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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. Ehlmann1, John F. Mustard1, Scott L. Murchie2, Francois Poulet3, Janice L. Bishop4, Adrian J. Brown4, Wendy M. Calvin5, Roger N. Clark6, David J. Des Marais7, Ralph E. Milliken8, Leah H. Roach1, Ted L. Roush7, Gregg A. Swayze6 and James J. Wray9

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.


Figure 1 Fig. 1. (A) Global map of carbonate detections by CRISM (green circles) on a Thermal Emission Imaging System (THEMIS) satellite day infrared (IR) image with Mars Orbiter Laser Altimeter elevation data (blue is low and red is high). (B) THEMIS day IR image of the Nili Fossae region with olivine mapped by using OMEGA orbits <4500 using the olivine parameter (43). Targeted CRISM images examined in this study are shown and outlined in yellow where Fe-Mg phyllosilicates were found and in white where they were not. Green circles indicate carbonate detections. Where image footprints have substantial overlap, only one circle is shown. [View Larger Version of this Image (61K GIF file)]
 

Figure 2 Fig. 2. CRISM and candidate mineral visible and near-IR spectra. CRISM data have been photometrically and atmospherically corrected as in (18) and then processed with the noise-reduction algorithm of (47). (A) CRISM reflectance spectra from hundreds of pixel regions of interest from image HRL000040FF. Black line, putative carbonite-bearing rock; gray line, spectrally unremarkable cap rock. (B) Spectral ratios, which highlight differences in composition between two units, are shown for the regions of interest in five different CRISM images (top to bottom, FRT00003FB9, FRT0000A09C, FRT000093BE, HRL000040FF, and FRT0000B072) that contain the spectral class. The bold spectrum is the ratio from (A). (C) Laboratory spectra (RELAB spectral database) (29) of (top to bottom) magnesite, the hydrated Fe-Mg carbonate brugnatellite, a hydromagnesite mixture, siderite, brucite, the zeolite analcime, nontronite, serpentine, and chlorite. (D) CRISM spectral ratios over the full wavelength range, using the same denominator, for terrains from HRL000040FF inferred to be carbonate-bearing (black) and olivine-bearing (gray). (E) Laboratory spectra for fayalitic olivine, magnesite, and a mixture of magnesite (80 wt %) and nontronite (5 wt %), sparsely covered with medium-grained olivine sand (15 wt %). [View Larger Version of this Image (37K GIF file)]
 

Figure 3 Fig. 3. Scatter plot of continuum-removed absorption band positions for anhydrous carbonates, hydrous carbonates, and other minerals with 2.3- and 2.5-µm absorptions. CRISM data that we identify as magnesite are shown as open circles within the larger dashed circle. Laboratory mineral spectra are shown from Gaffey (green circles) (22), Hunt and Salisbury (red squares) (21), RELAB spectra measured by E. Cloutis (orange triangles) (RELAB spectral database), and the USGS spectral library (purple circles) (29). Band centers of CRISM spectra are known to ±0.01 µm. [View Larger Version of this Image (23K GIF file)]
 

Figure 4 Fig. 4. Geomorphology and stratigraphy of the carbonate-bearing units. (A) CRISM false-color composite of FRT0000B072 (R, 2.38 µm; G, 1.80 µm; and B, 1.15 µm) where carbonate is green, olivine is yellow to brown, phyllosilicate is blue, and the mafic cap unit is purple. (B) subset of HiRISE PSP_002532_2020 from the white box in (A) showing a mafic knob overlying carbonate-bearing terrain. (C) FRT000093BE with colors as in (A). (D) Subset of HiRISE PSP_006778_1995 from the white box in (C), which shows the stratigraphy of carbonate-bearing units. (E) Schematic stratigraphy of the mineralogic units in the Nili Fossae region (not to scale). [View Larger Version of this Image (101K GIF file)]
 

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