Atmospheric conditions on early Mars and the missing layered carbonates

Widespread, massive layered sediments, studied by surface rovers and Mars-orbiting spectrometers, are found to be rich in sulfates. No similarly massive carbonates have been detected. We present the results of coupled atmospheric and geochemical calculations of the formation of the sulfate-rich Meridiani sediments, and offer an explanation for why extensive layered carbonates are not found there or elsewhere on Mars. Large scale volcanism from the build-up of Tharsis during the late Noachian would have injected large amounts of SO2 into the atmosphere. Efficient photochemical conversion of SO2 to H2SO4 would have caused widespread sulfuric acid/water clouds, similar to those seen on Venus today. Precipitation from these clouds and acidification of surface water would have sustained a thick, warm CO2 atmosphere via carbonate inhibition. Such an atmosphere could have been subjected to loss to space via impact erosion and sputtering. Once atmospheric SO2 gas production dropped and waters become more alkaline, the remaining CO2 in the atmosphere collapsed to form poorly consolidated carbonate patinas on rock surfaces and in open fractures.