The role of carbonates in the evolution of early martian oceans

Central to the question of Life on Mars is whether there has been liquid wa ter on the martian surface and how the planet could have evolved from possi ble initial warm and wet conditions to the cold and dry present state. Virt ually all models for this climatic evolution rely strongly on the removal o f an initial thick carbon dioxide atmosphere by precipitation of carbonate minerals from surface waters that may have been quite similar to those of H adean Eon Earth's oceans. In order for this to occur, a hydrologic cycle wo uld be necessary in which chemical weathering of silicate rocks consumes CO 2 that precipitates as carbonates in an acidic martian ocean which probably had a very high alkalinity. The consumption of atmospheric CO2 by this pro cess would result in a gradual decrease of the atmospheric greenhouse influ ence and cooling of the climate. Once the surface of Mars became cold enough so that freezing conditions pre vailed the hydrologic cycle would largely cease, and the uptake of CO2 by s ilicate rock weathering would greatly diminish. The alkalinity of the freez ing seawater would probably be sufficient to result in the removal of all c alcium as calcium carbonate. Some magnesium and sodium would also likely be removed as carbonates as well. The removal of these cations as carbonates has a major influence on the final temperature at which liquid brines would be able to persist on the surface of Mars. During the period of freezing, the oceans would act as a source of CO2 rather than a sink, further slowing the rate of climate change on Mars.