Time-variability of Polar Winter Snow Clouds on Mars

Abstract:

Carbon dioxide snow clouds are known to occur in the polar regions on Mars during the long polar night. Earlier studies have shown that a substantial fraction (up to ~20%) of the seasonal ice caps of Mars can be deposited as CO₂ snowfall. The presence of optically thick clouds can also strongly influence the polar energy balance, by scattering thermal radiation emitted by the surface and lower atmosphere. Furthermore, snow deposition is likely to affect the surface morphology and subsequent evolution of the seasonal caps. Therefore, both the spatial distribution and time variability of polar snow clouds are important for understanding their influence on the Martian CO₂cycle and climate. However, previous investigations have suffered from relatively coarse time resolution (typically days), coarse or incomplete spatial coverage, or both.

Here we report results of a dedicated campaign by the Mars Climate Sounder (MCS) onboard the Mars Reconnaissance Orbiter, to observe polar CO₂ clouds with an unprecedented time-resolution within the same spatial region. By scanning the MCS field of view, we acquired observations directly over the north pole for every ~2hr orbit over the course of several days. This was repeated during two separate periods in northern winter. The 2 hr sampling frequency enables the detailed study of cloud evolution. These observations were also compared to a cloud-free, control region just off the pole, which was sampled in the same way. Results from this experiment show that the north polar CO₂ clouds are dynamic, and appear to follow a consistent pattern: Beginning with a relatively clear atmosphere, the cloud rapidly grows to ~25 – 30 km altitude in < 2 hr. Then, the altitude of the cloud tops diminishes slowly, reaching near the surface after ~6 – 10 hr. We interpret this slow decay as the precipitation of snow particles, which constrains their size to be ~10 – 100 μm. Also pervasive in this season are water ice clouds, which may provide condensation nuclei for the CO₂. The interplay between these two atmospheric aerosol species on short timescales is a potentially fruitful area of future research, enabled by these unique observations.

Part of this work was performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.