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Sugar gliders are seasonally adaptive omnivores with a wide variety of foods in their diet, and mainly forage in the lower layers of the forest canopy. Sugar gliders may obtain up to half their daily water intake through drinking rainwater, with the remainder obtained through water held in its food. In summer they are primarily insectivorous, and in the winter when insects (and other arthropods) are scarce, they are mostly exudativorous (feeding on acacia gum, eucalyptus sap, manna, honeydew or lerp). Sugar gliders have an enlarged caecum to assist in digestion of complex carbohydrates obtained from gum and sap.
To obtain sap or gum from plants, sugar gliders will strip the bark off trees or open bore holes with their teeth to access stored liquid. Little time is spent foraging for insects, as it is an energetically expensive process, and sugar gliders will wait until insects fly into their habitat, or stop to feed on flowers.
Gliders consume approximately 11 g of dry food matter per day. This equates to roughly 8% and 9.5% of body weight for males and females, respectively.
They are opportunistic feeders and can be carnivorous, preying mostly on lizards and small birds. They eat many other foods when available, such as nectar, acacia seeds, bird eggs, pollen, fungi and native fruits. Pollen can make up a large portion of their diet, therefore sugar gliders are likely to be important pollinators of Banksia species.
Reproduction
Like most marsupials, female sugar gliders have two ovaries and two uteri; they are polyestrous, meaning they can go into heat several times a year. The female has a marsupium (pouch) in the middle of her abdomen to carry offspring. The pouch opens anteriorly, and two lateral pockets extend posteriorly when young are present. Four nipples are usually present in the pouch, although reports of individuals with two nipples have been recorded. Male sugar gliders have two pairs of bulbourethral glands and a bifurcated penis to correspond with the two uteri of females. | Sugar glider | Wikipedia | 445 | 42991 | https://en.wikipedia.org/wiki/Sugar%20glider | Biology and health sciences | Diprotodontia | Animals |
The age of sexual maturity in sugar gliders varies slightly between the males and females. Males reach maturity at 4 to 12 months of age, while females require from 8 to 12 months. In the wild, sugar gliders breed once or twice a year depending on the climate and habitat conditions, while they can breed multiple times a year in captivity as a result of consistent living conditions and proper diet.
A sugar glider female gives birth to one (19%) or two (81%) babies (joeys) per litter. The gestation period is 15 to 17 days, after which the tiny joey will crawl into a mother's pouch for further development. They are born largely undeveloped and furless, with only the sense of smell being developed. The mother has a scent gland in the external marsupium to attract the sightless joeys from the uterus. Joeys have a continuous arch of cartilage in their shoulder girdle which disappears soon after birth; this supports the forelimbs, assisting the climb into the pouch. Young are completely contained in the pouch for 60 days after birth, wherein mammae provide nourishment during the remainder of development. Eyes first open around 80 days after birth, and young will leave the nest around 110 days after birth. By the time young are weaned, the thermoregulatory system is developed, and in conjunction with a large body size and thicker fur, they are able to regulate their own body temperature.
Breeding is seasonal in southeast Australia, with young only born in winter and spring (June to November). Unlike animals that move along the ground, the sugar glider and other gliding species produce fewer, but heavier, offspring per litter. This allows female sugar gliders to retain the ability to glide when pregnant.
Socialisation
Sugar gliders are highly social animals. They live in family groups or colonies consisting of up to seven adults, plus the current season's young. Up to four age classes may exist within each group, although some sugar gliders are solitary, not belonging to a group. They engage in social grooming, which in addition to improving hygiene and health, helps bond the colony and establish group identity.
Within social communities, there are two codominant males who suppress subordinate males, but show no aggression towards each other. These co-dominant pairs are more related to each other than to subordinates within the group; and share food, nests, mates, and responsibility for scent marking of community members and territories. | Sugar glider | Wikipedia | 505 | 42991 | https://en.wikipedia.org/wiki/Sugar%20glider | Biology and health sciences | Diprotodontia | Animals |
Territory and members of the group are marked with saliva and a scent produced by separate glands on the forehead and chest of male gliders. Intruders who lack the appropriate scent marking are expelled violently. Rank is established through scent marking; and fighting does not occur within groups, but does occur when communities come into contact with each other. Within the colony, no fighting typically takes place beyond threatening behaviour. Each colony defends a territory of about where eucalyptus trees provide a staple food source.
Sugar gliders are one of the few species of mammals that exhibit male parental care. The oldest codominant male in a social community shows a high level of parental care, as he is the probable father of any offspring due to his social status.
This paternal care evolved in sugar gliders as young are more likely to survive when parental investment is provided by both parents. In the sugar glider, biparental care allows one adult to huddle with the young and prevent hypothermia while the other parent is out foraging, as young sugar gliders aren't able to thermoregulate until they are 100 days old (3.5 months).
Communication in sugar gliders is achieved through vocalisations, visual signals and complex chemical odours. Chemical odours account for a large part of communication in sugar gliders, similar to many other nocturnal animals. Odours may be used to mark territory, convey health status of an individual, and mark rank of community members. Gliders produce a number of vocalisations including barking and hissing.
Human relations | Sugar glider | Wikipedia | 315 | 42991 | https://en.wikipedia.org/wiki/Sugar%20glider | Biology and health sciences | Diprotodontia | Animals |
Conservation
Under the prior taxonomy, the sugar glider was not considered endangered, and its conservation rank was "Least Concern (LC)" on the IUCN Red List. However, with newer taxonomic studies indicating that it has a small and restricted range, it is now thought to be far more sensitive to potential threats. For example, the species' native range was hit hard by the 2019-20 Australian bushfires, which occurred just a few months prior to the publishing of the study indicating the true extent of its range. Sugar gliders use tree hollows, making them especially sensitive to intense fires. However, despite the loss of natural habitat in Australia over the last 200 years, it is adaptable and capable of living in small patches of remnant bush, particularly if it does not have to cross large expanses of cleared land to reach them. Sugar gliders may persist in areas that have undergone mild-moderate selective logging, as long as three to five hollow bearing trees are retained per hectare. Although not currently threatened by habitat loss, the ability of sugar gliders to forage and avoid predators successfully may be decreased in areas of high light pollution.
Conservation in Australia is enacted at the federal, state and local levels, where sugar gliders are protected as a native species. The central conservation law in Australia is the Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act). The National Parks and Wildlife Act 1974 is an example of conservation law in the state of South Australia, where it is legal to keep (only) one sugar glider without a permit, provided it was acquired legally from a source with a permit. A permit is required to obtain or possess more than one glider, or if one wants to sell or give away any glider in their possession. It is illegal to capture or sell wild sugar gliders without a permit.
In captivity | Sugar glider | Wikipedia | 367 | 42991 | https://en.wikipedia.org/wiki/Sugar%20glider | Biology and health sciences | Diprotodontia | Animals |
In captivity, the sugar glider can suffer from calcium deficiencies if not fed an adequate diet. A lack of calcium in the diet causes the body to leach calcium from the bones, with the hind legs first to show noticeable dysfunction. Calcium to phosphorus ratios should be 2:1 to prevent hypocalcemia, sometimes known as hind leg paralysis (HLP). Their diet should be 50% insects (gut-loaded) or other sources of protein, 25% fruit and 25% vegetables. Some of the more recognised diets are Bourbon's Modified Leadbeaters (BML), High Protein Wombaroo (HPW) and various calcium rich diets with Leadbeaters Mixture (LBM). Iron storage disease (hemochromatosis) is another dietary problem that has been reported in captive gliders and can lead to fatal complications if not diagnosed and treated early.
A large amount of attention and environmental enrichment may be required for the highly social species, especially for those kept as individuals. Inadequate social interaction can lead to depression and behavioural disorders such as loss of appetite, irritability and self-mutilation.
As a pet
In several countries, the sugar glider (or what was formerly considered to be the sugar glider) is popular as an exotic pet, and is sometimes referred to as a pocket pet. In Australia, there is opposition to keeping native animals as pets from Australia's largest wildlife rehabilitation organisation (WIRES), and concerns from Australian wildlife conservation organisations regarding animal welfare risks including neglect, cruelty and abandonment.
In Australia, sugar gliders can be kept in Victoria, South Australia, and the Northern Territory. However, they are not allowed to be kept as pets in Western Australia, New South Wales, the Australian Capital Territory, Queensland or Tasmania.
DNA analysis indicates that "the USA (sugar) glider population originates from West Papua, Indonesia with no illegal harvesting from other native areas such as Papua New Guinea or Australia". Given that the West Papuan gliders have been tentatively classified as Krefft's gliders (albeit to be changed in the future), this indicates that at least the captive gliders kept in the United States are Krefft's gliders, not sugar gliders. | Sugar glider | Wikipedia | 455 | 42991 | https://en.wikipedia.org/wiki/Sugar%20glider | Biology and health sciences | Diprotodontia | Animals |
A levee ( or ), dike (American English), dyke (British English; see spelling differences), embankment, floodbank, or stop bank is an elevated ridge, natural or artificial, alongside the banks of a river, often intended to protect against flooding of the area adjoining the river. It is usually earthen and often runs parallel to the course of a river in its floodplain or along low-lying coastlines.
Naturally occurring levees form on river floodplains following flooding, where sediment and alluvium is deposited and settles, forming a ridge and increasing the river channel's capacity. Alternatively, levees can be artificially constructed from fill, designed to regulate water levels. In some circumstances, artificial levees can be environmentally damaging.
Ancient civilizations in the Indus Valley, ancient Egypt, Mesopotamia and China all built levees. Today, levees can be found around the world, and failures of levees due to erosion or other causes can be major disasters, such as the catastrophic 2005 levee failures in Greater New Orleans that occurred as a result of Hurricane Katrina.
Etymology
Speakers of American English use the word levee, from the French word (from the feminine past participle of the French verb , 'to raise'). It originated in New Orleans a few years after the city's founding in 1718 and was later adopted by English speakers. The name derives from the trait of the levee's ridges being raised higher than both the channel and the surrounding floodplains.
The modern word dike or dyke most likely derives from the Dutch word , with the construction of dikes well attested as early as the 11th century. The Westfriese Omringdijk, completed by 1250, was formed by connecting existing older dikes. The Roman chronicler Tacitus mentions that the rebellious Batavi pierced dikes to flood their land and to protect their retreat (70 CE). The word originally indicated both the trench and the bank. It closely parallels the English verb to dig. | Levee | Wikipedia | 411 | 43024 | https://en.wikipedia.org/wiki/Levee | Technology | Hydraulic infrastructure | null |
In Anglo-Saxon, the word already existed and was pronounced as dick in northern England and as ditch in the south. Similar to Dutch, the English origins of the word lie in digging a trench and forming the upcast soil into a bank alongside it. This practice has meant that the name may be given to either the excavation or to the bank. Thus Offa's Dyke is a combined structure and Car Dyke is a trench – though it once had raised banks as well. In the English Midlands and East Anglia, and in the United States, a dike is what a ditch is in the south of England, a property-boundary marker or drainage channel. Where it carries a stream, it may be called a running dike as in Rippingale Running Dike, which leads water from the catchwater drain, Car Dyke, to the South Forty Foot Drain in Lincolnshire (TF1427). The Weir Dike is a soak dike in Bourne North Fen, near Twenty and alongside the River Glen, Lincolnshire. In the Norfolk and Suffolk Broads, a dyke may be a drainage ditch or a narrow artificial channel off a river or broad for access or mooring, some longer dykes being named, e.g., Candle Dyke.
In parts of Britain, particularly Scotland and Northern England, a dyke may be a field wall, generally made with dry stone.
Uses
The main purpose of artificial levees is to prevent flooding of the adjoining countryside and to slow natural course changes in a waterway to provide reliable shipping lanes for maritime commerce over time; they also confine the flow of the river, resulting in higher and faster water flow. Levees can be mainly found along the sea, where dunes are not strong enough, along rivers for protection against high floods, along lakes or along polders. Furthermore, levees have been built for the purpose of impoldering, or as a boundary for an inundation area. The latter can be a controlled inundation by the military or a measure to prevent inundation of a larger area surrounded by levees. Levees have also been built as field boundaries and as military defences. More on this type of levee can be found in the article on dry-stone walls.
Levees can be permanent earthworks or emergency constructions (often of sandbags) built hastily in a flood emergency. | Levee | Wikipedia | 475 | 43024 | https://en.wikipedia.org/wiki/Levee | Technology | Hydraulic infrastructure | null |
Some of the earliest levees were constructed by the Indus Valley civilization (in Pakistan and North India from ) on which the agrarian life of the Harappan peoples depended. Levees were also constructed over 3,000 years ago in ancient Egypt, where a system of levees was built along the left bank of the River Nile for more than , stretching from modern Aswan to the Nile Delta on the shores of the Mediterranean. The Mesopotamian civilizations and ancient China also built large levee systems. Because a levee is only as strong as its weakest point, the height and standards of construction have to be consistent along its length. Some authorities have argued that this requires a strong governing authority to guide the work and may have been a catalyst for the development of systems of governance in early civilizations. However, others point to evidence of large-scale water-control earthen works such as canals and/or levees dating from before King Scorpion in Predynastic Egypt, during which governance was far less centralized.
Another example of a historical levee that protected the growing city-state of Mēxihco-Tenōchtitlan and the neighboring city of Tlatelōlco, was constructed during the early 1400s, under the supervision of the tlahtoani of the altepetl Texcoco, Nezahualcoyotl. Its function was to separate the brackish waters of Lake Texcoco (ideal for the agricultural technique Chināmitls) from the fresh potable water supplied to the settlements. However, after the Europeans destroyed Tenochtitlan, the levee was also destroyed and flooding became a major problem, which resulted in the majority of The Lake being drained in the 17th century.
Levees are usually built by piling earth on a cleared, level surface. Broad at the base, they taper to a level top, where temporary embankments or sandbags can be placed. Because flood discharge intensity increases in levees on both river banks, and because silt deposits raise the level of riverbeds, planning and auxiliary measures are vital. Sections are often set back from the river to form a wider channel, and flood valley basins are divided by multiple levees to prevent a single breach from flooding a large area. A levee made from stones laid in horizontal rows with a bed of thin turf between each of them is known as a spetchel. | Levee | Wikipedia | 482 | 43024 | https://en.wikipedia.org/wiki/Levee | Technology | Hydraulic infrastructure | null |
Artificial levees require substantial engineering. Their surface must be protected from erosion, so they are planted with vegetation such as Bermuda grass in order to bind the earth together. On the land side of high levees, a low terrace of earth known as a banquette is usually added as another anti-erosion measure. On the river side, erosion from strong waves or currents presents an even greater threat to the integrity of the levee. The effects of erosion are countered by planting suitable vegetation or installing stones, boulders, weighted matting, or concrete revetments. Separate ditches or drainage tiles are constructed to ensure that the foundation does not become waterlogged.
River flood prevention
Prominent levee systems have been built along the Mississippi River and Sacramento River in the United States, and the Po, Rhine, Meuse River, Rhône, Loire, Vistula, the delta formed by the Rhine, Maas/Meuse and Scheldt in the Netherlands and the Danube in Europe. During the Chinese Warring States period, the Dujiangyan irrigation system was built by the Qin as a water conservation and flood control project. The system's infrastructure is located on the Min River, which is the longest tributary of the Yangtze River, in Sichuan, China.
The Mississippi levee system represents one of the largest such systems found anywhere in the world. It comprises over of levees extending some along the Mississippi, stretching from Cape Girardeau, Missouri, to the Mississippi delta. They were begun by French settlers in Louisiana in the 18th century to protect the city of New Orleans. The first Louisiana levees were about high and covered a distance of about along the riverside. The U.S. Army Corps of Engineers, in conjunction with the Mississippi River Commission, extended the levee system beginning in 1882 to cover the riverbanks from Cairo, Illinois to the mouth of the Mississippi delta in Louisiana. By the mid-1980s, they had reached their present extent and averaged in height; some Mississippi levees are as high as . The Mississippi levees also include some of the longest continuous individual levees in the world. One such levee extends southwards from Pine Bluff, Arkansas, for a distance of some . The scope and scale of the Mississippi levees has often been compared to the Great Wall of China. | Levee | Wikipedia | 462 | 43024 | https://en.wikipedia.org/wiki/Levee | Technology | Hydraulic infrastructure | null |
The United States Army Corps of Engineers (USACE) recommends and supports cellular confinement technology (geocells) as a best management practice. Particular attention is given to the matter of surface erosion, overtopping prevention and protection of levee crest and downstream slope. Reinforcement with geocells provides tensile force to the soil to better resist instability.
Artificial levees can lead to an elevation of the natural riverbed over time; whether this happens or not and how fast, depends on different factors, one of them being the amount and type of the bed load of a river. Alluvial rivers with intense accumulations of sediment tend to this behavior. Examples of rivers where artificial levees led to an elevation of the riverbed, even up to a point where the riverbed is higher than the adjacent ground surface behind the levees, are found for the Yellow River in China and the Mississippi in the United States.
Coastal flood prevention
Levees are very common on the marshlands bordering the Bay of Fundy in New Brunswick and Nova Scotia, Canada. The Acadians who settled the area can be credited with the original construction of many of the levees in the area, created for the purpose of farming the fertile tidal marshlands. These levees are referred to as dykes. They are constructed with hinged sluice gates that open on the falling tide to drain freshwater from the agricultural marshlands and close on the rising tide to prevent seawater from entering behind the dyke. These sluice gates are called "aboiteaux". In the Lower Mainland around the city of Vancouver, British Columbia, there are levees (known locally as dikes, and also referred to as "the sea wall") to protect low-lying land in the Fraser River delta, particularly the city of Richmond on Lulu Island. There are also dikes to protect other locations which have flooded in the past, such as the Pitt Polder, land adjacent to the Pitt River, and other tributary rivers.
Coastal flood prevention levees are also common along the inland coastline behind the Wadden Sea, an area devastated by many historic floods. Thus the peoples and governments have erected increasingly large and complex flood protection levee systems to stop the sea even during storm floods. The biggest of these are the huge levees in the Netherlands, which have gone beyond just defending against floods, as they have aggressively taken back land that is below mean sea level.
Spur dykes or groynes | Levee | Wikipedia | 496 | 43024 | https://en.wikipedia.org/wiki/Levee | Technology | Hydraulic infrastructure | null |
These typically man-made hydraulic structures are situated to protect against erosion. They are typically placed in alluvial rivers perpendicular, or at an angle, to the bank of the channel or the revetment, and are used widely along coastlines. There are two common types of spur dyke, permeable and impermeable, depending on the materials used to construct them.
Natural examples
Natural levees commonly form around lowland rivers and creeks without human intervention. They are elongated ridges of mud and/or silt that form on the river floodplains immediately adjacent to the cut banks. Like artificial levees, they act to reduce the likelihood of floodplain inundation.
Deposition of levees is a natural consequence of the flooding of meandering rivers which carry high proportions of suspended sediment in the form of fine sands, silts, and muds. Because the carrying capacity of a river depends in part on its depth, the sediment in the water which is over the flooded banks of the channel is no longer capable of keeping the same number of fine sediments in suspension as the main thalweg. The extra fine sediments thus settle out quickly on the parts of the floodplain nearest to the channel. Over a significant number of floods, this will eventually result in the building up of ridges in these positions and reducing the likelihood of further floods and episodes of levee building.
If aggradation continues to occur in the main channel, this will make levee overtopping more likely again, and the levees can continue to build up. In some cases, this can result in the channel bed eventually rising above the surrounding floodplains, penned in only by the levees around it; an example is the Yellow River in China near the sea, where oceangoing ships appear to sail high above the plain on the elevated river.
Levees are common in any river with a high suspended sediment fraction and thus are intimately associated with meandering channels, which also are more likely to occur where a river carries large fractions of suspended sediment. For similar reasons, they are also common in tidal creeks, where tides bring in large amounts of coastal silts and muds. High spring tides will cause flooding, and result in the building up of levees.
Failures and breaches | Levee | Wikipedia | 461 | 43024 | https://en.wikipedia.org/wiki/Levee | Technology | Hydraulic infrastructure | null |
Both natural and man-made levees can fail in a number of ways. Factors that cause levee failure include overtopping, erosion, structural failures, and levee saturation. The most frequent (and dangerous) is a levee breach. Here, a part of the levee actually breaks or is eroded away, leaving a large opening for water to flood land otherwise protected by the levee. A breach can be a sudden or gradual failure, caused either by surface erosion or by subsurface weakness in the levee. A breach can leave a fan-shaped deposit of sediment radiating away from the breach, described as a crevasse splay. In natural levees, once a breach has occurred, the gap in the levee will remain until it is again filled in by levee building processes. This increases the chances of future breaches occurring in the same location. Breaches can be the location of meander cutoffs if the river flow direction is permanently diverted through the gap.
Sometimes levees are said to fail when water overtops the crest of the levee. This will cause flooding on the floodplains, but because it does not damage the levee, it has fewer consequences for future flooding. | Levee | Wikipedia | 248 | 43024 | https://en.wikipedia.org/wiki/Levee | Technology | Hydraulic infrastructure | null |
Among various failure mechanisms that cause levee breaches, soil erosion is found to be one of the most important factors. Predicting soil erosion and scour generation when overtopping happens is important in order to design stable levee and floodwalls. There have been numerous studies to investigate the erodibility of soils. Briaud et al. (2008) used Erosion Function Apparatus (EFA) test to measure the erodibility of the soils and afterwards by using Chen 3D software, numerical simulations were performed on the levee to find out the velocity vectors in the overtopping water and the generated scour when the overtopping water impinges the levee. By analyzing the results from EFA test, an erosion chart to categorize erodibility of the soils was developed. Hughes and Nadal in 2009 studied the effect of combination of wave overtopping and storm surge overflow on the erosion and scour generation in levees. The study included hydraulic parameters and flow characteristics such as flow thickness, wave intervals, surge level above levee crown in analyzing scour development. According to the laboratory tests, empirical correlations related to average overtopping discharge were derived to analyze the resistance of levee against erosion. These equations could only fit to the situation, similar to the experimental tests, while they can give a reasonable estimation if applied to other conditions.
Osouli et al. (2014) and Karimpour et al. (2015) conducted lab scale physical modeling of levees to evaluate score characterization of different levees due to floodwall overtopping.
Another approach applied to prevent levee failures is electrical resistivity tomography (ERT). This non-destructive geophysical method can detect in advance critical saturation areas in embankments. ERT can thus be used in monitoring of seepage phenomena in earth structures and act as an early warning system, e.g., in critical parts of levees or embankments.
Negative impacts
Large scale structures designed to modify natural processes inevitably have some drawbacks or negative impacts.
Ecological impact
Levees interrupt floodplain ecosystems that developed under conditions of seasonal flooding. In many cases, the impact is two-fold, as reduced recurrence of flooding also facilitates land-use change from forested floodplain to farms. | Levee | Wikipedia | 464 | 43024 | https://en.wikipedia.org/wiki/Levee | Technology | Hydraulic infrastructure | null |
Increased height
In a natural watershed, floodwaters spread over a landscape and slowly return to the river. Downstream, the delivery of water from the area of flooding is spread out in time. If levees keep the floodwaters inside a narrow channel, the water is delivered downstream over a shorter time period. The same volume of water over a shorter time interval means higher river stage (height). As more levees are built upstream, the recurrence interval for high-water events in the river increases, often requiring increases in levee height.
Levee breaches produce high-energy flooding
During natural flooding, water spilling over banks rises slowly. When a levee fails, a wall of water held back by the levee suddenly pours out over the landscape, much like a dam break. Impacted areas far from a breach may experience flooding similar to a natural event, while damage near a breach can be catastrophic, including carving out deep holes and channels in the nearby landscape.
Prolonged flooding after levee failure
Under natural conditions, floodwaters return quickly to the river channel as water-levels drop. During a levee breach, water pours out into the floodplain and moves down-slope where it is blocked from return to the river. Flooding is prolonged over such areas, waiting for floodwater to slowly infiltrate and evaporate.
Subsidence and seawater intrusion
Natural flooding adds a layer of sediment to the floodplain. The added weight of such layers over many centuries makes the crust sink deeper into the mantle, much like a floating block of wood is pushed deeper into the water if another board is added on top. The momentum of downward movement does not immediately stop when new sediment layers stop being added, resulting in subsidence (sinking of land surface). In coastal areas, this results in land dipping below sea level, the ocean migrating inland, and salt-water intruding into freshwater aquifers. | Levee | Wikipedia | 390 | 43024 | https://en.wikipedia.org/wiki/Levee | Technology | Hydraulic infrastructure | null |
Coastal sediment loss
Where a large river spills out into the ocean, the velocity of the water suddenly slows and its ability to transport sand and silt decreases. Sediments begin to settle out, eventually forming a delta and extending to the coastline seaward. During subsequent flood events, water spilling out of the channel will find a shorter route to the ocean and begin building a new delta. Wave action and ocean currents redistribute some of the sediment to build beaches along the coast. When levees are constructed all the way to the ocean, sediments from flooding events are cut off, the river never migrates, and elevated river velocity delivers sediment to deep water where wave action and ocean currents cannot redistribute. Instead of a natural wedge shaped delta forming, a "birds-foot delta" extends far out into the ocean. The results for surrounding land include beach depletion, subsidence, salt-water intrusion, and land loss. | Levee | Wikipedia | 193 | 43024 | https://en.wikipedia.org/wiki/Levee | Technology | Hydraulic infrastructure | null |
In computing, endianness is the order in which bytes within a word of digital data are transmitted over a data communication medium or addressed (by rising addresses) in computer memory, counting only byte significance compared to earliness. Endianness is primarily expressed as big-endian (BE) or little-endian (LE), terms introduced by Danny Cohen into computer science for data ordering in an Internet Experiment Note published in 1980. The adjective endian has its origin in the writings of 18th century Anglo-Irish writer Jonathan Swift. In the 1726 novel Gulliver's Travels, he portrays the conflict between sects of Lilliputians divided into those breaking the shell of a boiled egg from the big end or from the little end. By analogy, a CPU may read a digital word big end first, or little end first.
Computers store information in various-sized groups of binary bits. Each group is assigned a number, called its address, that the computer uses to access that data. On most modern computers, the smallest data group with an address is eight bits long and is called a byte. Larger groups comprise two or more bytes, for example, a 32-bit word contains four bytes. There are two possible ways a computer could number the individual bytes in a larger group, starting at either end. Both types of endianness are in widespread use in digital electronic engineering. The initial choice of endianness of a new design is often arbitrary, but later technology revisions and updates perpetuate the existing endianness to maintain backward compatibility.
A big-endian system stores the most significant byte of a word at the smallest memory address and the least significant byte at the largest. A little-endian system, in contrast, stores the least-significant byte at the smallest address. Of the two, big-endian is thus closer to the way the digits of numbers are written left-to-right in English, comparing digits to bytes. Bi-endianness is a feature supported by numerous computer architectures that feature switchable endianness in data fetches and stores or for instruction fetches. Other orderings are generically called middle-endian or mixed-endian. | Endianness | Wikipedia | 447 | 43026 | https://en.wikipedia.org/wiki/Endianness | Technology | Computer architecture concepts | null |
Big-endianness is the dominant ordering in networking protocols, such as in the Internet protocol suite, where it is referred to as network order, transmitting the most significant byte first. Conversely, little-endianness is the dominant ordering for processor architectures (x86, most ARM implementations, base RISC-V implementations) and their associated memory. File formats can use either ordering; some formats use a mixture of both or contain an indicator of which ordering is used throughout the file.
Characteristics
Computer memory consists of a sequence of storage cells (smallest addressable units); in machines that support byte addressing, those units are called bytes. Each byte is identified and accessed in hardware and software by its memory address. If the total number of bytes in memory is n, then addresses are enumerated from 0 to n − 1.
Computer programs often use data structures or fields that may consist of more data than can be stored in one byte. In the context of this article where its type cannot be arbitrarily complicated, a "field" consists of a consecutive sequence of bytes and represents a "simple data value" which – at least potentially – can be manipulated by one single hardware instruction. On most systems, the address of a multi-byte simple data value is the address of its first byte (the byte with the lowest address). There are exceptions to this rule – for example, the Add instruction of the IBM 1401 addresses variable-length fields at their low-order (highest-addressed) position with their lengths being defined by a word mark set at their high-order (lowest-addressed) position. When an operation such as addition is performed, the processor begins at the low-order positions at the high addresses of the two fields and works its way down to the high-order.
Another important attribute of a byte being part of a "field" is its "significance".
These attributes of the parts of a field play an important role in the sequence the bytes are accessed by the computer hardware, more precisely: by the low-level algorithms contributing to the results of a computer instruction.
Numbers | Endianness | Wikipedia | 426 | 43026 | https://en.wikipedia.org/wiki/Endianness | Technology | Computer architecture concepts | null |
Positional number systems (mostly base 2, or less often base 10) are the predominant way of representing and particularly of manipulating integer data by computers. In pure form this is valid for moderate sized non-negative integers, e.g. of C data type unsigned. In such a number system, the value of a digit which it contributes to the whole number is determined not only by its value as a single digit, but also by the position it holds in the complete number, called its significance. These positions can be mapped to memory mainly in two ways:
Decreasing numeric significance with increasing memory addresses (or increasing time), known as big-endian and
Increasing numeric significance with increasing memory addresses (or increasing time), known as little-endian.
In these expressions, the term "end" is meant as the extremity where the big resp. little significance is written first, namely where the field starts.
The integer data that are directly supported by the computer hardware have a fixed width of a low power of 2, e.g. 8 bits ≙ 1 byte, 16 bits ≙ 2 bytes, 32 bits ≙ 4 bytes, 64 bits ≙ 8 bytes, 128 bits ≙ 16 bytes. The low-level access sequence to the bytes of such a field depends on the operation to be performed. The least-significant byte is accessed first for addition, subtraction and multiplication. The most-significant byte is accessed first for division and comparison. See .
Text
When character (text) strings are to be compared with one another, e.g. in order to support some mechanism like sorting, this is very frequently done lexicographically where a single positional element (character) also has a positional value. Lexicographical comparison means almost everywhere: first character ranks highest – as in the telephone book. Almost all machines which can do this using a single instruction are big-endian or at least mixed-endian.
Integer numbers written as text are always represented most significant digit first in memory, which is similar to big-endian, independently of text direction.
Byte addressing
When memory bytes are printed sequentially from left to right (e.g. in a hex dump), little-endian representation of integers has the significance increasing from right to left. In other words, it appears backwards when visualized, which can be counter-intuitive. | Endianness | Wikipedia | 482 | 43026 | https://en.wikipedia.org/wiki/Endianness | Technology | Computer architecture concepts | null |
This behavior arises, for example, in FourCC or similar techniques that involve packing characters into an integer, so that it becomes a sequence of specific characters in memory. For example, take the string "JOHN", stored in hexadecimal ASCII. On big-endian machines, the value appears left-to-right, coinciding with the correct string order for reading the result ("J O H N"). But on a little-endian machine, one would see "N H O J". Middle-endian machines complicate this even further; for example, on the PDP-11, the 32-bit value is stored as two 16-bit words "JO" "HN" in big-endian, with the characters in the 16-bit words being stored in little-endian, resulting in "O J N H".
Byte swapping
Byte-swapping consists of rearranging bytes to change endianness. Many compilers provide built-ins that are likely to be compiled into native processor instructions (/), such as . Software interfaces for swapping include:
Standard network endianness functions (from/to BE, up to 32-bit). Windows has a 64-bit extension in .
BSD and Glibc functions (from/to BE and LE, up to 64-bit).
macOS macros (from/to BE and LE, up to 64-bit).
The function in C++23.
Some CPU instruction sets provide native support for endian byte swapping, such as (x86 — 486 and later, i960 — i960Jx and later), and (ARMv6 and later).
Some compilers have built-in facilities for byte swapping. For example, the Intel Fortran compiler supports the non-standard specifier when opening a file, e.g.: . Other compilers have options for generating code that globally enables the conversion for all file IO operations. This permits the reuse of code on a system with the opposite endianness without code modification.
Considerations
Simplified access to part of a field | Endianness | Wikipedia | 444 | 43026 | https://en.wikipedia.org/wiki/Endianness | Technology | Computer architecture concepts | null |
On most systems, the address of a multi-byte value is the address of its first byte (the byte with the lowest address); little-endian systems of that type have the property that, for sufficiently low data values, the same value can be read from memory at different lengths without using different addresses (even when alignment restrictions are imposed). For example, a 32-bit memory location with content can be read at the same address as either 8-bit (value = 4A), 16-bit (004A), 24-bit (00004A), or 32-bit (0000004A), all of which retain the same numeric value. Although this little-endian property is rarely used directly by high-level programmers, it is occasionally employed by code optimizers as well as by assembly language programmers. While not allowed by C++, such type punning code is allowed as "implementation-defined" by the C11 standard and commonly used in code interacting with hardware.
Calculation order
Some operations in positional number systems have a natural or preferred order in which the elementary steps are to be executed. This order may affect their performance on small-scale byte-addressable processors and microcontrollers. However, high-performance processors usually fetch multi-byte operands from memory in the same amount of time they would have fetched a single byte, so the complexity of the hardware is not affected by the byte ordering.
Addition, subtraction, and multiplication start at the least significant digit position and propagate the carry to the subsequent more significant position. On most systems, the address of a multi-byte value is the address of its first byte (the byte with the lowest address). The implementation of these operations is marginally simpler using little-endian machines where this first byte contains the least significant digit.
Comparison and division start at the most significant digit and propagate a possible carry to the subsequent less significant digits. For fixed-length numerical values (typically of length 1,2,4,8,16), the implementation of these operations is marginally simpler on big-endian machines.
Some big-endian processors (e.g. the IBM System/360 and its successors) contain hardware instructions for lexicographically comparing varying length character strings.
The normal data transport by an assignment statement is in principle independent of the endianness of the processor.
Hardware | Endianness | Wikipedia | 498 | 43026 | https://en.wikipedia.org/wiki/Endianness | Technology | Computer architecture concepts | null |
Many historical and extant processors use a big-endian memory representation, either exclusively or as a design option. The IBM System/360 uses big-endian byte order, as do its successors System/370, ESA/390, and z/Architecture. The PDP-10 uses big-endian addressing for byte-oriented instructions. The IBM Series/1 minicomputer uses big-endian byte order. The Motorola 6800 / 6801, the 6809 and the 68000 series of processors use the big-endian format. Solely big-endian architectures include the IBM z/Architecture and OpenRISC. The PDP-11 minicomputer, however, uses little-endian byte order, as does its VAX successor.
The Datapoint 2200 used simple bit-serial logic with little-endian to facilitate carry propagation. When Intel developed the 8008 microprocessor for Datapoint, they used little-endian for compatibility. However, as Intel was unable to deliver the 8008 in time, Datapoint used a medium-scale integration equivalent, but the little-endianness was retained in most Intel designs, including the MCS-48 and the 8086 and its x86 successors, including IA-32 and x86-64 processors. The MOS Technology 6502 family (including Western Design Center 65802 and 65C816), the Zilog Z80 (including Z180 and eZ80), the Altera Nios II, the Atmel AVR, the Andes Technology NDS32, the Qualcomm Hexagon, and many other processors and processor families are also little-endian.
The Intel 8051, unlike other Intel processors, expects 16-bit addresses for LJMP and LCALL in big-endian format; however, xCALL instructions store the return address onto the stack in little-endian format.
Bi-endianness | Endianness | Wikipedia | 402 | 43026 | https://en.wikipedia.org/wiki/Endianness | Technology | Computer architecture concepts | null |
Some instruction set architectures feature a setting which allows for switchable endianness in data fetches and stores, instruction fetches, or both; those instruction set architectures are referred to as bi-endian. Architectures that support switchable endianness include PowerPC/Power ISA, SPARC V9, ARM versions 3 and above, DEC Alpha, MIPS, Intel i860, PA-RISC, SuperH SH-4, IA-64, C-Sky, and RISC-V. This feature can improve performance or simplify the logic of networking devices and software. The word bi-endian, when said of hardware, denotes the capability of the machine to compute or pass data in either endian format.
Many of these architectures can be switched via software to default to a specific endian format (usually done when the computer starts up); however, on some systems, the default endianness is selected by hardware on the motherboard and cannot be changed via software (e.g. Alpha, which runs only in big-endian mode on the Cray T3E).
IBM AIX and IBM i run in big-endian mode on bi-endian Power ISA; Linux originally ran in big-endian mode, but by 2019, IBM had transitioned to little-endian mode for Linux to ease the porting of Linux software from x86 to Power. SPARC has no relevant little-endian deployment, as both Oracle Solaris and Linux run in big-endian mode on bi-endian SPARC systems, and can be considered big-endian in practice. ARM, C-Sky, and RISC-V have no relevant big-endian deployments, and can be considered little-endian in practice.
The term bi-endian refers primarily to how a processor treats data accesses. Instruction accesses (fetches of instruction words) on a given processor may still assume a fixed endianness, even if data accesses are fully bi-endian, though this is not always the case, such as on Intel's IA-64-based Itanium CPU, which allows both. | Endianness | Wikipedia | 446 | 43026 | https://en.wikipedia.org/wiki/Endianness | Technology | Computer architecture concepts | null |
Some nominally bi-endian CPUs require motherboard help to fully switch endianness. For instance, the 32-bit desktop-oriented PowerPC processors in little-endian mode act as little-endian from the point of view of the executing programs, but they require the motherboard to perform a 64-bit swap across all 8 byte lanes to ensure that the little-endian view of things will apply to I/O devices. In the absence of this unusual motherboard hardware, device driver software must write to different addresses to undo the incomplete transformation and also must perform a normal byte swap.
Some CPUs, such as many PowerPC processors intended for embedded use and almost all SPARC processors, allow per-page choice of endianness.
SPARC processors since the late 1990s (SPARC v9 compliant processors) allow data endianness to be chosen with each individual instruction that loads from or stores to memory.
The ARM architecture supports two big-endian modes, called BE-8 and BE-32. CPUs up to ARMv5 only support BE-32 or word-invariant mode. Here any naturally aligned 32-bit access works like in little-endian mode, but access to a byte or 16-bit word is redirected to the corresponding address and unaligned access is not allowed. ARMv6 introduces BE-8 or byte-invariant mode, where access to a single byte works as in little-endian mode, but accessing a 16-bit, 32-bit or (starting with ARMv8) 64-bit word results in a byte swap of the data. This simplifies unaligned memory access as well as memory-mapped access to registers other than 32-bit.
Many processors have instructions to convert a word in a register to the opposite endianness, that is, they swap the order of the bytes in a 16-, 32- or 64-bit word.
Recent Intel x86 and x86-64 architecture CPUs have a MOVBE instruction (Intel Core since generation 4, after Atom), which fetches a big-endian format word from memory or writes a word into memory in big-endian format. These processors are otherwise thoroughly little-endian.
There are also devices which use different formats in different places. For instance, the BQ27421 Texas Instruments battery gauge uses the little-endian format for its registers and the big-endian format for its random-access memory. | Endianness | Wikipedia | 509 | 43026 | https://en.wikipedia.org/wiki/Endianness | Technology | Computer architecture concepts | null |
SPARC historically used big-endian until version 9, which is bi-endian. Similarly early IBM POWER processors were big-endian, but the PowerPC and Power ISA descendants are now bi-endian. The ARM architecture was little-endian before version 3 when it became bi-endian.
Floating point
Although many processors use little-endian storage for all types of data (integer, floating point), there are a number of hardware architectures where floating-point numbers are represented in big-endian form while integers are represented in little-endian form. There are ARM processors that have mixed-endian floating-point representation for double-precision numbers: each of the two 32-bit words is stored as little-endian, but the most significant word is stored first. VAX floating point stores little-endian 16-bit words in big-endian order. Because there have been many floating-point formats with no network standard representation for them, the XDR standard uses big-endian IEEE 754 as its representation. It may therefore appear strange that the widespread IEEE 754 floating-point standard does not specify endianness. Theoretically, this means that even standard IEEE floating-point data written by one machine might not be readable by another. However, on modern standard computers (i.e., implementing IEEE 754), one may safely assume that the endianness is the same for floating-point numbers as for integers, making the conversion straightforward regardless of data type. Small embedded systems using special floating-point formats may be another matter, however.
Variable-length data
Most instructions considered so far contain the size (lengths) of their operands within the operation code. Frequently available operand lengths are 1, 2, 4, 8, or 16 bytes. But there are also architectures where the length of an operand may be held in a separate field of the instruction or with the operand itself, e.g. by means of a word mark. Such an approach allows operand lengths up to 256 bytes or larger. The data types of such operands are character strings or BCD. Machines able to manipulate such data with one instruction (e.g. compare, add) include the IBM 1401, 1410, 1620, System/360, System/370, ESA/390, and z/Architecture, all of them of type big-endian.
Middle-endian | Endianness | Wikipedia | 498 | 43026 | https://en.wikipedia.org/wiki/Endianness | Technology | Computer architecture concepts | null |
Numerous other orderings, generically called middle-endian or mixed-endian, are possible.
The PDP-11 is in principle a 16-bit little-endian system. The instructions to convert between floating-point and integer values in the optional floating-point processor of the PDP-11/45, PDP-11/70, and in some later processors, stored 32-bit "double precision integer long" values with the 16-bit halves swapped from the expected little-endian order. The UNIX C compiler used the same format for 32-bit long integers. This ordering is known as PDP-endian.
UNIX was one of the first systems to allow the same code to be compiled for platforms with different internal representations. One of the first programs converted was supposed to print out , but on the Series/1 it printed instead.
A way to interpret this endianness is that it stores a 32-bit integer as two little-endian 16-bit words, with a big-endian word ordering:
Segment descriptors of IA-32 and compatible processors keep a 32-bit base address of the segment stored in little-endian order, but in four nonconsecutive bytes, at relative positions 2, 3, 4 and 7 of the descriptor start.
Software
Logic design
Hardware description languages (HDLs) used to express digital logic often support arbitrary endianness, with arbitrary granularity. For example, in SystemVerilog, a word can be defined as little-endian or big-endian.
Files and filesystems
The recognition of endianness is important when reading a file or filesystem created on a computer with different endianness.
Fortran sequential unformatted files created with one endianness usually cannot be read on a system using the other endianness because Fortran usually implements a record (defined as the data written by a single Fortran statement) as data preceded and succeeded by count fields, which are integers equal to the number of bytes in the data. An attempt to read such a file using Fortran on a system of the other endianness results in a run-time error, because the count fields are incorrect. | Endianness | Wikipedia | 457 | 43026 | https://en.wikipedia.org/wiki/Endianness | Technology | Computer architecture concepts | null |
Unicode text can optionally start with a byte order mark (BOM) to signal the endianness of the file or stream. Its code point is U+FEFF. In UTF-32 for example, a big-endian file should start with ; a little-endian should start with .
Application binary data formats, such as MATLAB .mat files, or the .bil data format, used in topography, are usually endianness-independent. This is achieved by storing the data always in one fixed endianness or carrying with the data a switch to indicate the endianness. An example of the former is the binary XLS file format that is portable between Windows and Mac systems and always little-endian, requiring the Mac application to swap the bytes on load and save when running on a big-endian Motorola 68K or PowerPC processor.
TIFF image files are an example of the second strategy, whose header instructs the application about the endianness of their internal binary integers. If a file starts with the signature it means that integers are represented as big-endian, while means little-endian. Those signatures need a single 16-bit word each, and they are palindromes, so they are endianness independent. stands for Intel and stands for Motorola. Intel CPUs are little-endian, while Motorola 680x0 CPUs are big-endian. This explicit signature allows a TIFF reader program to swap bytes if necessary when a given file was generated by a TIFF writer program running on a computer with a different endianness.
As a consequence of its original implementation on the Intel 8080 platform, the operating system-independent File Allocation Table (FAT) file system is defined with little-endian byte ordering, even on platforms using another endianness natively, necessitating byte-swap operations for maintaining the FAT on these platforms.
ZFS, which combines a filesystem and a logical volume manager, is known to provide adaptive endianness and to work with both big-endian and little-endian systems.
Networking
Many IETF RFCs use the term network order, meaning the order of transmission for bytes over the wire in network protocols. Among others, the historic defines the network order for protocols in the Internet protocol suite to be big-endian. | Endianness | Wikipedia | 480 | 43026 | https://en.wikipedia.org/wiki/Endianness | Technology | Computer architecture concepts | null |
However, not all protocols use big-endian byte order as the network order. The Server Message Block (SMB) protocol uses little-endian byte order. In CANopen, multi-byte parameters are always sent least significant byte first (little-endian). The same is true for Ethernet Powerlink.
The Berkeley sockets API defines a set of functions to convert 16- and 32-bit integers to and from network byte order: the (host-to-network-short) and (host-to-network-long) functions convert 16- and 32-bit values respectively from machine (host) to network order; the and functions convert from network to host order. These functions may be a no-op on a big-endian system.
While the high-level network protocols usually consider the byte (mostly meant as octet) as their atomic unit, the lowest layers of a network stack may deal with ordering of bits within a byte. | Endianness | Wikipedia | 195 | 43026 | https://en.wikipedia.org/wiki/Endianness | Technology | Computer architecture concepts | null |
Traffic comprises pedestrians, vehicles, ridden or herded animals, trains, and other conveyances that use public ways (roads/sidewalks) for travel and transportation.
Traffic laws govern and regulate traffic, while rules of the road include traffic laws and informal rules that may have developed over time to facilitate the orderly and timely flow of traffic. Organized traffic generally has well-established priorities, lanes, right-of-way, and traffic control at intersections. (International Regulations for Preventing Collisions at Sea govern the oceans and influence some laws for navigating domestic waters.)
Traffic is formally organized in many jurisdictions, with marked lanes, junctions, intersections, interchanges, traffic signals, cones, or signs. Traffic is often classified by type: heavy motor vehicle (e.g., car, truck), other vehicle (e.g., moped, bicycle), and pedestrian. Different classes may share speed limits and easement, or may be segregated. Some jurisdictions may have very detailed and complex rules of the road while others rely more on drivers' common sense and willingness to cooperate.
Organization typically produces a better combination of travel safety and efficiency. Events which disrupt the flow and may cause traffic to degenerate into a disorganized mess include road construction, collisions, and debris in the roadway. On particularly busy freeways, a minor disruption may persist in a phenomenon known as traffic waves. A complete breakdown of organization may result in traffic congestion and gridlock. Simulations of organized traffic frequently involve queuing theory, stochastic processes and equations of mathematical physics applied to traffic flow.
Etymology and types
The word traffic originally meant "trade" (as it still does) and comes from the Old Italian verb trafficare and noun traffico. The origin of the Italian words is unclear. Suggestions include Catalan trafegar "decant", an assumed Vulgar Latin verb transfricare 'rub across', an assumed Vulgar Latin combination of trans- and facere 'make or do', Arabic tafriq 'distribution', and Arabic taraffaqa, which can mean 'seek profit'. Broadly, the term covers many kinds of traffic including network traffic, air traffic, marine traffic and rail traffic, but it is often used narrowly to mean only road traffic.
Rules of the road | Traffic | Wikipedia | 462 | 43081 | https://en.wikipedia.org/wiki/Traffic | Technology | Basics_7 | null |
Rules of the road and driving etiquette are the general practices and procedures that road users are required to follow. These rules usually apply to all road users, though they are of special importance to motorists and cyclists. These rules govern interactions between vehicles and pedestrians. The basic traffic rules are defined by an international treaty under the authority of the United Nations, the 1968 Vienna Convention on Road Traffic. Not all countries are signatory to the convention and, even among signatories, local variations in practice may be found. There are also unwritten local rules of the road, which are generally understood by local drivers.
As a general rule, drivers are expected to avoid a collision with another vehicle and pedestrians, regardless of whether or not the applicable rules of the road allow them to be where they happen to be.
In addition to the rules applicable by default, traffic signs and traffic lights must be obeyed, and instructions may be given by a police officer, either routinely (on a busy crossing instead of traffic lights) or as road traffic control around a construction zone, accident, or other road disruption.
Directionality
Traffic heading in inverse ways ought to be isolated so as to not hinder each other's way. The most essential guideline is whether to utilize the left or right half of the street.
Traffic regulations
In many countries, the rules of the road are codified, setting out the legal requirements and punishments for breaking them.
In the United Kingdom, the rules are set out in the Highway Code, which includes not only obligations but also advice on how to drive sensibly and safely.
In the United States, traffic laws are regulated by the states and municipalities through their respective traffic codes. Most of these are based at least in part on the Uniform Vehicle Code, but there are variations from state to state. In states such as Florida, traffic law and criminal law are separate; therefore, unless someone flees the scene of an accident or commits vehicular homicide or manslaughter, they are only guilty of a minor traffic offense. However, states such as South Carolina have completely criminalised their traffic law, so, for example, one is guilty of a misdemeanor simply for travelling 5 miles over the speed limit.
Trail ethics (right of way)
Trail ethics are a set of informal rules for right of way for users of trails, including hikers, mountaineers, equestrians, cyclists, and mountain bikers.
Organised traffic
Passage priority (right of way) | Traffic | Wikipedia | 498 | 43081 | https://en.wikipedia.org/wiki/Traffic | Technology | Basics_7 | null |
Vehicles often come into conflict with other vehicles and pedestrians because their intended courses of travel intersect, and thus interfere with each other's routes. The general principle that establishes who has the right to go first is called "right of way" or "priority". It establishes who has the right to use the conflicting part of the road and who has to wait until the other does so.
Signs, signals, markings and other features are often used to make priority explicit. Some signs, such as the stop sign, are nearly universal. When there are no signs or markings, different rules are observed depending on the location. These default priority rules differ between countries, and may even vary within countries. Trends toward uniformity are exemplified at an international level by the Vienna Convention on Road Signs and Signals, which prescribes standardised traffic control devices (signs, signals, and markings) for establishing the right of way where necessary.
Crosswalks (or pedestrian crossings) are common in populated areas, and may indicate that pedestrians have priority over vehicular traffic. In most modern cities, the traffic signal is used to establish the right of way on the busy roads. Its primary purpose is to give each road a duration of time in which its traffic may use the intersection in an organised way. The intervals of time assigned for each road may be adjusted to take into account factors such as difference in volume of traffic, the needs of pedestrians, or other traffic signals. Pedestrian crossings may be located near other traffic control devices; if they are not also regulated in some way, vehicles must give priority to them when in use. Traffic on a public road usually has priority over other traffic such as traffic emerging from private access; rail crossings and drawbridges are typical exceptions.
Uncontrolled traffic
Uncontrolled traffic comes in the absence of lane markings and traffic control signals. On roads without marked lanes, drivers tend to keep to the appropriate side if the road is wide enough. Drivers frequently overtake others. Obstructions are
common.
Intersections have no signals or signage, and a particular road at a busy intersection may be dominant – that is, its traffic flows – until a break in traffic, at which time the dominance shifts to the other road where vehicles are queued. At the intersection of two perpendicular roads, a traffic jam may result if four vehicles face each other side-on. | Traffic | Wikipedia | 476 | 43081 | https://en.wikipedia.org/wiki/Traffic | Technology | Basics_7 | null |
Turning
Drivers often seek to turn onto another road or onto private property. The vehicle's blinking turn signals (commonly known as "blinkers" or "indicators") are often used as a way to announce one's intention to turn, thus alerting other drivers. The actual usage of directional signals varies greatly amongst countries, although its purpose is to indicate a driver's intention to depart from the current (and natural) flow of traffic well before the departure is executed (typically 3 seconds as a guideline).
This will usually mean that turning traffic must stop and wait for a breach to turn, and this might cause inconvenience for drivers that follow them but do not want to turn. This is why dedicated lanes and protected traffic signals for turning are sometimes provided. On busier intersections where a protected lane would be ineffective or cannot be built, turning may be entirely prohibited, and drivers will be required to "drive around the block" in order to accomplish the turn. Many cities employ this tactic quite often; in San Francisco, due to its common practice, making three right turns is known colloquially as a "San Francisco left turn". Likewise, as many intersections in Taipei City are too busy to allow direct left turns, signs often direct drivers to drive around the block to turn.
Turning rules are by no means universal. For example, in New Zealand (a drive-on-the-left country) between 1977 and 2012, left turning traffic had to give way to opposing right-turning traffic wishing to take the same road (unless there were multiple lanes, but then one must take care in case a vehicle jumped lanes). New Zealand abolished this particular rule on 25 March 2012, except at roundabouts or when denoted by a Give Way or Stop sign. Although the rule caused initial driver confusion, and many intersections required or still require modification, the change is predicted to eventually prevent one death and 13 serious injuries annually. | Traffic | Wikipedia | 389 | 43081 | https://en.wikipedia.org/wiki/Traffic | Technology | Basics_7 | null |
On roads with multiple lanes, turning traffic is generally expected to move to the lane closest to the direction they wish to turn. For example, traffic intending to turn right will usually move to the rightmost lane before the intersection. Likewise, left-turning traffic will move to the leftmost lane. Exceptions to this rule may exist where for example the traffic authority decides that the two rightmost lanes will be for turning right, in which case drivers may take whichever of them to turn. Traffic may adapt to informal patterns that rise naturally rather than by force of authority. For example, it is common for drivers to observe (and trust) the turn signals used by other drivers in order to make turns from other lanes. If several vehicles on the right lane are all turning right, a vehicle may come from the next-to-right lane and turn right as well, in parallel with the other right-turning vehicles.
Intersections
In most of Continental Europe, the default rule is to give priority to the right, but this may be overridden by signs or road markings. There, priority was initially given according to the social rank of each traveler, but early in the life of the automobile this rule was deemed impractical and replaced with the priorité à droite (priority to the right) rule, which still applies. At a traffic circle where priorité à droite is not overridden, traffic on what would otherwise be a roundabout gives way to traffic entering the circle. Most French roundabouts now have give-way signs for traffic entering the circle, but there remain some notable exceptions that operate on the old rule, such as the Place de l'Étoile around the Arc de Triomphe. Priority to the right where used in continental Europe may be overridden by an ascending hierarchy of markings, signs, signals, and authorized persons.
In the United Kingdom, priority is generally indicated by signs or markings, so that almost all junctions between public roads (except those governed by traffic signals) have a concept of a major road and minor road. The default give-way-to-the-right rule used in Continental Europe causes problems for many British and Irish drivers who are accustomed to having right of way by default unless otherwise indicated. A very small proportion of low-traffic junctions are unmarked – typically on housing estates or in rural areas. Here the rule is to "proceed with great care" i.e. slow the vehicle and check for traffic on the intersecting road. | Traffic | Wikipedia | 501 | 43081 | https://en.wikipedia.org/wiki/Traffic | Technology | Basics_7 | null |
Other countries use various methods similar to the above examples to establish the right of way at intersections. For example, in most of the United States, the default priority is to yield to traffic from the right, but this is usually overridden by traffic control devices or other rules, like the boulevard rule. This rule holds that traffic entering a major road from a smaller road or alley must yield to the traffic of the busier road, but signs are often still posted. The boulevard rule can be compared with the above concept of a major and minor road, or the priority roads that may be found in countries that are parties to the Vienna Convention on Road Signs and Signals.
Perpendicular intersections
Also known as a "four-way" intersection, this intersection is the most common configuration for roads that cross each other, and the most basic type.
If traffic signals do not control a four-way intersection, signs or other features are typically used to control movements and make clear priorities. The most common arrangement is to indicate that one road has priority over the other, but there are complex cases where all traffic approaching an intersection must yield and may be required to stop.
In the United States, South Africa, and Canada, there are four-way intersections with a stop sign at every entrance, called four-way stops. A failed signal or a flashing red light is equivalent to a four-way stop, or an all-way stop. Special rules for four-way stops may include:
In the countries that use four-way stops, pedestrians always have priority at crosswalks – even at unmarked ones, which exist as the logical continuations of the sidewalks at every intersection with approximately right angles – unless signed or painted otherwise.
Whichever vehicle first stops at the stop line – or before the crosswalk, if there is no stop line – has priority.
If two vehicles stop at the same time, priority is given to the vehicle on the right.
If several vehicles arrive at the same time, a right-of-way conflict may arise wherein no driver has the legal right-of-way. This may result in drivers informally signaling to other drivers to indicate their intent to yield, for example by waving or flashing headlights. | Traffic | Wikipedia | 442 | 43081 | https://en.wikipedia.org/wiki/Traffic | Technology | Basics_7 | null |
In Europe and other places, there are similar intersections. These may be marked by special signs (according to the Vienna Convention on Road Signs and Signals), a danger sign with a black X representing a crossroads. This sign informs drivers that the intersection is uncontrolled and that default rules apply. In Europe and in many areas of North America the default rules that apply at uncontrolled four-way intersections are almost identical:
Rules for pedestrians differ by country, in the United States and Canada pedestrians generally have priority at such an intersection.
All vehicles must give priority to any traffic approaching from their right,
Then, if the vehicle is turning right or continuing on the same road it may proceed.
Vehicles turning left must also give priority to traffic approaching from the opposite direction, unless that traffic is also turning left.
If the intersection is congested, vehicles must alternate directions and/or circulate priority to the right one vehicle at a time.
Protected intersection for bicycles
A number of features make this protected intersection. A corner refuge island, a setback crossing of the pedestrians and cyclists, generally between 1.5–7 metres of setback, a forward stop bar, which allows cyclists to stop for a traffic light well ahead of motor traffic who must stop behind the crosswalk. Separate signal staging or at least an advance green for cyclists and pedestrians is used to give cyclists and pedestrians no conflicts or a head start over traffic. The design makes a right turn on red, and sometimes left on red depending on the geometry of the intersection in question, possible in many cases, often without stopping.
This type of intersection is common in the bicycle-friendly Netherlands.
Pedestrian crossings
Pedestrians must often cross from one side of a road to the other, and in doing so may come into the way of vehicles traveling on the road. In many places pedestrians are entirely left to look after themselves, that is, they must observe the road and cross when they can see that no traffic will threaten them. Busier cities usually provide pedestrian crossings, which are strips of the road where pedestrians are expected to cross.
The actual appearance of pedestrian crossings varies greatly, but the two most common appearances are: (1) a series of lateral white stripes or (2) two longitudinal white lines. The former is usually preferred, as it stands out more conspicuously against the dark pavement. | Traffic | Wikipedia | 471 | 43081 | https://en.wikipedia.org/wiki/Traffic | Technology | Basics_7 | null |
Some pedestrian crossings accompany a traffic signal to make vehicles stop at regular intervals so pedestrians can cross. Some countries have "intelligent" pedestrian signals, where the pedestrian must push a button in order to assert their intention to cross. In some countries, approaching traffic is monitored by radar or by electromagnetic sensors buried in the road surface, and the pedestrian crossing lights are set to red if a speed infringement is detected. This has the effect of enforcing the local speed limit. See Speed Limits below.
Pedestrian crossings without traffic signals are also common. In this case, the traffic laws usually states that the pedestrian has the right of way when crossing, and that vehicles must stop when a pedestrian uses the crossing. Countries and driving cultures vary greatly as to the extent to which this is respected. In the state of Nevada the car has the right of way when the crosswalk signal specifically forbids pedestrian crossing. Traffic culture is a determinant factor for the behaviors of all road users’ traffic. Specifically, it has a main role in crashes.
Some jurisdictions forbid crossing or using the road anywhere other than at crossings, termed jaywalking. In other areas, pedestrians may have the right to cross where they choose, and have right of way over vehicular traffic while crossing.
In most areas, an intersection is considered to have a crosswalk, even if not painted, as long as the roads meet at approximate right angles. The United Kingdom and Croatia are among the exceptions.
Pedestrian crossings may also be located away from intersections.
Level crossings
A level crossing is an at-grade intersection of a railway by a road. Because of safety issues, they are often equipped with closable gates, crossing bells and warning signs.
Speed limits
The higher the speed of a vehicle, the more difficult collision avoidance becomes and the greater the damage if a collision does occur. Therefore, many countries of the world limit the maximum speed allowed on their roads. Vehicles are not supposed to be driven at speeds which are higher than the posted maximum. | Traffic | Wikipedia | 400 | 43081 | https://en.wikipedia.org/wiki/Traffic | Technology | Basics_7 | null |
To enforce speed limits, two approaches are generally employed. In the United States, it is common for the police to patrol the streets and use special equipment (typically a radar unit) to measure the speed of vehicles, and pull over any vehicle found to be in violation of the speed limit. In Brazil, Colombia and some European countries, there are computerized speed-measuring devices spread throughout the city, which will automatically detect speeding drivers and take a photograph of the license plate (or number plate), which is later used for applying and mailing the ticket. Many jurisdictions in the U.S. use this technology as well.
A mechanism that was developed in Germany is the Grüne Welle, or green wave, which is an indicator that shows the optimal speed to travel for the synchronized green lights along that corridor. Driving faster or slower than the speed set by the behavior of the lights causes the driver to encounter many red lights. This discourages drivers from speeding or impeding the flow of traffic. See related traffic wave and Pedestrian Crossings, above.
Overtaking
Overtaking (or passing) refers to a maneuver by which one or more vehicles traveling in the same direction are passed by another vehicle. On two-lane roads, when there is a split line or a dashed line on the side of the overtaker, drivers may overtake when it is safe. On multi-lane roads in most jurisdictions, overtaking is permitted in the "slower" lanes, though many require a special circumstance. See "Lanes" below.
In the United Kingdom and Canada, notably on extra-urban roads, a solid white or yellow line closer to the driver is used to indicate that no overtaking is allowed in that lane. A double white or yellow line means that neither side may overtake.
In the United States, a solid white line means that lane changes are discouraged and a double white line means that the lane change is prohibited.
Lanes
When a street is wide enough to accommodate several vehicles traveling side-by-side, it is usual for traffic to organize itself into lanes, that is, parallel corridors of traffic. Some roads have one lane for each direction of travel and others have multiple lanes for each direction. Most countries apply pavement markings to clearly indicate the limits of each lane and the direction of travel that it must be used for. In other countries lanes have no markings at all and drivers follow them mostly by intuition rather than visual stimulus. | Traffic | Wikipedia | 489 | 43081 | https://en.wikipedia.org/wiki/Traffic | Technology | Basics_7 | null |
On roads that have multiple lanes going in the same direction, drivers may usually shift amongst lanes as they please, but they must do so in a way that does not cause inconvenience to other drivers. Driving cultures vary greatly on the issue of "lane ownership": in some countries, drivers traveling in a lane will be very protective of their right to travel in it while in others drivers will routinely expect other drivers to shift back and forth.
Designation and overtaking
The usual designation for lanes on divided highways is the fastest lane is the one closest to the center of the road, and the slowest to the edge of the road. Drivers are usually expected to keep in the slowest lane unless overtaking, though with more traffic congestion all lanes are often used.
When driving on the left:
The lane designated for faster traffic is on the right.
The lane designated for slower traffic is on the left.
Most freeway exits are on the left.
Overtaking is permitted to the right, and sometimes to the left.
When driving on the right:
The lane designated for faster traffic is on the left.
The lane designated for slower traffic is on the right.
Most freeway exits are on the right.
Overtaking is permitted to the left, and sometimes to the right.
Countries party to the Vienna Convention on Road Traffic have uniform rules about overtaking and lane designation. The convention details (amongst other things) that "Every driver shall keep to the edge of the carriageway appropriate to the direction of traffic", and the "Drivers overtaking shall do so on the side opposite to that appropriate to the direction of traffic", notwithstanding the presence or absence of oncoming traffic. Allowed exceptions to these rules include turning or heavy traffic, traffic in lines, or situation in which signs or markings must dictate otherwise. These rules must be more strictly adhered to on roads with oncoming traffic, but still apply on multi-lane and divided highways. Many countries in Europe are party to the Vienna Conventions on traffic and roads. In Australia (which is not a contracting party), traveling in any lane other than the "slow" lane on a road with a speed limit at or above is an offence, unless signage is posted to the contrary or the driver is overtaking. | Traffic | Wikipedia | 455 | 43081 | https://en.wikipedia.org/wiki/Traffic | Technology | Basics_7 | null |
Many areas in North America do not have any laws about staying to the slowest lanes unless overtaking. In those areas, unlike many parts of Europe, traffic is allowed to overtake on any side, even in a slower lane. This practice is known as "passing on the right" in the United States and "overtaking on the inside" and "undertaking" in the United Kingdom.
When referring to individual lanes on dual carriageways, one does not consider traffic travelling the opposite direction. The inside lane (in the British English sense, i.e. the lane beside the hard shoulder) refers to the lane used for normal travel, while the middle lane is used for overtaking cars on the inside lane. The outside lane (i.e. closest to oncoming traffic) is used for overtaking vehicles in the middle lane. The same principle lies with dual carriageways with more than three lanes.
U.S.-state-specific practices
In some US states (such as Louisiana, Massachusetts and New York), although there are laws requiring all traffic on a public way to use the right-most lane unless overtaking, this rule is often ignored and seldom enforced on multi-lane roadways. Some states, such as Colorado, use a combination of laws and signs restricting speeds or vehicles on certain lanes to emphasize overtaking only on the left lane, and to avoid a psychological condition commonly called road rage.
In California, cars may use any lane on multi-lane roadways. Drivers moving slower than the general flow of traffic are required to stay in the right-most lanes (by California Vehicle Code (CVC) 21654) to keep the way clear for faster vehicles and thus speed up traffic. However, faster drivers may legally pass in the slower lanes if conditions allow (by CVC 21754). But the CVC also requires trucks to stay in the right lane, or in the right two lanes if the roadway has four or more lanes going in their direction. The oldest freeways in California, and some freeway interchanges, often have ramps on the left, making signs like "TRUCKS OK ON LEFT LANE" or "TRUCKS MAY USE ALL LANES" necessary to override the default rule. Lane splitting, or riding motorcycles in the space between cars in traffic, is permitted as long as it is done in a safe and prudent manner.
One-way roadways | Traffic | Wikipedia | 483 | 43081 | https://en.wikipedia.org/wiki/Traffic | Technology | Basics_7 | null |
In order to increase traffic capacity and safety, a route may have two or more separate roads for each direction of traffic. Alternatively, a given road might be declared one-way.
High-speed roads
In large cities, moving from one part of the city to another by means of ordinary streets and avenues can be time-consuming since traffic is often slowed by at-grade junctions, tight turns, narrow marked lanes and lack of a minimum speed limit. Therefore, it has become common practice for larger cities to build roads for faster through traffic. There are two different types of roads used to provide high-speed access across urban areas:
The controlled-access highway (freeway or motorway) is a divided multi-lane highway with fully controlled access and grade-separated intersections (no cross traffic). Some freeways are called expressways, super-highways, or turnpikes, depending on local usage. Access to freeways is fully controlled; entering and leaving the freeway is permitted only at grade-separated interchanges.
The limited-access road (often called expressway in areas where the name does not refer to a freeway or motorway) is a lower-grade type of road with some or many of the characteristics of a controlled-access highway: usually a broad multi-lane avenue, frequently divided, with some grade separation at intersections.
Motor vehicle drivers wishing to travel over great distances within the city will usually take the freeways or expressways in order to minimize travel time. When a crossing road is at the same grade as the freeway, a bridge (or, less often, an underpass) will be built for the crossing road. If the freeway is elevated, the crossing road will pass underneath it.
Minimum speed signs are sometimes posted (although increasingly rare) and usually indicate that any vehicle traveling slower than should indicate a slower speed of travel to other motor vehicles by engaging the vehicle's four-way flashing lights. Alternative slower-than-posted speeds may be in effect, based on the posted speed limit of the highway/freeway.
Systems of freeways and expressways are also built to connect distant and regional cities, notable systems include the Interstate highways, the Autobahnen and the Expressway Network of the People's Republic of China.
One-way streets | Traffic | Wikipedia | 452 | 43081 | https://en.wikipedia.org/wiki/Traffic | Technology | Basics_7 | null |
In more sophisticated systems such as large cities, this concept is further extended: some streets are marked as being one-way, and on those streets all traffic must flow in only one direction. Pedestrians on the sidewalks are generally not limited to one-way movement. Drivers wishing to reach a destination they have already passed must return via other streets. One-way streets, despite the inconveniences to some individual drivers, can greatly improve traffic flow since they usually allow traffic to move faster and tend to simplify intersections.
Congested traffic
In some places traffic volume is consistently, extremely large, either during periods of time referred to as rush hour or perpetually. Exceptionally, traffic upstream of a vehicular collision or an obstruction, such as construction, may also be constrained, resulting in a traffic jam. Such dynamics in relation to traffic congestion is known as traffic flow. Traffic engineers sometimes gauge the quality of traffic flow in terms of level of service.
In measured traffic data, common spatiotemporal empirical features of traffic congestion have been found that are qualitatively the same for different highways in different countries. Some of these common features distinguish the wide moving jam and synchronized flow phases of congested traffic in Kerner's three-phase traffic theory.
Rush hour
During business days in most major cities, traffic congestion reaches great intensity at predictable times of the day due to the large number of vehicles using the road at the same time. This phenomenon is called rush hour or peak hour, although the period of high traffic intensity often exceeds one hour. Since the advent of car radios, radio programming during rush hour is likely to be called drive time.
Congestion mitigation
Rush hour policies
Some cities adopt policies to reduce rush-hour traffic and pollution and encourage the use of public transportation. For example, in São Paulo, Manila and in Mexico City, each vehicle has a specific day of the week in which it is forbidden from traveling the roads during rush hour. The day for each vehicle is taken from the license plate number, and this rule is enforced by traffic police and also by hundreds of strategically positioned traffic cameras backed by computerized image-recognition systems that issue tickets to offending drivers. | Traffic | Wikipedia | 440 | 43081 | https://en.wikipedia.org/wiki/Traffic | Technology | Basics_7 | null |
In the United States and Canada, several expressways have a special lane (called an "HOV Lane" – High Occupancy Vehicle Lane) that can only be used by cars carrying two (some locations-three) or more people. Also, many major cities have instituted strict parking prohibitions during rush hour on major arterial streets leading to and from the central business district. During designated weekday hours, vehicles parked on these primary routes are subject to prompt ticketing and towing at owner expense. The purpose of these restrictions is to make available an additional traffic lane in order to maximize available traffic capacity. Additionally, several cities offer a public telephone service where citizens can arrange rides with others depending on where they live and work. The purpose of these policies is to reduce the number of vehicles on the roads and thus reduce rush-hour traffic intensity.
Metered freeways are also a solution for controlling rush hour traffic. In Phoenix, Arizona and Seattle, Washington, among other places, metered on-ramps have been implemented. During rush hour, traffic signals are used with green lights to allow one car per blink of the light to proceed on to the freeway.
Rush hour is typically caused by multiple cars all going to once place at the same time. There is no way to fix the issue because the economy has set times for work, school, and running errands all during the same hours. There is no avoiding this problem because it exists in every major metropolitan area in the world.
Pre-emption
In some areas, emergency responders are provided with specialized equipment, such as a Mobile Infrared Transmitter, which allows emergency response vehicles, particularly fire-fighting apparatus, to have high-priority travel by having the lights along their route change to green. The technology behind these methods has evolved, from panels at the fire department (which could trigger and control green lights for certain major corridors) to optical systems (which the individual fire apparatus can be equipped with to communicate directly with receivers on the signal head). In certain jurisdictions, public transport buses and government-operated winter service vehicles are permitted to use this equipment to extend the length of a green light. | Traffic | Wikipedia | 431 | 43081 | https://en.wikipedia.org/wiki/Traffic | Technology | Basics_7 | null |
During emergencies where evacuation of a heavily populated area is required, local authorities may institute contraflow lane reversal, in which all lanes of a road lead away from a danger zone regardless of their original flow. Aside from emergencies, contraflow may also be used to ease traffic congestion during rush hour or at the end of a sports event (where a large number of cars are leaving the venue at the same time). For example, the six lanes of the Lincoln Tunnel can be changed from three inbound and three outbound to a two/four configuration depending on traffic volume. The Brazilian highways Rodovia dos Imigrantes and Rodovia Anchieta connect São Paulo to the Atlantic coast. Almost all lanes of both highways are usually reversed during weekends to allow for heavy seaside traffic. The reversibility of the highways requires many additional highway ramps and complicated interchanges.
Intelligent transportation systems
An intelligent transportation system (ITS) is a system of hardware, software, and operators-in-the-loop that allow better monitoring and control of traffic in order to optimize traffic flow. As the number of vehicle lane miles traveled per year continues to increase dramatically, and as the number of vehicle lane miles constructed per year has not been keeping pace, this has led to ever-increasing traffic congestion. As a cost-effective solution toward optimizing traffic, ITS presents a number of technologies to reduce congestion by monitoring traffic flows through the use of sensors and live cameras or analysing cellular phone data travelling in cars (floating car data) and in turn rerouting traffic as needed through the use of variable message boards (VMS), highway advisory radio, on board or off board navigation devices and other systems through integration of traffic data with navigation systems. Additionally, the roadway network has been increasingly fitted with additional communications and control infrastructure to allow traffic operations personnel to monitor weather conditions, for dispatching maintenance crews to perform snow or ice removal, as well as intelligent systems such as automated bridge de-icing systems which help to prevent accidents. | Traffic | Wikipedia | 409 | 43081 | https://en.wikipedia.org/wiki/Traffic | Technology | Basics_7 | null |
Aviation
In aviation, right-of-way rules are established over the principle that the least maneuverable aircraft takes priority. In the United States, the Code of Federal Regulations ranks air traffic in the following passage order:
Any aircraft in distress
Air balloon
Glider
Airship
An aircraft towing or refueling other aircraft has the right-of-way over all other engine-driven aircraft
Powered parachute, weight-shift-control aircraft, airplane, and rotorcraft
In addition, head-on approaching aircraft shall alter course to the right. An aircraft being overtaken has the right-of-way. A landing aircraft has the right-of-way over other surface-operating aircraft. | Traffic | Wikipedia | 132 | 43081 | https://en.wikipedia.org/wiki/Traffic | Technology | Basics_7 | null |
Rutile is an oxide mineral composed of titanium dioxide (TiO2), the most common natural form of TiO2. Rarer polymorphs of TiO2 are known, including anatase, akaogiite, and brookite.
Rutile has one of the highest refractive indices at visible wavelengths of any known crystal and also exhibits a particularly large birefringence and high dispersion. Owing to these properties, it is useful for the manufacture of certain optical elements, especially polarization optics, for longer visible and infrared wavelengths up to about 4.5 micrometres. Natural rutile may contain up to 10% iron and significant amounts of niobium and tantalum.
Rutile derives its name from the Latin ('red'), in reference to the deep red color observed in some specimens when viewed by transmitted light. Rutile was first described in 1803 by Abraham Gottlob Werner using specimens obtained in Horcajuelo de la Sierra, Madrid (Spain), which is consequently the type locality.
Occurrence
Rutile is a common accessory mineral in high-temperature and high-pressure metamorphic rocks and in igneous rocks.
Thermodynamically, rutile is the most stable polymorph of TiO2 at all temperatures, exhibiting lower total free energy than metastable phases of anatase or brookite. Consequently, the transformation of the metastable TiO2 polymorphs to rutile is irreversible. As it has the lowest molecular volume of the three main polymorphs, it is generally the primary titanium-bearing phase in most high-pressure metamorphic rocks, chiefly eclogites.
Within the igneous environment, rutile is a common accessory mineral in plutonic igneous rocks, though it is also found occasionally in extrusive igneous rocks, particularly those such as kimberlites and lamproites that have deep mantle sources. Anatase and brookite are found in the igneous environment, particularly as products of autogenic alteration during the cooling of plutonic rocks; anatase is also found in placer deposits sourced from primary rutile. | Rutile | Wikipedia | 440 | 43085 | https://en.wikipedia.org/wiki/Rutile | Physical sciences | Minerals | Earth science |
The occurrence of large specimen crystals is most common in pegmatites, skarns, and granite greisens. Rutile is found as an accessory mineral in some altered igneous rocks, and in certain gneisses and schists. In groups of acicular crystals it is frequently seen penetrating quartz as in the from Graubünden, Switzerland. In 2005 the Republic of Sierra Leone in West Africa had a production capacity of 23% of the world's annual rutile supply, which rose to approximately 30% in 2008.
Crystal structure
Rutile has a tetragonal unit cell, with unit cell parameters a = b = 4.584 Å, and c = 2.953 Å. The titanium cations have a coordination number of 6, meaning they are surrounded by an octahedron of 6 oxygen atoms. The oxygen anions have a coordination number of 3, resulting in a trigonal planar coordination. Rutile also shows a screw axis when its octahedra are viewed sequentially. When formed under reducing conditions, oxygen vacancies can occur, coupled to Ti3+ centers. Hydrogen can enter these gaps, existing as an individual vacancy occupant (pairing as a hydrogen ion) or creating a hydroxide group with an adjacent oxygen.
Rutile crystals are most commonly observed to exhibit a prismatic or acicular growth habit with preferential orientation along their c axis, the [001] direction. This growth habit is favored as the {110} facets of rutile exhibit the lowest surface free energy and are therefore thermodynamically most stable. The c-axis oriented growth of rutile appears clearly in nanorods, nanowires and abnormal grain growth phenomena of this phase.
Application
In large enough quantities in beach sands, rutile forms an important constituent of heavy minerals and ore deposits. Miners extract and separate the valuable minerals – e.g., rutile, zircon, and ilmenite. The main uses for rutile are the manufacture of refractory ceramic, as a pigment, and for the production of titanium metal. | Rutile | Wikipedia | 429 | 43085 | https://en.wikipedia.org/wiki/Rutile | Physical sciences | Minerals | Earth science |
Finely powdered rutile is a brilliant white pigment and is used in paints, plastics, paper, foods, and other applications that call for a bright white color. Titanium dioxide pigment is the single greatest use of titanium worldwide. Nanoscale particles of rutile are transparent to visible light but are highly effective in the absorption of ultraviolet radiation (sunscreen). The UV absorption of nano-sized rutile particles is blue-shifted compared to bulk rutile so that higher-energy UV light is absorbed by the nanoparticles. Hence, they are used in sunscreens to protect against UV-induced skin damage.
Small rutile needles present in gems are responsible for an optical phenomenon known as asterism. Asteriated gems are known as "star" gems. Star sapphires, star rubies, and other star gems are highly sought after and are generally more valuable than their normal counterparts.
Rutile is widely used as a welding electrode covering. It is also used as a part of the ZTR index, which classifies highly weathered sediments.
Semiconductor
Rutile, as a large band-gap semiconductor, has in recent decades been the subject of significant research towards applications as a functional oxide for applications in photocatalysis and dilute magnetism. Research efforts typically utilize small quantities of synthetic rutile rather than mineral-deposit derived materials.
Synthetic rutile
Synthetic rutile was first produced in 1948 and is sold under a variety of names. It can be produced from the titanium ore ilmenite through the Becher process. Very pure synthetic rutile is transparent and almost colorless, being slightly yellow, in large pieces. Synthetic rutile can be made in a variety of colors by doping. The high refractive index gives an adamantine luster and strong refraction that leads to a diamond-like appearance. The near-colorless diamond substitute is sold as "Titania", which is the old-fashioned chemical name for this oxide. However, rutile is seldom used in jewellery because it is not very hard (scratch-resistant), measuring only about 6 on the Mohs hardness scale. | Rutile | Wikipedia | 426 | 43085 | https://en.wikipedia.org/wiki/Rutile | Physical sciences | Minerals | Earth science |
As the result of growing research interest in the photocatalytic activity of titanium dioxide, in both anatase and rutile phases (as well as biphasic mixtures of the two phases), rutile TiO2 in powder and thin film form is frequently fabricated in laboratory conditions through solution based routes using inorganic precursors (typically TiCl4) or organometallic precursors (typically alkoxides such as titanium isopropoxide, also known as TTIP). Depending on synthesis conditions, the first phase to crystallize may be the metastable anatase phase, which can then be converted to the equilibrium rutile phase through thermal treatment. The physical properties of rutile are often modified using dopants to impart improved photocatalytic activity through improved photo-generated charge carrier separation, altered electronic band structures and improved surface reactivity. | Rutile | Wikipedia | 177 | 43085 | https://en.wikipedia.org/wiki/Rutile | Physical sciences | Minerals | Earth science |
A flagellum (; : flagella) (Latin for 'whip' or 'scourge') is a hair-like appendage that protrudes from certain plant and animal sperm cells, from fungal spores (zoospores), and from a wide range of microorganisms to provide motility. Many protists with flagella are known as flagellates.
A microorganism may have from one to many flagella. A gram-negative bacterium Helicobacter pylori, for example, uses its flagella to propel itself through the stomach to reach the mucous lining where it may colonise the epithelium and potentially cause gastritis, and ulcers – a risk factor for stomach cancer. In some swarming bacteria, the flagellum can also function as a sensory organelle, being sensitive to wetness outside the cell.
Across the three domains of Bacteria, Archaea, and Eukaryota, the flagellum has a different structure, protein composition, and mechanism of propulsion but shares the same function of providing motility. The Latin word means "whip" to describe its lash-like swimming motion. The flagellum in archaea is called the archaellum to note its difference from the bacterial flagellum.
Eukaryotic flagella and cilia are identical in structure but have different lengths and functions. Prokaryotic fimbriae and pili are smaller, and thinner appendages, with different functions. Cilia are attached to the surface of flagella and are used to swim or move fluid from one region to another.
Types
The three types of flagella are bacterial, archaeal, and eukaryotic.
The flagella in eukaryotes have dynein and microtubules that move with a bending mechanism. Bacteria and archaea do not have dynein or microtubules in their flagella, and they move using a rotary mechanism.
Other differences among these three types are: | Flagellum | Wikipedia | 414 | 43093 | https://en.wikipedia.org/wiki/Flagellum | Biology and health sciences | Organelles and other cell parts | null |
Bacterial flagella are helical filaments, each with a rotary motor at its base which can turn clockwise or counterclockwise. They provide two of several kinds of bacterial motility.
Archaeal flagella (archaella) are superficially similar to bacterial flagella in that it also has a rotary motor, but are different in many details and considered non-homologous.
Eukaryotic flagella—those of animal, plant, and protist cells—are complex cellular projections that lash back and forth. Eukaryotic flagella and motile cilia are identical in structure, but have different lengths, waveforms, and functions. Primary cilia are immotile, and have a structurally different 9+0 axoneme rather than the 9+2 axoneme found in both flagella and motile cilia.
Bacterial flagella
Structure and composition
The bacterial flagellum is made up of protein subunits of flagellin. Its shape is a 20-nanometer-thick hollow tube. It is helical and has a sharp bend just outside the outer membrane; this "hook" allows the axis of the helix to point directly away from the cell. A shaft runs between the hook and the basal body, passing through protein rings in the cell's membrane that act as bearings. Gram-positive organisms have two of these basal body rings, one in the peptidoglycan layer and one in the plasma membrane. Gram-negative organisms have four such rings: the L ring associates with the lipopolysaccharides, the P ring associates with peptidoglycan layer, the M ring is embedded in the plasma membrane, and the S ring is directly attached to the cytoplasm. The filament ends with a capping protein.
The flagellar filament is the long, helical screw that propels the bacterium when rotated by the motor, through the hook. In most bacteria that have been studied, including the gram-negative Escherichia coli, Salmonella typhimurium, Caulobacter crescentus, and Vibrio alginolyticus, the filament is made up of 11 protofilaments approximately parallel to the filament axis. Each protofilament is a series of tandem protein chains. However, Campylobacter jejuni has seven protofilaments. | Flagellum | Wikipedia | 495 | 43093 | https://en.wikipedia.org/wiki/Flagellum | Biology and health sciences | Organelles and other cell parts | null |
The basal body has several traits in common with some types of secretory pores, such as the hollow, rod-like "plug" in their centers extending out through the plasma membrane. The similarities between bacterial flagella and bacterial secretory system structures and proteins provide scientific evidence supporting the theory that bacterial flagella evolved from the type-three secretion system (TTSS).
The atomic structure of both bacterial flagella as well as the TTSS injectisome have been elucidated in great detail, especially with the development of cryo-electron microscopy. The best understood parts are the parts between the inner and outer membrane, that is, the scaffolding rings of the inner membrane (IM), the scaffolding pairs of the outer membrane (OM), and the rod/needle (injectisome) or rod/hook (flagellum) sections.
Motor
The bacterial flagellum is driven by a rotary engine (Mot complex) made up of protein, located at the flagellum's anchor point on the inner cell membrane. The engine is powered by proton-motive force, i.e., by the flow of protons (hydrogen ions) across the bacterial cell membrane due to a concentration gradient set up by the cell's metabolism (Vibrio species have two kinds of flagella, lateral and polar, and some are driven by a sodium ion pump rather than a proton pump). The rotor transports protons across the membrane, and is turned in the process. The rotor alone can operate at 6,000 to 100,000 rpm, but with the flagellar filament attached usually only reaches 200 to 1000 rpm. The direction of rotation can be changed by the flagellar motor switch almost instantaneously, caused by a slight change in the position of a protein, FliG, in the rotor. The torque is transferred from the MotAB to the torque helix on FliG's D5 domain and with the increase in the requirement of the torque or speed more MotAB are employed. Because the flagellar motor has no on-off switch, the protein epsE is used as a mechanical clutch to disengage the motor from the rotor, thus stopping the flagellum and allowing the bacterium to remain in one place. | Flagellum | Wikipedia | 470 | 43093 | https://en.wikipedia.org/wiki/Flagellum | Biology and health sciences | Organelles and other cell parts | null |
The production and rotation of a flagellum can take up to 10% of an Escherichia coli cell's energy budget and has been described as an "energy-guzzling machine". Its operation generates reactive oxygen species that elevate mutation rates.
The cylindrical shape of flagella is suited to locomotion of microscopic organisms; these organisms operate at a low Reynolds number, where the viscosity of the surrounding water is much more important than its mass or inertia.
The rotational speed of flagella varies in response to the intensity of the proton-motive force, thereby permitting certain forms of speed control, and also permitting some types of bacteria to attain remarkable speeds in proportion to their size; some achieve roughly 60 cell lengths per second. At such a speed, a bacterium would take about 245 days to cover 1 km; although that may seem slow, the perspective changes when the concept of scale is introduced. In comparison to macroscopic life forms, it is very fast indeed when expressed in terms of number of body lengths per second. A cheetah, for example, only achieves about 25 body lengths per second.
Through use of their flagella, bacteria are able to move rapidly towards attractants and away from repellents, by means of a biased random walk, with runs and tumbles brought about by rotating its flagellum counterclockwise and clockwise, respectively. The two directions of rotation are not identical (with respect to flagellum movement) and are selected by a molecular switch. Clockwise rotation is called the traction mode with the body following the flagella. Counterclockwise rotation is called the thruster mode with the flagella lagging behind the body.
Assembly
During flagellar assembly, components of the flagellum pass through the hollow cores of the basal body and the nascent filament. During assembly, protein components are added at the flagellar tip rather than at the base. In vitro, flagellar filaments assemble spontaneously in a solution containing purified flagellin as the sole protein.
Evolution | Flagellum | Wikipedia | 419 | 43093 | https://en.wikipedia.org/wiki/Flagellum | Biology and health sciences | Organelles and other cell parts | null |
At least 10 protein components of the bacterial flagellum share homologous proteins with the type three secretion system (T3SS) found in many gram-negative bacteria, hence one likely evolved from the other. Because the T3SS has a similar number of components as a flagellar apparatus (about 25 proteins), which one evolved first is difficult to determine. However, the flagellar system appears to involve more proteins overall, including various regulators and chaperones, hence it has been argued that flagella evolved from a T3SS. However, it has also been suggested that the flagellum may have evolved first or the two structures evolved in parallel. Early single-cell organisms' need for motility (mobility) support that the more mobile flagella would be selected by evolution first, but the T3SS evolving from the flagellum can be seen as 'reductive evolution', and receives no topological support from the phylogenetic trees. The hypothesis that the two structures evolved separately from a common ancestor accounts for the protein similarities between the two structures, as well as their functional diversity.
Flagella and the intelligent design debate
Some authors have argued that flagella cannot have evolved, assuming that they can only function properly when all proteins are in place. In other words, the flagellar apparatus is "irreducibly complex". However, many proteins can be deleted or mutated and the flagellum still works, though sometimes at reduced efficiency. Moreover, with many proteins unique to some number across species, diversity of bacterial flagella composition was higher than expected. Hence, the flagellar apparatus is clearly very flexible in evolutionary terms and perfectly able to lose or gain protein components. For instance, a number of mutations have been found that increase the motility of E. coli. Additional evidence for the evolution of bacterial flagella includes the existence of vestigial flagella, intermediate forms of flagella and patterns of similarities among flagellar protein sequences, including the observation that almost all of the core flagellar proteins have known homologies with non-flagellar proteins. Furthermore, several processes have been identified as playing important roles in flagellar evolution, including self-assembly of simple repeating subunits, gene duplication with subsequent divergence, recruitment of elements from other systems ('molecular bricolage') and recombination. | Flagellum | Wikipedia | 481 | 43093 | https://en.wikipedia.org/wiki/Flagellum | Biology and health sciences | Organelles and other cell parts | null |
Flagellar arrangements
Different species of bacteria have different numbers and arrangements of flagella, named using the term tricho, from the Greek trichos meaning hair.
Monotrichous bacteria such as Vibrio cholerae have a single polar flagellum.
Amphitrichous bacteria have a single flagellum on each of two opposite ends (e.g., Campylobacter jejuni or Alcaligenes faecalis)—both flagella rotate but coordinate to produce coherent thrust.
Lophotrichous bacteria (lopho Greek combining term meaning crest or tuft) have multiple flagella located at the same spot on the bacterial surface such as Helicobacter pylori, which act in concert to drive the bacteria in a single direction. In many cases, the bases of multiple flagella are surrounded by a specialized region of the cell membrane, called the polar organelle.
Peritrichous bacteria have flagella projecting in all directions (e.g., E. coli).
Counterclockwise rotation of a monotrichous polar flagellum pushes the cell forward with the flagellum trailing behind, much like a corkscrew moving inside cork. Water on the microscopic scale is highly viscous, unlike usual water.
Spirochetes, in contrast, have flagella called endoflagella arising from opposite poles of the cell, and are located within the periplasmic space as shown by breaking the outer-membrane and also by electron cryotomography microscopy. The rotation of the filaments relative to the cell body causes the entire bacterium to move forward in a corkscrew-like motion, even through material viscous enough to prevent the passage of normally flagellated bacteria.
In certain large forms of Selenomonas, more than 30 individual flagella are organized outside the cell body, helically twining about each other to form a thick structure (easily visible with the light microscope) called a "fascicle".
In some Vibrio spp. (particularly Vibrio parahaemolyticus) and related bacteria such as Aeromonas, two flagellar systems co-exist, using different sets of genes and different ion gradients for energy. The polar flagella are constitutively expressed and provide motility in bulk fluid, while the lateral flagella are expressed when the polar flagella meet too much resistance to turn. These provide swarming motility on surfaces or in viscous fluids. | Flagellum | Wikipedia | 511 | 43093 | https://en.wikipedia.org/wiki/Flagellum | Biology and health sciences | Organelles and other cell parts | null |
Bundling
Bundling is an event that can happen in multi-flagellated cells, bundling the flagella together and causing them to rotate in a coordinated manner.
Flagella are left-handed helices, and when rotated counter-clockwise by their rotors, they can bundle and rotate together. When the rotors reverse direction, thus rotating clockwise, the flagellum unwinds from the bundle. This may cause the cell to stop its forward motion and instead start twitching in place, referred to as tumbling. Tumbling results in a stochastic reorientation of the cell, causing it to change the direction of its forward swimming.
It is not known which stimuli drive the switch between bundling and tumbling, but the motor is highly adaptive to different signals. In the model describing chemotaxis ("movement on purpose") the clockwise rotation of a flagellum is suppressed by chemical compounds favorable to the cell (e.g. food). When moving in a favorable direction, the concentration of such chemical attractants increases and therefore tumbles are continually suppressed, allowing forward motion; likewise, when the cell's direction of motion is unfavorable (e.g., away from a chemical attractant), tumbles are no longer suppressed and occur much more often, with the chance that the cell will be thus reoriented in the correct direction.
Even if all flagella would rotate clockwise, however, they often cannot form a bundle due to geometrical and hydrodynamic reasons.
Eukaryotic flagella
Terminology
Aiming to emphasize the distinction between the bacterial flagella and the eukaryotic cilia and flagella, some authors attempted to replace the name of these two eukaryotic structures with "undulipodia" (e.g., all papers by Margulis since the 1970s) or "cilia" for both (e.g., Hülsmann, 1992; Adl et al., 2012; most papers of Cavalier-Smith), preserving "flagella" for the bacterial structure. However, the discriminative usage of the terms "cilia" and "flagella" for eukaryotes adopted in this article (see below) is still common (e.g., Andersen et al., 1991; Leadbeater et al., 2000). | Flagellum | Wikipedia | 479 | 43093 | https://en.wikipedia.org/wiki/Flagellum | Biology and health sciences | Organelles and other cell parts | null |
Internal structure
The core of a eukaryotic flagellum, known as the axoneme is a bundle of nine fused pairs of microtubules known as doublets surrounding two central single microtubules (singlets). This 9+2 axoneme is characteristic of the eukaryotic flagellum. At the base of a eukaryotic flagellum is a basal body, "blepharoplast" or kinetosome, which is the microtubule organizing center for flagellar microtubules and is about 500 nanometers long. Basal bodies are structurally identical to centrioles. The flagellum is encased within the cell's plasma membrane, so that the interior of the flagellum is accessible to the cell's cytoplasm.
Besides the axoneme and basal body, relatively constant in morphology, other internal structures of the flagellar apparatus are the transition zone (where the axoneme and basal body meet) and the root system (microtubular or fibrilar structures that extend from the basal bodies into the cytoplasm), more variable and useful as indicators of phylogenetic relationships of eukaryotes. Other structures, more uncommon, are the paraflagellar (or paraxial, paraxonemal) rod, the R fiber, and the S fiber. For surface structures, see below.
Mechanism
Each of the outer 9 doublet microtubules extends a pair of dynein arms (an "inner" and an "outer" arm) to the adjacent microtubule; these produce force through ATP hydrolysis. The flagellar axoneme also contains radial spokes, polypeptide complexes extending from each of the outer nine microtubule doublets towards the central pair, with the "head" of the spoke facing inwards. The radial spoke is thought to be involved in the regulation of flagellar motion, although its exact function and method of action are not yet understood.
Flagella versus cilia
The regular beat patterns of eukaryotic cilia and flagella generate motion on a cellular level. Examples range from the propulsion of single cells such as the swimming of spermatozoa to the transport of fluid along a stationary layer of cells such as in the respiratory tract. | Flagellum | Wikipedia | 471 | 43093 | https://en.wikipedia.org/wiki/Flagellum | Biology and health sciences | Organelles and other cell parts | null |
Although eukaryotic cilia and flagella are ultimately the same, they are sometimes classed by their pattern of movement, a tradition from before their structures have been known. In the case of flagella, the motion is often planar and wave-like, whereas the motile cilia often perform a more complicated three-dimensional motion with a power and recovery stroke. Yet another traditional form of distinction is by the number of 9+2 organelles on the cell.
Intraflagellar transport
Intraflagellar transport, the process by which axonemal subunits, transmembrane receptors, and other proteins are moved up and down the length of the flagellum, is essential for proper functioning of the flagellum, in both motility and signal transduction.
Evolution and occurrence
Eukaryotic flagella or cilia, probably an ancestral characteristic, are widespread in almost all groups of eukaryotes, as a relatively perennial condition, or as a flagellated life cycle stage (e.g., zoids, gametes, zoospores, which may be produced continually or not).
The first situation is found either in specialized cells of multicellular organisms (e.g., the choanocytes of sponges, or the ciliated epithelia of metazoans), as in ciliates and many eukaryotes with a "flagellate condition" (or "monadoid level of organization", see Flagellata, an artificial group). | Flagellum | Wikipedia | 309 | 43093 | https://en.wikipedia.org/wiki/Flagellum | Biology and health sciences | Organelles and other cell parts | null |
Flagellated lifecycle stages are found in many groups, e.g., many green algae (zoospores and male gametes), bryophytes (male gametes), pteridophytes (male gametes), some gymnosperms (cycads and Ginkgo, as male gametes), centric diatoms (male gametes), brown algae (zoospores and gametes), oomycetes (assexual zoospores and gametes), hyphochytrids (zoospores), labyrinthulomycetes (zoospores), some apicomplexans (gametes), some radiolarians (probably gametes), foraminiferans (gametes), plasmodiophoromycetes (zoospores and gametes), myxogastrids (zoospores), metazoans (male gametes), and chytrid fungi (zoospores and gametes).
Flagella or cilia are completely absent in some groups, probably due to a loss rather than being a primitive condition. The loss of cilia occurred in red algae, some green algae (Zygnematophyceae), the gymnosperms except cycads and Ginkgo, angiosperms, pennate diatoms, some apicomplexans, some amoebozoans, in the sperm of some metazoans, and in fungi (except chytrids). | Flagellum | Wikipedia | 320 | 43093 | https://en.wikipedia.org/wiki/Flagellum | Biology and health sciences | Organelles and other cell parts | null |
Typology
A number of terms related to flagella or cilia are used to characterize eukaryotes. According to surface structures present, flagella may be:
whiplash flagella (= smooth, acronematic flagella): without hairs, e.g., in Opisthokonta
hairy flagella (= tinsel, flimmer, pleuronematic flagella): with hairs (= mastigonemes sensu lato), divided in:
with fine hairs (= non-tubular, or simple hairs): occurs in Euglenophyceae, Dinoflagellata, some Haptophyceae (Pavlovales)
with stiff hairs (= tubular hairs, retronemes, mastigonemes sensu stricto), divided in:
bipartite hairs: with two regions. Occurs in Cryptophyceae, Prasinophyceae, and some Heterokonta
tripartite (= straminipilous) hairs: with three regions (a base, a tubular shaft, and one or more terminal hairs). Occurs in most Heterokonta
stichonematic flagella: with a single row of hairs
pantonematic flagella: with two rows of hairs
acronematic: flagella with a single, terminal mastigoneme or flagellar hair (e.g., bodonids); some authors use the term as synonym of whiplash
with scales: e.g., Prasinophyceae
with spines: e.g., some brown algae
with undulating membrane: e.g., some kinetoplastids, some parabasalids
with proboscis (trunk-like protrusion of the cell): e.g., apusomonads, some bodonids | Flagellum | Wikipedia | 374 | 43093 | https://en.wikipedia.org/wiki/Flagellum | Biology and health sciences | Organelles and other cell parts | null |
According to the number of flagella, cells may be: (remembering that some authors use "ciliated" instead of "flagellated")
uniflagellated: e.g., most Opisthokonta
biflagellated: e.g., all Dinoflagellata, the gametes of Charophyceae, of most bryophytes and of some metazoans
triflagellated: e.g., the gametes of some Foraminifera
quadriflagellated: e.g., some Prasinophyceae, Collodictyonidae
octoflagellated: e.g., some Diplomonada, some Prasinophyceae
multiflagellated: e.g., Opalinata, Ciliophora, Stephanopogon, Parabasalida, Hemimastigophora, Caryoblastea, Multicilia, the gametes (or zoids) of Oedogoniales (Chlorophyta), some pteridophytes and some gymnosperms
According to the place of insertion of the flagella:
opisthokont: cells with flagella inserted posteriorly, e.g., in Opisthokonta (Vischer, 1945). In Haptophyceae, flagella are laterally to terminally inserted, but are directed posteriorly during rapid swimming.
akrokont: cells with flagella inserted apically
subakrokont: cells with flagella inserted subapically
pleurokont: cells with flagella inserted laterally
According to the beating pattern:
gliding: a flagellum that trails on the substrate
heterodynamic: flagella with different beating patterns (usually with one flagellum functioning in food capture and the other functioning in gliding, anchorage, propulsion or "steering")
isodynamic: flagella beating with the same patterns | Flagellum | Wikipedia | 407 | 43093 | https://en.wikipedia.org/wiki/Flagellum | Biology and health sciences | Organelles and other cell parts | null |
Other terms related to the flagellar type:
isokont: cells with flagella of equal length. It was also formerly used to refer to the Chlorophyta
anisokont: cells with flagella of unequal length, e.g., some Euglenophyceae and Prasinophyceae
heterokont: term introduced by Luther (1899) to refer to the Xanthophyceae, due to the pair of flagella of unequal length. It has taken on a specific meaning in referring to cells with an anterior straminipilous flagellum (with tripartite mastigonemes, in one or two rows) and a posterior usually smooth flagellum. It is also used to refer to the taxon Heterokonta
stephanokont: cells with a crown of flagella near its anterior end, e.g., the gametes and spores of Oedogoniales, the spores of some Bryopsidales. Term introduced by Blackman & Tansley (1902) to refer to the Oedogoniales
akont: cells without flagella. It was also used to refer to taxonomic groups, as Aconta or Akonta: the Zygnematophyceae and Bacillariophyceae (Oltmanns, 1904), or the Rhodophyceae (Christensen, 1962)
Archaeal flagella
The archaellum possessed by some species of Archaea is superficially similar to the bacterial flagellum; in the 1980s, they were thought to be homologous on the basis of gross morphology and behavior. Both flagella and archaella consist of filaments extending outside the cell, and rotate to propel the cell. Archaeal flagella have a unique structure which lacks a central channel. Similar to bacterial type IV pilins, the archaeal proteins (archaellins) are made with class 3 signal peptides and they are processed by a type IV prepilin peptidase-like enzyme. The archaellins are typically modified by the addition of N-linked glycans which are necessary for proper assembly or function.
Discoveries in the 1990s revealed numerous detailed differences between the archaeal and bacterial flagella. These include: | Flagellum | Wikipedia | 466 | 43093 | https://en.wikipedia.org/wiki/Flagellum | Biology and health sciences | Organelles and other cell parts | null |
Bacterial flagella rotation is powered by the proton motive force – a flow of H+ ions or occasionally by the sodium-motive force – a flow of Na+ ions; archaeal flagella rotation is powered by ATP.
While bacterial cells often have many flagellar filaments, each of which rotates independently, the archaeal flagellum is composed of a bundle of many filaments that rotates as a single assembly.
Bacterial flagella grow by the addition of flagellin subunits at the tip; archaeal flagella grow by the addition of subunits to the base.
Bacterial flagella are thicker than archaella, and the bacterial filament has a large enough hollow "tube" inside that the flagellin subunits can flow up the inside of the filament and get added at the tip; the archaellum is too thin (12-15 nm) to allow this.
Many components of bacterial flagella share sequence similarity to components of the type III secretion systems, but the components of bacterial flagella and archaella share no sequence similarity. Instead, some components of archaella share sequence and morphological similarity with components of type IV pili, which are assembled through the action of type II secretion systems (the nomenclature of pili and protein secretion systems is not consistent).
These differences support the theory that the bacterial flagella and archaella are a classic case of biological analogy, or convergent evolution, rather than homology. Research into the structure of archaella made significant progress beginning in the early 2010s, with the first atomic resolution structure of an archaella protein, the discovery of additional functions of archaella, and the first reports of archaella in Nanoarchaeota and Thaumarchaeota.
Fungal
The only fungi to have a single flagellum on their spores are the chytrids. In Batrachochytrium dendrobatidis the flagellum is 19–20 μm long. A nonfunctioning centriole lies adjacent to the kinetosome. Nine interconnected props attach the kinetosome to the plasmalemma, and a terminal plate is present in the transitional zone. An inner ring-like structure attached to the tubules of the flagellar doublets within the transitional zone has been observed in transverse section.
Additional images | Flagellum | Wikipedia | 484 | 43093 | https://en.wikipedia.org/wiki/Flagellum | Biology and health sciences | Organelles and other cell parts | null |
The cilium (: cilia; ; in Medieval Latin and in anatomy, cilium) is a short hair-like membrane protrusion from many types of eukaryotic cell. (Cilia are absent in bacteria and archaea.) The cilium has the shape of a slender threadlike projection that extends from the surface of the much larger cell body. Eukaryotic flagella found on sperm cells and many protozoans have a similar structure to motile cilia that enables swimming through liquids; they are longer than cilia and have a different undulating motion.
There are two major classes of cilia: motile and non-motile cilia, each with two subtypes, giving four types in all. A cell will typically have one primary cilium or many motile cilia. The structure of the cilium core, called the axoneme, determines the cilium class. Most motile cilia have a central pair of single microtubules surrounded by nine pairs of double microtubules called a 9+2 axoneme. Most non-motile cilia have a 9+0 axoneme that lacks the central pair of microtubules. Also lacking are the associated components that enable motility including the outer and inner dynein arms, and radial spokes. Some motile cilia lack the central pair, and some non-motile cilia have the central pair, hence the four types.
Most non-motile cilia, termed primary cilia or sensory cilia, serve solely as sensory organelles. Most vertebrate cell types possess a single non-motile primary cilium, which functions as a cellular antenna. Olfactory neurons possess a great many non-motile cilia. Non-motile cilia that have a central pair of microtubules are the kinocilia present on hair cells. | Cilium | Wikipedia | 384 | 43118 | https://en.wikipedia.org/wiki/Cilium | Biology and health sciences | Cell parts | Biology |
Motile cilia are found in large numbers on respiratory epithelial cells – around 200 cilia per cell, where they function in mucociliary clearance, and also have mechanosensory and chemosensory functions. Motile cilia on ependymal cells move the cerebrospinal fluid through the ventricular system of the brain. Motile cilia are also present in the oviducts (fallopian tubes) of female (therian) mammals, where they function in moving egg cells from the ovary to the uterus. Motile cilia that lack the central pair of microtubules are found in the cells of the embryonic primitive node; termed nodal cells, these nodal cilia are responsible for the left-right asymmetry of bilaterians.
Structure
A cilium is assembled and built from a basal body on the cell surface. From the basal body, the ciliary rootlet forms ahead of the transition plate and transition zone where the earlier microtubule triplets change to the microtubule doublets of the axoneme.
Basal body
The foundation of the cilium is the basal body, a term applied to the mother centriole when it is associated with a cilium. Mammalian basal bodies consist of a barrel of nine triplet microtubules, subdistal appendages and nine strut-like structures, known as distal appendages, which attach the basal body to the membrane at the base of the cilium. Two of each of the basal body's triplet microtubules extend during growth of the axoneme to become the doublet microtubules.
Ciliary rootlet
The ciliary rootlet is a cytoskeleton-like structure that originates from the basal body at the proximal end of a cilium. Rootlets are typically 80-100 nm in diameter and contain cross striae distributed at regular intervals of approximately 55-70 nm. A prominent component of the rootlet is rootletin a coiled coil rootlet protein coded for by the CROCC gene. | Cilium | Wikipedia | 434 | 43118 | https://en.wikipedia.org/wiki/Cilium | Biology and health sciences | Cell parts | Biology |
Transition zone
To achieve its distinct composition, the proximal-most region of the cilium consists of a transition zone, also known as the ciliary gate, that controls the entry and exit of proteins to and from the cilium. At the transition zone, Y-shaped structures connect the ciliary membrane to the underlying axoneme. Control of selective entry into cilia may involve a sieve-like function of transition zone. Inherited defects in components of the transition zone cause ciliopathies, such as Joubert syndrome. Transition zone structure and function is conserved across diverse organisms, including vertebrates, Caenorhabditis elegans, Drosophila melanogaster and Chlamydomonas reinhardtii. In mammals, disruption of the transition zone reduces the ciliary abundance of membrane-associated ciliary proteins, such as those involved in Hedgehog signal transduction, compromising Hedgehog-dependent embryonic development of digit number and central nervous system patterning.
Axoneme
Inside a cilium is a microtubule-based cytoskeletal core called the axoneme. The axoneme of a primary cilium typically has a ring of nine outer microtubule doublets (called a 9+0 axoneme), and the axoneme of a motile cilium has, in addition to the nine outer doublets, two central microtubule singlets (called a 9+2 axoneme). This is the same axoneme type of the flagellum. The axoneme in a motile cilium acts as a scaffold for the inner and outer dynein arms that move the cilium, and provides tracks for the microtubule motor proteins of kinesin and dynein. The transport of ciliary components is carried out by intraflagellar transport (IFT) which is similar to the axonal transport in a nerve fibre. Transport is bidirectional and cytoskeletal motor proteins kinesin and dynein transport ciliary components along the microtubule tracks; kinesin in an anterograde movement towards the ciliary tip and dynein in a retrograde movement towards the cell body. The cilium has its own ciliary membrane enclosed within the surrounding cell membrane.
Types | Cilium | Wikipedia | 482 | 43118 | https://en.wikipedia.org/wiki/Cilium | Biology and health sciences | Cell parts | Biology |
Non-motile cilia
In animals, non-motile primary cilia are found on nearly every type of cell, blood cells being a prominent exception. Most cells only possess one, in contrast to cells with motile cilia, an exception being olfactory sensory neurons, where the odorant receptors are located, which each possess about ten cilia. Some cell types, such as retinal photoreceptor cells, possess highly specialized primary cilia.
Although the primary cilium was discovered in 1898, it was largely ignored for a century and considered a vestigial organelle without important function. Recent findings regarding its physiological roles in chemosensation, signal transduction, and cell growth control, have revealed its importance in cell function. Its importance to human biology has been underscored by the discovery of its role in a diverse group of diseases caused by the dysgenesis or dysfunction of cilia, such as polycystic kidney disease, congenital heart disease, mitral valve prolapse, and retinal degeneration, called ciliopathies. The primary cilium is now known to play an important role in the function of many human organs. Primary cilia on pancreatic beta cells regulate their function and energy metabolism. Cilia deletion can lead to islet dysfunction and type 2 diabetes.
Cilia are assembled during the G1 phase and are disassembled before mitosis occurs. Disassembly of cilia requires the action of aurora kinase A.
The current scientific understanding of primary cilia views them as "sensory cellular antennae that coordinate many cellular signaling pathways, sometimes coupling the signaling to ciliary motility or alternatively to cell division and differentiation."
The cilium is composed of subdomains and enclosed by a plasma membrane continuous with the plasma membrane of the cell. For many cilia, the basal body, where the cilium originates, is located within a membrane invagination called the ciliary pocket. The cilium membrane and the basal body microtubules are connected by distal appendages (also called transition fibers). Vesicles carrying molecules for the cilia dock at the distal appendages. Distal to the transition fibers form a transition zone where entry and exit of molecules is regulated to and from the cilia. Some of the signaling with these cilia occur through ligand binding such as Hedgehog signaling. Other forms of signaling include G protein-coupled receptors including the somatostatin receptor 3 in neurons. | Cilium | Wikipedia | 509 | 43118 | https://en.wikipedia.org/wiki/Cilium | Biology and health sciences | Cell parts | Biology |
Modified non-motile cilia
Kinocilia that are found on hair cells in the inner ear are termed as specialized primary cilia, or modified non-motile cilia. They possess the 9+2 axoneme of the motile cilia but lack the inner dynein arms that give movement. They do move passively following the detection of sound, allowed by the outer dynein arms.
Motile cilia
Mammals also have motile cilia or secondary cilia that are usually present on a cell's surface in large numbers (multiciliate), and beat in coordinated metachronal waves. Multiciliated cells are found lining the respiratory tract where they function in mucociliary clearance sweeping mucus containing debris away from the lungs. Each cell in the respiratory epithelium has around 200 motile cilia.
In the reproductive tract, smooth muscle contractions help the beating of the cilia in moving the egg cell from the ovary to the uterus.
In the ventricles of the brain ciliated ependymal cells circulate the cerebrospinal fluid.
The functioning of motile cilia is strongly dependent on the maintenance of optimal levels of periciliary fluid bathing the cilia. Epithelial sodium channels (ENaCs) are specifically expressed along the entire length of cilia in the respiratory tract, and fallopian tube or oviduct that apparently serve as sensors to regulate the periciliary fluid.
Modified motile cilia
Motile cilia without the central pair of singlets (9+0) are found in early embryonic development. They are present as nodal cilia on the nodal cells of the primitive node. Nodal cells are responsible for the left-right asymmetry in bilateral animals. While lacking the central apparatus there are dynein arms present that allow the nodal cilia to move in a spinning fashion. The movement creates a current flow of the extraembryonic fluid across the nodal surface in a leftward direction that initiates the left-right asymmetry in the developing embryo. | Cilium | Wikipedia | 438 | 43118 | https://en.wikipedia.org/wiki/Cilium | Biology and health sciences | Cell parts | Biology |
Motile, multiple, 9+0 cilia are found on the epithelial cells of the choroid plexus. Cilia also can change structure when introduced to hot temperatures and become sharp. They are present in large numbers on each cell and move relatively slowly, making them intermediate between motile and primary cilia. In addition to 9+0 cilia that are mobile, there are also solitary 9+2 cilia that stay immobile found in hair cells.
Nodal cilia
Nodal cells have a single cilium called a monocilium. They are present in the very early development of the embryo on the primitive node. There are two areas of the node with different types of nodal cilia. On the central node are motile cilia, and on the peripheral area of the node the nodal cilia are modified motile.
The motile cilia on the central cells rotate to generate the leftward flow of extracellular fluid needed to initiate the left-right asymmetry.
Cilia versus flagella
The motile cilia on sperm cells and many protozoans enables swimming through liquids and are traditionally referred to as "flagella". As these protrusions are structurally identical to motile cilia, attempts at preserving this terminology include making a distinction by morphology ("flagella" are typically longer than ordinary cilia and have a different undulating motion) and by number.
Microorganisms
Ciliates are eukaryotic microorganisms that possess motile cilia exclusively and use them for either locomotion or to simply move liquid over their surface. A Paramecium for example is covered in thousands of cilia that enable its swimming. These motile cilia have been shown to be also sensory.
Ciliogenesis
Cilia are formed through the process of ciliogenesis. An early step is docking of the basal body to the growing ciliary membrane, after which the transition zone forms. The building blocks of the ciliary axoneme, such as tubulins, are added at the ciliary tips through a process that depends partly on intraflagellar transport (IFT). Exceptions include Drosophila sperm and Plasmodium falciparum flagella formation, in which cilia assemble in the cytoplasm. | Cilium | Wikipedia | 473 | 43118 | https://en.wikipedia.org/wiki/Cilium | Biology and health sciences | Cell parts | Biology |
At the base of the cilium where it attaches to the cell body is the microtubule organizing center, the basal body. Some basal body proteins as CEP164, ODF2
and CEP170, are required for the formation and the stability of the cilium.
In effect, the cilium is a nanomachine composed of perhaps over 600 proteins in molecular complexes, many of which also function independently as nanomachines. Flexible linkers allow the
mobile protein domains
connected by them to recruit their binding partners and induce
long-range allostery via
protein domain dynamics.
Function
The dynein in the axoneme – axonemal dynein forms bridges between neighbouring microtubule doublets. When ATP activates the motor domain of dynein, it attempts to walk along the adjoining microtubule doublet. This would force the adjacent doublets to slide over one another if not for the presence of nexin between the microtubule doublets. And thus the force generated by dynein is instead converted into a bending motion.
Sensing the extracellular environment
Some primary cilia on epithelial cells in eukaryotes act as cellular antennae, providing chemosensation, thermosensation and mechanosensation of the extracellular environment. These cilia then play a role in mediating specific signalling cues, including soluble factors in the external cell environment, a secretory role in which a soluble protein is released to have an effect downstream of the fluid flow, and mediation of fluid flow if the cilia are motile.
Some epithelial cells are ciliated, and they commonly exist as a sheet of polarized cells forming a tube or tubule with cilia projecting into the lumen. This sensory and signalling role puts cilia in a central role for maintaining the local cellular environment and may be why ciliary defects cause such a wide range of human diseases.
In the embryo, nodal cilia are used to direct the flow of extracellular fluid. This leftward movement is to generate left-right asymmetry across the midline of the embryo. Central cilia coordinate their rotational beating while the immotile cilia on the sides sense the direction of the flow. Studies in mice suggest a biophysical mechanism by which the direction of flow is sensed. | Cilium | Wikipedia | 474 | 43118 | https://en.wikipedia.org/wiki/Cilium | Biology and health sciences | Cell parts | Biology |
Axo-ciliary synapse
With axo-ciliary synapses, there is communication between serotonergic axons and primary cilia of CA1 pyramidal neurons that alters the neuron's epigenetic state in the nucleus – "a way to change what is being transcribed or made in the nucleus" via this signalling distinct from that at the plasma membrane which also is longer-term.
Clinical significance
Ciliary defects can lead to a number of human diseases. Defects in cilia adversely affect many critical signaling pathways essential to embryonic development and to adult physiology, and thus offer a plausible hypothesis for the often multi-symptom nature of diverse ciliopathies. Known ciliopathies include primary ciliary dyskinesia, Bardet–Biedl syndrome, polycystic kidney and liver disease, nephronophthisis, Alström syndrome, Meckel–Gruber syndrome, Sensenbrenner syndrome and some forms of retinal degeneration. Genetic mutations compromising the proper functioning of cilia, ciliopathies, can cause chronic disorders such as primary ciliary dyskinesia (PCD), nephronophthisis, and Senior–Løken syndrome. In addition, a defect of the primary cilium in the renal tubule cells can lead to polycystic kidney disease (PKD). In another genetic disorder called Bardet–Biedl syndrome (BBS), the mutant gene products are the components in the basal body and cilia. Defects in cilia cells are linked to obesity and often pronounced in type 2 diabetes. Several studies already showed impaired glucose tolerance and reduction in the insulin secretion in the ciliopathy models. Moreover, the number and length of cilia was decreased in the type 2 diabetes models.
Epithelial sodium channels (ENaCs) that are expressed along the length of cilia regulate periciliary fluid level. Mutations that decrease the activity of ENaCs result in multisystem pseudohypoaldosteronism, that is associated with fertility problems. In cystic fibrosis that results from mutations in the chloride channel CFTR, ENaC activity is enhanced leading to a severe reduction of the fluid level that causes complications and infections in the respiratory airways. | Cilium | Wikipedia | 482 | 43118 | https://en.wikipedia.org/wiki/Cilium | Biology and health sciences | Cell parts | Biology |
Since the flagellum of human sperm has the same internal structure of a cilium, ciliary dysfunction can also be responsible for male infertility.
There is an association of primary ciliary dyskinesia with left-right anatomic abnormalities such as situs inversus (a combination of findings is known as Kartagener syndrome), and situs ambiguus (also known as Heterotaxy syndrome). These left-right anatomic abnormalities can also result in congenital heart disease. It has been shown that proper cilial function is responsible for the normal left-right asymmetry in mammals.
The diverse outcomes caused by ciliary dysfunction may result from alleles of different strengths that compromise ciliary functions in different ways or to different extents. Many ciliopathies are inherited in a Mendelian manner, but specific genetic interactions between distinct functional ciliary complexes, such as transition zone and BBS complexes, can alter the phenotypic manifestations of recessive ciliopathies. Some mutations in transition zone proteins can cause specific serious ciliopathies.
Extracellular changes
Reduction of cilia function can also result from infection. Research into biofilms has shown that bacteria can alter cilia. A biofilm is a community of bacteria of either the same or multiple species of bacteria. The cluster of cells secretes different factors which form an extracellular matrix. Cilia in the respiratory system is known to move mucus and pathogens out of the airways. It has been found that patients with biofilm positive infections have impaired cilia function. The impairment may present as decreased motion or reduction in the number of cilia. Though these changes result from an external source, they still effect the pathogenicity of the bacteria, progression of infection, and how it is treated.
The transportation of the immature egg cell, and the embryo to the uterus for implantation depends on the combination of regulated smooth muscle contractions, and ciliary beating. Dysfunction in this transportation can result in an ectopic pregnancy where the embryo is implanted (usually) in the fallopian tube before reaching its proper destination of the uterus. Many factors can affect this stage including infection and menstrual cycle hormones. Smoking (causing inflammation), and infection can reduce the numbers of cilia, and the ciliary beat can be affected by hormonal changes. | Cilium | Wikipedia | 495 | 43118 | https://en.wikipedia.org/wiki/Cilium | Biology and health sciences | Cell parts | Biology |
Primary cilia in pancreatic cells
The pancreas is a mixture of highly differentiated exocrine and endocrine cells. Primary cilia are present in exocrine cells, which are centroacinar duct cells. Endocrine tissue is composed of different hormone-secreting cells. Insulin-secreting beta cells and glucagon-secreting alpha cells are highly ciliated. | Cilium | Wikipedia | 83 | 43118 | https://en.wikipedia.org/wiki/Cilium | Biology and health sciences | Cell parts | Biology |
Callisto ( ), or Jupiter IV, is the second-largest moon of Jupiter, after Ganymede. In the Solar System it is the third-largest moon after Ganymede and Saturn's largest moon Titan, and nearly as large as the smallest planet Mercury. Callisto is, with a diameter of , roughly a third larger than Earth's Moon and orbits Jupiter on average at a distance of , which is about five times further out than the Moon orbiting Earth. It is the outermost of the four large Galilean moons of Jupiter, which were discovered in 1610 with one of the first telescopes, and is today visible from Earth with common binoculars.
The surface of Callisto is the oldest and most heavily cratered in the Solar System. Its surface is completely covered with impact craters. It does not show any signatures of subsurface processes such as plate tectonics or volcanism, with no signs that geological activity in general has ever occurred, and is thought to have evolved predominantly under the influence of impacts. Prominent surface features include multi-ring structures, variously shaped impact craters, and chains of craters (catenae) and associated scarps, ridges and deposits. At a small scale, the surface is varied and made up of small, sparkly frost deposits at the tips of high spots, surrounded by a low-lying, smooth blanket of dark material. This is thought to result from the sublimation-driven degradation of small landforms, which is supported by the general deficit of small impact craters and the presence of numerous small knobs, considered to be their remnants. The absolute ages of the landforms are not known.
Callisto is composed of approximately equal amounts of rock and ice, with a density of about , the lowest density and surface gravity of Jupiter's major moons. Compounds detected spectroscopically on the surface include water ice, carbon dioxide, silicates and organic compounds. Investigation by the Galileo spacecraft revealed that Callisto may have a small silicate core and possibly a subsurface ocean of liquid water at depths greater than . | Callisto (moon) | Wikipedia | 422 | 43126 | https://en.wikipedia.org/wiki/Callisto%20%28moon%29 | Physical sciences | Solar System | null |
It is not in an orbital resonance like the three other Galilean satellites—Io, Europa and Ganymede—and is thus not appreciably tidally heated. Callisto's rotation is tidally locked to its orbit around Jupiter, so that it always faces the same direction, making Jupiter appear to hang directly overhead over its near-side. It is less affected by Jupiter's magnetosphere than the other inner satellites because of its more remote orbit, located just outside Jupiter's main radiation belt. Callisto is surrounded by an extremely thin atmosphere composed of carbon dioxide and probably molecular oxygen, as well as by a rather intense ionosphere. Callisto is thought to have formed by slow accretion from the disk of the gas and dust that surrounded Jupiter after its formation. Callisto's gradual accretion and the lack of tidal heating meant that not enough heat was available for rapid differentiation. The slow convection in the interior of Callisto, which commenced soon after formation, led to partial differentiation and possibly to the formation of a subsurface ocean at a depth of 100–150 km and a small, rocky core.
The likely presence of an ocean within Callisto leaves open the possibility that it could harbor life. However, conditions are thought to be less favorable than on nearby Europa. Various space probes from Pioneers 10 and 11 to Galileo and Cassini have studied Callisto. Because of its low radiation levels, Callisto has long been considered the most suitable to base possible future crewed missions on to study the Jovian system.
History
Discovery
Callisto was discovered independently by Simon Marius and Galileo Galilei in 1610, along with the three other large Jovian moons—Ganymede, Io and Europa.
Name
Callisto, like all of Jupiter's moons, is named after one of Zeus's many lovers or other sexual partners in Greek mythology. Callisto was a nymph (or, according to some sources, the daughter of Lycaon) who was associated with the goddess of the hunt, Artemis. The name was suggested by Simon Marius soon after Callisto's discovery. Marius attributed the suggestion to Johannes Kepler.
However, the names of the Galilean satellites fell into disfavor for a considerable time, and were not revived in common use until the mid-20th century. In much of the earlier astronomical literature, Callisto is referred to by its Roman numeral designation, a system introduced by Galileo, as or as "the fourth satellite of Jupiter". | Callisto (moon) | Wikipedia | 510 | 43126 | https://en.wikipedia.org/wiki/Callisto%20%28moon%29 | Physical sciences | Solar System | null |
There's no established English adjectival form of the name. The adjectival form of Greek Καλλιστῴ Kallistōi is Καλλιστῴος Kallistōi-os, from which one might expect Latin Callistōius and English *Callistóian (with 5 syllables), parallel to Sapphóian (4 syllables) for Sapphōi and Letóian for Lētōi. However, the iota subscript is often omitted from such Greek names (cf. Inóan from Īnōi and Argóan from Argōi), and indeed the analogous form Callistoan is found.
In Virgil, a second oblique stem appears in Latin: Callistōn-, but the corresponding Callistonian has rarely appeared in English. One also sees ad hoc forms, such as Callistan, Callistian and Callistean.
Orbit and rotation
Callisto is the outermost of the four Galilean moons of Jupiter. It orbits at a distance of approximately 1,880,000 km (26.3 times the 71,492 km radius of Jupiter itself). This is significantly larger than the orbital radius—1,070,000 km—of the next-closest Galilean satellite, Ganymede. As a result of this relatively distant orbit, Callisto does not participate in mean-motion resonance—in which the three inner Galilean satellites are locked—and probably never has. Callisto is expected to be captured into the resonance in about 1.5 billion years, completing the 1:2:4:8 chain.
Like most other regular planetary moons, Callisto's rotation is locked to be synchronous with its orbit. The length of Callisto's day, simultaneously its orbital period, is about 16.7 Earth days. Its orbit is very slightly eccentric and inclined to the Jovian equator, with the eccentricity and inclination changing quasi-periodically due to solar and planetary gravitational perturbations on a timescale of centuries. The ranges of change are 0.0072–0.0076 and 0.20–0.60°, respectively. These orbital variations cause the axial tilt (the angle between the rotational and orbital axes) to vary between 0.4 and 1.6°. | Callisto (moon) | Wikipedia | 477 | 43126 | https://en.wikipedia.org/wiki/Callisto%20%28moon%29 | Physical sciences | Solar System | null |
The dynamical isolation of Callisto means that it has never been appreciably tidally heated, which has important consequences for its internal structure and evolution. Its distance from Jupiter also means that the charged-particle flux from Jupiter's magnetosphere at its surface is relatively low—about 300 times lower than, for example, that at Europa. Hence, unlike the other Galilean moons, charged-particle irradiation has had a relatively minor effect on Callisto's surface. The radiation level at Callisto's surface is equivalent to a dose of about 0.01 rem (0.1 mSv) per day, which is just over ten times higher than Earth's average background radiation, but less than in Low Earth Orbit or on Mars.
Physical characteristics
Composition
The average density of Callisto, 1.83 g/cm3, suggests a composition of approximately equal parts of rocky material and water ice, with some additional volatile ices such as ammonia. The mass fraction of ices is 49–55%. The exact composition of Callisto's rock component is not known, but is probably close to the composition of L/LL type ordinary chondrites, which are characterized by less total iron, less metallic iron and more iron oxide than H chondrites. The weight ratio of iron to silicon is 0.9–1.3 in Callisto, whereas the solar ratio is around 1:8.
Callisto's surface has an albedo of about 20%. Its surface composition is thought to be broadly similar to its composition as a whole. Near-infrared spectroscopy has revealed the presence of water ice absorption bands at wavelengths of 1.04, 1.25, 1.5, 2.0 and 3.0 micrometers. Water ice seems to be ubiquitous on the surface of Callisto, with a mass fraction of 25–50%. The analysis of high-resolution, near-infrared and UV spectra obtained by the Galileo spacecraft and from the ground has revealed various non-ice materials: magnesium- and iron-bearing hydrated silicates, carbon dioxide, sulfur dioxide, and possibly ammonia and various organic compounds. Spectral data indicate that Callisto's surface is extremely heterogeneous at the small scale. Small, bright patches of pure water ice are intermixed with patches of a rock–ice mixture and extended dark areas made of a non-ice material. | Callisto (moon) | Wikipedia | 491 | 43126 | https://en.wikipedia.org/wiki/Callisto%20%28moon%29 | Physical sciences | Solar System | null |
The Callistoan surface is asymmetric: the leading hemisphere is darker than the trailing one. This is different from other Galilean satellites, where the reverse is true. The trailing hemisphere of Callisto appears to be enriched in carbon dioxide, whereas the leading hemisphere has more sulfur dioxide. Many fresh impact craters like Lofn also show enrichment in carbon dioxide. Overall, the chemical composition of the surface, especially in the dark areas, may be close to that seen on D-type asteroids, whose surfaces are made of carbonaceous material.
Internal structure
Callisto's battered surface lies on top of a cold, stiff and icy lithosphere that is between 80 and 150 km thick. A salty ocean 150–200 km deep may lie beneath the crust, indicated by studies of the magnetic fields around Jupiter and its moons. It was found that Callisto responds to Jupiter's varying background magnetic field like a perfectly conducting sphere; that is, the field cannot penetrate inside Callisto, suggesting a layer of highly conductive fluid within it with a thickness of at least 10 km. The existence of an ocean is more likely if water contains a small amount of ammonia or other antifreeze, up to 5% by weight. In this case the water+ice layer can be as thick as 250–300 km. Failing an ocean, the icy lithosphere may be somewhat thicker, up to about 300 km.
Beneath the lithosphere and putative ocean, Callisto's interior appears to be neither entirely uniform nor particularly variable. Galileo orbiter data (especially the dimensionless moment of inertia—0.3549 ± 0.0042—determined during close flybys) suggest that, if Callisto is in hydrostatic equilibrium, its interior is composed of compressed rocks and ices, with the amount of rock increasing with depth due to partial settling of its constituents. In other words, Callisto may be only partially differentiated. The density and moment of inertia for an equilibrium Callisto are compatible with the existence of a small silicate core in the center of Callisto. The radius of any such core cannot exceed 600 km, and the density may lie between 3.1 and 3.6 g/cm3. In this case, Callisto's interior would be in stark contrast to that of Ganymede, which appears to be fully differentiated. | Callisto (moon) | Wikipedia | 485 | 43126 | https://en.wikipedia.org/wiki/Callisto%20%28moon%29 | Physical sciences | Solar System | null |
However, a 2011 reanalysis of Galileo data suggests that Callisto is not in hydrostatic equilibrium. In that case, the gravity data may be more consistent with a more thoroughly differentiated Callisto with a hydrated silicate core.
Surface features
The ancient surface of Callisto is one of the most heavily cratered in the Solar System. In fact, the crater density is close to saturation: any new crater will tend to erase an older one. The large-scale geology is relatively simple; on Callisto there are no large mountains, volcanoes or other endogenic tectonic features. The impact craters and multi-ring structures—together with associated fractures, scarps and deposits—are the only large features to be found on the surface.
Callisto's surface can be divided into several geologically different parts: cratered plains, light plains, bright and dark smooth plains, and various units associated with particular multi-ring structures and impact craters. The cratered plains make up most of the surface area and represent the ancient lithosphere, a mixture of ice and rocky material. The light plains include bright impact craters like Burr and Lofn, as well as the effaced remnants of old large craters called palimpsests, the central parts of multi-ring structures, and isolated patches in the cratered plains. These light plains are thought to be icy impact deposits. The bright, smooth plains make up a small fraction of Callisto's surface and are found in the ridge and trough zones of the Valhalla and Asgard formations and as isolated spots in the cratered plains. They were thought to be connected with endogenic activity, but the high-resolution Galileo images showed that the bright, smooth plains correlate with heavily fractured and knobby terrain and do not show any signs of resurfacing. The Galileo images also revealed small, dark, smooth areas with overall coverage less than 10,000 km2, which appear to embay the surrounding terrain. They are possible cryovolcanic deposits. Both the light and the various smooth plains are somewhat younger and less cratered than the background cratered plains. | Callisto (moon) | Wikipedia | 435 | 43126 | https://en.wikipedia.org/wiki/Callisto%20%28moon%29 | Physical sciences | Solar System | null |
Impact crater diameters seen range from 0.1 km—a limit defined by the imaging resolution—to over 100 km, not counting the multi-ring structures. Small craters, with diameters less than 5 km, have simple bowl or flat-floored shapes. Those 5–40 km across usually have a central peak. Larger impact features, with diameters in the range 25–100 km, have central pits instead of peaks, such as Tindr crater. The largest craters with diameters over 60 km can have central domes, which are thought to result from central tectonic uplift after an impact; examples include Doh and Hár craters. A small number of very large—more than 100 km in diameter—and bright impact craters show anomalous dome geometry. These are unusually shallow and may be a transitional landform to the multi-ring structures, as with the Lofn impact feature. Callisto's craters are generally shallower than those on the Moon.
The largest impact features on Callisto's surface are multi-ring basins. Two are enormous. Valhalla is the largest, with a bright central region 600 km in diameter, and rings extending as far as 1,800 km from the center (see figure). The second largest is Asgard, measuring about 1,600 km in diameter. Multi-ring structures probably originated as a result of a post-impact concentric fracturing of the lithosphere lying on a layer of soft or liquid material, possibly an ocean. The catenae—for example Gomul Catena—are long chains of impact craters lined up in straight lines across the surface. They were probably created by objects that were tidally disrupted as they passed close to Jupiter prior to the impact on Callisto, or by very oblique impacts. A historical example of a disruption was Comet Shoemaker–Levy 9.
As mentioned above, small patches of pure water ice with an albedo as high as 80% are found on the surface of Callisto, surrounded by much darker material. High-resolution Galileo images showed the bright patches to be predominately located on elevated surface features: crater rims, scarps, ridges and knobs. They are likely to be thin water frost deposits. Dark material usually lies in the lowlands surrounding and mantling bright features and appears to be smooth. It often forms patches up to 5 km across within the crater floors and in the intercrater depressions. | Callisto (moon) | Wikipedia | 500 | 43126 | https://en.wikipedia.org/wiki/Callisto%20%28moon%29 | Physical sciences | Solar System | null |
On a sub-kilometer scale the surface of Callisto is more degraded than the surfaces of other icy Galilean moons. Typically there is a deficit of small impact craters with diameters less than 1 km as compared with, for instance, the dark plains on Ganymede. Instead of small craters, the almost ubiquitous surface features are small knobs and pits. The knobs are thought to represent remnants of crater rims degraded by an as-yet uncertain process. The most likely candidate process is the slow sublimation of ice, which is enabled by a temperature of up to 165 K, reached at a subsolar point. Such sublimation of water or other volatiles from the dirty ice that is the bedrock causes its decomposition. The non-ice remnants form debris avalanches descending from the slopes of the crater walls. Such avalanches are often observed near and inside impact craters and termed "debris aprons". Sometimes crater walls are cut by sinuous valley-like incisions called "gullies", which resemble certain Martian surface features. In the ice sublimation hypothesis, the low-lying dark material is interpreted as a blanket of primarily non-ice debris, which originated from the degraded rims of craters and has covered a predominantly icy bedrock.
The relative ages of the different surface units on Callisto can be determined from the density of impact craters on them. The older the surface, the denser the crater population. Absolute dating has not been carried out, but based on theoretical considerations, the cratered plains are thought to be ~4.5 billion years old, dating back almost to the formation of the Solar System. The ages of multi-ring structures and impact craters depend on chosen background cratering rates and are estimated by different authors to vary between 1 and 4 billion years.
Atmosphere and ionosphere
Callisto has a very tenuous atmosphere composed of carbon dioxide and probably oxygen. It was detected by the Galileo Near Infrared Mapping Spectrometer (NIMS) from its absorption feature near the wavelength 4.2 micrometers. The surface pressure is estimated to be 7.5 picobar (0.75 μPa) and particle density 4 cm−3. Because such a thin atmosphere would be lost in only about four years (see atmospheric escape), it must be constantly replenished, possibly by slow sublimation of carbon dioxide ice from Callisto's icy crust, which would be compatible with the sublimation–degradation hypothesis for the formation of the surface knobs. | Callisto (moon) | Wikipedia | 510 | 43126 | https://en.wikipedia.org/wiki/Callisto%20%28moon%29 | Physical sciences | Solar System | null |
Callisto's ionosphere was first detected during Galileo flybys; its high electron density of 7–17 cm−3 cannot be explained by the photoionization of the atmospheric carbon dioxide alone. Hence, it is suspected that the atmosphere of Callisto is actually dominated by molecular oxygen (in amounts 10–100 times greater than ). However, oxygen has not yet been directly detected in the atmosphere of Callisto. Observations with the Hubble Space Telescope (HST) placed an upper limit on its possible concentration in the atmosphere, based on lack of detection, which is still compatible with the ionospheric measurements. At the same time, HST was able to detect condensed oxygen trapped on the surface of Callisto.
Atomic hydrogen has also been detected in Callisto's atmosphere via recent analysis of 2001 Hubble Space Telescope data. Spectral images taken on 15 and 24 December 2001 were re-examined, revealing a faint signal of scattered light that indicates a hydrogen corona. The observed brightness from the scattered sunlight in Callisto's hydrogen corona is approximately two times larger when the leading hemisphere is observed. This asymmetry may originate from a different hydrogen abundance in both the leading and trailing hemispheres. However, this hemispheric difference in Callisto's hydrogen corona brightness is likely to originate from the extinction of the signal in Earth's geocorona, which is greater when the trailing hemisphere is observed.
Origin and evolution
The partial differentiation of Callisto (inferred e.g. from moment of inertia measurements) means that it has never been heated enough to melt its ice component. Therefore, the most favorable model of its formation is a slow accretion in the low-density Jovian subnebula—a disk of the gas and dust that existed around Jupiter after its formation. Such a prolonged accretion stage would allow cooling to largely keep up with the heat accumulation caused by impacts, radioactive decay and contraction, thereby preventing melting and fast differentiation. The allowable timescale for the formation of Callisto lies then in the range 0.1 million–10 million years. | Callisto (moon) | Wikipedia | 427 | 43126 | https://en.wikipedia.org/wiki/Callisto%20%28moon%29 | Physical sciences | Solar System | null |
The further evolution of Callisto after accretion was determined by the balance of the radioactive heating, cooling through thermal conduction near the surface, and solid state or subsolidus convection in the interior. Details of the subsolidus convection in the ice is the main source of uncertainty in the models of all icy moons. It is known to develop when the temperature is sufficiently close to the melting point, due to the temperature dependence of ice viscosity. Subsolidus convection in icy bodies is a slow process with ice motions of the order of 1 centimeter per year, but is, in fact, a very effective cooling mechanism on long timescales. It is thought to proceed in the so-called stagnant lid regime, where a stiff, cold outer layer of Callisto conducts heat without convection, whereas the ice beneath it convects in the subsolidus regime. For Callisto, the outer conductive layer corresponds to the cold and rigid lithosphere with a thickness of about 100 km. Its presence would explain the lack of any signs of the endogenic activity on the Callistoan surface. The convection in the interior parts of Callisto may be layered, because under the high pressures found there, water ice exists in different crystalline phases beginning from the ice I on the surface to ice VII in the center. The early onset of subsolidus convection in the Callistoan interior could have prevented large-scale ice melting and any resulting differentiation that would have otherwise formed a large rocky core and icy mantle. Due to the convection process, however, very slow and partial separation and differentiation of rocks and ices inside Callisto has been proceeding on timescales of billions of years and may be continuing to this day.
The current understanding of the evolution of Callisto allows for the existence of a layer or "ocean" of liquid water in its interior. This is connected with the anomalous behavior of ice I phase's melting temperature, which decreases with pressure, achieving temperatures as low as 251 K at 2,070 bar (207 MPa). In all realistic models of Callisto the temperature in the layer between 100 and 200 km in depth is very close to, or exceeds slightly, this anomalous melting temperature. The presence of even small amounts of ammonia—about 1–2% by weight—almost guarantees the liquid's existence because ammonia would lower the melting temperature even further. | Callisto (moon) | Wikipedia | 494 | 43126 | https://en.wikipedia.org/wiki/Callisto%20%28moon%29 | Physical sciences | Solar System | null |
Although Callisto is very similar in bulk properties to Ganymede, it apparently had a much simpler geological history. The surface appears to have been shaped mainly by impacts and other exogenic forces. Unlike neighboring Ganymede with its grooved terrain, there is little evidence of tectonic activity. Explanations that have been proposed for the contrasts in internal heating and consequent differentiation and geologic activity between Callisto and Ganymede include differences in formation conditions, the greater tidal heating experienced by Ganymede, and the more numerous and energetic impacts that would have been suffered by Ganymede during the Late Heavy Bombardment. The relatively simple geological history of Callisto provides planetary scientists with a reference point for comparison with other more active and complex worlds.
Habitability
It is speculated that there could be life in Callisto's subsurface ocean. Like Europa and Ganymede, as well as Saturn's moons Enceladus, Dione and Titan and Neptune's moon Triton, a possible subsurface ocean might be composed of salt water.
It is possible that halophiles could thrive in the ocean.
As with Europa and Ganymede, the idea has been raised that habitable conditions and even extraterrestrial microbial life may exist in the salty ocean under the Callistoan surface. However, the environmental conditions necessary for life appear to be less favorable on Callisto than on Europa. The principal reasons are the lack of contact with rocky material and the lower heat flux from the interior of Callisto. Callisto's ocean is heated only by radioactive decay, while Europa's is also heated by tidal energy, as it is much closer to Jupiter. It is thought that of all of Jupiter's moons, Europa has the greatest chance of supporting microbial life.
Exploration | Callisto (moon) | Wikipedia | 363 | 43126 | https://en.wikipedia.org/wiki/Callisto%20%28moon%29 | Physical sciences | Solar System | null |
Past
The Pioneer 10 and Pioneer 11 Jupiter encounters in the early 1970s contributed little new information about Callisto in comparison with what was already known from Earth-based observations. The real breakthrough happened later with the Voyager 1 and Voyager 2 flybys in 1979. They imaged more than half of the Callistoan surface with a resolution of 1–2 km, and precisely measured its temperature, mass and shape. A second round of exploration lasted from 1994 to 2003, when the Galileo spacecraft had eight close encounters with Callisto, the last flyby during the C30 orbit in 2001 came as close as 138 km to the surface. The Galileo orbiter completed the global imaging of the surface and delivered a number of pictures with a resolution as high as 15 meters of selected areas of Callisto. In 2000, the Cassini spacecraft en route to Saturn acquired high-quality infrared spectra of the Galilean satellites including Callisto. In February–March 2007, the New Horizons probe on its way to Pluto obtained new images and spectra of Callisto.
Future exploration
Callisto will be visited by three spacecraft in the near future.
The European Space Agency's Jupiter Icy Moons Explorer (JUICE), which launched on 14 April 2023, will perform 21 close flybys of Callisto between 2031 and 2034.
NASA's Europa Clipper, which launched on 14 October 2024, will conduct nine close flybys of Callisto beginning in 2030.
China's CNSA Tianwen-4 is planned to launch to Jupiter around 2030 before entering orbit around Callisto.
Old proposals
Formerly proposed for a launch in 2020, the Europa Jupiter System Mission (EJSM) was a joint NASA/ESA proposal for exploration of Jupiter's moons. In February 2009 it was announced that ESA/NASA had given this mission priority ahead of the Titan Saturn System Mission. At the time ESA's contribution still faced funding competition from other ESA projects. EJSM consisted of the NASA-led Jupiter Europa Orbiter, the ESA-led Jupiter Ganymede Orbiter and possibly a JAXA-led Jupiter Magnetospheric Orbiter.
Potential crewed exploration and habitation
In 2003 NASA conducted a conceptual study called Human Outer Planets Exploration (HOPE) regarding the future human exploration of the outer Solar System. The target chosen to consider in detail was Callisto. | Callisto (moon) | Wikipedia | 475 | 43126 | https://en.wikipedia.org/wiki/Callisto%20%28moon%29 | Physical sciences | Solar System | null |
The study proposed a possible surface base on Callisto that would produce rocket propellant for further exploration of the Solar System. Advantages of a base on Callisto include low radiation (due to its distance from Jupiter) and geological stability. Such a base could facilitate remote exploration of Europa, or be an ideal location for a Jovian system waystation servicing spacecraft heading farther into the outer Solar System, using a gravity assist from a close flyby of Jupiter after departing Callisto.
In December 2003, NASA reported that a crewed mission to Callisto might be possible in the 2040s. | Callisto (moon) | Wikipedia | 116 | 43126 | https://en.wikipedia.org/wiki/Callisto%20%28moon%29 | Physical sciences | Solar System | null |
Europa , or Jupiter II, is the smallest of the four Galilean moons orbiting Jupiter, and the sixth-closest to the planet of all the 95 known moons of Jupiter. It is also the sixth-largest moon in the Solar System. Europa was discovered independently by Simon Marius and Galileo Galilei and was named (by Marius) after Europa, the Phoenician mother of King Minos of Crete and lover of Zeus (the Greek equivalent of the Roman god Jupiter).
Slightly smaller than Earth's Moon, Europa is made of silicate rock and has a water-ice crust and probably an iron–nickel core. It has a very thin atmosphere, composed primarily of oxygen. Its geologically young white-beige surface is striated by light tan cracks and streaks, with very few impact craters. In addition to Earth-bound telescope observations, Europa has been examined by a succession of space-probe flybys, the first occurring in the early 1970s. In September 2022, the Juno spacecraft flew within about 320 km (200 miles) of Europa for a more recent close-up view.
Europa has the smoothest surface of any known solid object in the Solar System. The apparent youth and smoothness of the surface is due to a water ocean beneath the surface, which could conceivably harbor extraterrestrial life, although such life would most likely be that of single celled organisms and bacteria-like creatures. The predominant model suggests that heat from tidal flexing causes the ocean to remain liquid and drives ice movement similar to plate tectonics, absorbing chemicals from the surface into the ocean below. Sea salt from a subsurface ocean may be coating some geological features on Europa, suggesting that the ocean is interacting with the sea floor. This may be important in determining whether Europa could be habitable. In addition, the Hubble Space Telescope detected water vapor plumes similar to those observed on Saturn's moon Enceladus, which are thought to be caused by erupting cryogeysers. In May 2018, astronomers provided supporting evidence of water plume activity on Europa, based on an updated analysis of data obtained from the Galileo space probe, which orbited Jupiter from 1995 to 2003. Such plume activity could help researchers in a search for life from the subsurface Europan ocean without having to land on the moon. In March 2024, astronomers reported that the surface of Europa may have much less oxygen than previously inferred. | Europa (moon) | Wikipedia | 500 | 43127 | https://en.wikipedia.org/wiki/Europa%20%28moon%29 | Physical sciences | Solar System | null |
The Galileo mission, launched in 1989, provides the bulk of current data on Europa. No spacecraft has yet landed on Europa, although there have been several proposed exploration missions. The European Space Agency's Jupiter Icy Moon Explorer (JUICE) is a mission to Ganymede launched on 14 April 2023, that will include two flybys of Europa. NASA's Europa Clipper was launched on 14 October 2024.
Discovery and naming
Europa, along with Jupiter's three other large moons, Io, Ganymede, and Callisto, was discovered by Galileo Galilei on 8 January 1610, and possibly independently by Simon Marius. On 7 January, Galileo had observed Io and Europa together using a 20×-magnification refracting telescope at the University of Padua, but the low resolution could not separate the two objects. The following night, he saw Io and Europa for the first time as separate bodies.
The moon is the namesake of Europa, in Greek mythology the daughter of the Phoenician king of Tyre. Like all the Galilean satellites, Europa is named after a lover of Zeus, the Greek counterpart of Jupiter. Europa was courted by Zeus and became the queen of Crete. The naming scheme was suggested by Simon Marius, who attributed the proposal to Johannes Kepler:
The names fell out of favor for a considerable time and were not revived in general use until the mid-20th century. In much of the earlier astronomical literature, Europa is simply referred to by its Roman numeral designation as (a system also introduced by Galileo) or as the "second satellite of Jupiter". In 1892, the discovery of Amalthea, whose orbit lay closer to Jupiter than those of the Galilean moons, pushed Europa to the third position. The Voyager probes discovered three more inner satellites in 1979, so Europa is now counted as Jupiter's sixth satellite, though it is still referred to as .
The adjectival form has stabilized as Europan.
Orbit and rotation | Europa (moon) | Wikipedia | 403 | 43127 | https://en.wikipedia.org/wiki/Europa%20%28moon%29 | Physical sciences | Solar System | null |
Europa orbits Jupiter in just over three and a half days, with an orbital radius of about 670,900 km. With an orbital eccentricity of only 0.009, the orbit itself is nearly circular, and the orbital inclination relative to Jupiter's equatorial plane is small, at 0.470°. Like its fellow Galilean satellites, Europa is tidally locked to Jupiter, with one hemisphere of Europa constantly facing Jupiter. Because of this, there is a sub-Jovian point on Europa's surface, from which Jupiter would appear to hang directly overhead. Europa's prime meridian is a line passing through this point. Research suggests that tidal locking may not be full, as a non-synchronous rotation has been proposed: Europa spins faster than it orbits, or at least did so in the past. This suggests an asymmetry in internal mass distribution and that a layer of subsurface liquid separates the icy crust from the rocky interior.
The slight eccentricity of Europa's orbit, maintained by gravitational disturbances from the other Galileans, causes Europa's sub-Jovian point to oscillate around a mean position. As Europa comes slightly nearer to Jupiter, Jupiter's gravitational attraction increases, causing Europa to elongate towards and away from it. As Europa moves slightly away from Jupiter, Jupiter's gravitational force decreases, causing Europa to relax back into a more spherical shape, and creating tides in its ocean. The orbital eccentricity of Europa is continuously pumped by its mean-motion resonance with Io. Thus, the tidal flexing kneads Europa's interior and gives it a source of heat, possibly allowing its ocean to stay liquid while driving subsurface geological processes. The ultimate source of this energy is Jupiter's rotation, which is tapped by Io through the tides it raises on Jupiter and is transferred to Europa and Ganymede by the orbital resonance. | Europa (moon) | Wikipedia | 387 | 43127 | https://en.wikipedia.org/wiki/Europa%20%28moon%29 | Physical sciences | Solar System | null |
Analysis of the unique cracks lining Europa yielded evidence that it likely spun around a tilted axis at some point in time. If correct, this would explain many of Europa's features. Europa's immense network of crisscrossing cracks serves as a record of the stresses caused by massive tides in its global ocean. Europa's tilt could influence calculations of how much of its history is recorded in its frozen shell, how much heat is generated by tides in its ocean, and even how long the ocean has been liquid. Its ice layer must stretch to accommodate these changes. When there is too much stress, it cracks. A tilt in Europa's axis could suggest that its cracks may be much more recent than previously thought. The reason for this is that the direction of the spin pole may change by as much as a few degrees per day, completing one precession period over several months. A tilt could also affect estimates of the age of Europa's ocean. Tidal forces are thought to generate the heat that keeps Europa's ocean liquid, and a tilt in the spin axis would cause more heat to be generated by tidal forces. Such additional heat would have allowed the ocean to remain liquid for a longer time. However, it has not yet been determined when this hypothesized shift in the spin axis might have occurred.
Physical characteristics
Europa is slightly smaller than the Earth's moon. At just over in diameter, it is the sixth-largest moon and fifteenth-largest object in the Solar System. Though by a wide margin the least massive of the Galilean satellites, it is nonetheless more massive than all known moons in the Solar System smaller than itself combined. Its bulk density suggests that it is similar in composition to terrestrial planets, being primarily composed of silicate rock.
Internal structure
It is estimated that Europa has an outer layer of water around thick – a part frozen as its crust and a part as a liquid ocean underneath the ice. Recent magnetic-field data from the Galileo orbiter showed that Europa has an induced magnetic field through interaction with Jupiter's, which suggests the presence of a subsurface conductive layer. This layer is likely to be a salty liquid-water ocean. Portions of the crust are estimated to have undergone a rotation of nearly 80°, nearly flipping over (see true polar wander), which would be unlikely if the ice were solidly attached to the mantle. Europa probably contains a metallic iron core.
Surface features | Europa (moon) | Wikipedia | 495 | 43127 | https://en.wikipedia.org/wiki/Europa%20%28moon%29 | Physical sciences | Solar System | null |
Europa is the smoothest known object in the Solar System, lacking large-scale features such as mountains and craters. The prominent markings crisscrossing Europa appear to be mainly albedo features that emphasize low topography. There are few craters on Europa, because its surface is tectonically too active and therefore young. Its icy crust has an albedo (light reflectivity) of 0.64, one of the highest of any moon. This indicates a young and active surface: based on estimates of the frequency of cometary bombardment that Europa experiences, the surface is about 20 to 180 million years old. There is no scientific consensus about the explanation for Europa's surface features.
It has been postulated Europa's equator may be covered in icy spikes called penitentes, which may be up to 15 meters high. Their formation is due to direct overhead sunlight near the equator causing the ice to sublime, forming vertical cracks. Although the imaging available from the Galileo orbiter does not have the resolution for confirmation, radar and thermal data are consistent with this speculation.
The ionizing radiation level at Europa's surface is equivalent to a daily dose of about 5.4 Sv (540 rem), an amount that would cause severe illness or death in human beings exposed for a single Earth day (24 hours). A Europan day is about 3.5 times as long as an Earth day.
Lineae
Europa's most striking surface features are a series of dark streaks crisscrossing the entire globe, called lineae (). Close examination shows that the edges of Europa's crust on either side of the cracks have moved relative to each other. The larger bands are more than across, often with dark, diffuse outer edges, regular striations, and a central band of lighter material. | Europa (moon) | Wikipedia | 363 | 43127 | https://en.wikipedia.org/wiki/Europa%20%28moon%29 | Physical sciences | Solar System | null |
The most likely hypothesis is that the lineae on Europa were produced by a series of eruptions of warm ice as Europa's crust slowly spreads open to expose warmer layers beneath. The effect would have been similar to that seen on Earth's oceanic ridges. These various fractures are thought to have been caused in large part by the tidal flexing exerted by Jupiter. Because Europa is tidally locked to Jupiter, and therefore always maintains approximately the same orientation towards Jupiter, the stress patterns should form a distinctive and predictable pattern. However, only the youngest of Europa's fractures conform to the predicted pattern; other fractures appear to occur at increasingly different orientations the older they are. This could be explained if Europa's surface rotates slightly faster than its interior, an effect that is possible due to the subsurface ocean mechanically decoupling Europa's surface from its rocky mantle and the effects of Jupiter's gravity tugging on Europa's outer ice crust. Comparisons of Voyager and Galileo spacecraft photos serve to put an upper limit on this hypothetical slippage. A full revolution of the outer rigid shell relative to the interior of Europa takes at least 12,000 years. Studies of Voyager and Galileo images have revealed evidence of subduction on Europa's surface, suggesting that, just as the cracks are analogous to ocean ridges, so plates of icy crust analogous to tectonic plates on Earth are recycled into the molten interior. This evidence of both crustal spreading at bands and convergence at other sites suggests that Europa may have active plate tectonics, similar to Earth. However, the physics driving these plate tectonics are not likely to resemble those driving terrestrial plate tectonics, as the forces resisting potential Earth-like plate motions in Europa's crust are significantly stronger than the forces that could drive them.
Chaos and lenticulae
Other features present on Europa are circular and elliptical (Latin for "freckles"). Many are domes, some are pits and some are smooth, dark spots. Others have a jumbled or rough texture. The dome tops look like pieces of the older plains around them, suggesting that the domes formed when the plains were pushed up from below. | Europa (moon) | Wikipedia | 439 | 43127 | https://en.wikipedia.org/wiki/Europa%20%28moon%29 | Physical sciences | Solar System | null |
One hypothesis states that these lenticulae were formed by diapirs of warm ice rising up through the colder ice of the outer crust, much like magma chambers in Earth's crust. The smooth, dark spots could be formed by meltwater released when the warm ice breaks through the surface. The rough, jumbled lenticulae (called regions of "chaos"; for example, Conamara Chaos) would then be formed from many small fragments of crust, embedded in hummocky, dark material, appearing like icebergs in a frozen sea.
An alternative hypothesis suggests that lenticulae are actually small areas of chaos and that the claimed pits, spots and domes are artefacts resulting from the over-interpretation of early, low-resolution Galileo images. The implication is that the ice is too thin to support the convective diapir model of feature formation.
In November 2011, a team of researchers, including researchers at University of Texas at Austin, presented evidence suggesting that many "chaos terrain" features on Europa sit atop vast lakes of liquid water. These lakes would be entirely encased in Europa's icy outer shell and distinct from a liquid ocean thought to exist farther down beneath the ice shell. Full confirmation of the lakes' existence will require a space mission designed to probe the ice shell either physically or indirectly, e.g. using radar. Chaos features may also be a result of increased melting of the ice shell and deposition of marine ice at low latitudes as a result of heterogeneous heating.
Work published by researchers from Williams College suggests that chaos terrain may represent sites where impacting comets penetrated through the ice crust and into an underlying ocean.
Subsurface ocean | Europa (moon) | Wikipedia | 343 | 43127 | https://en.wikipedia.org/wiki/Europa%20%28moon%29 | Physical sciences | Solar System | null |
The scientific consensus is that a layer of liquid water exists beneath Europa's surface, and that heat from tidal flexing allows the subsurface ocean to remain liquid. Europa's surface temperature averages about at the equator and only at the poles, keeping Europa's icy crust as hard as granite. The first hints of a subsurface ocean came from theoretical considerations of tidal heating (a consequence of Europa's slightly eccentric orbit and orbital resonance with the other Galilean moons). Galileo imaging team members argue for the existence of a subsurface ocean from analysis of Voyager and Galileo images. The most dramatic example is "chaos terrain", a common feature on Europa's surface that some interpret as a region where the subsurface ocean has melted through the icy crust. This interpretation is controversial. Most geologists who have studied Europa favor what is commonly called the "thick ice" model, in which the ocean has rarely, if ever, directly interacted with the present surface. The best evidence for the thick-ice model is a study of Europa's large craters. The largest impact structures are surrounded by concentric rings and appear to be filled with relatively flat, fresh ice; based on this and on the calculated amount of heat generated by Europan tides, it is estimated that the outer crust of solid ice is approximately thick, including a ductile "warm ice" layer, which could mean that the liquid ocean underneath may be about deep. This leads to a volume of Europa's oceans of 3×1018m3, between two or three times the volume of Earth's oceans.
The thin-ice model suggests that Europa's ice shell may be only a few kilometers thick. However, most planetary scientists conclude that this model considers only those topmost layers of Europa's crust that behave elastically when affected by Jupiter's tides. One example is flexure analysis, in which Europa's crust is modeled as a plane or sphere weighted and flexed by a heavy load. Models such as this suggest the outer elastic portion of the ice crust could be as thin as . If the ice shell of Europa is really only a few kilometers thick, this "thin ice" model would mean that regular contact of the liquid interior with the surface could occur through open ridges, causing the formation of areas of chaotic terrain. Large impacts going fully through the ice crust would also be a way that the subsurface ocean could be exposed.
Composition | Europa (moon) | Wikipedia | 495 | 43127 | https://en.wikipedia.org/wiki/Europa%20%28moon%29 | Physical sciences | Solar System | null |
The Galileo orbiter found that Europa has a weak magnetic moment, which is induced by the varying part of the Jovian magnetic field. The field strength at the magnetic equator (about 120 nT) created by this magnetic moment is about one-sixth the strength of Ganymede's field and six times the value of Callisto's. The existence of the induced moment requires a layer of a highly electrically conductive material in Europa's interior. The most plausible candidate for this role is a large subsurface ocean of liquid saltwater.
Since the Voyager spacecraft flew past Europa in 1979, scientists have worked to understand the composition of the reddish-brown material that coats fractures and other geologically youthful features on Europa's surface. Spectrographic evidence suggests that the darker, reddish streaks and features on Europa's surface may be rich in salts such as magnesium sulfate, deposited by evaporating water that emerged from within. Sulfuric acid hydrate is another possible explanation for the contaminant observed spectroscopically. In either case, because these materials are colorless or white when pure, some other material must also be present to account for the reddish color, and sulfur compounds are suspected.
Another hypothesis for the colored regions is that they are composed of abiotic organic compounds collectively called tholins. The morphology of Europa's impact craters and ridges is suggestive of fluidized material welling up from the fractures where pyrolysis and radiolysis take place. In order to generate colored tholins on Europa, there must be a source of materials (carbon, nitrogen, and water) and a source of energy to make the reactions occur. Impurities in the water ice crust of Europa are presumed both to emerge from the interior as cryovolcanic events that resurface the body, and to accumulate from space as interplanetary dust. Tholins bring important astrobiological implications, as they may play a role in prebiotic chemistry and abiogenesis.
The presence of sodium chloride in the internal ocean has been suggested by a 450 nm absorption feature, characteristic of irradiated NaCl crystals, that has been spotted in HST observations of the chaos regions, presumed to be areas of recent subsurface upwelling. The subterranean ocean of Europa contains carbon and was observed on the surface ice as a concentration of carbon dioxide within Tara Regio, a geologically recently resurfaced terrain.
Sources of heat | Europa (moon) | Wikipedia | 493 | 43127 | https://en.wikipedia.org/wiki/Europa%20%28moon%29 | Physical sciences | Solar System | null |
Europa receives thermal energy from tidal heating, which occurs through the tidal friction and tidal flexing processes caused by tidal acceleration: orbital and rotational energy are dissipated as heat in the core of the moon, the internal ocean, and the ice crust.
Tidal friction
Ocean tides are converted to heat by frictional losses in the oceans and their interaction with the solid bottom and with the top ice crust. In late 2008, it was suggested Jupiter may keep Europa's oceans warm by generating large planetary tidal waves on Europa because of its small but non-zero obliquity. This generates so-called Rossby waves that travel quite slowly, at just a few kilometers per day, but can generate significant kinetic energy. For the current axial tilt estimate of 0.1 degree, the resonance from Rossby waves would contain 7.3 J of kinetic energy, which is two thousand times larger than that of the flow excited by the dominant tidal forces. Dissipation of this energy could be the principal heat source of Europa's ocean.
Tidal flexing
Tidal flexing kneads Europa's interior and ice shell, which becomes a source of heat. Depending on the amount of tilt, the heat generated by the ocean flow could be 100 to thousands of times greater than the heat generated by the flexing of Europa's rocky core in response to the gravitational pull from Jupiter and the other moons circling that planet. Europa's seafloor could be heated by the moon's constant flexing, driving hydrothermal activity similar to undersea volcanoes in Earth's oceans.
Experiments and ice modeling published in 2016, indicate that tidal flexing dissipation can generate one order of magnitude more heat in Europa's ice than scientists had previously assumed. Their results indicate that most of the heat generated by the ice actually comes from the ice's crystalline structure (lattice) as a result of deformation, and not friction between the ice grains. The greater the deformation of the ice sheet, the more heat is generated.
Radioactive decay
In addition to tidal heating, the interior of Europa could also be heated by the decay of radioactive material (radiogenic heating) within the rocky mantle. But the models and values observed are one hundred times higher than those that could be produced by radiogenic heating alone, thus implying that tidal heating has a leading role in Europa.
Plumes | Europa (moon) | Wikipedia | 473 | 43127 | https://en.wikipedia.org/wiki/Europa%20%28moon%29 | Physical sciences | Solar System | null |
The Hubble Space Telescope acquired an image of Europa in 2012 that was interpreted to be a plume of water vapour erupting from near its south pole. The image suggests the plume may be high, or more than 20 times the height of Mt. Everest., though recent observations and modeling suggest that typical Europan plumes may be much smaller. It has been suggested that if plumes exist, they are episodic and likely to appear when Europa is at its farthest point from Jupiter, in agreement with tidal force modeling predictions. Additional imaging evidence from the Hubble Space Telescope was presented in September 2016.
In May 2018, astronomers provided supporting evidence of water plume activity on Europa, based on an updated critical analysis of data obtained from the Galileo space probe, which orbited Jupiter between 1995 and 2003. Galileo flew by Europa in 1997 within of the moon's surface and the researchers suggest it may have flown through a water plume. Such plume activity could help researchers in a search for life from the subsurface Europan ocean without having to land on the moon.
The tidal forces are about 1,000 times stronger than the Moon's effect on Earth. The only other moon in the Solar System exhibiting water vapor plumes is Enceladus. The estimated eruption rate at Europa is about 7000 kg/s compared to about 200 kg/s for the plumes of Enceladus. If confirmed, it would open the possibility of a flyby through the plume and obtain a sample to analyze in situ without having to use a lander and drill through kilometres of ice. | Europa (moon) | Wikipedia | 321 | 43127 | https://en.wikipedia.org/wiki/Europa%20%28moon%29 | Physical sciences | Solar System | null |
In November 2020, a study was published in the peer-reviewed scientific journal Geophysical Research Letters suggesting that the plumes may originate from water within the crust of Europa as opposed to its subsurface ocean. The study's model, using images from the Galileo space probe, proposed that a combination of freezing and pressurization may result in at least some of the cryovolcanic activity. The pressure generated by migrating briny water pockets would thus, eventually, burst through the crust, thereby creating these plumes. The hypothesis that cryovolcanism on Europa could be triggered by freezing and pressurization of liquid pockets in the icy crust was first proposed by Sarah Fagents at the University of Hawai'i at Mānoa, who in 2003, was the first to model and publish work on this process. A press release from NASA's Jet Propulsion Laboratory referencing the November 2020 study suggested that plumes sourced from migrating liquid pockets could potentially be less hospitable to life. This is due to a lack of substantial energy for organisms to thrive off, unlike proposed hydrothermal vents on the subsurface ocean floor.
Atmosphere | Europa (moon) | Wikipedia | 232 | 43127 | https://en.wikipedia.org/wiki/Europa%20%28moon%29 | Physical sciences | Solar System | null |
The atmosphere of Europa can be categorized as thin and tenuous (often called an exosphere), primarily composed of oxygen and trace amounts of water vapor. However, this quantity of oxygen is produced in a non-biological manner. Given that Europa's surface is icy, and subsequently very cold; as solar ultraviolet radiation and charged particles (ions and electrons) from the Jovian magnetospheric environment collide with Europa's surface, water vapor is created and instantaneously separated into oxygen and hydrogen constituents. As it continues to move, the hydrogen is light enough to pass through the surface gravity of the atmosphere leaving behind only oxygen. The surface-bounded atmosphere forms through radiolysis, the dissociation of molecules through radiation. This accumulated oxygen atmosphere can get to a height of above the surface of Europa. Molecular oxygen is the densest component of the atmosphere because it has a long lifetime; after returning to the surface, it does not stick (freeze) like a water or hydrogen peroxide molecule but rather desorbs from the surface and starts another ballistic arc. Molecular hydrogen never reaches the surface, as it is light enough to escape Europa's surface gravity. Europa is one of the few moons in our solar system with a quantifiable atmosphere, along with Titan, Io, Triton, Ganymede and Callisto. Europa is also one of several moons in our solar system with very large quantities of ice (volatiles), otherwise known as "icy moons".Europa is also considered to be geologically active due to the constant release of hydrogen-oxygen mixtures into space. As a result of the moon's particle venting, the atmosphere requires continuous replenishment. Europa also contains a small magnetosphere (approximately 25% of Ganymede's). However, this magnetosphere varies in size as Europa orbits through Jupiter's magnetic field. This confirms that a conductive element, such as a large ocean, likely lies below its icy surface. As multiple studies have been conducted over Europa's atmosphere, several findings conclude that not all oxygen molecules are released into the atmosphere. This unknown percentage of oxygen may be absorbed into the surface and sink into the subsurface. Because the surface may interact with the subsurface ocean (considering the geological discussion above), this molecular oxygen may make its way to the ocean, where it could aid in biological processes. One estimate suggests that, given the turnover rate inferred from the apparent ~0 | Europa (moon) | Wikipedia | 502 | 43127 | https://en.wikipedia.org/wiki/Europa%20%28moon%29 | Physical sciences | Solar System | null |
5 Gyr maximum age of Europa's surface ice, subduction of radiolytically generated oxidizing species might well lead to oceanic free oxygen concentrations that are comparable to those in terrestrial deep oceans | Europa (moon) | Wikipedia | 40 | 43127 | https://en.wikipedia.org/wiki/Europa%20%28moon%29 | Physical sciences | Solar System | null |
Through the slow release of oxygen and hydrogen, a neutral torus around Europa's orbital plane is formed. This "neutral cloud" has been detected by both the Cassini and Galileo spacecraft, and has a greater content (number of atoms and molecules) than the neutral cloud surrounding Jupiter's inner moon Io. This torus was officially confirmed using Energetic Neutral Atom (ENA) imaging. Europa's torus ionizes through the process of neutral particles exchanging electrons with its charged particles. Since Europa's magnetic field rotates faster than its orbital velocity, these ions are left in the path of its magnetic field trajectory, forming a plasma. It has been hypothesized that these ions are responsible for the plasma within Jupiter's magnetosphere.
On 4 March 2024, astronomers reported that the surface of Europa may have much less oxygen than previously inferred.
Discovery of atmosphere
The atmosphere of Europa was first discovered in 1995 by astronomers D. T. Hall and collaborators using the Goddard High Resolution Spectrograph instrument of the Hubble Space Telescope. This observation was further supported in 1997 by the Galileo orbiter during its mission within the Jovian system. The Galileo orbiter performed three radio occultation events of Europa, where the probe's radio contact with Earth was temporarily blocked by passing behind Europa. By analyzing the effects Europa's sparse atmosphere had on the radio signal just before and after the occultation, for a total of six events, a team of astronomers led by A. J. Kliore established the presence of an ionized layer in Europa's atmosphere. | Europa (moon) | Wikipedia | 321 | 43127 | https://en.wikipedia.org/wiki/Europa%20%28moon%29 | Physical sciences | Solar System | null |
Climate and weather
Despite the presence of a gas torus, Europa has no weather producing clouds. As a whole, Europa has no wind, precipitation, or presence of sky color as its gravity is too low to hold an atmosphere substantial enough for those features. Europa's gravity is approximately 13% of Earth's. The temperature on Europa varies from −160 °C at the equator, to −220 °C at either of its poles. Europa's subsurface ocean is thought to be significantly warmer however. It is hypothesized that because of radioactive and tidal heating (as mentioned in the sections above), there are points in the depths of Europa's ocean that may be only slightly cooler than Earth's oceans. Studies have also concluded that Europa's ocean would have been rather acidic at first, with large concentrations of sulfate, calcium, and carbon dioxide. But over the course of 4.5 billion years, it became full of chloride, thus resembling our 1.94% chloride oceans on Earth.
Exploration
Exploration of Europa began with the Jupiter flybys of Pioneer 10 and 11 in 1973 and 1974, respectively. The first closeup photos were of low resolution compared to later missions. The two Voyager probes traveled through the Jovian system in 1979, providing more-detailed images of Europa's icy surface. The images caused many scientists to speculate about the possibility of a liquid ocean underneath.
Starting in 1995, the Galileo space probe orbited Jupiter for eight years, until 2003, and provided the most detailed examination of the Galilean moons to date. It included the "Galileo Europa Mission" and "Galileo Millennium Mission", with numerous close flybys of Europa. In 2007, New Horizons imaged Europa, as it flew by the Jovian system while on its way to Pluto. In 2022, the Juno orbiter flew by Europa at a distance of 352 km (219 mi).
In 2012, Jupiter Icy Moons Explorer (JUICE) was selected by the European Space Agency (ESA) as a planned mission. That mission includes two flybys of Europa, but is more focused on Ganymede. It was launched in 2023, and is expected to reach Jupiter in July 2031 after four gravity assists and eight years of travel. | Europa (moon) | Wikipedia | 462 | 43127 | https://en.wikipedia.org/wiki/Europa%20%28moon%29 | Physical sciences | Solar System | null |
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