1. Department of Geological Sciences, Brown University, Providence, Rhode Island 02912, USA
2. Johns Hopkins University/Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, Maryland 20723, USA
3. Jet Propulsion Laboratory, California Institute of Technology, Mail Stop 183-301, 4800 Oak Grove Drive, Pasadena, California 91109, USA
4. Center for Earth and Planetary Studies, National Air and Space Museum, Smithsonian Institution, Independence Avenue at 6th Street SW, Washington, DC 20560, USA
5. Institut d'Astrophysique Spatiale, Université Paris Sud 11, 91405 Orsay, France
6. National Aeronautics and Space Administration, Ames Research Center, 515 N. Whisman Road, Mountain View, California 94043, USA
7. Department of Earth and Planetary Sciences, Washington University, St Louis, Missouri 63130, USA
8. Applied Coherent Technology, 112 Elden Street Suite K, Herndon, Virginia 22070, USA
9. Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, Colorado 80301, USA
10. US Geological Survey, MS 964 Box 25046, Denver Federal Center, Denver, Colorado 80225, USA
11. ARES Code KR, National Aeronautics and Space Administration, Johnson Space Center, 2101 NASA Parkway, Houston, Texas 77058, USA
12. School of Earth and Space Exploration. Box 871404, Arizona State University, Tempe, Arizona 85287-1404, USA
13. National Aeronautics and Space Administration, Goddard Space Flight Center, Code 693.0, Greenbelt, Maryland 20771, USA
14. US Geological Survey, 2255 N. Gemini Drive, Flagstaff, Arizona, 86001, USA

Correspondence to: John F. Mustard¹ Correspondence and requests for materials should be addressed to J.F.M. (Email: john_mustard@brown.edu).

Abstract

Phyllosilicates, a class of hydrous mineral first definitively identified on Mars by the OMEGA (Observatoire pour la Mineralogie, L'Eau, les Glaces et l'Activitié) instrument^{1, 2}, preserve a record of the interaction of water with rocks on Mars. Global mapping showed that phyllosilicates are widespread but are apparently restricted to ancient terrains and a relatively narrow range of mineralogy (Fe/Mg and Al smectite clays). This was interpreted to indicate that phyllosilicate formation occurred during the Noachian (the earliest geological era of Mars), and that the conditions necessary for phyllosilicate formation (moderate to high pH and high water activity³) were specific to surface environments during the earliest era of Mars's history⁴. Here we report results from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM)⁴ of phyllosilicate-rich regions. We expand the diversity of phyllosilicate mineralogy with the identification of kaolinite, chlorite and illite or muscovite, and a new class of hydrated silicate (hydrated silica). We observe diverse Fe/Mg-OH phyllosilicates and find that smectites such as nontronite and saponite are the most common, but chlorites are also present in some locations. Stratigraphic relationships in the Nili Fossae region show olivine-rich materials overlying phyllosilicate-bearing units, indicating the cessation of aqueous alteration before emplacement of the olivine-bearing unit. Hundreds of detections of Fe/Mg phyllosilicate in rims, ejecta and central peaks of craters in the southern highland Noachian cratered terrain indicate excavation of altered crust from depth. We also find phyllosilicate in sedimentary deposits clearly laid by water. These results point to a rich diversity of Noachian environments conducive to habitability.

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