Mathematics – Logic
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p13c..06r&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P13C-06
Mathematics
Logic
[5410] Planetary Sciences: Solid Surface Planets / Composition, [5420] Planetary Sciences: Solid Surface Planets / Impact Phenomena, Cratering, [5464] Planetary Sciences: Solid Surface Planets / Remote Sensing, [5470] Planetary Sciences: Solid Surface Planets / Surface Materials And Properties
Scientific paper
Thermal infrared spectral variations reflect changes in the bulk composition of the Martian surface. Evaluating the role of igneous, mechanical alteration, and chemical weathering processes in producing changes in bulk composition is crucial for reconstructing the environmental history of Mars. Detailed spectral mapping in Iapygia Terra (study region 0-30°S, 50-100°E) with TES, THEMIS and CRISM data shows spectral variations corresponding with major geomorphic boundaries including crater fill, ejecta blankets and central peaks, and boundaries between degraded plains and younger intercrater plains. Features in Iapygia Terra have been divided into classes on the basis of near and thermal infrared spectral properties, thermal inertia, and geomorphic context. Detailed observations of the geologic context, spectral properties, and stratigraphy of exposed units in Iapygia Terra may help to resolve the role of both mechanical and chemical alteration processes in producing observed spectral units. For example, one feature class identified is thermally-distinct impact ejecta blankets; these are considered important because thermal contrast from surrounding materials helps to identify craters whose ejecta blankets are still relatively intact and exposed at the surface, and thus providing access for spectral study of subsurface materials. Approximately 60% of thermally-distinct crater ejecta blankets in Iapygia exhibit spectral differences from the surrounding plains, and in particular have a stronger spectral feature near 465 cm-1, suggesting elevated abundance of glass, secondary silica or phyllosilicates [Ruff and Christensen, 2007, GRL]. Possible reasons for the spectral differences include 1) an impact-related process such as shock or production of melt glass, 2) exposure of subsurface material that is more altered than the plains surface, or 3) exposure of a subsurface material that is less altered than the plains surface (where impacts into older degraded plains reveal a less weathered, more “original” composition). The first suggestion is less likely, because thermally distinct craters in Hesperian-aged surfaces in the region generally do not show spectral differences from the target material. Distinguishing between 2 and 3 is more difficult, however. The 465 cm-1 feature is not necessarily diagnostic of alteration. CRISM MRDR data over the spectrally-distinct craters show that ~30% of them have evidence of a 2.3 µm feature found in phyllosilicate minerals. In these cases, subsurface alteration is the best explanation. However, in the other 70% of craters, either explanation is possible. If it is the latter, it would indicate that (the limited) weathering has been largely surficial and constrained to the more ancient past. Detailed analyses of the full TES spectral range, rather than just the 465 cm-1 index maps, as well as examination of the regional distribution of these craters may help to distinguish these scenarios.
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