Material ejection by the cold jets and temperature evolution of the south seasonal polar cap of Mars from THEMIS/CRISM observations and implications for surface properties

As the seasonal CO2 ice polar caps of Mars retreat during spring, dark spots appear on the ice in some specific regions. These features are thought to result from basal sublimation of the transparent CO2 ice followed by ejection of regolith-type material, which then covers the ice. We have used Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) reflectance data, Thermal Emission Imaging System (THEMIS) visible images, and THEMIS-derived temperature retrievals along with a thermal numerical model to constrain the physical and compositional characteristics of the seasonal cap for several areas exhibiting dark spots at both high spatial and temporal resolutions. Data analysis suggests an active period of material ejection (before solar longitude (Ls) 200), accumulation around the ejection points, and spreading of part of the ejected material over the whole area, followed by a period where no significant amount of material is ejected, followed by complete defrosting (approximate to Ls 245). Dark material thickness on top of the CO2 ice is estimated to range from a few hundreds of microns to a few millimeters in the warmest spots, based on numerical modeling combined with the observed temperature evolution. The nature of the venting process and the amount of material that is moved lead to the conclusion that it could have an important impact on the surface physical properties.