Computer Science
Scientific paper
Apr 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003eaeja.....8784m&link_type=abstract
EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6 - 11 April 2003, abstract #8784
Computer Science
Scientific paper
Low albedo patches in Valles Marineris and neighboring Chasmata were often interpreted as volcanic rocks younger than the formation of the depressions. The data from the Imaging Spectrometer for Mars (ISM) onboard Mars Phobos showed that these layers are rich in pyroxene and contain a significant level of hydratation that were interpreted as hydro-volcanism. However, new MOC images and THEMIS IR images show that these low albedo regions are composed of dark sand dunes, sand sheets and eolian mantling. So, the material does not correspond to in-situ rock outcrops. Dark sands could correspond to the desegregation of the chasma walls or, alternatively, to accumulation of cinders due to late explosive activity of Tharsis Monses. To help address such questions, we have tried to determine the composition of these low albedo regions by modeling with a radiative transfer model several hundred spectra acquired in 1989 by the ISM instrument on the Phobos-II spacecraft Three types of surface were investigated: dust (mixture of particles of size << wavelength), sand (intimate mixture of particles of size >> wavelength), dust/sand mixture. Detailed results are presented in a companion paper (Poulet et al., this issue). Magnetite, hematite dust and pyroxenes sands are likely while obsidian, feldspars and phyllosilicates are unlikely. Such result favors sands accumulation resulting from chasma walls erosion. According to that hypothesis, the source rocks of the volcanic material is not younger than the chasmata and can be early Hesperian or Noachian. The hydratation could thus correspond to an early phase of hydromagmatism or explosive volcanism that was recorded in the several kilometers thick assemblage of Tharsis bulge lavas. The understanding of the nature of such layers is important to understand the erosional history of Valles Marineris but also in order to understand areas like Syrtis Major and Acidialia Planitia where the composition is still controversial (basalt-andesite-hydrated basalt end-members). Indeed, these regions are all covered by lot of dust or sand accumulation that could have a composition quantitatively different than the volcanic source rocks.
Erard Stephane
Gendrin Aline
Mangold Nicolas
Poulet François
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