Advance and Recession of the Southern Seasonal Polar Cap as Observed by the Mars Odyssey Neutron Spectrometer

Details Advance and Recession of the Southern Seasonal Polar Cap as Observed by the Mars Odyssey Neutron Spectrometer Advance and Recession of the Southern Seasonal Polar Cap as Observed by the Mars Odyssey Neutron Spectrometer

Dec 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufm.p21a..06p&link_type=abstract

American Geophysical Union, Fall Meeting 2003, abstract #P21A-06

Physics

5405 Atmospheres: Composition And Chemistry, 5409 Atmospheres: Structure And Dynamics, 5462 Polar Regions, 5494 Instruments And Techniques, 6225 Mars

Scientific paper

Data acquired by the neutron spectrometer aboard 2001 Mars Odyssey cover the advance and much of the recession of the seasonal CO2 cap in the southern hemisphere during 2002 and 2003 (from LS 0° to LS 250° ). We analyzed neutron counting data to determine the areal density (units of g/cm2) of the CO2 frost as a function of latitude on 5° LS intervals during southern winter. We determined that the seasonal inventory of surface CO2 achieved a maximum at 8x1018 g, roughly 30% of the total atmospheric mass, between 160° and 170° LS. The measured latitude dependence of the phase angle of the peak in CO2 frost inventory was generally consistent with predictions by the Ames Research Center General Circulation Model (GCM). The maximum depth of CO2, averaged over the footprint of the spectrometer (roughly 600 km full width at half maximum), was observed to be 105 g/cm2. Accompanying the build up of CO2 frost during winter was a local enrichment of the non-condensible portion of the polar atmosphere, which includes N2 and Ar. The enrichment of N2 and Ar was evident from loops traced in parametric plots of thermal versus epithermal count rates as a function of time. Thermal and epithermal neutron count rates depend on the amount of CO2 frost on the surface. However, while epithermal neutrons are not significantly affected by atmospheric composition, thermal neutrons are strongly absorbed by atmospheric N2 and Ar. We find that for a given epithermal count rate (or, equivalently, areal density of CO2 frost), the thermal neutron count rate is lower during the build up of frost during winter than during the recession. Based on this observation, we conclude that the areal density of non-condensibles in the polar atmosphere increases during winter as CO2 is deposited on the surface, and decreases during spring as the frost sublimates and replenishes the atmosphere with CO2. The observed variation in the thermal neutron count rates is consistent with roughly a factor-of-three seasonal variation in the abundance of N2+Ar at the south pole. In this study, we use epithermal and thermal neutron count rates to determine the amount of N2+Ar in the atmosphere and the areal density of the seasonal CO2 frost as a function of position and time. The distribution of CO2 frost determined from neutron spectroscopy is compared to orbital photography and telescopic observations of the seasonal cap boundary. The seasonal variation of N2+Ar is used to estimate the degree of horizontal mixing of the polar atmosphere. The measured distribution of surface frost and atmospheric composition are compared with predictions by the GCM.

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