Other
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p33c..08r&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #P33C-08
Other
5410 Composition (1060, 3672), 5464 Remote Sensing, 5470 Surface Materials And Properties, 5494 Instruments And Techniques
Scientific paper
For decades, evidence of phyllosilicate minerals on Mars was absent or inconclusive. With the advent of two visible/near infrared (VNIR) imaging spectrometers, the Mars Express OMEGA and Mars Reconnaissance Orbiter CRISM, spectral evidence has emerged that indicates hundreds to thousands of isolated occurrences of phyllosilicates around the planet. These locations appear to be limited to the oldest (Noachian) parts of the Martian crust. Dioctahedral and trioctahedral clay minerals, mica, and chlorite have been identified. In contrast to these discoveries made from orbit, no definitive identification of phyllosilicates has been made on the ground. The two Mars Exploration Rovers carry two instruments capable of such identifications: a Mössbauer spectrometer (MB) for the identification of Fe-bearing minerals and a thermal infrared spectrometer (Mini-TES) for the identification of a wide range of primary and secondary minerals. The bedrock at the Meridiani Planum landing site shows some evidence for nontronite (an Fe-smectite) based on Mini-TES results, but this was not corroborated by MB data. At the Gusev Crater landing site, highly altered rocks were encountered in the Columbia Hills, some of which show strong indications of phyllosilicates based upon chemical data from the Alpha Particle X-ray Spectrometer (APXS). However, neither Mini-TES nor MB could confirm their presence even though data from these instruments show clear evidence for other hydrated secondary phases in several locations. Laboratory thermal infrared (TIR) spectroscopy clearly shows sensitivity to even thin (<10 microns) coatings of phyllosilicates. Two features in the low wavenumber range covered by TES and Mini-TES (near 530 and 465 cm-1) are especially diagnostic of the presence of phyllosilicates and primary and secondary amorphous silicates. A feature near 465 cm-1 has been identified in Mini-TES spectra of rocks in the Columbia Hills that MB and APXS spectra indicate are highly altered. A similar feature has been mapped globally using TES spectra and may serve as a proxy for some of the rocks encountered in the Columbia Hills. I have now discovered that many of the locations identified by OMEGA and CRISM as phyllosilicate- bearing display this feature, including parts of Mawrth Vallis, Nili Fossae, and NE Tyrrhena Terra. Full TES spectra from one location adjacent to Nili Fossae look remarkably similar to the Assemblee-type rocks in the Columbia Hills that have the chemical signature of montmorillonite. Additionally, I have found that one of the locations in the Nili Fossae region identified as phyllosilicate-bearing by CRISM shows the TES 530 cm-1 feature along with the 465 cm-1 feature. Together, these are indicative of a dioctahedral smectite. The full spectra strongly resemble that of weathered Columbia River Basalt. Despite these examples of apparent agreement between TIR and VNIR observations, there are other examples in which phyllosilicates mapped by OMEGA and/or CRISM display no diagnostic features in TES spectra or conversely, the TES 465 cm-1 feature has no OMEGA/CRISM counterpart. The latter case may represent locations where unaltered volcanic glass or impact melt is present. The former case may indicate something about the mineralogy, abundance, or physical character of the identified phyllosilicates. In either case, the combination of TIR and VNIR observations provides a potential link between ground observations from the rovers and orbital observations around the globe. A more complete picture of Martian phyllosilicates is likely to emerge with these combined observations, which will become increasingly important in the landing site selection process for the upcoming Mars Science Laboratory rover.
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