A widespread low-latitude diurnal CO2 frost cycle on Mars revealed by Mars Climate Sounder observations

Abstract:

Surface observations by the Mars Climate Sounder (MCS) onboard the Mars Reconnaissance Orbiter document the widespread occurrence of atmospherically-corrected ground temperatures consistent with the presence of extensive CO₂ frost deposits at low latitudes at 3AM. While the detection of CO₂ ice has previously been reported outside the polar regions, in discrete locations associated with very high elevation terrains (i.e. associated with Elysium, Olympus Mons and the Tharsis Montes), our observations provide the first observational evidence of a global-scale diurnal CO₂ cycle involving the atmosphere and the surface.

MCS surface temperatures consistent with the presence of ephemeral diurnal CO₂ frost deposits are strongly correlated with geologic terrains characterized by high emissivity, high albedo, and low thermal inertia. All of these properties are conducive to low nighttime temperatures. Thermal modeling of these low inertia terrains (i.e. < 75 J m^-2K^-1s^-1/2), which cover a large fraction of the Martian surface, confirms that the regolith frequently reaches CO₂ frost point temperatures at all latitudes near 3AM, with seasonal variations noticed. In addition, models used in conjunction with MCS temperature retrievals also predict that right before sunrise, nearly all of the Martian dusty low thermal inertia units are frosted at some point during the Mars Year. Energy/mass balance modeling suggests deposits with depths of up to several hundreds of microns of CO₂ (<<0.1% of the Martian atmosphere). This is not sufficient to generate a measurable diurnal pressure cycle. The discovery of this diurnal CO₂ cycle may nonetheless represent an important step forward in our understanding of the way the Mars atmosphere interacts with the surface.