VOLATILE CONSUMPTION RESULTING FROM BASALT WEATHERING ON MARS

Weathering of fresh terrestrial rocks, and particularly of fresh terrestrial basalts, is a significant factor in drawdown of atmospheric CO₂. This weathering consumes carbonic acid dissolved in rainwater, releases dissolved ions that contribute to the salt content of the terrestrial oceans, and leaves behind secondary clay minerals with a high water content. These weathering reactions are known to exert important controls on the carbon content of the terrestrial atmosphere. Detailed studies of terrestrial weathering systems have shown that the mass of carbon fixed by weathering is linearly related to the runoff of base cations from the weathered rock. Other studies of terrestrial weathering have determined the extent of mass loss from weathering basalt to produce clay-bearing and clay-dominated systems.

Extensive clay mineral deposits have been detected on the martian surface and it has been proposed that some of these clays formed by surface weathering. Applying a mass balance approach to martian basalt compositions using weathering parameters from terrestrial basalts, it is possible to estimate the extent of carbon fixation, water sequestration, and dissolved ion leaching that occurred during formation of a given volume of pedogenic clay on Mars.

The results of these calculations suggest that production of one cubic meter of martian clay by basalt weathering would consume 0.6 – 1.5 tonnes of atmospheric CO₂ and 0.2-0.3 cubic meters of water and release 1.6-3.9 tonnes of dissolved basalt components. Weathering adequate to draw down 1 bar of martian CO₂ would produce the equivalent of a ~20 m thickness of clay over the entire martian surface, or proportionately thicker but less extensive layers. Formation of this amount of clay would consume ~6.7 * 10⁵ km³ of water.

Climate models suggest a thick atmosphere of up to several bars CO₂ (mixed with other gases) was necessary to stabilize liquid water on ancient Mars. The results presented here indicate that extensive surficial weathering of the martian crust could have consumed sufficient CO₂ to affect climate.