Mathematics – Logic
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p43d1457b&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P43D-1457
Mathematics
Logic
[1060] Geochemistry / Planetary Geochemistry
Scientific paper
Since the year 2002, the NASA Mars Odyssey spacecraft (MO) has mapped continuously the entire surface of Mars with its Gamma-Ray Spectrometer (GRS) instrument suite to obtain global concentration maps of different elements. Since January 2004, the NASA Mars Exploration Rovers (MER) Spirit and Opportunity have driven several kilometers at two different landing sites, Gusev crater and Meridiani Planum (180 degrees apart). Each rover made chemical measurements of a range of samples using its Alpha Particle X-Ray Spectrometer (APXS). Distinctive differences between the two instruments are that APXS has a penetration depth of micrometers and a field of view of centimeters vs. tens of centimeters and 500 km for the GRS, respectively. We compare the sulfur concentration determined by the APXS with its GRS counterpart at each MER site. In-situ APXS measurements of soils in Gusev and Meridiani show similar mean S mass fractions, 2.6 and 2.2 % respectively. Abraded rocks in Gusev have a mean S concentration of 1 %. Large enrichments of S were discovered in some subsurface soils in Gusev, which may be confined to smaller areas. In Meridiani, the abraded sedimentary rocks show high S concentrations (up to 12 wt-%). Chemical provinces were delineated - without a priori assumptions on their nature and extent - using GRS datasets. The GRS footprint at Gusev belongs to a chemical province defined by a striking enrichment of Cl and H. Meridiani is remarkable for Ca depletion and H enrichment. Renormalizing to a volatile-free basis, Meridiani is within a distinct province. At the MER landing sites, the large GRS footprint measures comparable S concentrations. The high S contents of the Meridiani sediments are not seen by the GRS. This may be due in part to the negligible area of crater walls relative to soil cover when projected vertically. When orbital data show regional deviation of S (or other elements) relative to the global average, in-situ inspection should reveal a geochemically unique regolith, either as new soil types or as local contributions to the soil from the bedrock. The deep penetration of gamma-rays is sensitive to composition beneath dust veneers, but has also enabled characterization of areas where a thick dust mantle may have been altered and reworked GRS data allows us to characterize large regions with distinctive chemical composition. As a consequence of orbital elemental mapping, chemically enriched or depleted areas can be identified and should be considered for the selection of future landing sites. Such regions would offer unique chemical and, by extension, mineralogical insight including areas where V/NIR instruments are blinded by dust. Orbital elemental measurements complement the findings by rovers and in situ analyses: one identifies a distinctive region, whereas the other takes a close look at one place in it.
Boynton Willam V.
Brueckner Johannes
Hamara Dave
Karunatillake Suniti
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