Physics
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p13a1244p&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P13A-1244
Physics
[3672] Mineralogy And Petrology / Planetary Mineralogy And Petrology, [5464] Planetary Sciences: Solid Surface Planets / Remote Sensing, [6225] Planetary Sciences: Solar System Objects / Mars
Scientific paper
Observations by CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) have revealed a range of minerals in Libya Montes including olivine, pyroxene, and phyllosilicate [1]. Here we extend our spectral analyses of CRISM images in Libya Montes to identify carbonates. We have also performed detailed characterization of the spectral signature of the phyllosilicate- and carbonate-bearing outcrops in order to constrain the types of phyllosilicates and carbonates present. Phyllosilicate-bearing rocks in Libya Montes have spectral bands at 1.42, 2.30 and 2.39 µm, consistent with Fe- and Mg- bearing smectites. The mixture of Fe and Mg in Libya Montes may be within the clay mineral structure or within the CRISM pixel. Because the pixels have 18 meter/pixel spatial resolution, it is possible that the bands observed are due to the mixing of nontronite and saponite rather than a smectite with both Fe and Mg. Carbonates found in Libya Montes are similar to those found in Nili Fossae [2]. The carbonates have bands centered at 2.30 and 2.52 µm. Libya Montes carbonates most closely resemble the Mg-carbonate, magnesite. Olivine spectra are seen throughout Libya Montes, characterized by a positive slope from 1.2-1.8 µm. Large outcrops of olivine are relatively rare on Mars [3]. This implies that fresh bedrock has been recently exposed because olivine weathers readily compared to pyroxene and feldspar. Pyroxene in Libya Montes resembles an Fe-bearing orthopyroxene with a broad band centered at 1.82 µm. The lowermost unit identified in Libya Montes is a clay-bearing unit. Overlying this is a carbonate-bearing unit with a clear unit division visible in at least one CRISM image. An olivine-bearing unit unconformably overlies these two units and may represent a drape related to the Isidis impact, as suggested for Nili Fossae [2]. However, it appears that the carbonate in Libya Montes is an integral portion of the rock underlying the olivine-bearing unit rather than an alteration product, contrasting with proposed stratigraphy for Nili Fossae. The uppermost unit identified is a pyroxene-bearing unit. Some spectra of clays and carbonates in this region present a slope from 1.2 to 1.8 µm similar to olivine. Laboratory experiments were conducted in attempt to understand the relationship of mixtures including olivine, nontronite, and magnesite. The characteristic olivine slope is evident in the spectra in any mixture, even with as little as 10% olivine. In ternary mixtures, the magnesite is almost completely overshadowed by the nontronite and olivine characteristics. The discovery of clays and carbonates in Libya Montes indicates that there was an aqueous environment with neutral pH in the past. In addition, water needs to be relatively still and deep for the small particles to precipitate out and form into the minerals. On Earth, this would be a still lake or deep ocean, and perhaps a similar environment may have been present here in Mars’ past. References [1] Bishop, J. L., et al. (2007) 7th Int'l Mars Conf. [2] Ehlmann, B. L., et al. (2008) Science, 322, 1828. [3] Mustard, J. F., et al. (2008) Nature, 454, 07305.
Bishop Janice L.
McKeown Nancy K.
Perry Karen A.
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