TES premapping data: Slab ice and snow flurries in the Martian north polarnight

In the 1970s, Mariner and Viking spacecraft observations of the north polar region of Mars revealed polar brightness temperatures that were significan tly below the expected kinetic temperatures for CO2 sublimation. For the pa st few decades, the scientific community has speculated as to the nature of these Martian polar cold spots. Thermal Emission Spectrometer (TES) therma l spectral data have shown these cold spots to result largely from fine-gra ined CO2 and have constrained most of these cold spots to the surface (or n ear-surface). Cold spot formation is strongly dependent on topography, form ing preferentially near craters and on polar slopes. TES data, combined wit h Mars Orbiter Laser Altimeter (MOLA) cloud data, suggest atmospheric conde nsates form a small fraction of the observed cold spots. TES observations o f spectra close to a blackbody indicate that another major component of the polar cap is slab CO2 ice; these spectrally bland regions commonly have a low albedo. The cause is uncertain but may result from most of the light be ing reflected toward the specular direction, from the slab ice being intrin sically dark, or from it being transparent. Regions of the cap where the di fference between the brightness temperatures at 18 mum (T-18) and 25 mum (T -25) is less than 5 degrees are taken to indicate deposits of slab ice. Sla b ice is the dominant component of the polar cap at latitudes outside of th e polar night.