Constraining the Physical and Spectral Properties of Martian Polar Clouds with Mars Climate Sounder Observations

The Martian poles are complex regions whose dynamics strongly influence Mars' atmospheric conditions on a global scale. Because the Martian atmosphere is 95% carbon dioxide, the interaction between the polar ice caps, which represent seasonal reservoirs of carbon dioxide ice on the Martian surface, and the atmosphere effectively regulates Mars' global mean surface pressure throughout the year. Previous work suggests that the stability and evolution of the Martian seasonal ice caps are strongly controlled by the optical properties and the amount of CO2 snowfall during polar winter (Hayne et al. 2012; Gary-Bicas et al. 2020). However, the aerosol size and bulk composition of the clouds from which this snowfall is derived are currently not well understood. Atmospheric retrievals such as those returned by the Mars Climate Sounder (MCS) onboard the Mars Reconnaissance Orbiter are also highly influenced by the composition and optical properties of aerosols suspended in the atmosphere. Constraints on these quantities are therefore critical for not only deepening our understanding of the seasonal ice caps, but also improving the accuracy of atmospheric retrievals in the Martian polar regions.

As part of previous work, CO2 ice opacity profiles were retrieved from a small subset of MCS observations during polar winter months using plausible optical constants for Martian CO2 ice, instead of dust. In this work, we compare the measured and calculated radiance profiles for this subset of observations across each of the MCS spectral channels in order to better constrain the optical parameters of Martian CO2 ice cloud particles. Significant variability in the shapes of these radiance profiles suggests a potentially high degree of compositional diversity within Martian polar clouds, especially surrounding the south pole. Other early results indicate possible cloud particle size variability with altitude and season.