37th DPS Meeting, 4-9 September 2005
Session 27 Mars IV
Oral, Tuesday, September 6, 2005, 4:20-6:00pm, Music Concert Hall

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[27.05] Atmospheric Conditions on Early Mars and the Lack of Carbonate Deposits

J.M. Moore (NASA Ames Research Center), M.A. Bullock (Southwest Research Institute)

Mars has had a complex aqueous history. Both MER rovers acquired data on aqueous processes that altered rocks at Gusev crater and Meridiani Planum. The massive sulfate beds at Meridiani, with cross-laminated bedforms and evidence for multiple episodes of aqueous activity, speak to a complex watery past. The most likely interpretation is that shallow seas or lakes came and went during the Noachian and perhaps early Hesperian. While finding exposed sulfate-rich sediments on Mars is not a surprise, the attendant lack of evidence for carbonate deposits remains a mystery. Fundamentally, a body of water in contact with a carbon dioxide atmosphere should have laid down massive carbonates, as happened on Earth during the Archean. So where are the carbonates on Mars?

More than 25 years before the MER mission, Clark and Baird proposed that high levels of atmospheric SO2 could have inhibited the formation of carbonates on Mars. Following up on their speculation, and with much more data on the nature of sulfate deposits, we have performed calculations that show what kind of atmospheric conditions must have existed during the Noachian in order to have suppressed the precipitation of carbonates on Mars. If the atmosphere of Noachian Mars were high in volcanically-derived sulfur gases it is reasonable to assume that open bodies of water would have been charged with both SO2 and CO2. High levels of volcanism and outgassing are implicated in the construction of Tharsis, built primarily during the late Noachian. Using a set of 10 coupled chemical equations for SO2 and CO2 solubility and reactivity, and sources and sinks of volcanically-derived sulfur gases, we have come up with constraints on the nature and composition of the early martian atmosphere, its role in maintaining the chemistry of martian water, and why carbonates never formed massive layered deposits on Mars.

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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.