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vations by the Global Surveyor and Odyssey orbiters, however, this view of Mars is still |
Mars almost resembles two different worlds that have been glued together. From lati- |
tudes around the equator to the south are ancient highlands pockmarked with craters |
from the solar system's early era, yet riddled with channels that attest to the flow of |
water. The northern third of the planet, however, overall is sunken and much smoother |
at kilometer (mile) scales. There is as yet no general agreement on how the northern |
plains got to be that way. At one end of the spectrum is the theory that it is the floor of |
an ancient sea; at the other, the notion that it is merely the end product of innumerable |
lava flows. New theories are emerging thanks to the discoveries of Mars Odyssey, and |
some scientists believe a giant ice sheet may be buried under much of the relatively |
smooth northern plains. Many scientists suspect that some unusual internal process |
not yet fully understood may have caused the northern plains to sink to relatively low |
elevations in relation to the southern uplands. |
Scientists today view Mars as having had three broad ages, each named for a geo- |
graphic area that typifies it: |
! The Noachian Era is the name given to the time spanning perhaps the first |
billion years of Mars' existence after the planet was formed 4.6 billion years ago. |
In this era, scientists suspect that Mars was quite active with periods of warm |
and wet environment, erupting volcanoes and some degree of tectonic activity. |
The planet may have had a thicker atmosphere to support running water, and it |
may have rained and snowed. |
! In the Hesperian Era, which lasted for about the next 500 million to 1.5 billion |
years, geologic activity was slowing down and near-surface water perhaps was |
freezing to form surface and buried ice masses. Plunging temperatures |
probably caused water pooled underground to erupt when heated by impacts in |
catastrophic floods that surged across vast stretches of the surface -- floods so |
powerful that they unleashed the force of thousands of Mississippi Rivers. |
Eventually, water became locked up as permafrost or subsurface ice, or was |
partially lost into outer space. |
! The Amazonian Era is the current age that began around 2 billion to 3 billion |
years ago. The planet is now a dry, desiccating environment with only a modest |
atmosphere in relation to Earth. In fact, the atmosphere is so thin that water |
can exist only as a solid or a gas, not as a liquid. |
Apart from that broad outline, there is lively debate and disagreement on the details of |
Mars' history. How wet was the planet, and how long ago? What eventually happened |
to all of the water? That is all a story that is still being written. |
In addition to studying the planet from above with orbiting spacecraft, NASA's Mars |
Exploration Program is putting robotic geologists on the surface in the form of instru- |
mented rovers. Both of the landing sites selected for the Mars Exploration Rovers |
show evidence of water activity in their past. The rovers will look at rocks to under- |
stand the types of minerals that they are made of, and hence the environments in |
which they formed. This, in turn, will offer clues about the environment in which the |
rocks formed. Some types of rocks, for example, might be of types that form in run- |
ning water, whereas others might be typical of the sediments that form on the beds of |
Even if we ultimately learn that Mars never harbored life as we know it here on Earth, |
scientific exploration of the Red Planet can assist in understanding the history and evo- |
lution of life on our own home world. Much if not all of the evidence for the origin of |
life here on Earth has been obliterated by the incredible pace of weathering and global |
tectonics that have operated over billions of years. Mars, by comparison, is a compos- |
ite world with some regions that may have histories similar to Earth's crust, while oth- |
Where We've Been and Where We're Going |
Building on scientific discoveries and lessons learned from past and ongoing missions, NASA's |
Mars Exploration Program will establish a sustained observational presence both around and on |
the surface of Mars in coming years. This will include orbiters that view the planet from above and |
act as telecommunications relays; surface-based mobile laboratories; robots that probe below the |
planet's surface; and, ultimately, missions that return soil and rock samples to Earth. With interna- |
tional cooperation, the long-term program will be guided by compelling questions that scientists are |
interested in answering about Mars, developing technologies to make missions possible within |
available resources. The program's strategy is to seek to uncover profound new insights into Mars' |
past environments, the history of its rocks and interior, the many roles and abundances of water |
and, quite possibly, evidence of past and present life. |
The following are the most recently completed, ongoing and near-term future Mars missions of |
exploration in the NASA program: |
! Mars Pathfinder (December 1996 - March 1998): The first completed mission in NASA's |
Discovery Program of low-cost, rapidly developed planetary missions with highly focused scientific |
goals, Mars Pathfinder far exceeded its expectations and outlived its primary design life. This lan- |
der, which released its Sojourner rover at the martian surface, returned 2.3 billion bits of informa- |
tion, including more than 17,000 images and more than 15 chemical analyses of rocks and soil and |
extensive data on winds and other types of weather. Investigations carried out by instruments on |
both the lander and the rover suggest that, in its past, Mars was warm and wet, with liquid water |
on its surface and a thicker atmosphere. The lander and rover functioned far beyond their planned |
lifetimes (30 days for the lander and 7 days for the rover), but eventually, after about three months |
on the martian surface, depletion of the lander's battery and a drop in the lander's operating tem- |
perature are thought to have ended the mission. |
! Mars Global Surveyor (November 1996 - present): During its primary mapping mission from |
March 1999 through January 2001, NASA's Mars Global Surveyor collected more information than |
any other previous Mars mission. Today the orbiter continues to gather data in a second extended |
mission. As of May 1, 2003, it has completed more than 20,000 orbits of Mars and returned more |
than 137,000 images, 671 million laser-altimeter shots and 151 million spectrometer measure- |
ments. Some of the mission's most significant findings include: evidence of possibly recent liquid |
water at the martian surface; evidence for layering of rocks that points to widespread ponds or |
lakes in the planet's early history; topographic evidence that most of the southern hemisphere is |
higher in elevation than most of the northern hemisphere, so that any downhill flow of water and |
sediments would have tended to be northward; identification of gray hematite, a mineral suggesting |
a wet environment when it was formed; and extensive evidence for the role of dust in reshaping |
the recent martian environment. Global Surveyor provided valuable details for evaluating the risks |
and attractions of potential landing sites for the Mars Exploration Rover missions, and it will serve |
as a communications relay for the rovers as they descend to land on Mars and afterwards. |
! Mars Climate Orbiter and Mars Polar Lander (1998-99): These spacecraft were both lost |
upon Mars arrival. |
! Mars Odyssey (April 2001 - present): This orbiter's prime mapping mission began in March |
2002. Its suite of gamma-ray spectrometer instruments has provided strong evidence for large |
quantities of frozen water mixed into the top layer of soil in the 20 percent of the planet near its |
north and south poles. By one estimate -- likely an underestimate -- the amount of water ice near |
the surface, if melted, would be enough water to fill Lake Michigan twice. Odyssey's infrared |
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camera system has also provided detailed maps of minerals in rocks and soils. A layer of olivine- |
rich rock in one canyon near Mars' equator suggests that site has been dry for a long time, since |
olivine is easily weathered by liquid water. Nighttime infrared imaging by Odyssey's camera sys- |
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