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Science 8 August 2008:
Vol. 321. no. 5890, pp. 830 - 833
DOI: 10.1126/science.1159699
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Reports

Phyllosilicate Diversity and Past Aqueous Activity Revealed at Mawrth Vallis, Mars

Janice L. Bishop,1* Eldar Z. Noe Dobrea,2 Nancy K. McKeown,3 Mario Parente,4 Bethany L. Ehlmann,5 Joseph R. Michalski,6 Ralph E. Milliken,2 Francois Poulet,6 Gregg A. Swayze,7 John F. Mustard,5 Scott L. Murchie,8 Jean-Pierre Bibring6

Observations by the Mars Reconnaissance Orbiter/Compact Reconnaissance Imaging Spectrometer for Mars in the Mawrth Vallis region show several phyllosilicate species, indicating a wide range of past aqueous activity. Iron/magnesium (Fe/Mg)–smectite is observed in light-toned outcrops that probably formed via aqueous alteration of basalt of the ancient cratered terrain. This unit is overlain by rocks rich in hydrated silica, montmorillonite, and kaolinite that may have formed via subsequent leaching of Fe and Mg through extended aqueous events or a change in aqueous chemistry. A spectral feature attributed to an Fe2+ phase is present in many locations in the Mawrth Vallis region at the transition from Fe/Mg-smectite to aluminum/silicon (Al/Si)–rich units. Fe2+-bearing materials in terrestrial sediments are typically associated with microorganisms or changes in pH or cations and could be explained here by hydrothermal activity. The stratigraphy of Fe/Mg-smectite overlain by a ferrous phase, hydrated silica, and then Al-phyllosilicates implies a complex aqueous history.

1 SETI Institute and NASA Ames Research Center, Mountain View, CA 94043, USA.
2 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA.
3 Department of Earth and Planetary Sciences, University of California Santa Cruz, Santa Cruz, CA 95064, USA.
4 Department of Electrical Engineering, Stanford University, Stanford, CA 94305, USA.
5 Department of Geological Sciences, Brown University, Providence, RI 02912, USA.
6 Institut d'Astrophysique Spatiale, Orsay, 91405, France.
7 U.S. Geological Survey, Denver, CO 80225, USA.
8 Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA.

* To whom correspondence should be addressed. E-mail: jbishop{at}seti.org

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