Spectral Mapping of Interior Layered Deposits of Western Candor Chasma by CRISM

Details Spectral Mapping of Interior Layered Deposits of Western Candor Chasma by CRISM Spectral Mapping of Interior Layered Deposits of Western Candor Chasma by CRISM

Dec 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufm.p21c..04m&link_type=abstract

American Geophysical Union, Fall Meeting 2007, abstract #P21C-04

Other

5220 Hydrothermal Systems And Weathering On Other Planets, 5410 Composition (1060, 3672), 5415 Erosion And Weathering, 5464 Remote Sensing, 5470 Surface Materials And Properties

Scientific paper

Western Candor Chasma contains a 3 km-thick sequence of interior layered deposits (ILDs) that may have been emplaced by sedimentary deposition subsequent to formation of Valles Marineris. Proposed genetic mechanisms include subaerial fluvial deposition or volcanism, accumulation of airfall dust, lacustrine evaporite precipitation, hydrovolcanism, or alternatively deep erosion of the chasma wall materials. Observations by the Mars Express/OMEGA spectrometer showed that the ILDs contain both monohydrated and polyhydrated sulfates in close spatial association with fine-grained ferric oxides having distinctive visible-infrared absorptions (Gendrin et al. 2005a,b). In general, OMEGA data also suggest that monohydrated sulfates are associated with steeper slopes and higher albedos than are polyhydrated sulfates (Mangold 2006). CRISM has observed the ILDs in western Candor using both its 200 m/pixel global mapping mode and targeted observations at 20 or 40 m/pixel. CRISM data show spatial heterogeneity in spectral properties to the spatial resolution limit of the instrument. Both monohydrated sulfates (indicated by 2130- and 2400-nm absorptions) and polyhydrated sulfates (indicated by 1450-, 1940-, and 2420-nm absorptions) are evident at all elevations in the ILDs. Polyhydrate signatures occur on intermediate-albedo, relatively intact exposures of stratified material and are rare, but not absent, in nearby erosional debris. Typically the polyhydrate outcrops are low-sloped and form erosion-resistant cap rocks. The monohydrated sulfate also occurs in intermediate- to high-albedo outcrops, but is more commonly distributed as dark, erosional debris on ledges and in depressions that has been modified by wind to form dunes. Only in rare cases can the dark debris be associated with a discrete, dark source layer. The erosional debris exhibits enhancements in sulfate absorptions as well as in 530-, 660-, and 860- to 900-nm absorptions due to ferric iron minerals; different debris deposits have band centers consistent with hematite and with one or more non- hematitic phases. To the spatial resolution limit of CRISM, there is no evidence for comparable sulfate- or ferric- containing materials in the chasma walls, whose spectra are instead dominated by high-Ca pyroxene. Preliminary interpretations of the CRISM data covering western Candor Chasma include: (a) the ILDs have a lithology distinct from the chasma walls; (b) interbedded layers weather to form surfaces with distinct absorptions due to polyhydrated and monohydrated sulfates; (c) the difference in sulfate absorptions may be attributable to deposition of different phases in response to environmental changes or to preferential dehydration / rehydration of sulfate phases in some exposures; and (d) enhancement of ferric iron absorptions in erosional debris is consistent with abrasion generating more optically active, finer-grained particles, possibly from gray hematite- bearing, sulfate-rich layers. An outstanding issue is whether the dark erosional debris is derived locally from thin, poorly resolved layers, is transported tens of kilometers from common sources, or represents a dark component that is sorted from higher-albedo source layers. References: A. Gendrin et al., Science 307, 1587-1591 (2005a); A. Gendrin et al., Lunar and Planetary Science XXXVI, 1378 (2005b); N. Mangold et al., in Martian Sulfates as Recorders of Atmospheric-Fluid-Rock Interactions, 7039 (2006).

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