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A. P. Zent (NASA Ames Research Center)
The return of organics remains one of the prime foci of NASA’s exploration of Mars. There has been concern that oxidants in the Martian environment will destroy any organics in the regolith, and a considerable effort has been expended to understand the nature of these oxidants. While definitive in situ tests remain to be conducted, numerous laboratory and modeling efforts have indicated that the penetration depth of oxidants is likely to be shallow. Hydrogen peroxide is likely to diffuse no more than centimeters before catalytically decomposing. The resultant hydroperoxyl radicals may complex with oxide or silicate surfaces, and may be stable for geologically significant periods of time, allowing aeolian mixing to fix the depth of strongly-oxidized regolith. Other oxidants related to the UV-flux are unlikely to penetrate further. Surface exposures of silt-sized, mafic particles show that the intra-crystalline diffusion of strong oxidants is insufficient to penetrate even microns into coherent particles. However, the SNC meteorites show evidence of oxidizing fluids percolating at depths well in excess of that reached by strong oxidants. Oxidizing groundwater occurs in consequence of the oxidizing nature of the atmosphere, which is in turn due to the loss of H2 on photolysis of water. Oxidizing fluids are therefore more likely to set limits on the persistence of native martian organics at depth. Aqueous activity in SNC meteorites dating from 11my or less, combined with evidence of recent surface seepage argue that the martian subsurface is periodically hydrologically active over much of the planet. A dual focus on areas where recent groundwater has flowed, and on older sites where aquicludes may have preserved older reducing material is indicated. This argues strongly for a reconnaissance phase with a focus on the structure and nature of the subsurface.