Constraints on the origin and evolution of the layered mound in Gale Crater, Mars using Mars Reconnaissance Orbiter data B.J. Thomson a,⇑ , N.T. Bridges a , R. Milliken b , A. Baldridge c , S.J. Hook d , J.K. Crowley e , G.M. Marion f , C.R. de Souza Filho g , A.J. Brown h , C.M. Weitz c a The Johns Hopkins Applied Physics Laboratory, 11100 John Hopkins Rd., Laurel, MD 20723, United States b University of Notre Dame, Notre Dame, IN 46556, United States c Planetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, AZ 85719, United States d Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Dr., Pasadena, CA 91109, United States e PO Box 344, Lovettsville, VA 20180, United States f Desert Research Institute, 2215 Raggio Parkway, Reno, NV 89512, United States g University of Campinas, PO Box 6152, 13083-970 Campinas, São Paulo, Brazil h SETI Institute, 189 Bernardo Ave., Mountain View, CA 94043, United States article info Article history: Received 15 August 2010 Revised 8 April 2011 Accepted 2 May 2011 Available online 14 May 2011 Keywords: Mars, Surface Geological processes Cratering Infrared observations abstract Gale Crater contains a 5.2 km-high central mound of layered material that is largely sedimentary in origin and has been considered as a potential landing site for both the MER (Mars Exploration Rover) and MSL (Mars Science Laboratory) missions. We have analyzed recent data from Mars Reconnaissance Orbiter to help unravel the complex geologic history evidenced by these layered deposits and other landforms in the crater. Results from imaging data from the High Resolution Imaging Science Experiment (HiRISE) and Context Camera (CTX) confirm geomorphic evidence for fluvial activity and may indicate an early lacustrine phase. Analysis of spectral data from the CRISM (Compact Reconnaissance Imaging Spectrom- eter for Mars) instrument shows clay-bearing units interstratified with sulfate-bearing strata in the lower member of the layered mound, again indicative of aqueous activity. The formation age of the layered mound, derived from crater counts and superposition relationships, is 3.6–3.8 Ga and straddles the Noachian–Hesperian time-stratigraphic boundary. Thus Gale provides a unique opportunity to investi- gate global environmental change on Mars during a period of transition from an environment that favored phyllosilicate deposition to a later one that was dominated by sulfate formation. Ó 2011 Elsevier Inc. All rights reserved. 1. Introduction Layered sedimentary sequences with repetitious bedding are of particular interest in planetary exploration because they constitute one of the key differences between bodies that posses an atmo- sphere or hydrosphere and those that lack them. Specifically, these types of sedimentary deposits require suitable transport media such as wind or liquid water to form. Thick, laterally extensive sed- imentary sequences with finely stratified materials may require an extended period of time for formation, particularly if sediment flux and accumulation rates were low during deposition. Therefore, such thick sequences have the potential to capture secular, epi- sodic, or cyclical environmental changes that may have occurred in their depositional settings. The overall objective in studying such deposits is to determine (or more broadly, place constraints upon) the environmental conditions that prevailed when the sedi- ments were laid down. Gale Crater hosts a sequence of layered deposits that exceeds several kilometers in thickness. Analysis of visible to near-infrared reflectance spectra (0.4–4 lm) from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) instrument indicates stra- ta in the lowermost section of the mound exhibit an upward tran- sition from clay- and clay/sulfate-bearing beds to predominantly sulfate-bearing beds (Milliken et al., 2010). Such a transition in mineralogy is broadly consistent with the proposed global environ- mental shift from circum-neutral/alkaline conditions to more acidic conditions inferred from analysis of OMEGA (Bibring et al., 2006) and CRISM spectral data (Murchie et al., 2009). In this paper, we analyze the impact crater population and stratigraphic relationships to better constrain the time frame with- in which the layered strata in Gale Crater were deposited. Images from the High Resolution Imaging Science Experiment (HiRISE) ta- ken at 25 cm/pixel scale and Context Camera (CTX) taken at 6 m/ pixel scale onboard the Mars Reconnaissance Orbiter (MRO) are 0019-1035/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2011.05.002 ⇑ Corresponding author. Present address: Boston University Center for Remote Sensing, 725 Commonwealth Ave., Rm. 433, Boston, MA 02155, United States. E-mail address: bjt@bu.edu (B.J. Thomson). Icarus 214 (2011) 413–432 Contents lists available at ScienceDirect Icarus journal homepage: www.elsevier.com/locate/icarus