On the Mystery of the Perennial Carbon Dioxide Cap at the South Pole of Mars

SOUTH POLE OF MARS. Alejandro Soto, Xin Guo, and Mark I. Richardson, Caltech, M/C 150-21, Pasadena, CA 91125. [email protected], [email protected], [email protected] Introduction: A perennial CO2 ice cap has long been observed near the south pole of Mars. The retention of a CO2 ice cap results from the surface energy balance of the latent heat, solar radiation, surface emission, and subsurface conduction. While models conventionally treat surface CO2 ice using constant ice albedos and emissivities, such an approach fails to predict the existence of a perennial cap. We explore the role of insolation-dependent ice albedo, which agrees well with Viking, Mars Global Surveyor and Mars Express albedo observations. Using a simple parameterization within a General Circulation Model, in which the albedo of CO2 ice responds linearly to the solar insolation, we are able to predict the existence of a perennial CO2 cap at the observed latitude and only in the southern hemisphere [1]. While the single insolation dependent function likely does not capture the full spatial and temporal variations of ice albedo, it provides a demonstration that insolation-dependence of albedo is probably the key factor in the existence of a residual cap. Future work on the microphysical and macrophysical mechanisms of CO2 ice albedo and emissivity is required, one application of which would be a physically-based parameterization of surface radiative effects of CO2 ice for climate models. Insolation-dependent albedo: We use the MarsWRF model, the Martian implementation of the planetWRF model [2]. We calibrate the GCM by tuning the albedos and the emissivities of the seasonal dry ice caps and the total CO2 inventory in the system to reproduce the VL1 surface pressure cycles. Water cycle and CO2 cloud microphysics, which are likely aliased to the model parameterization of frost albedo and emissivity, are not explicitly included. At the steady state, the model predicts a pressure cycle that matches the Viking Lander records very closely [3]. The predicted mass of the seasonal caps is consistent with other GCMs and observations[4]. However, like all the other models trying to fit the VL pressure records, a residual CO2 cap in the south pole is not predicted by MarsWRF with this setup. The Viking observations suggests that the albedo of the southern residual CO2 cap changes with time [5] [6]. More usefully, the relationship between the residual CO2 caps albedo and the incident solar flux is very linear (Figure 1). When we use a least square linear regression method, we obtain an empirical equation that predicts the surface CO2 frost albedo based on the insolation: