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W.V. Boynton (Univ. Arizona), G.J. Taylor (Univ. Hawaii), L.G. Evans (Computer Sciences Corp), W.C. Feldman (Los Alamos), I.G. Mitrofanov (Space Research Inst.), R.C. Reedy (Univ. New Mexico), S.W. Squyres (Cornell), R. Starr (Catholic Univ.), D.K. Hamara, D.M. Janes, K. Kerry (Univ. Arizona), Mars Odyssey GRS Team
The Mars Odyssey Gamma-Ray Spectrometer has been in orbit for slightly over one year since boom erection.
The distribution of hydrogen is highly variable over the martian surface. Expressed as H2O equivalent, it ranges from 100% at the north polar residual cap down to about 2% in the driest regions of the mid latitudes. H2O is clearly enriched at depth beneath the surface in the polar latitudes, but at this time we cannot say if that is the case in the mid latitudes. Hydrogen in the south polar region was found to be about 35% H2O equivalent, based on preliminary data with some assumptions about H2O at the Viking-1 lander site [1]. A new calibration results in even greater amounts of ice, with minimum values on the order of 50% H2O in the south and somewhat greater amounts in the north away from the residual cap. Converting this value to a volume percentage of ice yields 73% ice, far more than can be explained by simple formation of frost in free pore space in the regolith. This much ice argues for it being deposited in a past epoch under wetter conditions such that ice could be deposited at a rate higher than the deposition of dust, a process that could yield the equivalent of glaciers.
Interesting results have been seen in the distribution of K and Th. Normally these elements are not fractionated on planetary bodies, as most igneous fractionation processes do not separate them. On Mars, we see variations in the K/Th ratio of about a factor of two, which suggests that aqueous weathering may be playing a role in the distribution of these elements.
References: [1] Boynton et al. (2002) Science 297:81-84.
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.