Discovery of a widespread low‐latitude diurnal CO 2 frost cycle on Mars

While the detection of CO 2 ice has only been reported outside the Martian polar regions at very high elevation (i.e., Elysium, Olympus Mons, and the Tharsis Montes), nighttime surface observations by the Mars Climate Sounder on board the Mars Reconnaissance Orbiter document the widespread occurrence of atmospherically corrected ground temperatures consistent with the presence of extensive carbon dioxide frost deposits in the dusty low thermal inertia units at middle/low latitudes. Thermal infrared emissivities, interpreted in conjunction with mass balance modeling, suggest micrometer size CO 2 ice crystals forming optically thin layers never exceeding a few hundreds of microns in thickness (i.e., 10 −2  kg m −2 ) locally, which is insufficient to generate a measurable diurnal pressure cycle (<<0.1% of the Martian atmosphere). Atmospheric temperatures at middle/low latitudes are not consistent with precipitation of CO 2 ice, suggesting that condensation occurs on the surface. The recurring growth and sublimation of CO 2 ice on Martian dusty terrains may be an important process preventing soil induration and promoting dynamic phenomena (soil avalanching and fluidization and regolith gardening), maintaining a reservoir of micrometer size dust particles that are mobile and available for lifting. The discovery of this diurnal CO 2 cycle represents an important step forward in our understanding of the way the Martian atmosphere interacts with the surface.