HR: 09:15h
AN: P21A-06    [PDF]
TI: Advance and Recession of the Southern Seasonal Polar Cap as Observed by the Mars Odyssey Neutron Spectrometer
AU: * Prettyman, T H
EM: thp@lanl.gov
AF: Los Alamos National Laboratory, Space and Atmospheric Sciences group, Los Alamos, NM 87545 United States
AU: Wiens, R C
EM: rwiens@lanl.gov
AF: Los Alamos National Laboratory, Space and Atmospheric Sciences group, Los Alamos, NM 87545 United States
AU: Murphy, J R
EM: murphy@nmsu.edu
AF: New Mexico State University, Department of Astronomy, Las Cruces, NM 88011 United States
AU: Feldman, W C
EM: wfeldman@lanl.gov
AF: Los Alamos National Laboratory, Space and Atmospheric Sciences group, Los Alamos, NM 87545 United States
AU: Elphic, R C
EM: relphic@lanl.gov
AF: Los Alamos National Laboratory, Space and Atmospheric Sciences group, Los Alamos, NM 87545 United States
AU: Funsten, H O
EM: hfunsten@lanl.gov
AF: Los Alamos National Laboratory, Space and Atmospheric Sciences group, Los Alamos, NM 87545 United States
AU: Lawrence, D J
EM: djlawrence@lanl.gov
AF: Los Alamos National Laboratory, Space and Atmospheric Sciences group, Los Alamos, NM 87545 United States
AU: Linn, R R
EM: rrl@lanl.gov
AF: Los Alamos National Laboratory, Space and Atmospheric Sciences group, Los Alamos, NM 87545 United States
AU: Maurice, S
EM: maurice@obs-mip.fr
AF: Observatoire Midi-Pyrenees, 14 avenue Edouard Belin, Toulouse, 31400 France
AU: Tokar, R L
EM: rlt@lanl.gov
AF: Los Alamos National Laboratory, Space and Atmospheric Sciences group, Los Alamos, NM 87545 United States
AB: Data acquired by the neutron spectrometer aboard 2001 Mars Odyssey cover the advance and much of the recession of the seasonal CO$_{2}$ cap in the southern hemisphere during 2002 and 2003 (from L$_{S}$ $0\deg$ to L$_{S}$ $250\deg$). We analyzed neutron counting data to determine the areal density (units of g/cm$^{2}$) of the CO$_{2}$ frost as a function of latitude on $5\deg$ L$_{S}$ intervals during southern winter. We determined that the seasonal inventory of surface CO$_{2}$ achieved a maximum at 8x10$^{18}$ g, roughly 30% of the total atmospheric mass, between $160\deg$ and $170\deg$ L$_{S}$. The measured latitude dependence of the phase angle of the peak in CO$_{2}$ frost inventory was generally consistent with predictions by the Ames Research Center General Circulation Model (GCM). The maximum depth of CO$_{2}$, averaged over the footprint of the spectrometer (roughly 600 km full width at half maximum), was observed to be 105 g/cm$^{2}$. Accompanying the build up of CO$_{2}$ frost during winter was a local enrichment of the non-condensible portion of the polar atmosphere, which includes N$_{2}$ and Ar. The enrichment of N$_{2}$ 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 CO$_{2}$ frost on the surface. However, while epithermal neutrons are not significantly affected by atmospheric composition, thermal neutrons are strongly absorbed by atmospheric N$_{2}$ and Ar. We find that for a given epithermal count rate (or, equivalently, areal density of CO$_{2}$ 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 CO$_{2}$ is deposited on the surface, and decreases during spring as the frost sublimates and replenishes the atmosphere with CO$_{2}$. The observed variation in the thermal neutron count rates is consistent with roughly a factor-of-three seasonal variation in the abundance of N$_{2}$+Ar at the south pole. In this study, we use epithermal and thermal neutron count rates to determine the amount of N$_{2}$+Ar in the atmosphere and the areal density of the seasonal CO$_{2}$ frost as a function of position and time. The distribution of CO$_{2}$ frost determined from neutron spectroscopy is compared to orbital photography and telescopic observations of the seasonal cap boundary. The seasonal variation of N$_{2}$+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.
DE: 5405 Atmospheres--composition and chemistry
DE: 5409 Atmospheres--structure and dynamics
DE: 5462 Polar regions
DE: 5494 Instruments and techniques
DE: 6225 Mars
SC: Planetary Sciences [P]
MN: 2003 Fall Meeting