Growth and Decay of the South Polar Residual Cap of Mars

The southern residual ice cap (SRC) is composed of high-albedo solid CO2, is a few meters thick and has areas at its margins and in its interior where the underlying water ice of the polar layered deposits shows through. Previous observations show that pits within the SRC expose sections of layering in the CO2 ice (up to 10m thick) and are expanding by meters/year (Thomas et al., Icarus, 2005). The expansion of these pits is a problem as their spatial density and expansion rates indicate that there should be nothing left of the SRC within a century or so. Our analysis of pit expansion rates indicates that they increase during periods of dust-storm activity. The sensitivity of SRC evolution to climate indicates that its stratigraphy is likely also a valuable source of information about recent climate variability. We are using HiRISE data to constrain landscape evolution models of CO2 ice landscapes to quantify the history of the SRC and the connection of its stratigraphy to martian climate. Here we report on two investigations utilizing different models. 1. We use a landscape evolution model to simulate growth of an accumulating CO2 deposit. This model shows pits naturally form in this landscape, without any associated change in climate, due to increases in surface roughness with time and the feedback between slope and ice ablation. Comparison of model results with the current SRC indicates the thickest sections of stratigraphy likely represent up to 100 martian years of accumulation. 2. HiRISE data shows that some pits expand by faster retreat of the lower ice layers, undercutting of the upper bright ice and subsequent mass-wasting. A separate model that can handle topographic overhangs has been developed and is being used to investigate this behavior. Using this model to reproduce the non- circular aspects of the pit-shapes and their expansion rates (which HiRISE can measure as a function of azimuth) constrains properties of the lower CO2 ice layers such as density and albedo. HiRISE observations combined with these models allow us to start interpreting the SRC stratigraphy and history, both by assigning timescales and deducing how the lower strata differ from the upper. The very short timescales involved in the evolution of this landscape indicate that the SRC stratigraphy is an important record of the variability of the current climate rather than a record of climate change.