Other
Scientific paper
Dec 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufm.p31a0199s&link_type=abstract
American Geophysical Union, Fall Meeting 2005, abstract #P31A-0199
Other
0702 Permafrost (0475), 5422 Ices, 5462 Polar Regions, 6225 Mars
Scientific paper
Theoretical investigation of ground-ice stability and analysis of Mars Odyssey Gamma Ray Spectrometer (GRS) data have each been used to infer the depth of the ice-table (the boundary between dry and ice-cemented soil in the martian permafrost). Historically, both methods have assumed the martian soil to be homogeneous. However, imagery of the martian surface clearly shows a complex mixture of soils, rocks, and slopes. Remaining discrepancies between theoretical and GRS-inferred ice-table depths may be related to the natural heterogeneities of the surface layer (Mellon et al., 2004). We have therefore employed a new three-dimensional model to investigate the effects of surface rocks, dust, and albedo variations on Martian ground-ice stability. We find that these heterogeneities produce significant undulations/topography in the ice-table at horizontal length scales of a few meters. Near rocks, the ice-table is deeper than its equilibrium depth in homogeneous soil; dust lenses make the ice-table shallower in their vicinity. Decimeter-scale rocks produce a gross vertical deflection (10-30 cm) over a relatively small horizontal range (1-2 rock radii). Comparably sized dust lenses produce a weak vertical deflection (1-3 cm) over a more extended horizontal range (7-8 lens radii). Albedo variations slightly enhance the effects of dark rocks and bright dust. In general, ice-table depth can vary by 10s of cm under mixed surfaces containing rocks, dust, and average soil poleward of 60° N. We have also investigated the factor-of-two discrepancy between theoretical and observational estimates of ice-table depth noted by Mellon et al., but cannot fully resolve it. Ice-table depths derived from GRS neutron data can be up to 10% deeper than theoretical depths in areas where cumulative fractional rock abundance exceeds 20%. Thus, rocks play a significant but not dominant role in the remote sensing data . Other types of heterogeneities, such as slopes and the layered structure of the subsurface at high latitude may also contribute to theoretical/observational differences. Our results have considerable relevance to the selection of a landing site for the Mars Scout Mission Phoenix. They indicate that average ice-table depths inferred from GRS should be viewed as an upper limit, but theoretical depths should not be viewed as a lower limit. Under heterogeneous surfaces, the average geometrical ice-table depth may be shallower than indicated by GRS observations or theory. Additionally, at a heterogeneous landing site, the magnitude of local ice-table undulations may be greater than current differences between observation and theory. Reference: Mellon, M.T., W.C. Feldman, and T.H. Prettyman (2004), Icarus, 169, 324-340.
Mellon Michael T.
Sizemore Hanna G.
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