Atmospheric CO2 Ice in the Martian Polar Regions: Physical and Spectral Properties from Mars Climate Sounder Observation

CO2 ice clouds are important for the polar energy balance and carbon dioxide cycle on Mars. However, uncertainties remain regarding their physical and radiative properties, which control how polar CO2 clouds interact with the global Martian climate. Here, we use Mars Climate Sounder (MCS) observations of atmospheric radiance to estimate these bulk physical and radiative properties. We find that Martian CO2 clouds are typically composed of large particles from a narrow size distribution with an effective radius of 46 μm and an effective variance of 0.002 in the southern hemisphere, and an effective radius of 42 μm and an effective variance of 0.002 in the north. The similarity in sizes of CO2 ice particles in both hemispheres may be due to the fact that CO2 clouds tend to form near the same pressure level in each hemisphere, despite the higher surface pressures in the north. We use a simplified convective cooling model to show that the narrow effective variance we derive may be a consequence of the fact that CO2 is also the dominant atmospheric constituent on Mars, which allows CO2 ice particles to reach sizes of > 10 μm within seconds. At the same time, the fact that the Martian atmosphere is so thin means that large particles fall rapidly to the surface, reducing the range of particle sizes that can remain in the atmosphere for any extended period of time. This study is part of ongoing work to add CO2 ice opacity profiles to the MCS retrieval pipeline.