| Mars:
Where the Water Is By Andrew Chaikin Executive Editor, Space & Science posted: 02:45 pm ET 21 June 2000 |
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The Mars Global Surveyor (MGS) spacecraft has found evidence that water has seeped out of the walls of martian craters in the recent geologic past, a process that possibly continues today. One Mars specialist, who did not wish to be identified, said that if true, the findings would "cause big problems" for planetary geologists.The evidence centers on stream-like features and fan-shaped deposits seen within craters that are located in the planet's higher latitudes. One such image, published in 1998, shows a crater located in the southern highlands of Noachis Terra. V-shaped depressions in the crater wall bear a resemblance to features produced by seepage of water on Earth.
The big problem, the specialist said, is that only ice has been thought to exist in the upper layers of Mars' crust. Beneath the crust lies a so-called cryosphere in which conditions are too cold for liquid water to exist. This cryosphere is thought to extend to a depth of several miles (kilometers), on average. If water formed the features seen in the Global Surveyor images, however, it may have come from a much shallower depth -- perhaps only a few hundred yards (meters). The notion that water could exist in liquid form so near the surface of Mars -- where atmospheric pressure is only about 7 one-thousandths of that on Earth -- presents a daunting challenge to theorists.
One question raised by the specialist is whether a different substance -- specifically, carbon dioxide (CO2) -- might have been the cause of the features seen by Global Surveyor. Liquid carbon dioxide can exist at pressures around five times that at sea level here on Earth. However, like liquid water, liquid carbon dioxide would not be stable on the Martian surface. Another alternative to be studied, the specialist said, is the possibility that gaseous CO2, mixed with dust particles, might have acted as a fluid to form the observed features.
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