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

Abstract

${\text{CO}}_2$ ice clouds are important for polar energy balance and the carbon dioxide cycle on Mars. However, uncertainties remain regarding their physical and radiative properties, which control how polar ${\text{CO}}_2$ clouds interact with the global Martian climate. Here, we use Mars Climate Sounder (MCS) observations of atmospheric radiance to estimate these physical and radiative properties. We find that Martian ${\text{CO}}_2$ clouds are typically composed of large particles from a narrow size distribution with an effective radius of 46 $\mu$m and an effective variance of $2.0\times 10^{-3}$ in the southern hemisphere, and an effective radius of 42 $\mu$m and an effective variance of $2.0\times 10^{-3}$ in the north. The similarity in sizes of ${\text{CO}}_2$ ice particles in both hemispheres may be due to the fact that ${\text{CO}}_2$ 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 small effective variance we derive may be a consequence of the fact that ${\text{CO}}_2$ is also the dominant atmospheric constituent on Mars, which allows ${\text{CO}}_2$ ice particles to reach sizes upwards of 10 $\mu$m 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 ${\text{CO}}_2$ ice opacity profiles to the MCS retrieval pipeline.