CO2-H2O Phase Equilibria and Martian Volatile Evolution

Thee are now sufficient laboratory and observational data in the CO2-H2O system to predict atmospheric condensation sequences and phase stability in regolith and ice caps. Reports of sluggish nucleation of clathrate from CO2-H2O fluids in experiments as high as 10 bars suggest that clathrate is unlikely to condense from the present Martian atmosphere. Condensation on Mars thus takes 3 predictable forms: pure H2O-ice, a metastable eutectic mixture of solid-CO2 and H2O-ice (5,000 - 10,000 to 1 ratio), and under limited conditions pure solid-CO2. Seasonal frosts are likely to consist of thin layers of water ice overlain by layers of the metastable eutectic assemblage. Growing ice caps are likely to be layered, also, with sets of ice (lower) and eutectic (upper) layers interspersed with pure solid-CO2 layers. The saturated water content of a 1-bar CO2 atmosphere with surface temperatures higher than 273 K would have reached 20 precipitatible millimeters. Basal melting, triggered by trapped heat flow from the interior, limits the thickness and composition of the polar ice caps and is expected during periods of low obliquity, when there is extensive cooling and condensation at the poles. Introduction of liquid CO2 into the Martian regolith by basal melting at the poles may be the major mechanism for loss of the ancient greenhouse atmosphere and its long-term sequestration. Migration of carbonated groundwater away from the poles (particularly the south pole) may lead to extensive carbonate veining in the regolith at middle latitudes. Explosive release of CO2 fluid coexisting with groundwater may trigger outflow channel floods and reset atmospheric isotope ratios.