Modeling the Stability of the Large CO2 Deposits on Mars South Polar Cap

The Mars Reconnaissance Orbiter's Shallow Radar instrument (SHARAD) revealed that the geologic unit A_A3 (Tanaka et al. 2007) in Mars' South Polar Layered Deposits is composed of CO₂ ice (Phillips et al. 2011). This deposit is estimated to contain ~10,000 km³ of CO₂ ice, making its stability an important consideration in understanding recent climate change, as the deposit mass approaches that of the present atmosphere. Such a large mass would likely be deposited during a period of low obliquity, however different values for the emissivity and albedo of the CO₂ frost can dramatically change age estimates (Armstrong et al. 2004). Previous models have focused on the stability of the South Pole Residual Cap (SPRC) where the CO₂ ice is exposed to the atmosphere. A_A3 however, is largely covered by the SPRC and other thinner deposits. In this research we evaluate the age and long-term stability of the A_A3 deposit using standard thermal and sublimation modeling techniques. Our model incorporates diurnal and annual thermal variations that are allowed to propagate into the subsurface. In order to address the effect of the overlying layers on A_A3 we take account for both surface and subsurface sublimation. Any gas produced in the subsurface is then allowed to diffuse through the overlying material. Results are compared to a variety of spacecraft observations including visible imagery and thermal measurements. Additionally, these results should help guide interpretation of SHARAD data in the A_A3 region and point to locales for new observations. References: Tanaka et al. 2007, 'Recent advances in the stratigraphy of the polar regions of Mars.' Paper presented at the Seventh International Conference on Mars, Pasadena, CA, 9 to 13 July 2007, 3276. Phillips et al. 2011, Massive CO₂ Ice Deposits Sequestered in the South Polar Layered Deposits of Mars, Science 332: 838-841. Armstrong et al. 2004, A 1 Gyr climate model for Mars: new orbital statistics and the importance of seasonally resolved polar processes, Icarus, 171: 255-271.