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Science 19 December 2008:
Vol. 322. no. 5909, pp. 1828 - 1832
DOI: 10.1126/science.1164759
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Reports

Orbital Identification of Carbonate-Bearing Rocks on Mars

Bethany L. Ehlmann,1 John F. Mustard,1 Scott L. Murchie,2 Francois Poulet,3 Janice L. Bishop,4 Adrian J. Brown,4 Wendy M. Calvin,5 Roger N. Clark,6 David J. Des Marais,7 Ralph E. Milliken,8 Leah H. Roach,1 Ted L. Roush,7 Gregg A. Swayze,6 James J. Wray9

Geochemical models for Mars predict carbonate formation during aqueous alteration. Carbonate-bearing rocks had not previously been detected on Mars' surface, but Mars Reconnaissance Orbiter mapping reveals a regional rock layer with near-infrared spectral characteristics that are consistent with the presence of magnesium carbonate in the Nili Fossae region. The carbonate is closely associated with both phyllosilicate-bearing and olivine-rich rock units and probably formed during the Noachian or early Hesperian era from the alteration of olivine by either hydrothermal fluids or near-surface water. The presence of carbonate as well as accompanying clays suggests that waters were neutral to alkaline at the time of its formation and that acidic weathering, proposed to be characteristic of Hesperian Mars, did not destroy these carbonates and thus did not dominate all aqueous environments.

1 Department of Geological Sciences, Brown University, Providence, RI02912, USA.
2 Johns Hopkins University/Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, USA.
3 Institut d'Astrophysique Spatiale, Université Paris Sud 11, 91405 Orsay, France.
4 SETI Institute and NASA Ames Research Center, 515 North Whisman Road, Mountain View, CA 94043, USA.
5 Department of Geological Sciences and Engineering, University of Nevada, MS 172, 1664 North Virginia Street, Reno, NV 89557, USA.
6 U.S. Geological Survey, MS 964, Box 25046, Denver Federal Center, Denver, CO 80225, USA.
7 NASA Ames Research Center, Mountain View, CA 94043, USA.
8 Jet Propulsion Laboratory, California Institute of Technology, MS 183-301, 4800 Oak Grove Drive, Pasadena, CA 91109, USA.
9 Department of Astronomy, Cornell University, 610 Space Sciences Building, Ithaca, NY 14853, USA.

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THIS ARTICLE HAS BEEN CITED BY OTHER ARTICLES:
Evidence for Calcium Carbonate at the Mars Phoenix Landing Site.
W. V. Boynton, D. W. Ming, S. P. Kounaves, S. M. M. Young, R. E. Arvidson, M. H. Hecht, J. Hoffman, P. B. Niles, D. K. Hamara, R. C. Quinn, et al. (2009)
Science 325, 61-64
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Science. ISSN 0036-8075 (print), 1095-9203 (online)