Mars odyssey neutron sensing of the south residual polar cap

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Abstract
Measurements from the neutron spectrometer instrument of the Mars Odyssey Gamma‐Ray Spectrometer (GRS) package for solar aereocentric longitude between 340° and 360° are used to estimate the CO2 mass, dust content, and covered area of the south residual polar cap. Two models for the cap are explored; a cap uniformly covered in dust‐free CO2 and a “Swiss cheese” model with an exposed H2O ice substrate and 8m mesas composed of CO2 and dust. A range of 1.0 to 3.0 × 1017 g is found for the mass of residual CO2 on the cap, or about 3 to 9% of the total CO2 seasonally cycled in and out of the atmosphere. For the Swiss cheese model, if the CO2 is dust‐free then 60% of the cap area must be the exposed H2O ice substrate. This area decreases with increasing dust concentration in the CO2. For example, if 45% of the cap area is exposed water ice then 224 g/cm2 of dust is in the CO2 and for a typical dust deposition rate of 0.9 × 10−3 g/cm2/yr, the dust can be deposited in ∼25,000 yrs.