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B.M. Jakosky, M.T. Mellon, S. Varnes (Univ. of Colorado), W. Feldman (LANL), W. Boynton (Univ. of Arizona), R.M. Haberle (NASA/Ames)
The Mars Odyssey GRS/NS has provided indications of near-surface water in the equatorial and mid-latitudes, in amounts typically from 5-10 % and ranging up to 15-20 %. Although such high abundances could be present as adsorbed water in clays, other measurements suggest that clays are not present. The spatial pattern of where the water is located is not consistent with a dependence on composition (as would be the case if it were related to clay), topography, present atmospheric water abundance, latitude, or thermophysical properties. The “wave-number two” pattern of water abundance is very reminiscent of something related to an atmospheric phenomenon. We suggest that the high water abundances could be due to transient ground ice that is present in the top meter of the surface. Ice would be stable at tens-of-centimeters depth at these latitudes if the atmospheric water abundance were about twice the present value, much as ice is stable poleward of about 60 degrees latitude for current water abundances. Higher atmospheric water abundances could result under present-day orbital conditions if the south-polar cap were to lose its annual covering of CO2 ice; this would expose an underlying water-ice cap that could supply water to the atmosphere during southern summer. If this hypothesis is correct, then (i) the water ice is unstable today and is in the process of sublimating and diffusing back into the atmosphere, and (ii) the current configuration of perennial CO2 on the south and not the north might not be representative of the present epoch over the last ten thousand years.
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.