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A. S. Yen (Jet Propulsion Laboratory, California Institute of Technology)
Liquid water has not played a significant role in chemical weathering of the present martian surface. The detection of olivines and pyroxenes and the apparent absence of clays, carbonates, and other low-temperature aqueous sediments preclude extensive interactions between liquid water and surface materials.
Ferric minerals that dominate the visible color of Mars, generally attributed to precipitates following oxidation of ferrous iron in solution, could alternatively be exogenic in origin. A chondritic input to the surface could account for the elemental differences between rocks and soils at the Pathfinder landing site. This idea is supported by estimates of micrometeorite and interplanetary dust particle fluxes, which could be well in excess of 5 cm per billion years. Laboratory experiments show that metallic iron and poorly ordered samples of ferrous iron will oxidize to the ferric state under simulated martian surface conditions. Thus, the ferric oxide pigments could have formed in the absence of liquid water.
The hypothesis of meteoritic material in the martian soil is testable. Nickel is unlikely to be present at the surface of Mars in excess of a few hundred PPM unless there is a significant meteoritic component. If the elemental differences between Pathfinder rocks and soils were due entirely to meteoritic iron, 0.1% to 0.6% Ni would be present in the soil. The Alpha Particle X-ray Spectrometers (APXS) onboard the Mars Exploration Rovers will have a far greater sensitivity to Ni than instruments on Mars Pathfinder and the Viking Landers. Calibration testing of the flight hardware shows a clear signal from nickel in BE-N basalt standards (<300 PPM) with a 13 hour integration. Actual detection limits will be several times worse than the laboratory values, but a significant exogenic component can still be readily identified. Stay tuned for early 2004!
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.