Physics
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p31d1730g&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P31D-1730
Physics
[5410] Planetary Sciences: Solid Surface Planets / Composition, [5470] Planetary Sciences: Solid Surface Planets / Surface Materials And Properties, [5494] Planetary Sciences: Solid Surface Planets / Instruments And Techniques, [6225] Planetary Sciences: Solar System Objects / Mars
Scientific paper
The Alpha Particle X-ray Spectrometer (APXS) on board the MER rover Opportunity has documented the chemical composition of the bedrock at Meridiani Planum over the traverse of ~33 km so far. The bedrock is very high in sulfate, up to ~ 25 weight percents SO3, interpreted as sedimentary sandstone. The high precision and consistency of the acquired APXS data, mainly of the abraded samples, allowed the characterization of the homogeneous bedrock over the traverse. Inside Victoria and Endurance Crater the abundance of magnesium and sulfur dropped in a 1:1 molar ratio by about 30 % in parallel with an increase of chlorine by a factor of 3. This inferred the presence of magnesium sulfate and an unknown chlorine-compound. Moreover the identical change in soluble minerals between Victoria and Endurance craters (~6 km apart) might indicate a large scale change in a subsurface water table in the past. Using the scatter peak method in the APXS spectra, the excess of oxygen was determined to be equivalent to ~ 14% bound water for the average Meridiani outcrop. Besides bedrock, basaltic soil and a lag of hematitic concretions, the rover encountered several erratic rocks sitting on the plains. Both, iron-nickel meteorites and cobbles with a basaltic mineralogy are suggestive of emplacement as meteorites. The basaltic rocks, Bounce Rock and Marquette, are ejecta from different regions on Mars. The APXS data of even unbrushed surfaces, typically a mixture or airborne dust, soil and alteration rinds, clearly indicated significant differences to the dominating bedrock. Opportunity is expected to reach the rim of the Noachian-aged Endurance Crater in August 2011, where orbital instruments detected evidence for polyhydrated sulfates, Fe-Mg Smectites, as well as basaltic materials. It is unprecedented for a rover to drive into an area with these significant alteration minerals predicted from orbit. While the mineral spectrometers on the rover are now significantly degraded, the APXS will be able to detect even subtle changes in composition in this new terrain, possibly infer mineralogy from chemistry and shed new insights how to combine and reconcile orbital data with in-situ measurements. Polyhydrated sulfates are already implied by chemistry and mineralogy of the bedrock, but phyllosilicates would be a new mineral species encountered by a landed mission and could give a first taste of what is to be expected at the selected MSL landing site at Gale Crater.
Arvidson Ray E.
Campbell Lucy J.
Clark Ben C.
Gellert Ralf
Squyres Steve
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