The role of snowfall in forming the seasonal ice caps of Mars: Models and constraints from the Mars Climate Sounder

Abstract

Wintertime observations of the martian polar regions by orbiting spacecraft have provided evidence for carbon dioxide clouds, which measurably alter the polar energy budget and the annual CO2 cycle. However, it has remained unclear whether snowfall contributes a substantial quantity to the accumulating seasonal ice caps. We develop models to constrain precipitation rates based on observations of south polar CO2 clouds by the Mars Climate Sounder (MCS), and show that snowfall contributes between 3% and 20% by mass to the seasonal deposits at latitudes 70–90°S. The lower bound on this estimate depends on a minimum effective cloud particle size of ∼50μm, derived by comparing the short lifetimes (less than a few hours) of some clouds with calculated sedimentation velocities. Separate constraints from infrared spectra measured by MCS suggest CO2 cloud particles in the size range 10–100μm. Snow particles are not likely to re-sublime before reaching the surface, because the lower atmosphere in this region remains near saturation with respect to CO2. Based on cooling rate calculations, snowfall originating below 4km altitude likely contributes a comparable or greater amount to the seasonal deposits than the rest of the atmosphere. Due to the positive feedback between cloud particle number density and radiative cooling, CO2 snow clouds should propagate until they become limited by the availability of condensation nuclei or CO2 gas. Over the south polar residual cap, where cloud activity is greatest, atmospheric radiative cooling rates are high enough to offset heat advected into the polar regions and maintain consistent snowfall. At latitudes of 60–80°S the lower atmosphere tends to be slightly sub-saturated and rapid cooling by mechanical lift driven by orography or convergent flow may be required to initiate a snowstorm, consistent with the more sporadic clouds observed by MCS in this region, and their correlation with topographic features. Snowfall and accumulation at the surface are found to be inevitable consequences of the polar energy budget, unless advection redistributes heat from lower latitudes in much greater quantities than expected.