according to their relationship to human life. Animals as a category have several characteristics that generally set them apart from other living things. Animals are eukaryotic and usually multicellular, which separates them from bacteria, archaea, and most protists. They are heterotrophic, generally digesting food in an internal chamber, which separates them from plants and algae. They are also distinguished from plants, algae, and fungi by lacking cell walls. With a few exceptions—most notably the two phyla consisting of sponges and placozoans—animals have bodies that are differentiated into tissues. These include muscles, which are able to contract and control locomotion, and a nervous system, which sends and processes signals. There is also typically an internal digestive chamber. The eukaryotic cells possessed by all animals are surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins. This may be calcified to form structures like shells, bones, and spicules, a framework upon which cells can move about and be reorganized during development and maturation, and which supports the complex anatomy required for mobility.[citation needed] Human interrelationship Although humans comprise a minuscule proportion of the total living biomass on Earth, the human effect on nature is disproportionately large. Because of the extent of human influence, the boundaries between what humans regard as nature and "made environments" is not clear cut except at the extremes. Even at the extremes, the amount of natural environment that is free of discernible human influence is diminishing at an increasingly rapid pace. A 2020 study published in Nature found that anthropogenic mass (human-made materials) outweighs all living biomass on earth, with plastic alone exceeding the mass of all land and marine animals combined. And according to a 2021 study published in Frontiers in Forests and Global Change, only about 3% of the planet's terrestrial surface is ecologically and faunally intact, with a low human footprint and healthy populations of native animal species. Philip Cafaro, professor of philosophy at the School of Global Environmental Sustainability at Colorado State University, wrote in 2022 that "the cause of global biodiversity loss is clear: other species are being displaced by a rapidly growing human economy." The development of technology by the human race has allowed the greater exploitation of natural resources and has helped to alleviate some of the risk from natural hazards. However, in spite of this progress, the fate of human civilization remains closely linked to changes in the environment. There exists a highly complex feedback loop between the use of advanced technology and changes to the environment. Human-made threats to the Earth's natural environment include pollution, deforestation, and disasters such as oil spills. Humans have contributed to the extinction of many plants and animals, with roughly 1 million species threatened with extinction within decades. The loss of biodiversity and ecosystem functions over the last half century have impacted the extent that nature can contribute to human quality of life, and continued declines could pose a major threat to the existence of human civilization, unless a rapid course correction is made. The value of natural resources to society is often poorly reflected in market prices, because whilst there are extraction costs, natural resources themselves are typically available free of charge. This distorts market pricing of natural resources and at the same time leads to underinvestment in our natural assets. The annual global cost of public subsidies that damage nature is conservatively estimated at $4–6 trillion (million million). Institutional protections of these natural goods, such as the oceans and rainforests, are lacking. Governments have not prevented these economic externalities. Humans employ nature for both leisure and economic activities. The acquisition of natural resources for industrial use remains a sizable component of the world's economic system. Some activities, such as hunting and fishing, are used for both sustenance and leisure, often by different people. Agriculture was first adopted around the 9th millennium BCE. Ranging from food production to energy, nature influences economic wealth. Although early humans gathered uncultivated plant materials for food and employed the medicinal properties of vegetation for healing, most modern human use of plants is through agriculture. The clearance of large tracts of land for crop growth has led to a significant reduction in the amount available of forestation and wetlands, resulting in the loss of habitat for many plant and animal species as well as increased erosion. Beauty in nature has historically been a prevalent theme in art and books, filling large sections of libraries and bookstores. That nature has been depicted and celebrated by so much art, photography, poetry, and other literature shows the strength with which many people associate nature and beauty. Reasons why this association exists, and what the association consists of, are studied by the branch of philosophy called aesthetics. Beyond certain basic characteristics that many philosophers agree about to explain what is seen as beautiful, the opinions are virtually endless. Nature and wildness have been important subjects in various eras of world history. An early tradition of landscape art began in China during the Tang Dynasty (618–907). The tradition of representing nature as it is became one of the aims of Chinese painting and was a significant influence in Asian art.[citation needed] Although natural wonders are celebrated in the Psalms and the Book of Job, in the West, wilderness portrayals in art became more prevalent in the 1800s, especially in the works of the Romantic movement. British artists John Constable and J. M. W. Turner turned their attention to capturing the beauty of the natural world in their paintings. Before that, paintings had been primarily of religious scenes or of human beings.[citation needed] William Wordsworth's poetry described the wonder of the natural world, which had formerly been viewed as a threatening place. Increasingly the valuing of nature became an aspect of Western culture. This artistic movement also coincided with the Transcendentalist movement in the Western world. A common classical idea of beautiful art involves the word mimesis, the imitation of nature. Also in the realm of ideas about beauty in nature is that the perfect is implied through perfect mathematical forms and more generally by patterns in nature. As David Rothenburg writes, "The beautiful is the root of science and the goal of art, the highest possibility that humanity can ever hope to see".: 281 Matter and energy Matter is defined as a substance that has mass and takes up a volume of space, while energy is a property that can make matter perform work. At the quantum mechanical scale of the very tiny, both matter and energy exibit the property of wave–particle duality, and they are related to each other through mass–energy equivalence. Matter constitutes the observable universe, which is made visible by the radiation of energy waves. The visible components of the universe are now believed to compose only 4.9 percent of the total mass. The remainder is in an unknown form that is believed to consist of 26.8 percent cold dark matter and 68.3 percent dark energy. The exact nature of these unseen components is under intensive investigation by physicists. The behaviour of matter and energy throughout the observable universe appears to follow well-defined physical laws, or laws of nature, which scientists seek to understand. These laws have been employed to produce cosmological models that successfully explain the structure and the evolution of the universe we can observe. The mathematical expressions of the laws of physics employ a set of twenty physical constants that appear to be static across the observable universe. The values of these constants have been carefully measured, but the reason for their specific values remains a mystery. The anthropic principle argues that the physical constants have the observed values precisely because intelligent life is here to observe them. Beyond Earth Outer space, also simply called space, refers to the relatively empty regions of the universe outside the atmospheres of celestial bodies. Outer space is used to distinguish it from airspace (and terrestrial locations). There is no discrete boundary between Earth's atmosphere and space, as the atmosphere gradually attenuates with increasing altitude. Outer space within the Solar System is called interplanetary space, which passes over into interstellar space at what is known as the heliopause. Outer space is saturated by blackbody radiation left over from the Big Bang and the origin of the universe. It contains a near-perfect vacuum of predominantly hydrogen and helium plasma, and is permeated by electromagnetic radiation, magnetic fields, and cosmic rays; the latter include various ionized atomic nuclei and subatomic particles. Regions enriched by matter expelled by stars is sparsely filled with dust and numerous types of organic molecules discovered to date by microwave spectroscopy. Near the Earth, there are signs of human life in outer space today, such as material left over from previous crewed and uncrewed launches which are a potential hazard to spacecraft. Some of this debris re-enters the atmosphere periodically. At the largest scale, the visible universe follows the Cosmological principle, appearing uniformly isotropic and homogeneous in all directions. On smaller scales, observable matter is organized in a hierarchy of structures due to the cumulative effect of gravity. Stars are formed in galaxy structures that typically span up to 100,000 light years in scale. These in turn are organized in larger scale galaxy clusters and groups spanning tens of millions of light years, then superclusters that extend hundreds of millions of light years across. The largest known structures are the galaxy filaments that link together superclusters. In the open regions between these structures are vast, nearly empty voids. Individual galaxies have numerous groupings of stars called clusters. All stars can appear individually or in hierarchical systems of co-orbiting stars. Each star can have orbiting sub-stellar bodies at various scales: brown dwarfs, exoplanets, moons, asteroids and comets, down to meteoroids. A major question in astronomy concerns the existence of life elsewhere in the universe. Although Earth is the only body within the Solar System known to support life, evidence suggests that in the distant past the planet Mars possessed bodies of liquid water on the surface. For a brief period in Mars' history, it may have also been capable of forming life. At present though, most of the water remaining on Mars is frozen. If life exists at all on Mars, it is most likely to be located underground where liquid water can still exist. Conditions on the other terrestrial planets, Mercury and Venus, appear to be too harsh to support life as we know it. But it has been conjectured that Europa, the fourth-largest moon of Jupiter, may possess a sub-surface ocean of liquid water and could potentially host life. Astronomers have discovered extrasolar Earth analogs – planets that lie in the habitable zone of space surrounding a star, and therefore could possibly host life. However the requirements for life are not completely known and astronomical observations provide limited information. See also Media: Organizations: Philosophy: Notes and references Further reading External links |
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Contents Gravity of Earth The gravity of Earth, denoted by g, is the net acceleration that is imparted to objects due to the combined effect of gravitation (from mass distribution within Earth) and the centrifugal force (from the Earth's rotation). It is a vector quantity, whose direction coincides with a plumb bob and strength or magnitude is given by the norm g = ‖ g ‖ {\displaystyle g=\|{\mathit {\mathbf {g} }}\|} . In SI units, this acceleration is expressed in metres per second squared (in symbols, m/s2 or m·s−2) or equivalently in newtons per kilogram (N/kg or N·kg−1). Near Earth's surface, the acceleration due to gravity, accurate to 2 significant figures, is 9.8 m/s2 (32 ft/s2). This means that, ignoring the effects of air resistance, the vertical component of velocity of an object falling freely will increase in the downwards direction by about 9.8 metres per second (32 ft/s) every second. The precise strength of Earth's gravity varies with location. The conventional value for standard gravity is 9.80665 m⋅s−2 by definition, originally adopted by the CGPM in 1901.: 159 This quantity is denoted variously as gn, ge, g0, or simply g (which is also used for the variable local value). The weight of an object on Earth's surface is the downwards force on that object, given by Newton's second law of motion, or F = m a (force = mass × acceleration). Gravitational acceleration contributes to the total gravity acceleration, but other factors, such as the rotation of Earth, also contribute, and, therefore, affect the weight of the object. Gravity does not normally include the gravitational pull of the Moon and Sun, which are accounted for in terms of tidal effects. Variation in magnitude A non-rotating perfect sphere of uniform mass density, or whose density varies solely with distance from the centre (spherical symmetry), would produce a gravitational field of uniform magnitude at all points on its surface. The Earth is rotating and is also not spherically symmetric; rather, it is slightly flatter at the poles while bulging at the Equator: an oblate spheroid. There are consequently slight deviations in the magnitude of gravity across its surface. Gravity on the Earth's surface varies by around 0.7%, from 9.7639 m/s2 on the Nevado Huascarán mountain in Peru to 9.8337 m/s2 at the surface of the Arctic Ocean. In large cities, it ranges from 9.7806 m/s2 in Kuala Lumpur, Mexico City, and Singapore to 9.825 m/s2 in Oslo and Helsinki. In 1901, the third General Conference on Weights and Measures defined a standard gravitational acceleration for the surface of the Earth: gn = 9.80665 m/s2. It was based on measurements at the Pavillon de Breteuil near Paris in 1888, with a theoretical correction applied in order to convert to a latitude of 45° at sea level. This definition is thus not a value of any particular place or carefully worked out average, but an agreement for a value to use if a better actual local value is not known or not important. It is also used to define the units kilogram force and pound force. The surface of the Earth is rotating, so it is not an inertial frame of reference. At latitudes nearer the Equator, the outward centrifugal force produced by Earth's rotation is larger than at polar latitudes. This counteracts the Earth's gravity to a small degree – up to a maximum of 0.3% at the Equator – and reduces the apparent downward acceleration of falling objects. The second major reason for the difference in gravity at different latitudes is that the Earth's equatorial bulge (itself also caused by centrifugal force from rotation) causes objects at the Equator to be further from the planet's center than objects at the poles. The force due to gravitational attraction between two masses (a piece of the Earth and the object being weighed) varies inversely with the square of the distance between them. The distribution of mass is also different below someone on the equator and below someone at a pole. The net result is that an object at the Equator experiences a weaker gravitational pull than an object on one of the poles. In combination, the equatorial bulge and the effects of the surface centrifugal force due to rotation mean that sea-level gravity increases from about 9.780 m/s2 at the Equator to about 9.832 m/s2 at the poles, so an object will weigh approximately 0.5% more at the poles than at the Equator. Gravity decreases with altitude as one rises above the Earth's surface because greater altitude means greater distance from the Earth's centre. All other things being equal, an increase in altitude from sea level to 9,000 metres (30,000 ft) causes a weight decrease of about 0.29%. An additional factor affecting apparent weight is the decrease in air density at altitude, which lessens an object's buoyancy. This would increase a person's apparent weight at an altitude of 9,000 metres by about 0.08%. It is a common misconception that astronauts in orbit are weightless because they have flown high enough to escape the Earth's gravity. In fact, at an altitude of 400 kilometres (250 mi), equivalent to a typical orbit of the ISS, gravity is still nearly 90% as strong as at the Earth's surface. Weightlessness actually occurs because orbiting objects are in free-fall. The effect of ground elevation depends on the density of the ground (see Local geology). A person flying at 9,100 m (30,000 ft) above sea level over mountains will feel more gravity than someone at the same elevation but over the sea. However, a person standing on the Earth's surface feels less gravity when the elevation is higher. The following formula approximates the Earth's gravity variation with altitude: where The formula treats the Earth as a perfect sphere with a radially symmetric distribution of mass; a more accurate mathematical treatment is discussed below. An approximate value for gravity at a distance r from the center of the Earth can be obtained by assuming that the Earth's density is spherically symmetric. The force of gravity at a radius r depends only on the mass inside the sphere of that radius. All the contributions from outside cancel out as a consequence of the inverse-square law of gravitation. Another consequence is that the gravity is the same as if all the mass were concentrated at the center. Thus, the gravitational acceleration at this radius is where G is the gravitational constant and M is the total mass enclosed within radius r. This result is known as the Shell theorem; it took Isaac Newton 20 years to prove this result, delaying his work on gravity.: 13 If the Earth had a constant density ρ, the mass would be M(r) = (4/3)πρr3 and the dependence of gravity on depth would be The gravity g′ at depth d is given by g′ = g(1 − d/R) where g is acceleration due to gravity on the surface of the Earth, d is depth and R is the radius of the Earth. If the density decreased linearly with increasing radius from a density ρ0 at the center to ρ1 at the surface, then ρ(r) = ρ0 − (ρ0 − ρ1) r / R, and the dependence would be The actual depth dependence of density and gravity, inferred from seismic travel times (see Adams–Williamson equation), is shown in the graphs below. Local differences in topography (such as the presence of mountains), geology (such as the density of rocks in the vicinity), and deeper tectonic structure cause local and regional differences in the Earth's gravitational field, known as gravity anomalies. Some of these anomalies can be very extensive, resulting in bulges in sea level, and throwing pendulum clocks out of synchronisation. The study of these anomalies forms the basis of gravitational geophysics. The fluctuations are measured with highly sensitive gravimeters, the effect of topography and other known factors is subtracted, and from the resulting data conclusions are drawn. Such techniques are now used by prospectors to find oil and mineral deposits. Denser rocks (often containing mineral ores) cause higher than normal local gravitational fields on the Earth's surface. Less dense sedimentary rocks cause the opposite. There is a strong correlation between the gravity derivation map of earth from NASA GRACE with positions of recent volcanic activity, ridge spreading and volcanos: these regions have a stronger gravitation than theoretical predictions. In air or water, objects experience a supporting buoyancy force which reduces the apparent strength of gravity (as measured by an object's weight). The magnitude of the effect depends on the air density (and hence air pressure) or the water density respectively; see Apparent weight for details. The gravitational effects of the Moon and the Sun (also the cause of the tides) have a very small effect on the apparent strength of Earth's gravity, depending on their relative positions; typical variations are 2 μm/s2 (0.2 mGal) over the course of a day. Direction Gravity acceleration is a vector quantity, with direction in addition to magnitude. In a spherically symmetric Earth, gravity would point directly towards the sphere's centre. As the Earth's figure is slightly flatter, there are consequently significant deviations in the direction of gravity: essentially the difference between geodetic latitude and geocentric latitude. Smaller deviations, called vertical deflection, are caused by local mass anomalies, such as mountains. Comparative values worldwide Tools exist for calculating the strength of gravity at various cities around the world. The effect of latitude can be clearly seen with gravity in high-latitude cities: Anchorage (9.826 m/s2), Helsinki (9.825 m/s2), being about 0.5% greater than that in cities near the equator: Kuala Lumpur (9.776 m/s2). The effect of altitude can be seen in Mexico City (9.776 m/s2; altitude 2,240 metres (7,350 ft)), and by comparing Denver (9.798 m/s2; 1,616 metres (5,302 ft)) with Washington, D.C. (9.801 m/s2; 30 metres (98 ft)), both of which are near 39° N. Measured values can be obtained from Physical and Mathematical Tables by T.M. Yarwood and F. Castle, Macmillan, revised edition 1970. Mathematical models If the terrain is at sea level, we can estimate, for the Geodetic Reference System 1980, g { ϕ } {\displaystyle g\{\phi \}} , the acceleration at latitude ϕ {\displaystyle \phi } : This is the International Gravity Formula 1967, the 1967 Geodetic Reference System Formula, Helmert's equation or Clairaut's formula. An alternative formula for g as a function of latitude is the WGS (World Geodetic System) 84 Ellipsoidal Gravity Formula: where then, where G p = 9.8321849378 m ⋅ s − 2 {\displaystyle \mathbb {G} _{\text{p}}=9.8321849378\,\,\mathrm {m{\cdot }s} ^{-2}} , where the semi-axes of the earth are: The difference between the WGS-84 formula and Helmert's equation is less than 0.68 μm·s−2. Further reductions are applied to obtain gravity anomalies (see Gravity anomaly § Computation). Estimating g from the law of universal gravitation From the law of universal gravitation, the force on a body acted upon by Earth's gravitational force is given by where r is the distance between the centre of the Earth and the body (see below), and here we take M ⊕ {\displaystyle M_{\oplus }} to be the mass of the Earth and m to be the mass of the body. Additionally, Newton's second law, F = ma, where m is mass and a is acceleration, here tells us that Comparing the two formulas it is seen that: So, to find the acceleration due to gravity at sea level, substitute the values of the gravitational constant, G, the Earth's mass (in kilograms), m1, and the Earth's radius (in metres), r, to obtain the value of g: This formula works because the gravity of a uniform spherical body, as measured on or above its surface, is the same as if all its mass were concentrated at a point at its centre. This is what allows us to use the Earth's radius for r. The value obtained agrees approximately with the measured value of g. The difference may be attributed to several factors, mentioned above under "Variation in magnitude": There are significant uncertainties in the values of r and m1 as used in this calculation, and the value of G is also rather difficult to measure precisely. If G, g and r are known then a reverse calculation will give an estimate of the mass of the Earth. This method was used by Henry Cavendish. Measurement The measurement of Earth's gravity is called gravimetry. Currently, the static and time-variable Earth's gravity field parameters are determined using modern satellite missions, such as GOCE, CHAMP, Swarm, GRACE and GRACE-FO. The lowest-degree parameters, including the Earth's oblateness and geocenter motion are best determined with satellite laser ranging. Large-scale gravity anomalies can be detected from space, as a by-product of satellite gravity missions. The satellite missions aim at the generation of a detailed gravity field model of the Earth, typically presented in the form of a spherical-harmonic expansion of the Earth's gravitational potential, but alternative presentations, such as maps of geoid undulations or gravity anomalies, are also produced. Gravity Recovery and Climate Experiment (GRACE) consisted of two satellites that detected gravitational changes across the Earth. These changes could be presented as gravity anomaly temporal variations. Gravity Recovery and Interior Laboratory (GRAIL) also consisted of two spacecraft orbiting the Moon for three years before their deorbit in 2015. See also References External links |
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[SOURCE: https://en.wikipedia.org/wiki/Earth%27s_energy_budget] | [TOKENS: 4509] |
Contents Earth's energy budget Earth's energy budget (or Earth's energy balance) is the balance between the energy that Earth receives from the Sun and the energy the Earth loses back into outer space. Smaller energy sources, such as Earth's internal heat, are taken into consideration, but make a tiny contribution compared to solar energy. The energy budget also takes into account how energy moves through the climate system.: 2227 The Sun heats the equatorial tropics more than the polar regions. Therefore, the amount of solar irradiance received by a certain region is unevenly distributed. As the energy seeks equilibrium across the planet, it drives interactions in Earth's climate system, i.e., Earth's water, ice, atmosphere, rocky crust, and all living things.: 2224 The result is Earth's climate. Earth's energy budget depends on many factors, such as atmospheric aerosols, greenhouse gases, surface albedo, clouds, and land use patterns. When the incoming and outgoing energy fluxes are in balance, Earth is in radiative equilibrium and the climate system will be relatively stable. Global warming occurs when earth receives more energy than it gives back to space, and global cooling takes place when the outgoing energy is greater. Multiple types of measurements and observations show a warming imbalance since at least year 1970. The rate of heating from this human-caused event is without precedent.: 54 The main origin of changes in the Earth's energy is from human-induced changes in the composition of the atmosphere. During 2005 to 2019 the Earth's energy imbalance (EEI) averaged about 460 TW or globally 0.90±0.15 W/m2. It takes time for any changes in the energy budget to result in any significant changes in the global surface temperature. This is due to the thermal inertia of the oceans, land and cryosphere. Most climate models make accurate calculations of this inertia, energy flows and storage amounts. Definition Earth's energy budget includes the "major energy flows of relevance for the climate system". These are "the top-of-atmosphere energy budget; the surface energy budget; changes in the global energy inventory and internal flows of energy within the climate system".: 2227 Earth's energy flows In spite of the enormous transfers of energy into and from the Earth, it maintains a relatively constant temperature because, as a whole, there is little net gain or loss: Earth emits via atmospheric and terrestrial radiation (shifted to longer electromagnetic wavelengths) to space about the same amount of energy as it receives via solar insolation (all forms of electromagnetic radiation). The main origin of changes in the Earth's energy is from human-induced changes in the composition of the atmosphere, amounting to about 460 TW or globally 0.90±0.15 W/m2. The total amount of energy received per second at the top of Earth's atmosphere (TOA) is measured in watts and is given by the solar constant times the cross-sectional area of the Earth corresponded to the radiation. Because the surface area of a sphere is four times the cross-sectional area of a sphere (i.e. the area of a circle), the globally and yearly averaged TOA flux is one quarter of the solar constant and so is approximately 340 watts per square meter (W/m2). Since the absorption varies with location as well as with diurnal, seasonal and annual variations, the numbers quoted are multi-year averages obtained from multiple satellite measurements. Of the ~340 W/m2 of solar radiation received by the Earth, an average of ~77 W/m2 is reflected back to space by clouds and the atmosphere and ~23 W/m2 is reflected by the surface albedo, leaving ~240 W/m2 of solar energy input to the Earth's energy budget. This amount is called the absorbed solar radiation (ASR). It implies a value of about 0.3 for the mean net albedo of Earth, also called its Bond albedo (A): Energy leaves the planet in the form of outgoing longwave radiation (OLR). Longwave radiation is electromagnetic thermal radiation emitted by Earth's surface and atmosphere. Longwave radiation is in the infrared band, but the terms are not synonymous, as infrared radiation can be either shortwave or longwave. Sunlight contains significant amounts of shortwave infrared radiation. A threshold wavelength of 4 microns is sometimes used to distinguish longwave and shortwave radiation. Generally, absorbed solar energy is converted to different forms of heat energy. Some of the solar energy absorbed by the surface is converted to thermal radiation at wavelengths in the "atmospheric window"; this radiation is able to pass through the atmosphere unimpeded and directly escape to space, contributing to OLR. The remainder of absorbed solar energy is transported upwards through the atmosphere through a variety of heat transfer mechanisms, until some of that energy is also able to escape to space, again contributing to OLR. For example, heat is transported into the atmosphere via evapotranspiration and latent heat fluxes or conduction/convection processes, as well as via radiative heat transport. Ultimately, all outgoing energy is radiated into space in the form of longwave radiation. The transport of longwave radiation from Earth's surface through its multi-layered atmosphere is governed by radiative transfer equations such as Schwarzschild's equation for radiative transfer (or more complex equations if scattering is present) and obeys Kirchhoff's law of thermal radiation. A one-layer model produces an approximate description of OLR which yields temperatures at the surface (Ts=288 Kelvin) and at the middle of the troposphere (Ta=242 K) that are close to observed average values: In this expression σ is the Stefan–Boltzmann constant and ε represents the emissivity of the atmosphere, which is less than 1 because the atmosphere does not emit within the wavelength range known as the atmospheric window. Aerosols, clouds, water vapor, and trace greenhouse gases contribute to an effective value of about ε = 0.78. The strong (fourth-power) temperature sensitivity maintains a near-balance of the outgoing energy flow to the incoming flow via small changes in the planet's absolute temperatures. As viewed from Earth's surrounding space, greenhouse gases influence the planet's atmospheric emissivity (ε). Changes in atmospheric composition can thus shift the overall radiation balance. For example, an increase in heat trapping by a growing concentration of greenhouse gases (i.e. an enhanced greenhouse effect) forces a decrease in OLR and a warming (restorative) energy imbalance. Ultimately when the amount of greenhouse gases increases or decreases, in-situ surface temperatures rise or fall until the absorbed solar radiation equals the outgoing longwave radiation, or ASR equals OLR. The geothermal heat flow from the Earth's interior is estimated to be 47 terawatts (TW) and split approximately equally between radiogenic heat and heat left over from the Earth's formation. This corresponds to an average flux of 0.087 W/m2 and represents only 0.027% of Earth's total energy budget at the surface, being dwarfed by the 173000 TW of incoming solar radiation. Human production of energy is even lower at an average 18 TW, corresponding to an estimated 160,000 TW-hr, for all of year 2019. However, consumption is growing rapidly and energy production with fossil fuels also produces an increase in atmospheric greenhouse gases, leading to a more than 20 times larger imbalance in the incoming/outgoing flows that originate from solar radiation. Photosynthesis also has a significant effect: An estimated 140 TW (or around 0.08%) of incident energy gets captured by photosynthesis, giving energy to plants to produce biomass. A similar flow of heat is released over the course of a year when plants are used as food or fuel. Other minor sources of energy are usually ignored in the calculations, including accretion of interplanetary dust and solar wind, light from stars other than the Sun and the thermal radiation from space. Earlier, Joseph Fourier had claimed that deep space radiation was significant in a paper often cited as the first on the greenhouse effect. Budget analysis In simplest terms, Earth's energy budget is balanced when the incoming flow equals the outgoing flow. Since a portion of incoming energy is directly reflected, the balance can also be stated as absorbed incoming solar (shortwave) radiation equal to outgoing longwave radiation: To describe some of the internal flows within the budget, let the insolation received at the top of the atmosphere be 100 units (= 340 W/m2), as shown in the accompanying Sankey diagram. Called the albedo of Earth, around 35 units in this example are directly reflected back to space: 27 from the top of clouds, 2 from snow and ice-covered areas, and 6 by other parts of the atmosphere. The 65 remaining units (ASR = 220 W/m2) are absorbed: 14 within the atmosphere and 51 by the Earth's surface. The 51 units reaching and absorbed by the surface are emitted back to space through various forms of terrestrial energy: 17 directly radiated to space and 34 absorbed by the atmosphere (19 through latent heat of vaporisation, 9 via convection and turbulence, and 6 as absorbed infrared by greenhouse gases). The 48 units absorbed by the atmosphere (34 units from terrestrial energy and 14 from insolation) are then finally radiated back to space. This simplified example neglects some details of mechanisms that recirculate, store, and thus lead to further buildup of heat near the surface. Ultimately the 65 units (17 from the ground and 48 from the atmosphere) are emitted as OLR. They approximately balance the 65 units (ASR) absorbed from the sun in order to maintain a net-zero gain of energy by Earth. Land, ice, and oceans are active material constituents of Earth's climate system along with the atmosphere. They have far greater mass and heat capacity, and thus much more thermal inertia. When radiation is directly absorbed or the surface temperature changes, energy will flow as sensible heat either into or out of the bulk mass of these components via conduction/convection heat transfer processes. The transformation of water between its solid/liquid/vapor states also acts as a source or sink of potential energy in the form of latent heat. These processes buffer the surface conditions against some of the rapid radiative changes in the atmosphere. As a result, the daytime versus nighttime difference in surface temperatures is relatively small. Likewise, Earth's climate system as a whole shows a slow response to shifts in the atmospheric radiation balance. The top few meters of Earth's oceans harbor more energy than its entire atmosphere. Like atmospheric gases, fluidic ocean waters transport vast amounts of energy over the planet's surface. Sensible heat also moves into and out of great depths under conditions that favor downwelling or upwelling. Scientists observe these large-scale energy transfers by measuring changes in oceanic enthalpy. Over 90 percent of the extra energy that has accumulated on Earth from ongoing global warming since 1970 has been stored in the ocean. About one-third has propagated to depths below 700 meters. The overall rate of growth has also risen during recent decades, reaching close to 500 TW (1 W/m2) as of 2020. That led to about 14 zettajoules (ZJ) of heat gain for the year, exceeding the 570 exajoules (=160,000 TW-hr) of total primary energy consumed by humans by a factor of at least 20. Generally speaking, changes to Earth's energy flux balance can be thought of as being the result of external forcings (both natural and anthropogenic, radiative and non-radiative), system feedbacks, and internal system variability. Such changes are primarily expressed as observable shifts in temperature (T), clouds (C), water vapor (W), aerosols (A), trace greenhouse gases (G), land/ocean/ice surface reflectance (S), and as minor shifts in insolation (I) among other possible factors. Earth's heating/cooling rate can then be analyzed over selected timeframes (Δt) as the net change in energy (ΔE) associated with these attributes: Here the term ΔET, corresponding to the Planck response, is negative-valued when temperature rises due to its strong direct influence on OLR. The recent increase in trace greenhouse gases produces an enhanced greenhouse effect, and thus a positive ΔEG forcing term. By contrast, a large volcanic eruption (e.g. Mount Pinatubo 1991, El Chichón 1982) can inject sulfur-containing compounds into the upper atmosphere. High concentrations of stratospheric sulfur aerosols may persist for up to a few years, yielding a negative forcing contribution to ΔEA. Various other types of anthropogenic aerosol emissions make both positive and negative contributions to ΔEA. Solar cycles produce ΔEI smaller in magnitude than those of recent ΔEG trends from human activity. Climate forcings are complex since they can produce direct and indirect feedbacks that intensify (positive feedback) or weaken (negative feedback) the original forcing. These often follow the temperature response. Water vapor trends as a positive feedback with respect to temperature changes due to evaporation shifts and the Clausius-Clapeyron relation. An increase in water vapor results in positive ΔEW due to further enhancement of the greenhouse effect. A slower positive feedback is the ice-albedo feedback. For example, the loss of Arctic ice due to rising temperatures makes the region less reflective, leading to greater absorption of energy and even faster ice melt rates, thus positive influence on ΔES. Collectively, feedbacks excluding the Planck response tend to amplify global warming or cooling.: 94 Clouds are responsible for about half of Earth's albedo and are powerful expressions of internal variability of the climate system. They may also act as feedbacks to forcings, and could be forcings themselves if for example a result of cloud seeding activity. Contributions to ΔEC vary regionally and depending upon cloud type. Measurements from satellites are gathered in concert with simulations from models in an effort to improve understanding and reduce uncertainty. Earth's energy imbalance (EEI) The Earth's energy imbalance (EEI) is defined as "the persistent and positive (downward) net top of atmosphere energy flux associated with greenhouse gas forcing of the climate system".: 2227 If Earth's incoming energy flux (ASR) is larger or smaller than the outgoing energy flux (OLR), then the planet will gain (warm) or lose (cool) net heat energy in accordance with the law of energy conservation: Positive EEI thus defines the overall rate of planetary heating and is typically expressed as watts per square meter (W/m2). During 2005 to 2019 the Earth's energy imbalance averaged about 460 TW or globally 0.90 ± 0.15 W per m2. When Earth's energy imbalance (EEI) shifts by a sufficiently large amount, the shift is measurable by orbiting satellite-based instruments. Imbalances that fail to reverse over time will also drive long-term temperature changes in the atmospheric, oceanic, land, and ice components of the climate system. Temperature, sea level, ice mass and related shifts thus also provide measures of EEI. The biggest changes in EEI arise from changes in the composition of the atmosphere through human activities, thereby interfering with the natural flow of energy through the climate system. The main changes are from increases in carbon dioxide and other greenhouse gases, that produce heating (positive EEI), and pollution. The latter refers to atmospheric aerosols of various kinds, some of which absorb energy while others reflect energy and produce cooling (or lower EEI). Square brackets show 90% confidence intervals It is not (yet) possible to measure the absolute magnitude of EEI directly at top of atmosphere, although changes over time as observed by satellite-based instruments are thought to be accurate. The only practical way to estimate the absolute magnitude of EEI is through an inventory of the changes in energy in the climate system. The biggest of these energy reservoirs is the ocean. The planetary heat content that resides in the climate system can be compiled given the heat capacity, density and temperature distributions of each of its components. Most regions are now reasonably well sampled and monitored, with the most significant exception being the deep ocean. Estimates of the absolute magnitude of EEI have likewise been calculated using the measured temperature changes during recent multi-decadal time intervals. For the 2006 to 2020 period EEI was about +0.76±0.2 W/m2 and showed a significant increase above the mean of +0.48±0.1 W/m2 for the 1971 to 2020 period. EEI has been positive because temperatures have increased almost everywhere for over 50 years. Global surface temperature (GST) is calculated by averaging temperatures measured at the surface of the sea along with air temperatures measured over land. Reliable data extending to at least 1880 shows that GST has undergone a steady increase of about 0.18 °C per decade since about year 1970. Ocean waters are especially effective absorbents of solar energy and have a far greater total heat capacity than the atmosphere. Research vessels and stations have sampled sea temperatures at depth and around the globe since before 1960. Additionally, after the year 2000, an expanding network of nearly 4000 Argo robotic floats has measured the temperature anomaly, or equivalently the ocean heat content change (ΔOHC). Since at least 1990, OHC has increased at a steady or accelerating rate. ΔOHC represents the largest portion of EEI since oceans have thus far taken up over 90% of the net excess energy entering the system over time (Δt): Earth's outer crust and thick ice-covered regions have taken up relatively little of the excess energy. This is because excess heat at their surfaces flows inward only by means of thermal conduction, and thus penetrates only several tens of centimeters on the daily cycle and only several tens of meters on the annual cycle. Much of the heat uptake goes either into melting ice and permafrost or into evaporating more water from soils. Several satellites measure the energy absorbed and radiated by Earth, and thus by inference the energy imbalance. These are located top of atmosphere (TOA) and provide data covering the globe. The NASA Earth Radiation Budget Experiment (ERBE) project involved three such satellites: the Earth Radiation Budget Satellite (ERBS), launched October 1984; NOAA-9, launched December 1984; and NOAA-10, launched September 1986. NASA's Clouds and the Earth's Radiant Energy System (CERES) instruments are part of its Earth Observing System (EOS) since March 2000. CERES is designed to measure both solar-reflected (short wavelength) and Earth-emitted (long wavelength) radiation. The CERES data showed increases in EEI from +0.42±0.48 W/m2 in 2005 to +1.12±0.48 W/m2 in 2019. Contributing factors included more water vapor, less clouds, increasing greenhouse gases, and declining ice that were partially offset by rising temperatures. Subsequent investigation of the behavior using the GFDL CM4/AM4 climate model concluded there was a less than 1% chance that internal climate variability alone caused the trend. Other researchers have used data from CERES, AIRS, CloudSat, and other EOS instruments to look for trends of radiative forcing embedded within the EEI data. Their analysis showed a forcing rise of +0.53±0.11 W/m2 from years 2003 to 2018. About 80% of the increase was associated with the rising concentration of greenhouse gases which reduced the outgoing longwave radiation. Further satellite measurements including TRMM and CALIPSO data have indicated additional precipitation, which is sustained by increased energy leaving the surface through evaporation (the latent heat flux), offsetting some of the increase in the longwave greenhouse flux to the surface. It is noteworthy that radiometric calibration uncertainties limit the capability of the current generation of satellite-based instruments, which are otherwise stable and precise. As a result, relative changes in EEI are quantifiable with an accuracy which is not also achievable for any single measurement of the absolute imbalance. Observations since 1994 show that ice has retreated from every part of Earth at an accelerating rate. Mean global sea level has likewise risen as a consequence of the ice melt in combination with the overall rise in ocean temperatures. These shifts have contributed measurable changes to the geometric shape and gravity of the planet. Changes to the mass distribution of water within the hydrosphere and cryosphere have been deduced using gravimetric observations by the GRACE satellite instruments. These data have been compared against ocean surface topography and further hydrographic observations using computational models that account for thermal expansion, salinity changes, and other factors. Estimates thereby obtained for ΔOHC and EEI have agreed with the other (mostly) independent assessments within uncertainties. Climate scientists Kevin Trenberth, James Hansen, and colleagues have identified the monitoring of Earth's energy imbalance as an important metric to help policymakers guide the pace for mitigation and adaptation measures. Because of climate system inertia, longer-term EEI (Earth's energy imbalance) trends can forecast further changes that are "in the pipeline". Scientists found that the EEI is the most important metric related to climate change. It is the net result of all the processes and feedbacks in play in the climate system. Knowing how much extra energy affects weather systems and rainfall is vital to understand the increasing weather extremes. In 2012, NASA scientists reported that to stop global warming atmospheric CO2 concentration would have to be reduced to 350 ppm or less, assuming all other climate forcings were fixed. As of 2020, atmospheric CO2 reached 415 ppm and all long-lived greenhouse gases exceeded a 500 ppm CO2-equivalent concentration due to continued growth in human emissions. See also References External links |
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[SOURCE: https://en.wikipedia.org/wiki/Navigation] | [TOKENS: 8252] |
Contents Navigation Navigation is a field of study that focuses on the process of monitoring and controlling the movement of a craft or vehicle from one place to another. The field of navigation includes four general categories: land navigation, marine navigation, aeronautic navigation, and space navigation. It is also the term of art used for the specialized knowledge used by navigators to perform navigation tasks. All navigational techniques involve locating the navigator's position compared to known locations or patterns. Navigation, in a broader sense, can refer to any skill or study that involves the determination of position and direction. In this sense, navigation includes orienteering and pedestrian navigation. For marine navigation, this involves the safe movement of ships, boats and other nautical craft either on or underneath the water using positions from navigation equipment with appropriate nautical charts (electronic and paper). Navigation equipment for ships is mandated under the requirements of the SOLAS Convention, depending on ship size. For land navigation, this involves the movement of persons, animals and vehicles from one place to another by means of navigation equipment (such as a compass or GNSS receivers), maps and visual navigation marks across urban or rural environments. Aeronautic (air) navigation involves piloting an aircraft from one geographic position to another position while monitoring the position as the flight progresses. Etymology The term stems from the 1530s, from Latin navigationem (nom. navigatio), from navigatus, pp. of navigare "to sail, sail over, go by sea, steer a ship," from navis "ship" and the root of agere "to drive". History Polynesian navigation is probably the earliest form of open-ocean navigation; it was based on memory and observation recorded on scientific instruments like the Marshall Islands Stick Charts of Ocean Swells. Early Pacific Polynesians used the motion of stars, weather, the position of certain wildlife species, or the size of waves to find the path from one island to another. Among the first proper navigational instruments was the compass, with one of the oldest Chinese in origin from the Han dynasty (since c. 206 BC). The compass was later adopted for sea navigation by the Song dynasty Chinese during the 11th century. The first usage of a compass recorded in Western Europe and the Islamic world occurred around 1190. Maritime navigation using scientific instruments such as the mariner's astrolabe first occurred in the Mediterranean during the Middle Ages. Although land astrolabes were invented in the Hellenistic period and existed in classical antiquity and the Islamic Golden Age, the oldest record of a sea astrolabe is that of Spanish astronomer Ramon Llull dating from 1295. The perfecting of this navigation instrument is attributed to Portuguese navigators during early Portuguese discoveries in the Age of Discovery. The earliest known description of how to make and use a sea astrolabe comes from Spanish cosmographer Martín Cortés de Albacar's Arte de Navegar (The Art of Navigation) published in 1551, based on the principle of the archipendulum used in constructing the Egyptian pyramids. However, the first altitude measuring instrument to navigate extensively used at sea was the quadrant. This was reintroduced by Leonardo of Pisa in the 13th century. Its first recorded use was in 1461 by Diogo Gomes. As well as astrolabes and quadrants, the first cross-staff used in navigation was known from the 14th century onwards, believed to have come from early Arab navigators. However, it had many errors and was also difficult to use as it required squinting at the sun. These disadvantages were overcome with the invention of the backstaff in 1595 by John Davis. Widespread open-seas navigation using the astrolabe, quadrant, backstaff and the compass started during the Age of Discovery in the 15th century. The Portuguese began systematically exploring the Atlantic coast of Africa from 1418, under the sponsorship of Prince Henry. In 1488 Bartolomeu Dias reached the Indian Ocean by this route. In 1492 the Spanish monarchs funded Christopher Columbus's expedition to sail west to reach the Indies by crossing the Atlantic, which resulted in the Discovery of the Americas. In 1498, a Portuguese expedition commanded by Vasco da Gama reached India by sailing around Africa, opening up direct trade with Asia. Soon, the Portuguese sailed further eastward, to the Spice Islands in 1512, landing in China one year later. The first circumnavigation of the earth was completed in 1522 with the Magellan-Elcano expedition, a Spanish voyage of discovery led by Portuguese explorer Ferdinand Magellan and completed by Spanish navigator Juan Sebastián Elcano after the former's death in the Philippines in 1521. For sailing ships, other developments took place with charting and methods to record courses. One of the oldest surviving marine charts is the Carta Pisana, drawn on a sheepskin, dating to 1275. On land, improvements in the production of maps led to improved navigation by armies, traders and other travellers. For sailing ships, navigation by dead reckoning requires frequent recording of course changes and the ship tacks with the wind. To prevent paper charts, which were expensive and in the early days, rare, from being worn out, other methods were used, including the Traverse board and traverse tables (the oldest traverse tables, dates back to 1428). Quadrants were further developed by inventors such as Robert Hooke, Isaac Newton and John Hadley leading to the invention of the octant. Developments in mathematics were also important in the history of navigation. These include initially meridional parts, then developments in spherical trigonometry and logarithms enabled navigators from the 1700s onwards to navigate more accurately. On land, mathematical and new instruments led to developments in Surveying and triangulation which further improved maps, as well as the construction of better roads, paths, canals and eventually railways. Development of an accurate marine chronometer under John Harrison and others ensured accurate timekeeping for calculating longitude. Further improvements in ocean navigation led to the first proper sextant in 1757, the parts and usage developed by various inventors including Pierre Vernier and John Campbell. Various methods for calculation with sextant and chronometer evolved over time, beginning with the Duller method (1728) but reached their most accessible with the Douwes method (1821), the Sumner method (1837), modified by Henry Raper (1844) and the Marc St Hilaire or intercept method (1877). Modifications to the magnetic compass and better methods of determining course were also important, include developments in the compass by Matthew Flinders, Lord Kelvin and others. The sextant, together with the chronometer, compass and astronomical calculations became the most widely used methods of maritime navigation until developments in the 20th century with radio-navigation and gyrocompasses. These in turn were superseded with the advent of computers, electronic calculators and later satellite navigation in the 20th century. On land, the development of handheld GPS occurred in the 1980s and with the advent of smartphones, with in-build compassess and satellite receivers, navigation is now widely achieved through technology globally. Basic concepts In terrestrial navigation, the location of a person, ship, plane, etc is defined as a position using a reference point/coordinates (see Cartesian coordinate system). Positions can either be referenced as latitude/longitude or a distance and direction from a fixed reference point (bearing). Lines of position can be derived from a variety of methods and equipment. By determining and monitoring positions it is possible to find and direct a person, ship, plane, etc in a scientific way from one place to another. This often involves the use of maps or charts from which if desired, courses can be calculated or followed depending on the projection or methods used (Rhumb line, Great circle, etc). Roughly, the latitude of a place on Earth is its angular distance north or south of the equator. Latitude is usually expressed in degrees (marked with °) ranging from 0° at the Equator to 90° at the North and South poles. The latitude of the North Pole is 90° N, and the latitude of the South Pole is 90° S. Mariners calculated latitude in the Northern Hemisphere by sighting the pole star (Polaris) with a sextant and using sight reduction tables to correct for height of eye and atmospheric refraction. The height of Polaris in degrees above the horizon is the latitude of the observer, within a degree or so. Similar to latitude, the longitude of a place on Earth is the angular distance east or west of the prime meridian or Greenwich meridian. Longitude is usually expressed in degrees (marked with °) ranging from 0° at the Greenwich meridian to 180° east and west. Sydney, for example, has a longitude of about 151° east. New York City has a longitude of 74° west. For most of history, mariners struggled to determine longitude. Longitude can be calculated if the precise time of a sighting is known. Lacking that, one can use a sextant to take a lunar distance (also called the lunar observation, or "lunar" for short) that, with a nautical almanac, can be used to calculate the time at zero longitude (see Greenwich Mean Time). Reliable marine chronometers were unavailable until the late 18th century and not affordable until the 19th century. For about a hundred years, from about 1767 until about 1850, mariners lacking a chronometer used the method of lunar distances to determine Greenwich time to find their longitude. A mariner with a chronometer could check its reading using a lunar determination of Greenwich time. In navigation, a rhumb line (or loxodrome) is a line crossing all meridians of longitude at the same angle, i.e. a path derived from a defined initial bearing. That is, upon taking an initial bearing, one proceeds along the same bearing, without changing the direction as measured relative to true or magnetic north. Methods of navigation Most modern navigation relies primarily on positions determined electronically by receivers collecting information from satellites. Most other modern techniques rely on finding intersecting lines of position or LOP. A line of position can refer to two different things, either a line on a chart or a line between the observer and an object in real life. A bearing is a measure of the direction to an object. If the navigator measures the direction in real life, the angle can then be drawn on a nautical chart and the navigator will be somewhere on that bearing line on the chart. In addition to bearings, navigators also often measure distances to objects. On the chart, a distance produces a circle or arc of position. Circles, arcs, and hyperbolae of positions are often referred to as lines of position. If the navigator draws two lines of position, and they intersect he must be at that position. A fix is the intersection of two or more LOPs. If only one line of position is available, this may be evaluated against the dead reckoning position to establish an estimated position. Lines (or circles) of position can be derived from a variety of sources: There are some methods seldom used today such as the maritime method of "dipping a light" to calculate the geographic range from observer to lighthouse, where the height of the lighthouse is known (from a list of lights or from a chart). Methods of navigation have changed through history. Each new method has enhanced the mariner's ability to complete his voyage. One of the most important judgments the navigator must make is the best method to use. Some types of navigation are depicted in the table. The practice of navigation usually involves a combination of these different methods. By mental navigation checks, a pilot or a navigator estimates tracks, distances, and altitudes which will then help the pilot avoid gross navigation errors. Piloting (also called pilotage) involves navigating an aircraft by visual reference to landmarks, or a water vessel in restricted waters and fixing its position as precisely as possible at frequent intervals. More so than in other phases of navigation, proper preparation and attention to detail are important. Procedures vary from vessel to vessel, and between military, commercial, and private vessels. As pilotage takes place in shallow waters, it typically involves following courses to ensure sufficient under keel clearance, ensuring a sufficient depth of water below the hull as well as a consideration for squat. It may also involve navigating a ship within a river, canal or channel in close proximity to land. A military navigation team will nearly always consist of several people. A military navigator might have bearing takers stationed at the gyro repeaters on the bridge wings for taking simultaneous bearings, while the civilian navigator on a merchant ship or leisure craft must often take and plot their position themselves, typically with the aid of electronic position fixing. While the military navigator will have a bearing book and someone to record entries for each fix, the civilian navigator will simply pilot the bearings on the chart as they are taken and not record them at all. If the ship is equipped with an ECDIS, it is reasonable for the navigator to simply monitor the progress of the ship along the chosen track, visually ensuring that the ship is proceeding as desired, checking the compass, sounder and other indicators only occasionally. If a pilot is aboard, as is often the case in the most restricted of waters, his judgement can generally be relied upon, further easing the workload. But should the ECDIS fail, the navigator will have to rely on his skill in the manual and time-tested procedures. Celestial navigation systems are based on observation of the positions of the Sun, Moon, planets and navigational stars using a sextant or similar navigation instrument. By knowing which point on the rotating Earth a celestial object is above and measuring its height above the observer's horizon, the navigator can determine his distance from that subpoint using mathematical calculation. A nautical almanac and a source of time, typically a marine chronometer are used to compute the subpoint on Earth a celestial body is over, and a sextant is used to measure the body's angular height above the horizon. That height can then be used to compute distance from the subpoint to create a circular line of position. Alternatively sight reduction tables can be used. A navigator shoots a number of stars in succession to give a series of overlapping lines of position. Where they intersect is the celestial fix. The Moon and Sun may also be used. The Sun can also be used by itself to shoot a succession of lines of position (best done around local noon) to determine a position. Since the advent of GNSS, celestial navigation is less used for marine and air navigation, though it remains useful as a backup or as another method to cross-check the accuracy of electronic systems, particularly in the open ocean. In order to accurately measure longitude, the precise time is required of a sextant sighting (down to the second, if possible) which is then recorded for subsequent calculation. Each second of error is equivalent to 15 seconds of longitude error, which at the equator is a position error of .25 of a nautical mile, about the accuracy limit of manual celestial navigation. The spring-driven marine chronometer is a precision timepiece used aboard ship to provide accurate time for celestial observations. A chronometer differs from a spring-driven watch principally in that it contains a variable lever device to maintain even pressure on the mainspring, and a special balance designed to compensate for temperature variations. A spring-driven chronometer is set approximately to Greenwich mean time (GMT) and is not reset until the instrument is overhauled and cleaned, usually at three-year intervals. The difference between GMT and chronometer time is carefully determined and applied as a correction to all chronometer readings. Spring-driven chronometers must be wound at about the same time each day. Quartz crystal marine chronometers have replaced spring-driven chronometers onboard modern ships because of their greater accuracy. They are maintained on GMT directly from radio time signals. This eliminates chronometer error and watch error corrections. Should the second hand be in error by a readable amount, it can be reset electrically. The basic element for time generation is a quartz crystal oscillator. The quartz crystal is temperature compensated and is hermetically sealed in an evacuated envelope. A calibrated adjustment capability is provided to adjust for the aging of the crystal. The chronometer is typically designed to operate for a minimum of one year on a single set of batteries. Observations may be timed and ship's clocks set with a comparing watch, which is set to chronometer time and taken to the bridge wing for recording sight times. In practice, a wrist watch coordinated to the nearest second with the chronometer will be adequate. A stop watch, either spring wound or digital, may also be used for celestial observations. In this case, the watch is started at a known GMT by chronometer, and the elapsed time of each sight added to this to obtain GMT of the sight. All chronometers and watches should be checked regularly with a radio time signal. Times and frequencies of radio time signals are listed in publications such as Radio Navigational Aids. The second critical component of celestial navigation is to measure the angle formed at the observer's eye between the celestial body and the sensible horizon. The sextant, an optical instrument, is used to perform this function. The sextant consists of two primary assemblies. The frame is a rigid triangular structure with a pivot at the top and a graduated segment of a circle, referred to as the "arc", at the bottom. The second component is the index arm, which is attached to the pivot at the top of the frame. At the bottom is an endless vernier which clamps into teeth on the bottom of the "arc". The optical system consists of two mirrors and, generally, a low power telescope. One mirror, referred to as the "index mirror" is fixed to the top of the index arm, over the pivot. As the index arm is moved, this mirror rotates, and the graduated scale on the arc indicates the measured angle ("altitude"). The second mirror, referred to as the "horizon glass", is fixed to the front of the frame. One half of the horizon glass is silvered and the other half is clear. Light from the celestial body strikes the index mirror and is reflected to the silvered portion of the horizon glass, then back to the observer's eye through the telescope. The observer manipulates the index arm so the reflected image of the body in the horizon glass is just resting on the visual horizon, seen through the clear side of the horizon glass. There are three main errors that must be corrected in order to each usage for navigation. The main errors are perpendicular error, side error and index error. Adjustment of the sextant consists of checking and aligning all the optical elements to eliminate the overall "index error" (or index correction). Index correction should be checked, using the horizon or more preferably a star, each time the sextant is used. The practice of taking celestial observations from the deck of a rolling ship, often through cloud cover and with a hazy horizon, is by far the most challenging part of celestial navigation. Until the widespread usage of technologies such as inertial navigation systems, VHF omnidirectional range and GNSS, air navigators used the Bubble octant or bubble sextant. Using this instrument to take sights, mathematical calculations could then be carried out to determine the past position of the aircraft. Inertial navigation system (INS) is a dead reckoning type of navigation system that computes its position based on motion sensors. Before actually navigating, the initial latitude and longitude and the INS's physical orientation relative to the Earth (e.g., north and level) are established. After alignment, an INS receives impulses from motion detectors that measure (a) the acceleration along three axes (accelerometers), and (b) rate of rotation about three orthogonal axes (gyroscopes). These enable an INS to continually and accurately calculate its current latitude and longitude (and often velocity). Advantages over other navigation systems are that, once aligned, an INS does not require outside information. An INS is not affected by adverse weather conditions and it cannot be detected or jammed. Its disadvantage is that since the current position is calculated solely from previous positions and motion sensors, its errors are cumulative, increasing at a rate roughly proportional to the time since the initial position was input. Inertial navigation systems must therefore be frequently corrected with a location 'fix' from some other type of navigation system. The first inertial system is considered to be the V-2 guidance system deployed by the Germans in 1942. However, inertial sensors are traced to the early 19th century. The advantages INSs led their use in aircraft, missiles, surface ships and submarines. For example, the U.S. Navy developed the Ships Inertial Navigation System (SINS) during the Polaris missile program to ensure a reliable and accurate navigation system to initial its missile guidance systems. Inertial navigation systems were in wide use until satellite navigation systems (GPS) became available. INSs are still in common use on submarines (since GPS reception or other fix sources are not possible while submerged) and long-range missiles but are not now widely found elsewhere. Gravity-aided navigation originated in the 1990s and provides a technology to obtain a position fix for navigation. It utilises the concept that an onboard sensor measures elements of the gravitational vector while the platform is in motion and then these measurements are referenced to a map of the Earth's gravitational field to determine a position. Not to be confused with satellite navigation, which depends upon satellites to function, space navigation refers to the navigation of spacecraft themselves. This has historically been achieved (during the Apollo program) via a navigational computer, an Inertial navigation system, and via celestial inputs entered by astronauts which were recorded by sextant and telescope. Space rated navigational computers, like those found on Apollo and later missions, are designed to be hardened against possible data corruption from radiation. Navigation in space has three main components: the use of a suitable reference trajectory which describes the planned flight path of the spacecraft, monitoring the actual spacecraft position while the mission is in flight (orbit determination) and creating maneuvers to bring the spacecraft back to the reference trajectory as required (flight path control). Another possibility that has been explored for deep space navigation is Pulsar navigation, which compares the X-ray bursts from a collection of known pulsars in order to determine the position of a spacecraft. This method has been tested by multiple space agencies, such as NASA and ESA. Radars can be used for navigation and marine radars are commonly fitted to ships for navigation at sea. Radar is an effective aid to navigation because it provides ranges and bearings to objects within range of the radar scanner. When a vessel (ship or boat) is within radar range of land or fixed objects (such as special radar aids to navigation and navigation marks) the navigator can take distances and angular bearings to charted objects and use these to establish arcs of position and lines of position on a chart. A fix consisting of only radar information is called a radar fix. Types of radar fixes include "range and bearing to a single object," "two or more bearings," "tangent bearings," and "two or more ranges." Radar can also be used with ECDIS as a means of position fixing with the radar image or distance/bearing overlaid onto an Electronic nautical chart. Parallel indexing is a technique defined by William Burger in the 1957 book The Radar Observer's Handbook. This technique involves creating a line on the screen that is parallel to the ship's course, but offset to the left or right by some distance. This parallel line allows the navigator to maintain a given distance away from hazards. The line on the radar screen is set to a specific distance and angle, then the ship's position relative to the parallel line is observed. This can provide an immediate reference to the navigator as to whether the ship is on or off its intended course for navigation. Other techniques that are less used in general navigation have been developed for special situations. One, known as the "contour method," involves marking a transparent plastic template on the radar screen and moving it to the chart to fix a position. Another special technique, known as the Franklin Continuous Radar Plot Technique, involves drawing the path a radar object should follow on the radar display if the ship stays on its planned course. During the transit, the navigator can check that the ship is on track by checking that the pip lies on the drawn line. A radio direction finder or RDF is a device for finding the direction to a radio source. Due to radio's ability to travel very long distances "over the horizon", it makes a particularly good navigation system for ships and aircraft that might be flying at a distance from land. RDFs works by rotating a directional antenna and listening for the direction in which the signal from a known station comes through most strongly. This sort of system was widely used in the 1930s and 1940s. RDF antennas are easy to spot on German World War II aircraft, as loops under the rear section of the fuselage, whereas most US aircraft enclosed the antenna in a small teardrop-shaped fairing. In navigational applications, RDF signals are provided in the form of radio beacons, the radio version of a lighthouse. The signal is typically a simple AM broadcast of a morse code series of letters, which the RDF can tune in to see if the beacon is "on the air". Most modern detectors can also tune in any commercial radio stations, which is particularly useful due to their high power and location near major cities. Decca, OMEGA, and LORAN-C are three similar hyperbolic navigation systems. Decca was a hyperbolic low frequency radio navigation system (also known as multilateration) that was first deployed during World War II when the Allied forces needed a system which could be used to achieve accurate landings. As was the case with Loran C, its primary use was for ship navigation in coastal waters. Fishing vessels were major post-war users, but it was also used on aircraft, including a very early (1949) application of moving-map displays. The system was deployed in the North Sea and was used by helicopters operating to oil platforms. The OMEGA Navigation System was the first truly global radio navigation system for aircraft, operated by the United States in cooperation with six partner nations. OMEGA was developed by the United States Navy for military aviation users. It was approved for development in 1968 and promised a true worldwide oceanic coverage capability with only eight transmitters and the ability to achieve a four-mile (6 km) accuracy when fixing a position. Initially, the system was to be used for navigating nuclear bombers across the North Pole to Russia. Later, it was found useful for submarines. Due to the success of the Global Positioning System the use of Omega declined during the 1990s, to a point where the cost of operating Omega could no longer be justified. Omega was terminated on September 30, 1997, and all stations ceased operation. LORAN is a terrestrial navigation system using low frequency radio transmitters that use the time interval between radio signals received from three or more stations to determine the position of a ship or aircraft. The current version of LORAN in common use is LORAN-C, which operates in the low frequency portion of the EM spectrum from 90 to 110 kHz. Many nations are users of the system, including the United States, Japan, and several European countries. Russia uses a nearly exact system in the same frequency range, called CHAYKA. LORAN use is in steep decline, with GPS being the primary replacement. However, there are attempts to enhance and re-popularize LORAN. LORAN signals are less susceptible to interference and can penetrate better into foliage and buildings than GPS signals. A GNSS allow small electronic receivers to determine their location (longitude, latitude, and altitude) within a few meters using time signals transmitted along a line of sight by radio from satellites. Positions derived can then be used with maps and charts for satellite navigation. Since the first experimental satellite was launched in 1978, GNSS have become an indispensable aid to navigation around the world, and an important tool for map-making and land surveying. GNSS also provides a precise time reference used in many applications including scientific study of earthquakes, and synchronization of telecommunications networks. Global Navigation Satellite System or GNSS is the term for satellite navigation systems that provide positioning with global coverage. The first system, GPS was developed by the United States Department of Defense and officially named NAVSTAR GPS (NAVigation Satellite Timing And Ranging Global Positioning System). The satellite constellation is managed by the United States Air Force 50th Space Wing. The cost of maintaining the system is approximately US$750 million per year, including the replacement of aging satellites, and research and development. Despite this fact, GPS is free for civilian use as a public good. With improvements in technology and developments globally, as of 2024, there are several different operational GNSS now available for navigation by the public. These include the United States NAVSTAR Global Positioning System (GPS), the Russian GLONASS, the European Union's Galileo positioning system and the Beidou navigation system of China. The different global systems have varying differences in accuracy but stated positions are normally in the range of between 1 and 10 metres accuracy depending on system and on that system's satellite coverage. As a result over 100 satellites are in medium Earth orbit, transmitting signals allowing GNSS receivers to determine the receiver's location, speed and direction. There are also several regional GNSS systems available for navigation, including the Indian Regional Navigation Satellite System and the Quasi-Zenith Satellite System. However, not all GNSS receivers are capable of operating with these systems and older GNSS receivers, such as on old ships may not be capable of receiving all of the GNSS now available to users. Modern smartphones act as personal GNSS navigators for civilians who own them. Overuse of these devices, whether in the vehicle or on foot, can lead to a relative inability to learn about navigated environments, resulting in sub-optimal navigation abilities when and if these devices become unavailable. Typically a compass is also provided to determine direction when not moving. Acoustic location is a method of navigation by the use of acoustic positioning systems which determine the position of an object by using sound waves. It is primarily used by submarines and ships fitted with sonar and similar transducer based technologies. Underwater acoustic positioning systems are also commonly used by divers and Remotely operated underwater vehicles, specifically the Long baseline acoustic positioning system, the Short baseline acoustic positioning system and the Ultra-short baseline acoustic positioning system. Navigation processes Passage planning or voyage planning is a procedure to develop a complete description of vessel's voyage from start to finish. The plan includes leaving the dock and harbor area, the en route portion of a voyage, approaching the destination, and mooring. According to international law, a vessel's captain is legally responsible for passage planning, however on larger vessels, the task will be delegated to the ship's navigator. Studies show that human error is a factor in 80 percent of navigational accidents and that in many cases the human making the error had access to information that could have prevented the accident. The practice of voyage planning has evolved from penciling lines on nautical charts to a process of risk management. Passage planning consists of four stages: appraisal, planning, execution, and monitoring, which are specified in International Maritime Organization Resolution A.893(21), Guidelines For Voyage Planning, and these guidelines are reflected in the local laws of IMO signatory countries (for example, Title 33 of the U.S. Code of Federal Regulations), and a number of professional books or publications. There are some fifty elements of a comprehensive passage plan depending on the size and type of vessel. The appraisal stage deals with the collection of information relevant to the proposed voyage as well as ascertaining risks and assessing the key features of the voyage. This will involve considering the type of navigation required e.g. Ice navigation, the region the ship will be passing through and the hydrographic information on the route. In the next stage, the written plan is created. The third stage is the execution of the finalised voyage plan, taking into account any special circumstances which may arise such as changes in the weather, which may require the plan to be reviewed or altered. The final stage of passage planning consists of monitoring the vessel's progress in relation to the plan and responding to deviations and unforeseen circumstances. Electronic integrated bridge concepts are driving future navigation system planning. Integrated systems take inputs from various ship sensors, electronically display positioning information, and provide control signals required to maintain a vessel on a preset course. The navigator becomes a system manager, choosing system presets, interpreting system output, and monitoring vessel response. In traditional marine navigation, one day's work in navigation is a minimal set of tasks consistent with prudent celestial navigation. The definition and processes vary on military and civilian vessels, and from ship to ship, but the traditional method takes a form resembling: Navigation on ships is usually always conducted on the bridge. It may also take place in adjacent space, where chart tables and publications are available. However, increasingly traditional navigation processes have been replaced with technological processes for marine navigation using GNSS and marine radar. Navigation for cars and other land-based travel typically uses maps, landmarks, and in recent times computer navigation ("satnav", short for satellite navigation), as well as any means available on water. Computerized navigation commonly relies on GPS for current location information, a navigational map database of roads and navigable routes, and uses algorithms related to the shortest path problem to identify optimal routes. Pedestrian navigation is involved in orienteering, land navigation (military), and wayfinding. Submariners, divers, remotely operated underwater vehicles (ROVs) and other underwater craft carry out underwater navigation by a variety of methods and processes including GNSS, radar navigation and sonar/acoustic position fixing. Artificial intelligence can be utilised to assist with planning, problem-serving and decision-making processes in navigation. This includes using AI in navigation systems such as GNSS as well as in general computing to assist with position fixing and monitoring from one position to another such as in vehicles, planes and cars. Standards, training and organisations Professional standards for navigation depend on the type of navigation and vary by country. For marine navigation, Merchant Navy deck officers are trained and internationally certified according to the STCW Convention. Leisure and amateur mariners may undertake lessons in navigation at local/regional training schools. Naval officers receive navigation training as part of their naval training. In land navigation, courses and training is often provided to young persons as part of general or extra-curricular education. Land navigation is also an essential part of army training. Additionally, organisations such as the Scouts and DoE programme teach navigation to their students. Orienteering organisations are a type of sports that require navigational skills using a map and compass to navigate from point to point in diverse and usually unfamiliar terrain whilst moving at speed. In aviation, pilots undertake air navigation training as part of learning to fly. Professional organisations also assist to encourage improvements in navigation or bring together navigators in learned environments. The Royal Institute of Navigation (RIN) is a learned society with charitable status, aimed at furthering the development of navigation on land and sea, in the air and in space. It was founded in 1947 as a forum for mariners, pilots, engineers and academics to compare their experiences and exchange information. In the US, the Institute of Navigation (ION) is a non-profit professional organisation advancing the art and science of positioning, navigation and timing. Numerous nautical publications are available on navigation, which are published by professional sources all over the world. In the UK, the United Kingdom Hydrographic Office, the Witherby Publishing Group and the Nautical Institute provide numerous navigational publications, including the comprehensive Admiralty Manual of Navigation. In the US, Bowditch's American Practical Navigator is a free available encyclopedia of navigation issued by the US Government. Navigation in spatial cognition Navigation is an essential everyday activity that involves a series of abilities that help humans and animals to locate, track, and follow paths in order to arrive at different destinations. Navigation, in spatial cognition, allows for acquiring information about the environment by using the body and landmarks of the environment as frames of references to create mental representations of our environment, also known as a cognitive map. Humans navigate by transitioning between different spaces and coordinating both egocentric and allocentric frames of reference. Navigation can be distinguished into two sptial components: locomotion and wayfinding. Locomotion is the process of movement from one place to another, both in humans and in animals. Locomotion helps you understand an environment by moving through a space in order to create a mental representation of it. Wayfinding is defined as an active process of following or deciding upon a path between one place to another through mental representations. It involves processes such as representation, planning and decision which help to avoid obstacles, to stay on course or to regulate pace when approaching particular objects. Navigation and wayfinding can be approached in the environmental space. According to Dan Montello’s space classification, there are four levels of space with the third being the environmental space. The environmental space represents a very large space, like a city, and can only be fully explored through movement since all objects and space are not directly visible. Also Barbara Tversky systematized the space, but this time taking into consideration the three dimensions that correspond to the axes of the human body and its extensions: above/below, front/back and left/right. Tversky ultimately proposed a fourfold classification of navigable space: space of the body, space around the body, space of navigation and space of graphics. There are two types of wayfinding in navigation: aided and unaided. Aided wayfinding requires a person to use various types of media, such as maps, GPS, directional signage, etc., in their navigation process which generally involves low spatial reasoning and is less cognitively demanding. Unaided wayfinding involves no such devices for the person who is navigating. Unaided wayfinding can be subdivided into a taxonomy of tasks depending on whether it is undirected or directed, which basically makes the distinction of whether there is a precise destination or not: undirected wayfinding means that a person is simply exploring an environment for pleasure without any set destination. Directed wayfinding, instead, can be further subdivided into search vs. target approximation. Search means that a person does not know where the destination is located and must find it either in an unfamiliar environment, which is labeled as an uninformed search, or in a familiar environment, labeled as an informed search. In target approximation, on the other hand, the location of the destination is known to the navigator but a further distinction is made based on whether the navigator knows how to arrive or not to the destination. Path following means that the environment, the path, and the destination are all known which means that the navigator simply follows the path they already know and arrive at the destination without much thought. For example, when you are in your city and walking on the same path as you normally take from your house to your job or university. However, path finding means that the navigator knows where the destination is but does not know the route they have to take to arrive at the destination: you know where a specific store is but you do not know how to arrive there or what path to take. If the navigator does not know the environment, it is called path search which means that only the destination is known while neither the path nor the environment is: you are in a new city and need to arrive at the train station but do not know how to get there. Path planning, on the other hand, means that the navigator knows both where the destination is and is familiar with the environment so they only need to plan the route or path that they should take to arrive at their target. For example, if you are in your city and need to get to a specific store that you know the destination of but do not know the specific path you need to take to get there. See also References Bibliography External links |
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Contents Cartography Cartography (/kɑːrˈtɒɡrəfi/)[a] is the study and practice of making and using maps. Combining science, aesthetics and technique, cartography builds on the premise that reality (or an imagined reality) can be modeled in ways that communicate spatial information effectively. The fundamental objectives of traditional cartography are to: Modern cartography constitutes many theoretical and practical foundations of geographic information systems (GIS) and geographic information science (GISc). History What is the earliest known map is a matter of some debate, both because the term "map" is not well-defined and because some artifacts that might be maps might actually be something else. A wall painting that might depict the ancient Anatolian city of Çatalhöyük (previously known as Catal Huyuk or Çatal Hüyük) has been dated to the late 7th millennium BCE. Among the prehistoric alpine rock carvings of Mount Bego (France) and Valcamonica (Italy), dated to the 4th millennium BCE, geometric patterns consisting of dotted rectangles and lines are widely interpreted in archaeological literature as depicting cultivated plots. Other known maps of the ancient world include the Minoan "House of the Admiral" wall painting from c. 1600 BCE, showing a seaside community in an oblique perspective, and an engraved map of the holy Babylonian city of Nippur, from the Kassite period (14th – 12th centuries BCE). The oldest surviving world maps are from 9th century BCE Babylonia. One shows Babylon on the Euphrates, surrounded by Assyria, Urartu and several cities, all, in turn, surrounded by a "bitter river" (Oceanus). Another depicts Babylon as being north of the center of the world. The ancient Greeks and Romans created maps from the time of Anaximander in the 6th century BCE. In the 2nd century CE, Ptolemy wrote his treatise on cartography, Geographia. This contained Ptolemy's world map – the world then known to Western society (Ecumene). As early as the 8th century, Arab scholars were translating the works of the Greek geographers into Arabic. Roads were essential in the Roman world, motivating the creation of maps, called itinerarium, that portrayed the world as experienced via the roads. The Tabula Peutingeriana is the only surviving example. In ancient China, geographical literature dates to the 5th century BCE. The oldest extant Chinese maps come from the State of Qin, dated back to the 4th century BCE, during the Warring States period. In the book Xin Yi Xiang Fa Yao, published in 1092 by the Chinese scientist Su Song, a star map on the equidistant cylindrical projection. Although this method of charting seems to have existed in China even before this publication and scientist, the greatest significance of the star maps by Su Song is that they represent the oldest existent star maps in printed form. Early forms of cartography of India included depictions of the pole star and surrounding constellations. These charts may have been used for navigation. Mappae mundi ('maps of the world') are the medieval European maps of the world. About 1,100 of these are known to have survived: of these, some 900 are found illustrating manuscripts, and the remainder exist as stand-alone documents. The Arab geographer Muhammad al-Idrisi produced his medieval atlas Tabula Rogeriana (Book of Roger) in 1154. By combining the knowledge of Africa, the Indian Ocean, Europe, and the Far East (which he learned through contemporary accounts from Arab merchants and explorers) with the information he inherited from the classical geographers, he was able to write detailed descriptions of a multitude of countries. Along with the substantial text he had written, he created a world map influenced mostly by the Ptolemaic conception of the world, but with significant influence from multiple Arab geographers. It remained the most accurate world map for the next three centuries. The map was divided into seven climatic zones, with detailed descriptions of each zone. As part of this work, a smaller, circular map depicting the south on top and Arabia in the center was made. Al-Idrisi also made an estimate of the circumference of the world, accurate to within 10%. In the Age of Discovery, from the 15th century to the 17th century, European cartographers both copied earlier maps (some of which had been passed down for centuries) and drew their own based on explorers' observations and new surveying techniques. The invention of the magnetic compass, telescope and sextant enabled increasing accuracy. In 1492, Martin Behaim, a German cartographer and advisor to the king John II of Portugal, made the oldest extant globe of the Earth. In 1507, Martin Waldseemüller produced a globular world map and a large 12-panel world wall map (Universalis Cosmographia) bearing the first use of the name "America." Portuguese cartographer Diogo Ribero was the author of the first known planisphere with a graduated Equator (1527). Italian cartographer Battista Agnese produced at least 71 manuscript atlases of sea charts. Johannes Werner refined and promoted the Werner projection. This was an equal-area, heart-shaped world map projection (generally called a cordiform projection) that was used in the 16th and 17th centuries. Over time, other iterations of this map type arose; most notable are the sinusoidal projection and the Bonne projection. The Werner projection places its standard parallel at the North Pole; a sinusoidal projection places its standard parallel at the equator; and the Bonne projection is intermediate between the two. In 1569, mapmaker Gerardus Mercator first published a map based on his Mercator projection, which uses equally-spaced parallel vertical lines of longitude and parallel latitude lines spaced farther apart as they get farther away from the equator. By this construction, courses of constant bearing are conveniently represented as straight lines for navigation. The same property limits its value as a general-purpose world map because regions are shown as increasingly larger than they actually are the further from the equator they are. Mercator is also credited as the first to use the word "atlas" to describe a collection of maps. In the later years of his life, Mercator resolved to create his Atlas, a book filled with many maps of different regions of the world, as well as a chronological history of the world from the Earth's creation by God until 1568. He was unable to complete it to his satisfaction before he died. Still, some additions were made to the Atlas after his death, and new editions were published after his death. In 1570, the Brabantian cartographer Abraham Ortelius, strongly encouraged by Gillis Hooftman, created the first true modern atlas, Theatrum Orbis Terrarum. In a rare move, Ortelius credited mapmakers who contributed to the atlas, the list of which grew to 183 individuals by 1603. In the Renaissance, maps were used to impress viewers and establish the owner's reputation as sophisticated, educated, and worldly. Because of this, towards the end of the Renaissance, maps were displayed with equal importance of painting, sculptures, and other pieces of art. In the sixteenth century, maps were becoming increasingly available to consumers through the introduction of printmaking, with about 10% of Venetian homes having some sort of map by the late 1500s. There were three main functions of maps in the Renaissance: In medieval times, written directions of how to get somewhere were more common than the use of maps. With the Renaissance, cartography began to be seen as a metaphor for power. Political leaders could lay claim to territories through the use of maps, and this was greatly aided by the religious and colonial expansion of Europe. The Holy Land and other religious places were the most commonly mapped during the Renaissance. In the late 1400s to the late 1500s, Rome, Florence, and Venice dominated map-making and trade. It started in Florence in the mid-to late 1400s. Map trade quickly shifted to Rome and Venice but then was overtaken by atlas makers in the late 16th century. Map publishing in Venice was completed with humanities and book publishing in mind, rather than just informational use. There were two main printmaking technologies in the Renaissance: woodcut and copper-plate intaglio, referring to the medium used to transfer the image onto paper. In woodcut, the map image is created as a relief chiseled from medium-grain hardwood. The areas intended to be printed are inked and pressed against the sheet. Being raised from the rest of the block, the map lines cause indentations in the paper that can often be felt on the back of the map. There are advantages to using relief to make maps. For one, a printmaker doesn't need a press because the maps could be developed as rubbings. Woodblock is durable enough to be used many times before defects appear. Existing printing presses can be used to create the prints rather than having to create a new one. On the other hand, it is hard to achieve fine detail with the relief technique. Inconsistencies in linework are more apparent in woodcut than in intaglio. To improve quality in the late fifteenth century, a style of relief craftsmanship developed using fine chisels to carve the wood, rather than the more commonly used knife.[citation needed] In intaglio, lines are engraved into workable metals, typically copper but sometimes brass. The engraver spreads a thin sheet of wax over the metal plate and uses ink to draw the details. Then, the engraver traces the lines with a stylus to etch them into the plate beneath. The engraver can also use styli to prick holes along the drawn lines, trace along them with colored chalk, and then engrave the map. Lines going in the same direction are carved at the same time, and then the plate is turned to carve lines going in a different direction. To print from the finished plate, ink is spread over the metal surface and scraped off such that it remains only in the etched channels. Then the plate is pressed forcibly against the paper so that the ink in the channels is transferred to the paper. The pressing is so forceful that it leaves a "plate mark" around the border of the map at the edge of the plate, within which the paper is depressed compared to the margins. Copper and other metals were expensive at the time, so the plate was often reused for new maps or melted down for other purposes. Whether woodcut or intaglio, the printed map is hung out to dry. Once dry, it is usually placed in another press to flatten the paper. Any type of paper that was available at the time could be used to print the map, but thicker paper was more durable. Both relief and intaglio were used about equally by the end of the fifteenth century. Lettering in mapmaking is important for denoting information. Fine lettering is difficult in woodcut, where it often turned out square and blocky, contrary to the stylized, rounded writing style popular in Italy at the time. To improve quality, mapmakers developed fine chisels to carve the relief. Intaglio lettering did not suffer the troubles of a coarse medium and so was able to express the looping cursive that came to be known as cancellaresca. There were custom-made reverse punches that were also used in metal engraving alongside freehand lettering. The first use of color in map-making cannot be narrowed down to one reason. There are arguments that color started as a way to indicate information on the map, with aesthetics coming second. There are also arguments that color was first used on maps for aesthetics but then evolved into conveying information. Either way, many maps of the Renaissance left the publisher without being colored, a practice that continued all the way into the 1800s. However, most publishers accepted orders from their patrons to have their maps or atlases colored if they wished. Because all coloring was done by hand, the patron could request simple, cheap color, or more expensive, elaborate color, even going so far as silver or gold gilding. The simplest coloring was merely outlines, such as of borders and along rivers. Wash color meant painting regions with inks or watercolors. Limning meant adding silver and gold leaf to the map to illuminate lettering, heraldic arms, or other decorative elements. The early modern period saw the convergence of cartographical techniques across Eurasia and the exchange of mercantile mapping techniques via the Indian Ocean. In the early seventeenth century, the Selden map was created by a Chinese cartographer. Historians have put its date of creation around 1620, but there is debate in this regard. This map's significance draws from historical misconceptions of East Asian cartography, the main one being that East Asians did not do cartography until Europeans arrived. The map's depiction of trading routes, a compass rose, and scale bar points to the culmination of many map-making techniques incorporated into Chinese mercantile cartography. In 1689, representatives of the Russian tsar and Qing Dynasty met near the border town of Nerchinsk, which was near the disputed border of the two powers, in eastern Siberia. The two parties, with the Qing negotiation party bringing Jesuits as intermediaries, managed to work a treaty which placed the Amur River as the border between the Eurasian powers, and opened up trading relations between the two. This treaty's significance draws from the interaction between the two sides, and the intermediaries who were drawn from a wide variety of nationalities. Maps of the Enlightenment period practically universally used copper plate intaglio, having abandoned the fragile, coarse woodcut technology. Use of map projections evolved, with the double hemisphere being very common and Mercator's prestigious navigational projection gradually making more appearances.[citation needed] Due to the paucity of information and the immense difficulty of surveying during the period, mapmakers frequently plagiarized material without giving credit to the original cartographer. For example, a famous map of North America known as the "Beaver Map" was published in 1715 by Herman Moll. This map is a close reproduction of a 1698 work by Nicolas de Fer. De Fer, in turn, had copied images that were first printed in books by Louis Hennepin, published in 1697, and François Du Creux, in 1664. By the late 18th century, mapmakers often credited the original publisher with something along the lines of, "After [the original cartographer]" in the map's title or cartouche. In cartography, technology has continually changed in order to meet the demands of new generations of mapmakers and map users. The first maps were produced manually, with brushes and parchment; so they varied in quality and were limited in distribution. The advent of magnetic devices, such as the compass and much later, magnetic storage devices, allowed for the creation of far more accurate maps and the ability to store and manipulate them digitally. Advances in mechanical devices such as the printing press, quadrant, and vernier allowed the mass production of maps and the creation of accurate reproductions from more accurate data. Hartmann Schedel was one of the first cartographers to use the printing press to make maps more widely available. Optical technology, such as the telescope, sextant, and other devices that use telescopes, allowed accurate land surveys and allowed mapmakers and navigators to find their latitude by measuring angles to the North Star at night or the Sun at noon. Advances in photochemical technology, such as the lithographic and photochemical processes, make possible maps with fine details, which do not distort in shape and which resist moisture and wear. This also eliminated the need for engraving, which further speeded up map production. In the 20th century, aerial photography, satellite imagery, and remote sensing provided efficient, precise methods for mapping physical features, such as coastlines, roads, buildings, watersheds, and topography. The United States Geological Survey has devised multiple new map projections, notably the Space Oblique Mercator for interpreting satellite ground tracks for mapping the surface. The use of satellites and space telescopes now allows researchers to map other planets and moons in outer space. Advances in electronic technology ushered in another revolution in cartography: ready availability of computers and peripherals such as monitors, plotters, printers, scanners (remote and document) and analytic stereo plotters, along with computer programs for visualization, image processing, spatial analysis, and database management, have democratized and greatly expanded the making of maps. The ability to superimpose spatially located variables onto existing maps has created new uses for maps and new industries to explore and exploit these potentials. See also digital raster graphic. In the early years of the new millennium, three key technological advances transformed cartography: the removal of Selective Availability in the Global Positioning System (GPS) in May 2000, which improved locational accuracy for consumer-grade GPS receivers to within a few metres; the invention of OpenStreetMap in 2004, a global digital counter-map that allowed anyone to contribute and use new spatial data without complex licensing agreements; and the launch of Google Earth in 2005 as a development of the virtual globe EarthViewer 3D (2004), which revolutionised accessibility of accurate world maps, as well as access to satellite and aerial imagery. These advances brought more accuracy to geographical and location-based data and widened the range of applications for cartography, for example in the development of satnav devices. Today most commercial-quality maps are made using software of three main types: CAD, GIS and specialized illustration software. Spatial information can be stored in a database, from which it can be extracted on demand. These tools lead to increasingly dynamic, interactive maps that can be manipulated digitally. On the other hand, we can observe a reverse trend. In contemporary times, there is a resurgence of interest in the most beautiful periods of cartography, with various maps being created using, for example, Renaissance-style aesthetics. We encounter imitators or continuators of Renaissance traditions that merge the realms of science and art. Among them are figures such as Luther Phillips (1891–1960) and Ruth Rhoads Lepper Gardner (1905–2011), who still operated using traditional cartographic methods, as well as creators utilizing modern developments based on GIS solutions and those employing techniques that combine advanced GIS/CAD methods with traditional artistic forms. Field-rugged computers, GPS, and laser rangefinders make it possible to create maps directly from measurements made on site. Deconstruction There are technical and cultural aspects to producing maps. In this sense, maps can sometimes be said to be biased. The study of bias, influence, and agenda in making a map is what comprise a map's deconstruction. A central tenet of deconstructionism is that maps have power. Other assertions are that maps are inherently biased and that we search for metaphor and rhetoric in maps. It is claimed that the Europeans promoted an "epistemological" understanding of the map as early as the 17th century. An example of this understanding is that "[European reproduction of terrain on maps] reality can be expressed in mathematical terms; that systematic observation and measurement offer the only route to cartographic truth…". A common belief is that science heads in a direction of progress, and thus leads to more accurate representations of maps. In this belief, European maps must be superior to others, which necessarily employed different map-making skills. "There was a 'not cartography' land where lurked an army of inaccurate, heretical, subjective, valuative, and ideologically distorted images. Cartographers developed a 'sense of the other' in relation to nonconforming maps." Depictions of Africa are a common target of deconstructionism. According to deconstructionist models, cartography was used for strategic purposes associated with imperialism and as instruments and representations of power during the conquest of Africa. The depiction of Africa and the low latitudes in general on the Mercator projection has been interpreted as imperialistic and as symbolic of subjugation due to the diminished proportions of those regions compared to higher latitudes where the European powers were concentrated. Maps furthered imperialism and colonization of Africa in practical ways by showing basic information like roads, terrain, natural resources, settlements, and communities. Through this, maps made European commerce in Africa possible by showing potential commercial routes and made natural resource extraction possible by depicting locations of resources. Such maps also enabled military conquests and made them more efficient, and imperial nations further used them to put their conquests on display. These same maps were then used to cement territorial claims, such as at the Berlin Conference of 1884–1885. Before 1749, maps of the African continent had African kingdoms drawn with assumed or contrived boundaries, with unknown or unexplored areas having drawings of animals, imaginary physical geographic features, and descriptive texts. In 1748, Jean B. B. d'Anville created the first map of the African continent that had blank spaces to represent the unknown territory. Map types In understanding basic maps, the field of cartography can be divided into two general categories: general cartography and thematic cartography. General cartography involves those maps that are constructed for a general audience and thus contain a variety of features. General maps exhibit many reference and location systems and often are produced in a series. For example, the 1:24,000 scale topographic maps of the United States Geological Survey (USGS) are a standard as compared to the 1:50,000 scale Canadian maps. The government of the UK produces the classic 1:50,000 (replacing the older 1 inch to 1 mile) "Ordnance Survey" maps of the entire UK and with a range of correlated larger- and smaller-scale maps of great detail. Many private mapping companies have also produced thematic map series. Thematic cartography involves maps of specific geographic themes, oriented toward specific audiences. A couple of examples might be a dot map showing corn production in Indiana or a shaded area map of Ohio counties, divided into numerical choropleth classes. As the volume of geographic data has exploded over the last century, thematic cartography has become increasingly useful and necessary to interpret spatial, cultural and social data. A third type of map is known as an "orienteering," or special purpose map. This type of map falls somewhere between thematic and general maps. They combine general map elements with thematic attributes in order to design a map with a specific audience in mind. Oftentimes, the type of audience an orienteering map is made for is in a particular industry or occupation. An example of this kind of map would be a municipal utility map. A topographic map is primarily concerned with the topographic description of a place, including (especially in the 20th and 21st centuries) the use of contour lines showing elevation. Terrain or relief can be shown in a variety of ways (see Cartographic relief depiction). In the present era, one of the most widespread and advanced methods used to form topographic maps is to use computer software to generate digital elevation models which show shaded relief. Before such software existed, cartographers had to draw shaded relief by hand. One cartographer who is respected as a master of hand-drawn shaded relief is the Swiss professor Eduard Imhof whose efforts in hill shading were so influential that his method became used around the world despite it being so labor-intensive. A topological map is a very general type of map, the kind one might sketch on a napkin. It often disregards scale and detail in the interest of clarity of communicating specific route or relational information. Beck's London Underground map is an iconic example. Although the most widely used map of "The Tube," it preserves little of reality: it varies scale constantly and abruptly, it straightens curved tracks, and it contorts directions. The only topography on it is the River Thames, letting the reader know whether a station is north or south of the river. That and the topology of station order and interchanges between train lines are all that is left of the geographic space. Yet those are all a typical passenger wishes to know, so the map fulfills its purpose. Map design Modern technology, including advances in printing, the advent of geographic information systems and graphics software, and the Internet, has vastly simplified the process of map creation and increased the palette of design options available to cartographers. This has led to a decreased focus on production skill, and an increased focus on quality design, the attempt to craft maps that are both aesthetically pleasing and practically useful for their intended purposes. A map has a purpose and an audience. Its purpose may be as broad as teaching the major physical and political features of the entire world, or as narrow as convincing a neighbor to move a fence. The audience may be as broad as the general public or as narrow as a single person. Mapmakers use design principles to guide them in constructing a map that is effective for its purpose and audience. The cartographic process spans many stages, starting from conceiving the need for a map and extending all the way through its consumption by an audience. Conception begins with a real or imagined environment. As the cartographer gathers information about the subject, they consider how that information is structured and how that structure should inform the map's design. Next, the cartographers experiment with generalization, symbolization, typography, and other map elements to find ways to portray the information so that the map reader can interpret the map as intended. Guided by these experiments, the cartographer settles on a design and creates the map, whether in physical or electronic form. Once finished, the map is delivered to its audience. The map reader interprets the symbols and patterns on the map to draw conclusions and perhaps to take action. By the spatial perspectives they provide, maps help shape how we view the world. Designing a map involves bringing together a number of elements and making a large number of decisions. The elements of design fall into several broad topics, each of which has its own theory, its own research agenda, and its own best practices. That said, there are synergistic effects between these elements, meaning that the overall design process is not just working on each element one at a time, but an iterative feedback process of adjusting each to achieve the desired gestalt. Deliberate cartographic errors Some maps contain deliberate errors or distortions, either as propaganda or as a "watermark" to help the copyright owner identify infringement if the error appears in competitors' maps. The latter often come in the form of nonexistent, misnamed, or misspelled "trap streets". Other names and forms for this are paper towns, fictitious entries, and copyright easter eggs. Another motive for deliberate errors is cartographic "vandalism": a mapmaker wishing to leave their mark on the work. Mount Richard, for example, was a fictitious peak on the Rocky Mountains' continental divide that appeared on a Boulder County, Colorado map in the early 1970s. It is believed to be the work of draftsman Richard Ciacci. The fiction was not discovered until two years later.[citation needed] In Switzerland's official maps, mapmakers have hidden illustrations of a hiker, fish, marmot, and more for over a century. Sandy Island in New Caledonia is an example of a fictitious location that stubbornly survives, reappearing on new maps copied from older maps while being deleted from other new editions. With the emergence of the internet and Web mapping, technologies allow for the creation and distribution of maps by people without proper cartographic training are readily available. This has led to maps that ignore cartographic conventions and are potentially misleading. Professional and learned societies Professional and learned societies include: Journals related to cartography, as well as GIS, GISc, include: See also References Further reading Mapmaking History Meanings External links |
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[SOURCE: https://en.wikipedia.org/wiki/History_of_life] | [TOKENS: 12921] |
Contents History of life The history of life on Earth traces the processes by which living and extinct organisms evolved, from the earliest emergence of life to the present day. Earth formed about 4.54 ± 0.05 billion years ago (abbreviated as Ga, for gigaannum) and evidence suggests that life emerged prior to 3.7 Ga. The similarities among all known present-day species indicate that they have diverged through the process of evolution from a common ancestor. The earliest clear evidence of life comes from biogenic carbon signatures and stromatolite fossils discovered in 3.7 billion-year-old metasedimentary rocks from western Greenland. In 2015, possible "remains of biotic life" were found in 4.1 billion-year-old rocks in Western Australia. There is further evidence of possibly the oldest forms of life in the form of fossilized microorganisms in hydrothermal vent precipitates from the Nuvvuagittuq Belt, that may have lived as early as 4.28 billion years ago, not long after the oceans formed 4.4 billion years ago, and after the Earth formed 4.54 ± 0.05 billion years ago. These earliest fossils, however, may have originated from non-biological processes. Microbial mats of coexisting bacteria and archaea were the dominant form of life in the early Archean eon, and many of the major steps in early evolution are thought to have taken place in this environment. The evolution of photosynthesis by cyanobacteria, around 3.5 Ga, eventually led to a buildup of its waste product, oxygen, in the oceans. After free oxygen saturated all available reductant substances on the Earth's surface, it built up in the atmosphere, leading to the Great Oxygenation Event around 2.4 Ga. The earliest evidence of eukaryotes (complex cells with organelles) dates from 1.85 Ga, likely due to symbiogenesis between anaerobic archaea and aerobic proteobacteria in co-adaptation against the new oxidative stress. While eukaryotes may have been present earlier, their diversification accelerated when aerobic cellular respiration by the endosymbiont mitochondria provided a more abundant source of biological energy. Around 1.6 Ga, some eukaryotes gained the ability to photosynthesize via endosymbiosis with cyanobacteria, and gave rise to various algae that eventually overtook cyanobacteria as the dominant primary producers. At around 1.7 Ga, multicellular organisms began to appear, with differentiated cells performing specialized functions. While early organisms reproduced asexually, the primary method of reproduction for the vast majority of macroscopic organisms, including almost all eukaryotes (which includes animals and plants), is sexual reproduction, the fusion of male and female reproductive cells (gametes) to create a zygote. The origin and evolution of sexual reproduction remain a puzzle for biologists, though it is thought to have evolved from a single-celled eukaryotic ancestor. While microorganisms formed the earliest terrestrial ecosystems at least 2.7 Ga, the evolution of plants from freshwater green algae dates back to about 1 billion years ago. Microorganisms are thought to have paved the way for the inception of land plants in the Ordovician period. Land plants were so successful that they are thought to have contributed to the Late Devonian extinction event as early tree Archaeopteris drew down CO2 levels, leading to global cooling and lowered sea levels, while their roots increased rock weathering and nutrient run-offs which may have triggered algal bloom anoxic events. Bilateria, animals having a left and a right side that are mirror images of each other, appeared by 555 Ma (million years ago). Ediacara biota appeared during the Ediacaran period, while vertebrates, along with most other modern phyla originated about 525 Ma during the Cambrian explosion. During the Permian period, synapsids, including the ancestors of mammals, dominated the land. The Permian–Triassic extinction event killed most complex species of its time, 252 Ma. During the recovery from this catastrophe, archosaurs became the most abundant land vertebrates; one archosaur group, the dinosaurs, dominated the Jurassic and Cretaceous periods. After the Cretaceous–Paleogene extinction event 66 Ma killed off the non-avian dinosaurs, mammals increased rapidly in size and diversity. Such mass extinctions may have accelerated evolution by providing opportunities for new groups of organisms to diversify. Only a very small percentage of species have been identified: one estimate claims that Earth may have 1 trillion species, because "identifying every microbial species on Earth presents a huge challenge." Only 1.75–1.8 million species have been named and 1.8 million documented in a central database. The currently living species represent less than one percent of all species that have ever lived on Earth. Earliest history of Earth The oldest meteorite fragments found on Earth are about 4.54 billion years old; this, coupled primarily with the dating of ancient lead deposits, has put the estimated age of Earth at around that time. The Moon has the same composition as Earth's crust but does not contain an iron-rich core like the Earth's. Many scientists think that about 60–110 million years (period of late stage accretion) after the formation of Solar System, the proto-Earth collided with Theia, a co-orbital, Mars-sized protoplanet originating from the inner Solar System. This collision ejected massive amounts of proto-Earth silicate mantle material into orbit that accreted to form the Moon. Another hypothesis is that the Earth and Moon started to coalesce at the same time but the Earth, having a much stronger gravity than the early Moon, attracted almost all the iron particles in the area. Until 2001, the oldest rocks found on Earth were about 3.8 billion years old, leading scientists to estimate that the Earth's surface had been molten until then. Accordingly, they named this part of Earth's history the Hadean. However, analysis of zircons formed 4.4 Ga indicates that Earth's crust solidified about 100 million years after the planet's formation and that the planet quickly acquired oceans and an atmosphere, which may have been capable of supporting life. Evidence from the Moon indicates that, from 4 to 3.8 Ga, it suffered a Late Heavy Bombardment by debris that was left over from the formation of the Solar System, and the Earth should have experienced an even heavier bombardment due to its stronger gravity. While there is no direct evidence of conditions on Earth 4 to 3.8 Ga, there is no reason to think that the Earth was not also affected by this late heavy bombardment. This event may well have stripped away any previous atmosphere and oceans; in this case gases and water from comet impacts may have contributed to their replacement, although outgassing from volcanoes on Earth would have supplied at least half. However, if subsurface microbial life had evolved by this point, it would have survived the bombardment. Earliest evidence for life on Earth The earliest identified organisms were minute and relatively featureless, and their fossils looked like small rods that are very difficult to tell apart from structures that arise through abiotic physical processes. The oldest undisputed evidence of life on Earth, interpreted as fossilized bacteria, dates to 3 Ga. Other finds in rocks dated to about 3.5 Ga have been interpreted as bacteria, with geochemical evidence also seeming to show the presence of life 3.8 Ga. However, these analyses were closely scrutinized, and non-biological processes were found which could produce all of the "signatures of life" that had been reported. While this does not prove that the structures found had a non-biological origin, they cannot be taken as clear evidence for the presence of life. Geochemical signatures from rocks deposited 3.4 Ga have been interpreted as evidence for life. Evidence for fossilized microorganisms considered to be 3.77 billion to 4.28 billion years old was found in the Nuvvuagittuq Greenstone Belt in Quebec, Canada, although the evidence is disputed as inconclusive. Origins of life on Earth Most biologists reason that all living organisms on Earth must share a single last universal ancestor, because it would be virtually impossible that two or more separate lineages could have independently developed the many complex biochemical mechanisms common to all living organisms. According to a different scenario a single last universal ancestor, e.g. a "first cell" or a first individual precursor cell has never existed. Instead, the early biochemical evolution of life led to diversification through the development of a multiphenotypic population of pre-cells from which the precursor cells (protocells) of the three domains of life emerged. Thus, the formation of cells was a successive process. See § Metabolism first: Pre-cells, successive cellularisation, below. Life on Earth is based on carbon and water. Carbon provides stable frameworks for complex chemicals and can be easily extracted from the environment, especially from carbon dioxide. There is no other chemical element whose properties are similar enough to carbon's to be called an analogue; silicon, the element directly below carbon on the periodic table, does not form very many complex stable molecules, and because most of its compounds are water-insoluble and because silicon dioxide is a hard and abrasive solid in contrast to carbon dioxide at temperatures associated with living things, it would be more difficult for organisms to extract. The elements boron and phosphorus have more complex chemistries but suffer from other limitations relative to carbon. Water is an excellent solvent and has two other useful properties: the fact that ice floats enables aquatic organisms to survive beneath it in winter; and its molecules have electrically negative and positive ends, which enables it to form a wider range of compounds than other solvents can. Other good solvents, such as ammonia, are liquid only at such low temperatures that chemical reactions may be too slow to sustain life, and lack water's other advantages. Organisms based on alternative biochemistry may, however, be possible on other planets. Research on how life might have emerged from non-living chemicals focuses on three possible starting points: self-replication, an organism's ability to produce offspring that are very similar to itself; metabolism, its ability to feed and repair itself; and external cell membranes, which allow food to enter and waste products to leave, but exclude unwanted substances. Research on abiogenesis still has a long way to go, since theoretical and empirical approaches are only beginning to make contact with each other. Even the simplest members of the three modern domains of life use DNA to record their "recipes" and a complex array of RNA and protein molecules to "read" these instructions and use them for growth, maintenance and self-replication. The discovery that some RNA molecules can catalyze both their own replication and the construction of proteins led to the hypothesis of earlier life-forms based entirely on RNA. These ribozymes could have formed an RNA world in which there were individuals but no species, as mutations and horizontal gene transfers would have meant that offspring were likely to have different genomes from their parents, and evolution occurred at the level of genes rather than organisms. RNA would later have been replaced by DNA, which can build longer, more stable genomes, strengthening heritability and expanding the capabilities of individual organisms. Ribozymes remain as the main components of ribosomes, the "protein factories" in modern cells. Evidence suggests the first RNA molecules formed on Earth prior to 4.17 Ga. Although short self-replicating RNA molecules have been artificially produced in laboratories, doubts have been raised about whether natural non-biological synthesis of RNA is possible. The earliest "ribozymes" may have been formed of simpler nucleic acids such as PNA, TNA or GNA, which would have been replaced later by RNA. In 2003, it was proposed that porous metal sulfide precipitates would assist RNA synthesis at about 100 °C (212 °F) and ocean-bottom pressures near hydrothermal vents. Under this hypothesis, lipid membranes would be the last major cell components to appear and, until then, the protocells would be confined to the pores. It has been suggested that double-walled "bubbles" of lipids like those that form the external membranes of cells may have been an essential first step. Experiments that simulated the conditions of the early Earth have reported the formation of lipids, and these can spontaneously form liposomes, double-walled "bubbles", and then reproduce themselves. Although they are not intrinsically information-carriers as nucleic acids are, they would be subject to natural selection for longevity and reproduction. Nucleic acids such as RNA might then have formed more easily within the liposomes than outside. RNA is complex and there are doubts about whether it can be produced non-biologically in the wild. Some clays, notably montmorillonite, have properties that make them plausible accelerators for the emergence of an RNA world: they grow by self-replication of their crystalline pattern; they are subject to an analogue of natural selection, as the clay "species" that grows fastest in a particular environment rapidly becomes dominant; and they can catalyze the formation of RNA molecules. Although this idea has not become the scientific consensus, it still has active supporters. Research in 2003 reported that montmorillonite could also accelerate the conversion of fatty acids into "bubbles" and that the "bubbles" could encapsulate RNA attached to the clay. These "bubbles" can then grow by absorbing additional lipids and then divide. The formation of the earliest cells may have been aided by similar processes. A similar hypothesis presents self-replicating iron-rich clays as the progenitors of nucleotides, lipids and amino acids. A series of experiments starting in 1997 showed that early stages in the formation of proteins from inorganic materials including carbon monoxide and hydrogen sulfide could be achieved by using iron sulfide and nickel sulfide as catalysts. Most of the steps required temperatures of about 100 °C (212 °F) and moderate pressures, although one stage required 250 °C (482 °F) and a pressure equivalent to that found under 7 kilometres (4.3 mi) of rock. Hence it was suggested that self-sustaining synthesis of proteins could have occurred near hydrothermal vents. In this scenario, the biochemical evolution of life led to diversification through the development of a multiphenotypic population of pre-cells, i.e. evolving entities of primordial life with different characteristics and widespread horizontal gene transfer. From this pre-cell population the founder groups A, B, C and then, from them, the precursor cells (here named proto-cells) of the three domains of life arose successively, leading first to the domain Bacteria, then to the domain Archea and finally to the domain Eucarya. For the development of cells (cellularization), the pre-cells had to be protected from their surroundings by envelopes (i.e. membranes, walls). For instance, the development of rigid cell walls by the invention of peptidoglycan in bacteria (domain Bacteria) may have been a prerequisite for their successful survival, radiation and colonization of virtually all habitats of the geosphere and hydrosphere. This scenario may explain the quasi-random distribution of evolutionarily important features among the three domains and, at the same time, the existence of the most basic biochemical features (genetic code, set of protein amino acids etc.) in all three domains (unity of life), as well as the close relationship between the Archaea and the Eucarya. A scheme of the pre-cell scenario is shown in the adjacent figure, where important evolutionary improvements are indicated by numbers. Wet-dry cycles at geothermal springs are shown to solve the problem of hydrolysis and promote the polymerization and vesicle encapsulation of biopolymers. The temperatures of geothermal springs are suitable for biomolecules. Silica minerals and metal sulfides in these environments have photocatalytic properties to catalyze biomolecules. Solar UV exposure also promotes the synthesis of biomolecules like RNA nucleotides. An analysis of hydrothermal veins at a 3.5 Gya (giga years ago or 1 billion years) geothermal spring setting were found to have elements required for the origin of life, which are potassium, boron, hydrogen, sulfur, phosphorus, zinc, nitrogen, and oxygen. Mulkidjanian and colleagues find that such environments have identical ionic concentrations to the cytoplasm of modern cells. Fatty acids in acidic or slightly alkaline geothermal springs assemble into vesicles after wet-dry cycles as there is a lower concentration of ionic solutes at geothermal springs since they are freshwater environments, in contrast to seawater which has a higher concentration of ionic solutes. For organic compounds to be present at geothermal springs, they would have likely been transported by carbonaceous meteors. The molecules that fell from the meteors were then accumulated in geothermal springs. Geothermal springs can accumulate aqueous phosphate in the form of phosphoric acid. Based on lab-run models, these concentrations of phosphate are insufficient to facilitate biosynthesis. As for the evolutionary implications, freshwater heterotrophic cells that depended upon synthesized organic compounds later evolved photosynthesis because of the continuous exposure to sunlight as well as their cell walls with ion pumps to maintain their intracellular metabolism after they entered the oceans. Catalytic mineral particles and transition metal sulfides at these environments are capable of catalyzing organic compounds. Scientists simulated laboratory conditions that were identical to white smokers and successfully oligomerized RNA, measured to be 4 units long. Long chain fatty acids can be synthesized via Fischer-Tropsch synthesis. Another experiment that replicated conditions also similar white smokers, with long chain fatty acids present resulted in the assembly of vesicles. Exergonic reactions at hydrothermal vents are suggested to have been a source of free energy that promoted chemical reactions, synthesis of organic molecules, and are inducive to chemical gradients. In small rock pore systems, membranous structures between alkaline seawater and the acidic ocean would be conducive to natural proton gradients. Nucleobase synthesis could occur by following universally conserved biochemical pathways by using metal ions as catalysts. RNA molecules of 22 bases can be polymerized in alkaline hydrothermal vent pores. Thin pores are shown to only accumulate long polynucleotides whereas thick pores accumulate both short and long polynucleotides. Small mineral cavities or mineral gels could have been a compartment for abiogenic processes. A genomic analysis supports this hypothesis as they found 355 genes that likely traced to LUCA upon 6.1 million sequenced prokaryotic genes. They reconstruct LUCA as a thermophilic anaerobe with a Wood-Ljungdahl pathway, implying an origin of life at white smokers. LUCA would also have exhibited other biochemical pathways such as gluconeogenesis, reverse incomplete Krebs cycle, glycolysis, and the pentose phosphate pathway, including biochemical reactions such as reductive amination and transamination. One theory traces the origins of life to the abundant carbonate-rich lakes which would have dotted the early Earth. Phosphate would have been an essential cornerstone to the origin of life since it is a critical component of nucleotides, phospholipids, and adenosine triphosphate. Phosphate is often depleted in natural environments due to its uptake by microbes and its affinity for calcium ions. In a process called 'apatite precipitation', free phosphate ions react with the calcium ions abundant in water to precipitate out of solution as apatite minerals. When attempting to simulate prebiotic phosphorylation, scientists have only found success when using phosphorus levels far above modern day natural concentrations. This problem of low phosphate is solved in carbonate-rich environments. When in the presence of carbonate, calcium readily reacts to form calcium carbonate instead of apatite minerals. With the free calcium ions removed from solution, phosphate ions are no longer precipitated from solution. This is specifically seen in lakes with no inflow, since no new calcium is introduced into the water body. After all of the calcium is sequestered into calcium carbonate (calcite), phosphate concentrations are able to increase to levels necessary for facilitating biomolecule creation. Though carbonate-rich lakes have alkaline chemistry in modern times, models suggest that carbonate lakes had a pH low enough for prebiotic synthesis when placed in the acidifying context of Earth's early carbon dioxide rich atmosphere. Rainwater rich in carbonic acid weathered the rock on the surface of the Earth at rates far greater than today. With high phosphate influx, no phosphate precipitation, and no microbial usage of phosphate at this time, models show phosphate reached concentrations approximately 100 times greater than they are today. Modeled pH and phosphate levels of early Earth carbonate-rich lakes nearly match the conditions used in current laboratory experiments on the origin of life. Similar to the process predicted by geothermal hot spring hypotheses, changing lake levels and wave action deposited phosphorus-rich brine onto dry shore and marginal pools. This drying of the solution promotes polymerization reactions and removes enough water to promote phosphorylation, a process integral to biological energy storage and transfer. When washed away by further precipitation and wave action, researchers concluded these newly formed biomolecules may have washed back into the lake - allowing the first prebiotic syntheses on Earth to occur. The Panspermia hypothesis does not explain how life arose originally, but simply examines the possibility of its coming from somewhere other than Earth. The idea that life on Earth was "seeded" from elsewhere in the Universe dates back at least to the Greek philosopher Anaximander in the sixth century BCE. In the twentieth century it was proposed by the physical chemist Svante Arrhenius, by the astronomers Fred Hoyle and Chandra Wickramasinghe, and by molecular biologist Francis Crick and chemist Leslie Orgel. There are three main versions of the "seeded from elsewhere" hypothesis: from elsewhere in the Solar System via fragments knocked into space by a large meteor impact, in which case the most credible sources are Mars and Venus; by alien visitors, possibly as a result of accidental contamination by microorganisms that they brought with them; and from outside the Solar System but by natural means. Experiments in low Earth orbit, such as EXOSTACK, have demonstrated that some microorganism spores can survive the shock of being catapulted into space and some can survive exposure to outer space radiation for at least 5.7 years. Meteorite ALH84001, which was once part of the Martian crust, shows evidence of carbonate-globules with texture and size indicative of terrestrial bacterial activity. Scientists are divided over the likelihood of life arising independently on Mars, or on other planets in our galaxy. Environmental and evolutionary impact of microbial mats Microbial mats are multi-layered, multi-species colonies of bacteria and other organisms that are generally only a few millimeters thick, but still contain a wide range of chemical environments, each of which favors a different set of microorganisms. To some extent each mat forms its own food chain, as the by-products of each group of microorganisms generally serve as "food" for adjacent groups. Stromatolites are stubby pillars built as microorganisms in mats slowly migrate upwards to avoid being smothered by sediment deposited on them by water. There has been vigorous debate about the validity of alleged stromatolite fossils from before 3 Ga, with critics arguing that they could have been formed by non-biological processes. In 2006, another find of stromatolites was reported from the same part of Australia, in rocks dated to 3.5 Ga. In modern underwater mats the top layer often consists of photosynthesizing cyanobacteria which create an oxygen-rich environment, while the bottom layer is oxygen-free and often dominated by hydrogen sulfide emitted by the organisms living there. Oxygen is toxic to organisms that are not adapted to it, but greatly increases the metabolic efficiency of oxygen-adapted organisms; oxygenic photosynthesis by bacteria in mats increased biological productivity by a factor of between 100 and 1,000. The source of hydrogen atoms used by oxygenic photosynthesis is water, which is much more plentiful than the geologically produced reducing agents required by the earlier non-oxygenic photosynthesis. From this point onwards life itself produced significantly more of the resources it needed than did geochemical processes. Oxygen became a significant component of Earth's atmosphere about 2.4 Ga. Although eukaryotes may have been present much earlier, the oxygenation of the atmosphere was a prerequisite for the evolution of the most complex eukaryotic cells, from which all multicellular organisms are built. The boundary between oxygen-rich and oxygen-free layers in microbial mats would have moved upwards when photosynthesis shut down overnight, and then downwards as it resumed on the next day. This would have created selection pressure for organisms in this intermediate zone to acquire the ability to tolerate and then to use oxygen, possibly via endosymbiosis, where one organism lives inside another and both of them benefit from their association. Cyanobacteria have the most complete biochemical "toolkits" of all the mat-forming organisms. Hence they are the most self-sufficient, well-adapted to strike out on their own both as floating mats and as the first of the phytoplankton, provide the basis of most marine food chains. Diversification of eukaryotes Eukaryotes may have been present long before the oxygenation of the atmosphere, but most modern eukaryotes require oxygen, which is used by their mitochondria to fuel the production of ATP, the internal energy supply of all known cells. In the 1970s, a vigorous debate concluded that eukaryotes emerged as a result of a sequence of endosymbiosis between prokaryotes. For example: a predatory microorganism invaded a large prokaryote, probably an archaean, but instead of killing its prey, the attacker took up residence and evolved into mitochondria; one of these chimeras later tried to swallow a photosynthesizing cyanobacterium, but the victim survived inside the attacker and the new combination became the ancestor of plants; and so on. After each endosymbiosis, the partners eventually eliminated unproductive duplication of genetic functions by rearranging their genomes, a process which sometimes involved transfer of genes between them. Another hypothesis proposes that mitochondria were originally sulfur- or hydrogen-metabolizing endosymbionts, and became oxygen-consumers later. On the other hand, mitochondria might have been part of eukaryotes' original equipment. There is a debate about when eukaryotes first appeared: the presence of steranes in Australian shales may indicate eukaryotes at 2.7 Ga; however, an analysis in 2008 concluded that these chemicals infiltrated the rocks less than 2.2 Ga and prove nothing about the origins of eukaryotes. Fossils of the algae Grypania have been reported in 1.85 billion-year-old rocks (originally dated to 2.1 Ga but later revised), indicating that eukaryotes with organelles had already evolved. A diverse collection of fossil algae were found in rocks dated between 1.5 and 1.4 Ga. The earliest known fossils of fungi date from 1.43 Ga. Plastids, the superclass of organelles of which chloroplasts are the best-known exemplar, are thought to have originated from endosymbiotic cyanobacteria. The symbiosis evolved around 1.5 Ga and enabled eukaryotes to carry out oxygenic photosynthesis. Three evolutionary lineages of photosynthetic plastids have since emerged: chloroplasts in green algae and plants, rhodoplasts in red algae and cyanelles in the glaucophytes. Not long after this primary endosymbiosis of plastids, rhodoplasts, and chloroplasts were passed down to other bikonts, establishing a eukaryotic assemblage of phytoplankton by the end of the Neoproterozoic Eon. Sexual reproduction and multicellular organisms The defining characteristics of sexual reproduction in eukaryotes are meiosis and fertilization, resulting in genetic recombination, giving offspring 50% of their genes from each parent. By contrast, in asexual reproduction there is no recombination, but occasional horizontal gene transfer. Bacteria also exchange DNA by bacterial conjugation, enabling the spread of resistance to antibiotics and other toxins, and the ability to utilize new metabolites. However, conjugation is not a means of reproduction, and is not limited to members of the same species – there are cases where bacteria transfer DNA to plants and animals. On the other hand, bacterial transformation is clearly an adaptation for transfer of DNA between bacteria of the same species. This is a complex process involving the products of numerous bacterial genes and can be regarded as a bacterial form of sex. This process occurs naturally in at least 67 prokaryotic species (in seven different phyla). Sexual reproduction in eukaryotes may have evolved from bacterial transformation. The disadvantages of sexual reproduction are well-known: the genetic reshuffle of recombination may break up favorable combinations of genes; and since males do not directly increase the number of offspring in the next generation, an asexual population can out-breed and displace in as little as 50 generations a sexual population that is equal in every other respect. Nevertheless, the great majority of animals, plants, fungi and protists reproduce sexually. There is strong evidence that sexual reproduction arose early in the history of eukaryotes and that the genes controlling it have changed very little since then. How sexual reproduction evolved and survived is an unsolved puzzle. The Red Queen hypothesis suggests that sexual reproduction provides protection against parasites, because it is easier for parasites to evolve means of overcoming the defenses of genetically identical clones than those of sexual species that present moving targets, and there is some experimental evidence for this. However, there is still doubt about whether it would explain the survival of sexual species if multiple similar clone species were present, as one of the clones may survive the attacks of parasites for long enough to out-breed the sexual species. Furthermore, contrary to the expectations of the Red Queen hypothesis, Kathryn A. Hanley et al. found that the prevalence, abundance and mean intensity of mites was significantly higher in sexual geckos than in asexuals sharing the same habitat. In addition, biologist Matthew Parker, after reviewing numerous genetic studies on plant disease resistance, failed to find a single example consistent with the concept that pathogens are the primary selective agent responsible for sexual reproduction in the host. Alexey Kondrashov's deterministic mutation hypothesis (DMH) assumes that each organism has more than one harmful mutation and that the combined effects of these mutations are more harmful than the sum of the harm done by each individual mutation. If so, sexual recombination of genes will reduce the harm that bad mutations do to offspring and at the same time eliminate some bad mutations from the gene pool by isolating them in individuals that perish quickly because they have an above-average number of bad mutations. However, the evidence suggests that the DMH's assumptions are shaky because many species have on average less than one harmful mutation per individual and no species that has been investigated shows evidence of synergy between harmful mutations. The random nature of recombination causes the relative abundance of alternative traits to vary from one generation to another. This genetic drift is insufficient on its own to make sexual reproduction advantageous, but a combination of genetic drift and natural selection may be sufficient. When chance produces combinations of good traits, natural selection gives a large advantage to lineages in which these traits become genetically linked. On the other hand, the benefits of good traits are neutralized if they appear along with bad traits. Sexual recombination gives good traits the opportunities to become linked with other good traits, and mathematical models suggest this may be more than enough to offset the disadvantages of sexual reproduction. Other combinations of hypotheses that are inadequate on their own are also being examined. The adaptive function of sex remains a major unresolved issue in biology. The competing models to explain it were reviewed by John A. Birdsell and Christopher Wills. The hypotheses discussed above all depend on the possible beneficial effects of random genetic variation produced by genetic recombination. An alternative view is that sex arose and is maintained as a process for repairing DNA damage, and that the genetic variation produced is an occasionally beneficial byproduct. The simplest definitions of "multicellular", for example "having multiple cells", could include colonial cyanobacteria like Nostoc. Even a technical definition such as "having the same genome but different types of cell" would still include some genera of the green algae Volvox, which have cells that specialize in reproduction. Multicellularity evolved independently in organisms as diverse as sponges and other animals, fungi, plants, brown algae, cyanobacteria, slime molds and myxobacteria. For the sake of brevity, this article focuses on the organisms that show the greatest specialization of cells and variety of cell types, although this approach to the evolution of biological complexity could be regarded as "rather anthropocentric". The initial advantages of multicellularity may have included: more efficient sharing of nutrients that are digested outside the cell, increased resistance to predators, many of which attacked by engulfing; the ability to resist currents by attaching to a firm surface; the ability to reach upwards to filter-feed or to obtain sunlight for photosynthesis; the ability to create an internal environment that gives protection against the external one; and even the opportunity for a group of cells to behave "intelligently" by sharing information. These features would also have provided opportunities for other organisms to diversify, by creating more varied environments than flat microbial mats could. Multicellularity with differentiated cells is beneficial to the organism as a whole but disadvantageous from the point of view of individual cells, most of which lose the opportunity to reproduce themselves. In an asexual multicellular organism, rogue cells which retain the ability to reproduce may take over and reduce the organism to a mass of undifferentiated cells. Sexual reproduction eliminates such rogue cells from the next generation and therefore appears to be a prerequisite for complex multicellularity. The available evidence indicates that eukaryotes evolved much earlier but remained inconspicuous until a rapid diversification around 1 Ga. The only respect in which eukaryotes clearly surpass bacteria and archaea is their capacity for variety of forms, and sexual reproduction enabled eukaryotes to exploit that advantage by producing organisms with multiple cells that differed in form and function. By comparing the composition of transcription factor families and regulatory network motifs between unicellular organisms and multicellular organisms, scientists found there are many novel transcription factor families and three novel types of regulatory network motifs in multicellular organisms, and novel family transcription factors are preferentially wired into these novel network motifs which are essential for multicellular development. These results propose a plausible mechanism for the contribution of novel-family transcription factors and novel network motifs to the origin of multicellular organisms at transcriptional regulatory level. Fungi-like fossils were found in vesicular basalt dating back to the Paleoproterozoic Era around 2.4 billion years ago. The controversial Francevillian biota fossils, dated to 2.1 Ga, are the earliest known fossil organisms that are clearly multicellular, if they are indeed fossils. They may have had differentiated cells. Another early multicellular fossil, Qingshania, dated to 1.7 Ga, appears to consist of virtually identical cells. The red algae called Bangiomorpha, dated at 1.2 Ga, is the earliest known organism that certainly has differentiated, specialized cells, and is also the oldest known sexually reproducing organism. The 1.43 billion-year-old fossils interpreted as fungi appear to have been multicellular with differentiated cells. The "string of beads" organism Horodyskia, found in rocks dated from 1.5 Ga to 900 Ma, may have been an early metazoan; however, it has also been interpreted as a colonial foraminifera. Emergence of animals Deuterostomes Ecdysozoa Spiralia Xenacoelomorpha Cnidaria Placozoa Ctenophora Porifera Animals are multicellular eukaryotes,[note 1] and are distinguished from plants, algae, and fungi by lacking cell walls. All animals are motile, if only at certain life stages. All animals except sponges have bodies differentiated into separate tissues, including muscles, which move parts of the animal by contracting, and nerve tissue, which transmits and processes signals. In November 2019, researchers reported the discovery of Caveasphaera, a multicellular organism found in 609-million-year-old rocks, that is not easily defined as an animal or non-animal, which may be related to one of the earliest instances of animal evolution. Fossil studies of Caveasphaera have suggested that animal-like embryonic development arose much earlier than the oldest clearly defined animal fossils. and may be consistent with studies suggesting that animal evolution may have begun about 750 million years ago. Nonetheless, the earliest widely accepted animal fossils are the rather modern-looking cnidarians (the group that includes jellyfish, sea anemones and Hydra), possibly from around 580 Ma, although fossils from the Doushantuo Formation can only be dated approximately. Their presence implies that the cnidarian and bilaterian lineages had already diverged. The Ediacara biota, which flourished for the last 40 million years before the start of the Cambrian, were the first animals more than a very few centimeters long. Many were flat and had a "quilted" appearance, and seemed so strange that there was a proposal to classify them as a separate kingdom, Vendozoa. Others, however, have been interpreted as early molluscs (Kimberella), echinoderms (Arkarua), and arthropods (Spriggina, Parvancorina). There is still debate about the classification of these specimens, mainly because the diagnostic features which allow taxonomists to classify more recent organisms, such as similarities to living organisms, are generally absent in the Ediacarans. However, there seems little doubt that Kimberella was at least a triploblastic bilaterian animal, in other words, an animal significantly more complex than the cnidarians. The small shelly fauna are a very mixed collection of fossils found between the Late Ediacaran and Middle Cambrian periods. The earliest, Cloudina, shows signs of successful defense against predation and may indicate the start of an evolutionary arms race. Some tiny Early Cambrian shells almost certainly belonged to molluscs, while the owners of some "armor plates", Halkieria and Microdictyon, were eventually identified when more complete specimens were found in Cambrian lagerstätten that preserved soft-bodied animals. In the 1970s there was already a debate about whether the emergence of the modern phyla was "explosive" or gradual but hidden by the shortage of Precambrian animal fossils. A re-analysis of fossils from the Burgess Shale lagerstätte increased interest in the issue when it revealed animals, such as Opabinia, which did not fit into any known phylum. At the time these were interpreted as evidence that the modern phyla had evolved very rapidly in the Cambrian explosion and that the Burgess Shale's "weird wonders" showed that the Early Cambrian was a uniquely experimental period of animal evolution. Later discoveries of similar animals and the development of new theoretical approaches led to the conclusion that many of the "weird wonders" were evolutionary "aunts" or "cousins" of modern groups—for example that Opabinia was a member of the lobopods, a group which includes the ancestors of the arthropods, and that it may have been closely related to the modern tardigrades. Nevertheless, there is still much debate about whether the Cambrian explosion was really explosive and, if so, how and why it happened and why it appears unique in the history of animals. Most of the animals at the heart of the Cambrian explosion debate were protostomes, one of the two main groups of complex animals. The other major group, the deuterostomes, contains invertebrates such as starfish and sea urchins (echinoderms), as well as chordates (see below). Many echinoderms have hard calcite "shells", which are fairly common from the Early Cambrian small shelly fauna onwards. Other deuterostome groups are soft-bodied, and most of the significant Cambrian deuterostome fossils come from the Chengjiang fauna, a lagerstätte in China. The chordates are another major deuterostome group: animals with a distinct dorsal nerve cord. Chordates include soft-bodied invertebrates such as tunicates as well as vertebrates—animals with a backbone. While tunicate fossils predate the Cambrian explosion, the Chengjiang fossils Haikouichthys and Myllokunmingia appear to be true vertebrates, and Haikouichthys had distinct vertebrae, which may have been slightly mineralized. Vertebrates with jaws, such as the acanthodians, first appeared in the Late Ordovician. Colonization of land Adaptation to life on land is a major challenge: all land organisms need to avoid drying-out and all those above microscopic size must create special structures to withstand gravity; respiration and gas exchange systems have to change; reproductive systems cannot depend on water to carry eggs and sperm towards each other. Although the earliest good evidence of land plants and animals dates back to the Ordovician period (488 to 444 Ma), and a number of microorganism lineages made it onto land much earlier, modern land ecosystems only appeared in the Late Devonian, about 385 to 359 Ma. In May 2017, evidence of the earliest known life on land may have been found in 3.48-billion-year-old geyserite and other related mineral deposits (often found around hot springs and geysers) uncovered in the Pilbara Craton of Western Australia. In July 2018, scientists reported that the earliest life on land may have been bacteria living on land 3.22 billion years ago. In May 2019, scientists reported the discovery of a fossilized fungus, named Ourasphaira giraldae, in the Canadian Arctic, that may have grown on land a billion years ago, well before plants were living on land. Oxygen began to accumulate in Earth's atmosphere over 3 Ga, as a by-product of photosynthesis in cyanobacteria (blue-green algae). However, oxygen produces destructive chemical oxidation which was toxic to most previous organisms. Protective endogenous antioxidant enzymes and exogenous dietary antioxidants helped to prevent oxidative damage. For example, brown algae accumulate inorganic mineral antioxidants such as rubidium, vanadium, zinc, iron, copper, molybdenum, selenium and iodine, concentrated more than 30,000 times more than in seawater. Most marine mineral antioxidants act in the cells as essential trace elements in redox and antioxidant metalloenzymes.[citation needed] When plants and animals began to enter rivers and land about 500 Ma, environmental deficiency of these marine mineral antioxidants was a challenge to the evolution of terrestrial life. Terrestrial plants slowly optimized the production of new endogenous antioxidants such as ascorbic acid, polyphenols, flavonoids, tocopherols, etc. A few of these appeared more recently, in the last 200–50 Ma, in fruits and flowers of angiosperm plants.[citation needed] In fact, angiosperms (the dominant type of plant today) and most of their antioxidant pigments evolved during the Late Jurassic period. Plants employ antioxidants to defend their structures against reactive oxygen species produced during photosynthesis. Animals are exposed to the same oxidants, and they have evolved endogenous enzymatic antioxidant systems. Iodine in the form of the iodide ion I− is the most primitive and abundant electron-rich essential element in the diet of marine and terrestrial organisms; it acts as an electron donor and has this ancestral antioxidant function in all iodide-concentrating cells, from primitive marine algae to terrestrial vertebrates. Before the colonization of land there was no soil, a combination of mineral particles and decomposed organic matter. Land surfaces were either bare rock or shifting sand produced by weathering. Water and dissolved nutrients would have drained away very quickly. In the Sub-Cambrian peneplain in Sweden, for example, maximum depth of kaolinitization by Neoproterozoic weathering is about 5 m, while nearby kaolin deposits developed in the Mesozoic are much thicker. It has been argued that in the late Neoproterozoic sheet wash was a dominant process of erosion of surface material due to the lack of plants on land. Films of cyanobacteria, which are not plants but use the same photosynthesis mechanisms, have been found in modern deserts in areas unsuitable for vascular plants. This suggests that microbial mats may have been the first organisms to colonize dry land, possibly in the Precambrian. Mat-forming cyanobacteria could have gradually evolved resistance to desiccation as they spread from the seas to intertidal zones and then to land. Lichens, which are symbiotic combinations of a fungus (almost always an ascomycete) and one or more photosynthesizers (green algae or cyanobacteria), are also important colonizers of lifeless environments, and their ability to break down rocks contributes to soil formation where plants cannot survive. The earliest known ascomycete fossils date from 423 to 419 Ma in the Silurian. Soil formation would have been very slow until the appearance of burrowing animals, which mix the mineral and organic components of soil and whose feces are a major source of organic components. Burrows have been found in Ordovician sediments, and are attributed to annelids (worms) or arthropods. In aquatic algae, almost all cells are capable of photosynthesis and are nearly independent. Life on land requires plants to become internally more complex and specialized: photosynthesis is most efficient at the top; roots extract water and nutrients from the ground; and the intermediate parts support and transport. Spores of land plants resembling liverworts have been found in Middle Ordovician rocks from 476 Ma. Middle Silurian rocks from 430 Ma contain fossils of true plants, including clubmosses such as Baragwanathia; most were under 10 centimetres (3.9 in) high, and some appear closely related to vascular plants, the group that includes trees. By the Late Devonian 370 Ma, abundant trees such as Archaeopteris bound the soil so firmly that they changed river systems from mostly braided to mostly meandering. This caused the "Late Devonian wood crisis" because: Animals had to change their feeding and excretory systems, and most land animals developed internal fertilization of their eggs. The difference in refractive index between water and air required changes in their eyes. On the other hand, in some ways movement and breathing became easier, and the better transmission of high-frequency sounds in the air encouraged the development of hearing. The oldest animal with evidence of air-breathing, although not being the oldest myriapod fossil record, is Pneumodesmus, an archipolypodan millipede from the Early Devonian, about 414 Ma. Its air-breathing, terrestrial nature is evidenced by the presence of spiracles, the openings to tracheal systems. However, some earlier trace fossils from the Cambrian-Ordovician boundary about 490 Ma are interpreted as the tracks of large amphibious arthropods on coastal sand dunes, and may have been made by euthycarcinoids, which are thought to be evolutionary "aunts" of myriapods. Other trace fossils from the Late Ordovician a little over 445 Ma probably represent land invertebrates, and there is clear evidence of numerous arthropods on coasts and alluvial plains shortly before the Silurian-Devonian boundary, about 415 Ma, including signs that some arthropods ate plants. Arthropods were well pre-adapted to colonize land, because their existing jointed exoskeletons provided protection against desiccation, support against gravity and a means of locomotion that was not dependent on water. The fossil record of other major invertebrate groups on land is poor: none at all for non-parasitic flatworms, nematodes or nemerteans; some parasitic nematodes have been fossilized in amber; annelid worm fossils are known from the Carboniferous, but they may still have been aquatic animals; the earliest fossils of gastropods on land date from the Late Carboniferous, and this group may have had to wait until leaf litter became abundant enough to provide the moist conditions they need. The earliest confirmed fossils of flying insects date from the Late Carboniferous, but it is thought that insects developed the ability to fly in the Early Carboniferous or even Late Devonian. This gave them a wider range of ecological niches for feeding and breeding, and a means of escape from predators and from unfavorable changes in the environment. About 99% of modern insect species fly or are descendants of flying species. Osteolepiformes Panderichthyidae Obruchevichthidae Acanthostega Ichthyostega Tulerpeton Early labyrinthodonts Anthracosauria Amniotes Tetrapods, vertebrates with four limbs, evolved from other rhipidistian fish over a relatively short timespan during the Late Devonian (370 to 360 Ma). The early groups are grouped together as Labyrinthodontia. They retained aquatic, fry-like tadpoles, a system still seen in modern amphibians. Iodine and T4/T3 stimulate the amphibian metamorphosis and the evolution of nervous systems transforming the aquatic, vegetarian tadpole into a "more developed" terrestrial, carnivorous frog with better neurological, visuospatial, olfactory and cognitive abilities for hunting. The new hormonal action of T3 was made possible by the formation of T3-receptors in the cells of vertebrates. First, about 600–500 million years ago, the alpha T3-receptors with a metamorphosing action appeared in primitive chordates and then, about 250–150 million years ago, the beta T3-receptors with metabolic and thermogenetic actions appeared in birds and mammals. From the 1950s to the early 1980s it was thought that tetrapods evolved from fish that had already acquired the ability to crawl on land, possibly in order to go from a pool that was drying out to one that was deeper. However, in 1987, nearly complete fossils of Acanthostega from about 363 Ma showed that this Late Devonian transitional animal had legs and both lungs and gills, but could never have survived on land: its limbs and its wrist and ankle joints were too weak to bear its weight; its ribs were too short to prevent its lungs from being squeezed flat by its weight; its fish-like tail fin would have been damaged by dragging on the ground. The current hypothesis is that Acanthostega, which was about 1 metre (3.3 ft) long, was a wholly aquatic predator that hunted in shallow water. Its skeleton differed from that of most fish, in ways that enabled it to raise its head to breathe air while its body remained submerged, including: its jaws show modifications that would have enabled it to gulp air; the bones at the back of its skull are locked together, providing strong attachment points for muscles that raised its head; the head is not joined to the shoulder girdle and it has a distinct neck. The Devonian proliferation of land plants may help to explain why air breathing would have been an advantage: leaves falling into streams and rivers would have encouraged the growth of aquatic vegetation; this would have attracted grazing invertebrates and small fish that preyed on them; they would have been attractive prey but the environment was unsuitable for the big marine predatory fish; air-breathing would have been necessary because these waters would have been short of oxygen, since warm water holds less dissolved oxygen than cooler marine water and since the decomposition of vegetation would have used some of the oxygen. Later discoveries revealed earlier transitional forms between Acanthostega and completely fish-like animals. Unfortunately, there is then a gap (Romer's gap) of about 30 Ma between the fossils of ancestral tetrapods and Middle Carboniferous fossils of vertebrates that look well-adapted for life on land, during which only some fossils are found, which had five digits in the terminating point of the four limbs, showing true or crown tetrapods appeared in the gap around 350 Ma. Some of the fossils after this gap look as if the animals which they belonged to were early relatives of modern amphibians, all of which need to keep their skins moist and to lay their eggs in water, while others are accepted as early relatives of the amniotes, whose waterproof skin and egg membranes enable them to live and breed far from water. The Carboniferous Rainforest Collapse may have paved the way for amniotes to become dominant over amphibians. Early synapsids (extinct) Extinct pelycosaurs Extinct therapsids Extinct mammaliaforms Mammals Anapsids; whether turtles belong here is debated Captorhinidae and Protorothyrididae (extinct) Araeoscelidia (extinct) Squamata (lizards and snakes) Extinct archosaurs Crocodilians Pterosaurs (extinct) Extincttheropods Birds Sauropods(extinct) Ornithischians (extinct) Amniotes, whose eggs can survive in dry environments, probably evolved in the Late Carboniferous period (330 to 298.9 Ma). The earliest fossils of the two surviving amniote groups, synapsids and sauropsids, date from around 313 Ma. The synapsid pelycosaurs and their descendants the therapsids are the most common land vertebrates in the best-known Permian (298.9 to 251.9 Ma) fossil beds. However, at the time these were all in temperate zones at middle latitudes, and there is evidence that hotter, drier environments nearer the Equator were dominated by sauropsids and amphibians. The Permian–Triassic extinction event wiped out almost all land vertebrates, as well as the great majority of other life. During the slow recovery from this catastrophe, estimated to have taken 30 million years, a previously obscure sauropsid group became the most abundant and diverse terrestrial vertebrates: a few fossils of archosauriformes ("ruling lizard forms") have been found in Late Permian rocks, but, by the Middle Triassic, archosaurs were the dominant land vertebrates. Dinosaurs distinguished themselves from other archosaurs in the Late Triassic, and became the dominant land vertebrates of the Jurassic and Cretaceous periods (201.4 to 66 Ma). During the Late Jurassic, birds evolved from small, predatory theropod dinosaurs. The first birds inherited teeth and long, bony tails from their dinosaur ancestors, but some had developed horny, toothless beaks by the very Late Jurassic and short pygostyle tails by the Early Cretaceous. While the archosaurs and dinosaurs were becoming more dominant in the Triassic, the mammaliaform successors of the therapsids evolved into small, mainly nocturnal insectivores. This ecological role may have promoted the evolution of mammals, for example nocturnal life may have accelerated the development of endothermy ("warm-bloodedness") and hair or fur. By 195 Ma in the Early Jurassic there were animals that were very like today's mammals in a number of respects. Unfortunately, there is a gap in the fossil record throughout the Middle Jurassic. However, fossil teeth discovered in Madagascar indicate that the split between the lineage leading to monotremes and the one leading to other living mammals had occurred by 167 Ma. After dominating land vertebrate niches for about 150 Ma, the non-avian dinosaurs perished in the Cretaceous–Paleogene extinction event (66 Ma) along with many other groups of organisms. Mammals throughout the time of the dinosaurs had been restricted to a narrow range of taxa, sizes and shapes, but increased rapidly in size and diversity after the extinction, with bats taking to the air within 13 million years, and cetaceans to the sea within 15 million years. Gnetales(gymnosperm) Welwitschia(gymnosperm) Ephedra(gymnosperm) Bennettitales Angiosperms(flowering plants) Angiosperms(flowering plants) Cycads(gymnosperm) Bennettitales Ginkgo Gnetales(gymnosperm) Conifers(gymnosperm) The first flowering plants appeared around 130 Ma. The 250,000 to 400,000 species of flowering plants outnumber all other ground plants combined, and are the dominant vegetation in most terrestrial ecosystems. There is fossil evidence that flowering plants diversified rapidly in the Early Cretaceous, from 130 to 90 Ma, and that their rise was associated with that of pollinating insects. Among modern flowering plants Magnolia are thought to be close to the common ancestor of the group. However, paleontologists have not succeeded in identifying the earliest stages in the evolution of flowering plants. The social insects are remarkable because the great majority of individuals in each colony are sterile. This appears contrary to basic concepts of evolution such as natural selection and the selfish gene. In fact, there are very few eusocial insect species: only 15 out of approximately 2,600 living families of insects contain eusocial species, and it seems that eusociality has evolved independently only 12 times among arthropods, although some eusocial lineages have diversified into several families. Nevertheless, social insects have been spectacularly successful; for example although ants and termites account for only about 2% of known insect species, they form over 50% of the total mass of insects. Their ability to control a territory appears to be the foundation of their success. The sacrifice of breeding opportunities by most individuals has long been explained as a consequence of these species' unusual haplodiploid method of sex determination, which has the paradoxical consequence that two sterile worker daughters of the same queen share more genes with each other than they would with their offspring if they could breed. However, E. O. Wilson and Bert Hölldobler argue that this explanation is faulty: for example, it is based on kin selection, but there is no evidence of nepotism in colonies that have multiple queens. Instead, they write, eusociality evolves only in species that are under strong pressure from predators and competitors, but in environments where it is possible to build "fortresses"; after colonies have established this security, they gain other advantages through co-operative foraging. In support of this explanation they cite the appearance of eusociality in bathyergid mole rats, which are not haplodiploid. The earliest fossils of insects have been found in Early Devonian rocks from about 400 Ma, which preserve only a few varieties of flightless insect. The Mazon Creek lagerstätten from the Late Carboniferous, about 300 Ma, include about 200 species, some gigantic by modern standards, and indicate that insects had occupied their main modern ecological niches as herbivores, detritivores and insectivores. Social termites and ants first appeared in the Early Cretaceous, and advanced social bees have been found in Late Cretaceous rocks but did not become abundant until the Middle Cenozoic. The idea that, along with other life forms, modern-day humans evolved from an ancient, common ancestor was proposed by Robert Chambers in 1844 and taken up by Charles Darwin in 1871. Modern humans evolved from a lineage of upright-walking apes that has been traced back over 6 Ma to Sahelanthropus. The first known stone tools were made about 2.5 Ma, apparently by Australopithecus garhi, and were found near animal bones that bear scratches made by these tools. The earliest hominines had chimpanzee-sized brains, but there has been a fourfold increase in the last 3 Ma; a statistical analysis suggests that hominine brain sizes depend almost completely on the date of the fossils, while the species to which they are assigned has only slight influence. There is a long-running debate about whether modern humans evolved all over the world simultaneously from existing advanced hominines or are descendants of a single small population in Africa, which then migrated all over the world less than 200,000 years ago and replaced previous hominine species. There is also debate about whether anatomically modern humans had an intellectual, cultural and technological "Great Leap Forward" under 40,000–50,000 years ago and, if so, whether this was due to neurological changes that are not visible in fossils. Mass extinctions Life on Earth has suffered occasional mass extinctions at least since 542 Ma. Although they were disasters at the time, mass extinctions have sometimes accelerated the evolution of life on Earth. When dominance of particular ecological niches passes from one group of organisms to another, it is rarely because the new dominant group is "superior" to the old and usually because an extinction event eliminates the old dominant group and makes way for the new one. The fossil record appears to show that the gaps between mass extinctions are becoming longer and that the average and background rates of extinction are decreasing. Both of these phenomena could be explained in one or more ways: Biodiversity in the fossil record, which is "...the number of distinct genera alive at any given time; that is, those whose first occurrence predates and whose last occurrence postdates that time" shows a different trend: a fairly swift rise from 542 to 400 Ma; a slight decline from 400 to 200 Ma, in which the devastating Permian–Triassic extinction event is an important factor; and a swift rise from 200 Ma to the present. See also Footnotes References Bibliography Further reading External links |
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[SOURCE: https://en.wikipedia.org/wiki/Magnetosphere] | [TOKENS: 2298] |
Contents Magnetosphere In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object, such as a planet or other object, in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior dynamo. In the space environment close to a planetary body with a dipole magnetic field such as Earth, the field lines resemble a simple magnetic dipole. Farther out, field lines can be significantly distorted by the flow of electrically conducting plasma, as emitted from the Sun (i.e., the solar wind) or a nearby star. Planets having active magnetospheres, like the Earth, are capable of mitigating or blocking the effects of solar radiation or cosmic radiation. Interactions of particles and atmospheres with magnetospheres are studied under the specialized scientific subjects of plasma physics, space physics, and aeronomy. History Study of Earth's magnetosphere began in 1600, when William Gilbert discovered that the magnetic field on the surface of Earth resembled that of a terrella, a small, magnetized sphere. In the 1940s, Walter M. Elsasser proposed the model of dynamo theory, which attributes Earth's magnetic field to the motion of Earth's iron outer core. Through the use of magnetometers, scientists were able to study the variations in Earth's magnetic field as functions of both time and latitude and longitude. Beginning in the late 1940s, rockets were used to study cosmic rays. In 1958, Explorer 1, the first of the Explorer series of space missions, was launched to study the intensity of cosmic rays above the atmosphere and measure the fluctuations in this activity. This mission observed the existence of the Van Allen radiation belt (located in the inner region of Earth's magnetosphere), with the follow-up Explorer 3 later that year definitively proving its existence. Also during 1958, Eugene Parker proposed the idea of the solar wind, with the term 'magnetosphere' being proposed by Thomas Gold in 1959 to explain how the solar wind interacted with the Earth's magnetic field. The later mission of Explorer 12 in 1961 led by the Cahill and Amazeen observation in 1963 of a sudden decrease in magnetic field strength near the noon-time meridian, later was named the magnetopause. By 1983, the International Cometary Explorer observed the magnetotail, or the distant magnetic field. Structure and behavior The structure of magnetospheres are dependent on several factors: the type of astronomical object, the nature of sources of plasma and momentum, the period of the object's spin, the nature of the axis about which the object spins, the axis of the magnetic dipole, and the magnitude and direction of the flow of solar wind. The planetary distance where the magnetosphere can withstand the solar wind pressure is called the Chapman–Ferraro distance R C F {\displaystyle R_{\rm {CF}}} . This is usefully modeled by the formula wherein R P {\displaystyle R_{\rm {P}}} represents the radius of the planet, B s u r f {\displaystyle B_{\rm {surf}}} represents the magnetic field on the surface of the planet at the equator, V S W {\displaystyle V_{\rm {SW}}} represents the velocity of the solar wind, ρ {\displaystyle \rho } is the particle density of solar wind, and μ 0 {\displaystyle \mu _{0}} is the vacuum permeability constant: A magnetosphere is classified as "intrinsic" when R C F ≫ R P {\displaystyle R_{\rm {CF}}\gg R_{\rm {P}}} , or when the primary opposition to the flow of solar wind is the magnetic field of the object. Mercury, Earth, Jupiter, Ganymede, Saturn, Uranus, and Neptune, for example, exhibit intrinsic magnetospheres. A magnetosphere is classified as "induced" when R C F ≪ R P {\displaystyle R_{\rm {CF}}\ll R_{\rm {P}}} , or when the solar wind is not opposed by the object's magnetic field. In this case, the solar wind interacts with the atmosphere or ionosphere of the planet (or surface of the planet, if the planet has no atmosphere). Venus has an induced magnetic field, which means that because Venus appears to have no internal dynamo effect, the only magnetic field present is that formed by the solar wind's wrapping around the physical obstacle of Venus (see also Venus' induced magnetosphere). When R C F ≈ R P {\displaystyle R_{\rm {CF}}\approx R_{\rm {P}}} , the planet itself and its magnetic field both contribute. It is possible that Mars is of this type. When viewed from the Sun, a celestial body's orbital motion can compress its otherwise symmetrical magnetosphere slightly, and stretch it out in the direction opposite its motion (in Earth's example, from west to east). This is known as dawn-dusk asymmetry. Structure The bow shock forms the outermost layer of the magnetosphere; the boundary between the magnetosphere and the surrounding medium. For stars, this is usually the boundary between the stellar wind and interstellar medium; for planets, the speed of the solar wind there decreases as it approaches the magnetopause. Due to interactions with the bow shock, the stellar wind plasma gains a substantial anisotropy, leading to various plasma instabilities upstream and downstream of the bow shock. The magnetosheath is the region of the magnetosphere between the bow shock and the magnetopause. It is formed mainly from shocked solar wind, though it contains a small amount of plasma from the magnetosphere. It is an area exhibiting high particle energy flux, where the direction and magnitude of the magnetic field varies erratically. This is caused by the collection of solar wind gas that has effectively undergone thermalization. It acts as a cushion that transmits the pressure from the flow of the solar wind and the barrier of the magnetic field from the object. The magnetopause is the area of the magnetosphere wherein the pressure from the planetary magnetic field is balanced with the pressure from the solar wind. It is the convergence of the shocked solar wind from the magnetosheath with the magnetic field of the object and plasma from the magnetosphere. Because both sides of this convergence contain magnetized plasma, the interactions between them are complex. The structure of the magnetopause depends upon the Mach number and beta ratio of the plasma, as well as the magnetic field. The magnetopause changes size and shape as the pressure from the solar wind fluctuates. Opposite the compressed magnetic field is the magnetotail, where the magnetosphere extends far beyond the astronomical object. It contains two lobes, referred to as the northern and southern tail lobes. Magnetic field lines in the northern tail lobe point towards the object while those in the southern tail lobe point away. The tail lobes are almost empty, with few charged particles opposing the flow of the solar wind. The two lobes are separated by a plasma sheet, an area where the magnetic field is weaker, and the density of charged particles is higher. Over Earth's equator, the magnetic field lines become almost horizontal, then return to reconnect at high latitudes. However, at high altitudes, the magnetic field is significantly distorted by the solar wind and its solar magnetic field. On the dayside of Earth, the magnetic field is significantly compressed by the solar wind to a distance of approximately 65,000 kilometers (40,000 mi). Earth's bow shock is about 17 kilometers (11 mi) thick and located about 90,000 kilometers (56,000 mi) from Earth. The dayside magnetopause exists at a distance of about 30,000–60,000 kilometers above Earth's surface. Earth's magnetopause has been compared to a sieve because it allows solar wind particles to enter. Kelvin–Helmholtz instabilities occur when large swirls of plasma travel along the edge of the magnetosphere at different velocities from the magnetosphere, causing the plasma to slip past. This results in magnetic reconnection, and as the magnetic field lines break and reconnect, solar wind particles are able to enter the magnetosphere. On Earth's nightside, the magnetic field extends in the magnetotail, which lengthwise exceeds 6,300,000 kilometers (3,900,000 mi). Earth's magnetotail is the primary source of the polar aurora. Also, NASA scientists have suggested that Earth's magnetotail might cause "dust storms" on the Moon by creating a potential difference between the day side and the night side. The center of the tail's plasma sheet, referred to as the neutral sheet, is the region in which the magnetic field lines from each lobe can meet. It is therefore an important site of reconnection in the tail. Far from quiescent, the plasma sheet is known to exhibit bulk motions that tilt the neutral sheet relative to the ecliptic plane, producing oscillations referred to as flapping motions. These motions consist of oscillations of the plasma sheet in the north-south direction. An analogy with windsocks may be helpful in visualising these movements of the plasma sheet. Many astronomical objects generate and maintain magnetospheres. In the Solar System this includes the Sun, Mercury, Earth, Jupiter, Saturn, Uranus, Neptune, and Ganymede. The magnetosphere of Jupiter is the largest planetary magnetosphere in the Solar System, extending up to 7,000,000 kilometers (4,300,000 mi) on the dayside and almost to the orbit of Saturn on the nightside. Jupiter's magnetosphere is stronger than Earth's by an order of magnitude, and its magnetic moment is approximately 18,000 times larger. Venus, Mars, and Pluto, on the other hand, have no intrinsic magnetic field. This may have had significant effects on their geological history. It is hypothesized that Venus and Mars may have lost their primordial water to photodissociation and the solar wind. A strong magnetosphere, were it present, would greatly slow down this process. Alfvén Mach number Magnetospheres generated by exoplanets are thought to be common, though the first discoveries did not come until the 2010s. In 2014, a magnetic field around HD 209458 b was inferred from the way hydrogen was evaporating from the planet. In 2019, the strength of the surface magnetic fields of 4 hot Jupiters were estimated and ranged between 20 and 120 gauss compared to Jupiter's surface magnetic field of 4.3 gauss. In 2020, a radio emission in the 14-30 MHz band was detected from the Tau Boötis system, likely associated with cyclotron radiation from the poles of Tau Boötis b which might be a signature of a planetary magnetic field. In 2021 a magnetic field generated by the hot Neptune HAT-P-11b became the first to be confirmed. The first unconfirmed detection of a magnetic field generated by a terrestrial exoplanet was found in 2023 on YZ Ceti b. See also References |
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[SOURCE: https://en.wikipedia.org/wiki/Planetary_science] | [TOKENS: 2678] |
Contents Planetary science Planetary science (or more rarely, planetology) is the scientific study of planets (including Earth), celestial bodies (such as moons, asteroids, comets) and planetary systems (in particular those of the Solar System) and the processes of their formation. It studies objects ranging in sizes from micrometeoroids to huge gas giants, with the aim of determining their composition, dynamics, formation, interrelations and history. It is a strongly interdisciplinary field, which originally grew from astronomy and Earth science, and now incorporates many disciplines, including planetary geology, cosmochemistry, atmospheric science, physics, oceanography, hydrology, theoretical planetary science, glaciology, and exoplanetology. Allied disciplines include space physics, when concerned with the effects of the Sun on the bodies of the Solar System, and astrobiology. There are interrelated observational and theoretical branches of planetary science. Observational research can involve combinations of space exploration, predominantly with robotic spacecraft missions using remote sensing, and comparative, experimental work in Earth-based laboratories. The theoretical component involves considerable computer simulation and mathematical modelling. Planetary scientists are generally located in the astronomy and physics or Earth sciences departments of universities or research centres, though there are several purely planetary science institutes worldwide. Generally, planetary scientists study one of the Earth sciences, astronomy, astrophysics, geophysics, or physics at the graduate level and concentrate their research in planetary science disciplines. There are several major conferences each year, and a wide range of peer-reviewed journals. Some planetary scientists work at private research centres and often initiate partnership research tasks. History The history of planetary science may be said to have begun with the Ancient Greek philosopher Democritus, who is reported by Hippolytus as saying The ordered worlds are boundless and differ in size, and that in some there is neither sun nor moon, but that in others, both are greater than with us, and yet with others more in number. And that the intervals between the ordered worlds are unequal, here more and there less, and that some increase, others flourish and others decay, and here they come into being and there they are eclipsed. But that they are destroyed by colliding with one another. And that some ordered worlds are bare of animals, plants, and all water. These planets are rocky planets with nothing else, sometimes with an atmosphere, but inhospitable. In more modern times, planetary science began in astronomy, from studies of the unresolved planets. In this sense, the original planetary astronomer would be Galileo, who discovered the four largest moons of Jupiter, the mountains on the Moon, and first observed the rings of Saturn, all objects of intense later study. Galileo's study of the lunar mountains in 1609 also began the study of extraterrestrial landscapes: his observation "that the Moon certainly does not possess a smooth and polished surface" suggested that it and other worlds might appear "just like the face of the Earth itself". Advances in telescope construction and instrumental resolution gradually allowed increased identification of the atmospheric as well as surface details of the planets. The Moon was initially the most heavily studied, due to its proximity to the Earth, as it always exhibited elaborate features on its surface, and the technological improvements gradually produced more detailed lunar geological knowledge. In this scientific process, the main instruments were astronomical optical telescopes (and later radio telescopes) and finally robotic exploratory spacecraft, such as space probes. The Solar System has now been relatively well-studied, and a good overall understanding of the formation and evolution of this planetary system exists. However, there are large numbers of unsolved questions, and the rate of new discoveries is very high, partly due to the large number of interplanetary spacecraft currently exploring the Solar System. Disciplines Planetary science studies observational and theoretical astronomy, geology (astrogeology), atmospheric science, and an emerging subspecialty in planetary oceans, called planetary oceanography. This is both an observational and a theoretical science. Observational researchers are predominantly concerned with the study of the small bodies of the Solar System: those that are observed by telescopes, both optical and radio, so that characteristics of these bodies such as shape, spin, surface materials and weathering are determined, and the history of their formation and evolution can be understood. Theoretical planetary astronomy is concerned with dynamics: the application of the principles of celestial mechanics to the Solar System and extrasolar planetary systems. Observing exoplanets and determining their physical properties, exoplanetology, is a major area of research besides Solar System studies. Every planet has its own branch. In planetary science, the term geology is used in its broadest sense, to mean the study of the surface and interior parts of planets and moons, from their core to their magnetosphere. The best-known research topics of planetary geology deal with the planetary bodies in the near vicinity of the Earth: the Moon, and the two neighboring planets: Venus and Mars. Of these, the Moon was studied first, using methods developed earlier on the Earth. Planetary geology focuses on celestial objects that exhibit a solid surface or have significant solid physical states as part of their structure. Planetary geology applies geology, geophysics and geochemistry to planetary bodies. Geomorphology studies the features on planetary surfaces and reconstructs the history of their formation, inferring the physical processes that acted on the surface. Planetary geomorphology includes the study of several classes of surface features: The history of a planetary surface can be deciphered by mapping features from top to bottom according to their deposition sequence, as first determined on terrestrial strata by Nicolas Steno. For example, stratigraphic mapping prepared the Apollo astronauts for the field geology they would encounter on their lunar missions. Overlapping sequences were identified on images taken by the Lunar Orbiter program, and these were used to prepare a lunar stratigraphic column and geological map of the Moon. One of the main problems when generating hypotheses on the formation and evolution of objects in the Solar System is the lack of samples that can be analyzed in the laboratory, where a large suite of tools are available, and the full body of knowledge derived from terrestrial geology can be brought to bear. Direct samples from the Moon, asteroids and Mars are present on Earth, removed from their parent bodies, and delivered as meteorites. Some of these have suffered contamination from the oxidising effect of Earth's atmosphere and the infiltration of the biosphere, but those meteorites collected in the last few decades from Antarctica are almost entirely pristine. The different types of meteorites that originate from the asteroid belt cover almost all parts of the structure of differentiated bodies: meteorites even exist that come from the core-mantle boundary (pallasites). The combination of geochemistry and observational astronomy has also made it possible to trace the HED meteorites back to a specific asteroid in the main belt, 4 Vesta. The comparatively few known Martian meteorites have provided insight into the geochemical composition of the Martian crust, although the unavoidable lack of information about their points of origin on the diverse Martian surface has meant that they do not provide more detailed constraints on theories of the evolution of the Martian lithosphere. As of July 24, 2013, 65 samples of Martian meteorites have been discovered on Earth. Many were found in either Antarctica or the Sahara Desert. During the Apollo era, in the Apollo program, 384 kilograms of lunar samples were collected and transported to the Earth, and three Soviet Luna robots also delivered regolith samples from the Moon. These samples provide the most comprehensive record of the composition of any Solar System body besides the Earth. The numbers of lunar meteorites are growing quickly in the last few years – as of April 2008 there are 54 meteorites that have been officially classified as lunar. Eleven of these are from the US Antarctic meteorite collection, 6 are from the Japanese Antarctic meteorite collection and the other 37 are from hot desert localities in Africa, Australia, and the Middle East. The total mass of recognized lunar meteorites is close to 50 kg. Space probes made it possible to collect data in not only the visible light region but in other areas of the electromagnetic spectrum. The planets can be characterized by their force fields: gravity and their magnetic fields, which are studied through geophysics and space physics. Measuring the changes in acceleration experienced by spacecraft as they orbit has allowed fine details of the gravity fields of the planets to be mapped. For example, in the 1970s, the gravity field disturbances above lunar maria were measured through lunar orbiters, which led to the discovery of concentrations of mass, mascons, beneath the Imbrium, Serenitatis, Crisium, Nectaris and Humorum basins. If a planet's magnetic field is sufficiently strong, its interaction with the solar wind forms a magnetosphere around a planet. Early space probes discovered the gross dimensions of the terrestrial magnetic field, which extends about 10 Earth radii towards the Sun. The solar wind, a stream of charged particles, streams out and around the terrestrial magnetic field, and continues behind the magnetic tail, hundreds of Earth radii downstream. Inside the magnetosphere, there are relatively dense regions of solar wind particles, the Van Allen radiation belts. Planetary geophysics includes, but is not limited to, seismology and tectonophysics, geophysical fluid dynamics, mineral physics, geodynamics, mathematical geophysics, and geophysical surveying. Planetary geodesy (also known as planetary geodetics) deals with the measurement and representation of the planets of the Solar System, their gravitational fields and geodynamic phenomena (polar motion in three-dimensional, time-varying space). The science of geodesy has elements of both astrophysics and planetary sciences. The shape of the Earth is to a large extent the result of its rotation, which causes its equatorial bulge, and the competition of geologic processes such as the collision of plates and of vulcanism, resisted by the Earth's gravity field. These principles can be applied to the solid surface of Earth (orogeny; Few mountains are higher than 10 km (6 mi), few deep sea trenches deeper than that because quite simply, a mountain as tall as, for example, 15 km (9 mi), would develop so much pressure at its base, due to gravity, that the rock there would become plastic, and the mountain would slump back to a height of roughly 10 km (6 mi) in a geologically insignificant time. Some or all of these geologic principles can be applied to other planets besides Earth. For instance on Mars, whose surface gravity is much less, the largest volcano, Olympus Mons, is 27 km (17 mi) high at its peak, a height that could not be maintained on Earth. The Earth geoid is essentially the figure of the Earth abstracted from its topographic features. Therefore, the Mars geoid (areoid) is essentially the figure of Mars abstracted from its topographic features. Surveying and mapping are two important fields of application of geodesy. An atmosphere is an important transitional zone between the solid planetary surface and the higher rarefied ionizing and radiation belts. Not all planets have atmospheres: their existence depends on the mass of the planet, and the planet's distance from the Sun – too distant and frozen atmospheres occur. Besides the four giant planets, three of the four terrestrial planets (Earth, Venus, and Mars) have significant atmospheres. Two moons have significant atmospheres: Saturn's moon Titan and Neptune's moon Triton. A tenuous atmosphere exists around Mercury. The effects of the rotation rate of a planet about its axis can be seen in atmospheric streams and currents. Seen from space, these features show as bands and eddies in the cloud system and are particularly visible on Jupiter and Saturn. Exoplanetology studies exoplanets, the planets existing outside of the Solar System. Until recently, the means of studying exoplanets have been extremely limited, but with the current rate of innovation in research technology, exoplanetology has become a rapidly developing subfield of astronomy. Comparative planetary science Planetary science frequently makes use of the method of comparison to give a greater understanding of the object of study. This can involve comparing the dense atmospheres of Earth and Saturn's moon Titan, the evolution of outer Solar System objects at different distances from the Sun, or the geomorphology of the surfaces of the terrestrial planets, to give only a few examples. The main comparison that can be made is to features on the Earth, as it is much more accessible and allows a much greater range of measurements to be made. Earth analog studies are particularly common in planetary geology, geomorphology, and also in atmospheric science. The use of terrestrial analogs was first described by Gilbert (1886). In fiction Professional activity This non-exhaustive list includes those institutions and universities with major groups of people working in planetary science. Alphabetical order is used. Smaller workshops and conferences on particular fields occur worldwide throughout the year. See also References Further reading External links |
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[SOURCE: https://en.wikipedia.org/wiki/Category:Earth] | [TOKENS: 51] |
Category:Earth Subcategories This category has the following 23 subcategories, out of 23 total. Pages in category "Earth" The following 58 pages are in this category, out of 58 total. This list may not reflect recent changes. |
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[SOURCE: https://en.wikipedia.org/wiki/Geologic_record] | [TOKENS: 981] |
Contents Geologic record The geologic record in stratigraphy, paleontology and other natural sciences refers to the entirety of the layers of rock strata. That is, deposits laid down by volcanism or by deposition of sediment derived from weathering detritus (clays, sands etc.). This includes all its fossil content and the information it yields about the history of the Earth: its past climate, geography, geology and the evolution of life on its surface. According to the law of superposition, sedimentary and volcanic rock layers are deposited on top of each other. They harden over time to become a solidified (competent) rock column, that may be intruded by igneous rocks and disrupted by tectonic events. Correlating the rock record At a certain locality on the Earth's surface, the rock column provides a cross section of the natural history in the area during the time covered by the age of the rocks. This is sometimes called the rock history and gives a window into the natural history of the location that spans many geological time units such as ages, epochs, or in some cases even multiple major geologic periods—for the particular geographic region or regions. The geologic record is in no one place entirely complete for where geologic forces one age provide a low-lying region accumulating deposits much like a layer cake, in the next may have uplifted the region, and the same area is instead one that is weathering and being torn down by chemistry, wind, temperature, and water. This is to say that in a given location, the geologic record can be and is quite often interrupted as the ancient local environment was converted by geological forces into new landforms and features. Sediment core data at the mouths of large riverine drainage basins, some of which go 7 miles (11 km) deep thoroughly support the law of superposition.[clarification needed] However using broadly occurring deposited layers trapped within differently located rock columns, geologists have pieced together a system of units covering most of the geologic time scale using the law of superposition, for where tectonic forces have uplifted one ridge newly subject to erosion and weathering in folding and faulting the strata, they have also created a nearby trough or structural basin region that lies at a relative lower elevation that can accumulate additional deposits. By comparing overall formations, geologic structures and local strata, calibrated by those layers which are widespread, a nearly complete geologic record has been constructed since the 17th century. Discordant strata example Correcting for discordancies can be done in a number of ways and utilizing a number of technologies or field research results from studies in other disciplines. In this example, the study of layered rocks and the fossils they contain is called biostratigraphy and utilizes amassed geobiology and paleobiological knowledge. Fossils can be used to recognize rock layers of the same or different geologic ages, thereby coordinating locally occurring geologic stages to the overall geologic timeline. The pictures of the fossils of monocellular algae in this USGS figure were taken with a scanning electron microscope and have been magnified 250 times. In the U.S. state of South Carolina three marker species of fossil algae are found in a core of rock whereas in Virginia only two of the three species are found in the Eocene Series of rock layers spanning three stages and the geologic ages from 37.2–55.8 MA. Comparing the record about the discordance in the record to the full rock column shows the non-occurrence of the missing species and that portion of the local rock record, from the early part of the middle Eocene is missing there. This is one form of discordancy and the means geologists use to compensate for local variations in the rock record. With the two remaining marker species it is possible to correlate rock layers of the same age (early Eocene and latter part of the middle Eocene) in both South Carolina and Virginia, and thereby "calibrate" the local rock column into its proper place in the overall geologic record. Lithology vs paleontology Consequently, as the picture of the overall rock record emerged, and discontinuities and similarities in one place were cross-correlated to those in others, it became useful to subdivide the overall geologic record into a series of component sub-sections representing different sized groups of layers within known geologic time, from the shortest time span stage to the largest thickest strata eonothem and time spans eon. Concurrent work in other natural science fields required a time continuum be defined, and earth scientists decided to coordinate the system of rock layers and their identification criteria with that of the geologic time scale. This gives the pairing between the physical layers of the left column and the time units of the center column in the table at right. Gallery References |
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[SOURCE: https://en.wikipedia.org/wiki/Template:World_topic] | [TOKENS: 57] |
Contents Template:World topic Basic usage Prefix is what is added before each country listed. Thus "prefix=Music of" creates "Music of Africa", etc. Additional parameters Title is the table heading (top). Group1 is the row heading (left). Overriding the style See also |
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