HR: 0800h
AN: P11A-0956    [Abstracts]
TI: Evidence for high-silica, sulfate-bearing sedimentary rocks in Acidalia Planitia from TES and THEMIS data
AU: * Michalski, J R
EM: michalski@asu.edu
AF: Dept. of Geological Sciences, Arizona State University, Tempe, AZ 85287-1404 United States
AU: Rogers, D
EM: adr@asu.edu
AF: Dept. of Geological Sciences, Arizona State University, Tempe, AZ 85287-1404 United States
AU: Kraft, M D
EM: mdkraft@asu.edu
AF: Dept. of Geological Sciences, Arizona State University, Tempe, AZ 85287-1404 United States
AU: Christensen, P R
EM: phil.christensen@asu.edu
AF: Dept. of Geological Sciences, Arizona State University, Tempe, AZ 85287-1404 United States
AU: Sharp, T G
EM: tom.sharp@asu.edu
AF: Dept. of Geological Sciences, Arizona State University, Tempe, AZ 85287-1404 United States
AB: Thermal infrared spectra of some layered materials in Acidalia Planitia suggest a mineralogy dominated by high-silica-poorly crystalline materials, intermediate-composition feldspars, and sulfates. These spectra are similar in shape to the Acidalia Planitia-type global spectral component (ST2) previously identified by Bandfield et al. [2000], but are more sulfate- and silica-rich than ST2. The mineralogy in Acidalia Planitia is equivocal and could be explained through a variety of contexts, including igneous rock compositions, volcanicastic sedimentary rocks, or alteration products related to rock surfaces (rather than bulk-rock compositions). If the materials indicate unaltered, primary igneous mineralogy, it would suggest the presence of intermediate-felsic (dacitic) lavas or pyroclastics. While a volcanic origin cannot be ruled out, the observation of widespread layering at a vertical scale of 10s of meters, lack of observed volcanic landforms, and location in a basin that is down-slope from fluvial channels all point to a sedimentary origin for these units. The spectral signature of surfaces in Acidalia Planitia require a high-silica (Si/O $>$0.4) material to be present which, though spectrally similar to felsic volcanic glass, could also be explained as aluminous-ferric opaline silica or an intimate mixture of pure opal with allophane, clay minerals, zeolites, or possibly other aluminosilicates. The high-silica materials in Acidalia Planitia appear to be tied to relatively high thermal inertia surfaces, which are overlain by a lower thermal inertia, discontinuous, dissected, and possibly ice-rich surface layer. We propose that the observed surface compositions are related to either: 1) chemical weathering products, possibly related to recent periglacial environmental conditions, or 2) a bulk component (cement?) within a sedimentary layer that was deposited during episodic flooding. The poorly crystalline nature of high-silica materials and lack of evidence for abundant crystalline, indurated clay minerals are both consistent with formation in a short-lived aqueous sedimentary system or with formation of weathering products in low-water surface conditions. In either case, the explanation of these surface materials has major implications for the interpretation of global TES surface compositions. It is intriguing that the silicate and sulfate spectral components of some Acidalia Planitia surfaces are similar to those of the sulfate-rich rocks at the Mars Exploration Rover landing site in Meridiani Planum, which are interpreted as aqueous sediments. It is possible that Meridiani-type rocks (minus the hematite) are present in the northern plains of Mars and related to episodic flooding throughout Mars' history.
DE: 5410 Composition
DE: 5415 Erosion and weathering
DE: 5470 Surface materials and properties
DE: 5480 Volcanism (8450)
DE: 6225 Mars
SC: Planetary Sciences [P]
MN: 2004 AGU Fall Meeting