Dix Planetary Science Seminar
A Massive CO2 Ice Deposit (MCID) that rivals the mass of Mars' current, 96% CO2 atmosphere was recently discovered to overlie part of Mars' southern H2O cap. The MCID is layered: a top layer of 1-10 m of CO2, the Residual South Polar Cap (RSPC), is underlain by ~10-20 m of H2O ice, followed by up to three layers of CO2 ice that are each up to several hundred meters thick, separated by two layers of ~20-40 m of H2O ice. Previous studies invoked orbital cycles to explain the layering, assuming the H2O ice insulates and seals in the CO2, allowing it to survive periods of high obliquity. We also invoke orbital cycles, but instead assert the surface of the MCID is in continuous vapor equilibrium with the atmosphere rather than sealed. Using currently observed albedo and emissivity properties of martian polar CO2 ice deposits, we model that the present MCID is a remnant of larger CO2 ice deposits laid down during periods of decreasing obliquity that are eroded, liberating a residual lag layer of H2O ice, when obliquity increases. Fractions of prior CO2 deposits remain because the amplitudes of the obliquity maxima have been mostly decreasing during the past ~500 kyr. With these simple assumptions, our energy balance model explains why only the south polar cap hosts an MCID, why the RSPC exists, and the observed MCID stratigraphy. We use our model to calculate Mars' pressure history and the age of the MCID.