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"While water flowed south to north in general, the data clearly reveal localized areas [in Valles Marineris] where water may have once formed ponds, " explained Dr. Maria Zuber of the Massachusetts Institute of Technology, Cambridge, MA, and Goddard. If there once were ponds and lakes in Valles Marineris, they're dry now. Physical conditions on the surface of Mars, namely low atmospheric pressure and low temperature, conspire to make liquid water unstable. The average atmospheric pressure on Mars is only about 6 millibars compared to the Earth's average pressure of 1013 millibars. The average surface temperature on Mars is about -60 deg C compared to the Earth's 15 deg C. At certain locations and times on Mars, when the air pressure is high enough and the temperature is above freezing (greater than 0 deg C), liquid water is theoretically possible; but the rate of evaporation would be so great that liquid water (if it were present) would rapidly vaporize. 
Left:
High resolution Mars Global Surveyor images were combined with Viking Orbiter color data to produce this
stunning, detailed view of a Martian canyon's edge. The area pictured is about 6 miles wide and represents a tiny part of the
northern edge of the canyon Valles Marineris, whose total length is about 2,500 miles. Details 20 to 30 feet across can be
seen in the high resolution data.
What
processes caused the well-defined layers in the steep canyon walls? In the Grand Canyon on planet Earth, sedimentary processes have resulted in spectacular
rock layers. But similar layers of rock in canyons of the Hawaiian Islands were created by volcanoes. Regardless of the origin
of layering on Mars, its extent suggests that early Mars was geologically active and complex.
More information.
Nevertheless, there is widespread evidence of dried-up valleys and channels thought to have been eroded by liquid water. Many Martian outflow channels strongly resemble flood channels on Eath, like those in eastern Washington in the USA. On Mars they may have formed when groundwater or subsurface slush was catastrophically brought to the surface, perhaps triggered by large impacts or "marsquakes". On the other hand, geological studies of the valley networks suggest that these must have been gradually eroded by running water: some show morphology suggesting formation by groundwater sapping (i.e. when a river is fed by a spring and the valley grows by headward erosion); others seem to have been produced by precipitation runoff. The valley networks are almost completely (but not quite) restricted to ancient upper highlands, dated as 3.5 to 4.0 billion years old from the quantity of impact craters, so it is postulated that environmental conditions on Mars must have been conducive to liquid water at this time. The upper limit on the present amount of water on the martian surface is 800,000 to 1.2 million cubic miles (3.2 to 4.7 million cubic kilometers), or about 1.5 times the amount of ice covering Greenland. If both caps are composed completely of water, the combined volumes are equivalent to a global layer 66 to 100 feet (22 to 33 meters) deep, about one-third the minimum volume of a proposed ancient ocean on Mars. 
Right: A false-color animation of Mars showing four
hemispheric views at 90 degree intervals. Colors correspond
to elevations measured by the Mars Orbiter Laser Altimeter
on the Mars Global Surveyor spacecraft. Red and white
colors denote high elevations; blue denotes low.
This new global 3D map was compiled from 27 million
elevation measurements gathered in 1998 and 1999.
The data were assembled
into a global grid with each point spaced 37 miles (60 kilometers) apart at the
equator, and less elsewhere. Each elevation point is known with an accuracy of
42 feet (13 meters) in general, with large areas of the flat northern hemisphere
known to better than six feet (two meters).
During the ongoing Mars Global Surveyor mission, the MOLA instrument is collecting about 900,000 measurements of elevation every day. These data will further improve the global model, help engineers assess the area where NASA's Mars Polar Lander mission will set down on Dec. 3, and aid the selection of future landing sites. MOLA was designed and built by the Laser Remote Sensing Branch of the Laboratory for Terrestrial Physics at Goddard. The Mars Global Surveyor mission is managed for NASA's Office of Space Science, Washington, DC, by the Jet Propulsion Laboratory, Pasadena, CA, a division of the California Institute of Technology. |
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FIRST GLOBAL 3-D VIEW OF MARS REVEALS DEEP BASIN AND PATHWAYS FOR WATER FLOW - NASA HQ Press Release The Planet Mars - from the SEDS Nine Planets web site Life on Mars - A review of evidence of signs of life in the Allen Hills meteorite Mars Global Surveyor - home page Mars - by Percival Lowell, 1895 Related Stories:
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