A Cold, Intermittently Wet Mars

The evolution of an early warm, wet Mars to the present-day cold, dry Mars can be explained, in principle, by assuming high atmospheric concentrations of CO2 early in Martian history whose removal as carbonate minerals led to 

Initially, this Martian brine was equilibrated at Pco2 = 50 mbars within each of the ten layers. Under these conditions, virtually all the Fe (99.99%) precipitates as siderite (FeCOo), and 77.6 to 96.6% of the Ca precipitates as calcite (CaCOo) (Fig. 5.18). After equilibration of each 0.5-km layer separately, we then froze the profile from the top down, layer by layer, assuming that the freezing process would be sufficiently slow that all soluble salts would be ejected into the lower, unfrozen, layers. This process leads to increasing salt concentrations with depth (Fig. 5.18). These freezing simulations were done assuming that all Fe was removed in the initial evaporative concentration. Freeze concentration leads to the additional precipitation of calcite and 

A cold early climate and a thin CO2 atmosphere are consistent with the ubiquity of primary igneous minerals and the apparent absence of secondary minerals and copious carbonates on the surface. Aqueous eruptions introduced soluble chloride and sulfate salts into Martian soils, leaving clays and carbonates within the crust. If the Martian crust hosts a deep biosphere, aqueous eruptions could bring organisms or their chemical signature to the surface. The biotic component of any recent eruptions may be preserved in transient ice in cold traps on the surface. 

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