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Discovery of alunite in Cross crater, Terra Sirenum, Mars: Evidence for acidic, sulfurous waters

Ehlmann, Bethany L. and Swayze, Gregg A. and Milliken, Ralph E. and Mustard, John F. and Clark, Roger N. and Murchie, Scott L. and Breit, George N. and Wray, James J. and Gondet, Brigitte and Poulet, Francois and Carter, John and Calvin, Wendy M. and Benzel, William M. and Seelos, Kimberly D. (2016) Discovery of alunite in Cross crater, Terra Sirenum, Mars: Evidence for acidic, sulfurous waters. American Mineralogist, 101 (7). pp. 1527-1542. ISSN 0003-004X. doi:10.2138/am-2016-5574.

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Cross crater is a 65 km impact crater, located in the Noachian highlands of the Terra Sirenum region of Mars (30°S, 158°W), which hosts aluminum phyllosilicate deposits first detected by the Observatoire pour la Minéralogie, L’Eau, les Glaces et l’Activitié (OMEGA) imaging spectrometer on Mars Express. Using high-resolution data from the Mars Reconnaissance Orbiter, we examine Cross crater’s basin-filling sedimentary deposits. Visible/shortwave infrared (VSWIR) spectra from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) show absorptions diagnostic of alunite. Combining spectral data with high-resolution images, we map a large (10 km × 5 km) alunite-bearing deposit in southwest Cross crater, widespread kaolin-bearing sediments with variable amounts of alunite that are layered in <10 m scale beds, and silica- and/or montmorillonite-bearing deposits that occupy topographically lower, heavily fractured units. The secondary minerals are found at elevations ranging from 700 to 1550 m, forming a discontinuous ring along the crater wall beneath darker capping materials. The mineralogy inside Cross crater is different from that of the surrounding terrains and other martian basins, where Fe/Mg-phyllosilicates and Ca/Mg-sulfates are commonly found. Alunite in Cross crater indicates acidic, sulfurous waters at the time of its formation. Waters in Cross crater were likely supplied by regionally upwelling groundwaters as well as through an inlet valley from a small adjacent depression to the east, perhaps occasionally forming a lake or series of shallow playa lakes in the closed basin. Like nearby Columbus crater, Cross crater exhibits evidence for acid sulfate alteration, but the alteration in Cross is more extensive/complete. The large but localized occurrence of alunite suggests a localized, high-volume source of acidic waters or vapors, possibly supplied by sulfurous (H_2S- and/or SO_2-bearing) waters in contact with a magmatic source, upwelling steam or fluids through fracture zones. The unique, highly aluminous nature of the Cross crater deposits relative to other martian acid sulfate deposits indicates acid waters, high water throughput during alteration, atypically glassy and/or felsic materials, or a combination of these conditions.

Item Type:Article
Related URLs:
URLURL TypeDescription
Ehlmann, Bethany L.0000-0002-2745-3240
Milliken, Ralph E.0000-0003-3240-4918
Murchie, Scott L.0000-0002-1616-8751
Carter, John0000-0002-2698-6926
Additional Information:© 2016 Mineralogical Society of America. Open Access, thanks to the authors’ funding. Article available to all readers via GSW ( and the MSA web site. Manuscript received October 1, 2015; Manuscript accepted March 9, 2016; First Published on July 01, 2016. Manuscript handled by William Farrand. Many thanks to the science operations teams of CRISM, HiRISE, and CTX for their efforts in collecting this data set. Thanks to R. Rye for key discussions on mineralogy, C. Fassett for developing a program to access MOLA point shot data, T. Goudge for helpful discussions of martian impact crater volcanism and lacustrine deposits, P. Schultz for discussion of crater morphology and west Tharsis regional hydrology, J. Crowley for providing samples of lacustrine alunite, N. Pearson for research on prevailing wind directions, and S. Mattson for preparation of the Cross crater HiRISE DEM. Portions of this work were funded by investigator grants to the NASA MRO CRISM science team and participating scientists. Any use of trade, firm, or product names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government.
Funding AgencyGrant Number
Subject Keywords:Alunite, phyllosilicates, hydrothermal activity, lakes, groundwater, Mars, sediments, infrared spectroscopy, Invited Centennial article
Issue or Number:7
Record Number:CaltechAUTHORS:20160708-103531201
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Official Citation:Discovery of alunite in Cross crater, Terra Sirenum, Mars: Evidence for acidic, sulfurous waters Bethany L. Ehlmann, Gregg A. Swayze, Ralph E. Milliken, John F. Mustard, Roger N. Clark, Scott L. Murchie, George N. Breit, James J. Wray, Brigitte Gondet, Francois Poulet, John Carter, Wendy M. Calvin, William M. Benzel, Kimberly D. Seelos American Mineralogist Jul 2016, 101 (7) 1527-1542; DOI: 10.2138/am-2016-5574
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:68920
Deposited By: Tony Diaz
Deposited On:08 Jul 2016 23:43
Last Modified:11 Nov 2021 04:06

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