Asymmetries in Snowfall, Emissivity, and Albedo of Mars' Seasonal Polar Caps: Mars Climate Sounder Observations. Issue 5 (5th May 2020) - Gary‐Bicas, C. E.
Abstract: The stability of the residual carbon dioxide cap near the south pole of Mars is currently not well understood. The cap's survival depends on its radiation budget, controlled by the visible albedo and infrared emissivity. We investigated the role of CO2 snowfall in altering the albedo and emissivity, leading to the observed asymmetry in the net CO2 accumulation at the two poles. Uncontaminated snowfall increases albedo, and lowers emissivity, due to scattering by optically thick clouds and granular surface deposits. Data from the Mars Climate Sounder (MCS) show that fall and winter snowfall is correlated with higher springtime albedo at both poles. For the seasonal CO2 deposits in each polar region >60° latitude, we find mean albedo values of 0.39 in the north and 0.51 in the south, and winter 32‐μm emissivity values of 0.84 in the north and 0.87 in the south. Using a radiative transfer model and the MCS data, we find that the north polar deposits have ∼10× higher dust content than those in the south, explaining the ∼31% lower albedo of the north seasonal cap during spring. Our model shows that greater amounts of snowfall can explain the ∼4% lower emissivity of the north polar seasonal cap. These findings demonstrate that winter snowfall and dust transport affect the composition of Mars' seasonal ice caps and polar energy balance. Snowfall and dust loading are therefore important in modeling the CO2 cycle on Mars, as well as the planet's long‐term climate variations. Plain Language Summary: The permanent carbon dioxide (CO2, also known as dry ice) deposits on Mars control the planet's global atmospheric pressure. Seasonal fluctuations of the CO2 cycle drive pressure variations measured at the surface anywhere on the planet. These variations are buffered by the stable permanent CO2 deposit at the south pole. The north pole has the more favorable altitude and pressure to be in equilibrium with the atmosphere, but satellite observations showed that the residual cap at the south pole was in equilibrium and not the north. Other studies found evidence of carbon dioxide snowfall in both polar regions. Using data from Mars Climate Sounder on board the Mars Reconnaissance Orbiter, we found that there is an asymmetry between the optical properties of the northern and the southern permanent caps. We attribute this asymmetry to dust and snowfall quantities, which promote more CO2 accumulation in the south, relative to the north. Key Points: Infrared emissivity and visible albedo of Mars' seasonal ice caps are correlated The north seasonal cap has greater snowfall, lower emissivity, and lower albedo Higher dust loading in the northern fall and winter seasons causes lower ice cap albedo