Transport of Water Ice into the Polar Regions of Mars through Scavenging by CO2 Snowfall

Abstract:

The Martian water cycle is tightly coupled to the polar regions, where the residual polar caps exchange water with the atmosphere on seasonal and multi-annual timescales. Understanding the exchange of water into and out of the polar regions is thus crucial in understanding the water cycle on Mars. While there is no evidence that water ice snowfall contributes significant amounts to the seasonal ice caps, previous studies have shown that CO₂ snowfall is frequent in both hemispheres and deposits material onto the polar caps. Due to the abundance of H₂O ice particles in the wintertime atmosphere above the polar region, we hypothesize that these micron-sized water ice particles act as condensation nuclei (CN) for the CO₂ ice particles and are deposited onto the polar caps during the CO₂ snowfall process. H₂O ice cloud particles may thus contribute a significant amount of water to the seasonal and residual polar caps through incorporation as CN in CO₂ snowfall.

In this study we utilize atmospheric retrievals of temperature and CO₂ ice cloud opacity from the Mars Climate Sounder (MCS) on board NASA’s Mars Reconnaissance Orbiter coupled with simple models to determine sedimentation rates of CO₂ ice onto the surface of the caps. Since the MCS data show prevalent H₂O ice clouds in the polar winter atmosphere, we can then use these snowfall rates to quantify the possible amount of water ice CN contributed to the seasonal polar caps via CO₂ snowfall.

Our results show sedimentation rates of CO₂ ranging from 10^-6 to 10^-4 kg m^-2 s^-1 within a winter season, with anywhere from 10¹⁰ to 10¹² kg of water deposited on the poles per season via CO₂ snowfall depending on particle size (r = 1 to 4 µm). This is equivalent to a water ice layer 1-100 microns thick distributed evenly over the pole, which is comparable to the amount incorporated into the seasonal deposits as determined by OMEGA, CRISM, and MCS surface observations, thus showing that this process plays a key role in the seasonal exchange of volatiles on Mars.