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 water on the martian surface and how the planet could have evolved from possible initial warm and wet conditions to the cold and dry present state. Virtually all models for this climatic evolution rely strongly on the removal of an initial thick carbon dioxide atmosphere by precipitation of carbonate minerals from surface waters that may have been quite similar to those of Hadean Eon Earth’s oceans. In order for this to occur, a hydrologic cycle would be necessary in which chemical weathering of silicate rocks consumes CO2 that precipitates as carbonates in an acidic martian ocean which probably had a very high alkalinity. The consumption of atmospheric CO2 by this process would result in a gradual decrease of the atmospheric greenhouse influence and cooling of the climate. Once the surface of Mars became cold enough so that freezing conditions prevailed, the hydrologic cycle would largely cease, and the uptake of CO2 by silicate rock weathering would greatly diminish. The alkalinity of the freezing seawater would probably be sufficient to result in the removal of all calcium 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.