Planetary and Space Science 70 (2012) 84–95 Contents lists available at SciVerse ScienceDirect Planetary and Space Science journal homepage: www.elsevier.com/locate/pss Gale Crater: Formation and post-impact hydrous environments Schwenzer S.P.a,h,n, Abramov O.a,i, Allen C.C.b, Bridges J.C.c, Clifford S.M.a,1, Filiberto J.d,2, Kring D.A.a,3, Lasue J.a,e,j, McGovern P.J.a,4, Newsom H.E.f, Treiman A.H.a,5, Vaniman D.T.g, Wiens R.C.e, Wittmann A.a,k,6 a Lunar and Planetary Institute, USRA, 3600 Bay Area Blvd., Houston TX 77058, USA b ARES, NASA JSC, Mail code: KA, 2101 NASA Road One, Houston, TX, 77058, USA c Space Research Centre, Dept. of Physics & Astronomy, University of Leicester, LE1 7RH, UK d Southern Illinois University, Geology Department—MC 4234, 1259 Lincoln Dr, Carbondale, IL 62901, USA e Los Alamos National Laboratory, Space Remote Sensing, ISR-2, Mail Stop D-466, Los Alamos, NM 87545, USA f Institute of Meteoritics and Dept. of Earth and Planetary Sciences MSC03-2050, University of New Mexico, Albuquerque, NM 87131, USA g Planetary Science Institute, 1700 East Fort Lowell Rd., Tucson, AZ 85719, USA h The Open University, Earth and Environmental Sciences, Walton Hall, Milton Keynes, MK7 &AA, United Kingdom i U.S. Geological Survey, Astrogeology Research Program, 2255N. Gemini Dr., Flagstaff, AZ 86001, USA j Universite´ de Toulouse; UPS-OMP; IRAP; Toulouse, France k Department of Earth and Planetary Sciences, Washington University St. Louis, Campus Box 1169, 1 Brookings Dr., St. Louis, MO 63130-4899, USA article info abstract Article history: Gale Crater, the landing site of the 2011 Mars Science Laboratory mission, formed in the Late Noachian. Received 1 March 2012 It is a 150 km diameter complex impact structure with a central mound (Mount Sharp), the original Received in revised form features of which may be transitional between a central peak and peak ring impact structure. The 1 May 2012 impact might have melted portions of the substrate to a maximum depth of 17 km and produced a Accepted 2 May 2012 minimum of 3600 km3 of impact melt, half of which likely remained within the crater. The bulk of this Available online 6 June 2012 impact melt would have pooled in an annular depression surrounding the central uplift, creating an Keywords: impact melt pool as thick as 0.5–1 km. The ejecta blanket surrounding Gale may have been as thick as Gale Crater 600 m, which has implications for the amount of erosion that has occurred since Gale Crater formed. Impact-processes After the impact, a hydrothermal system may have been active for several hundred thousand years and Hydrothermal a crater lake with associated sediments is likely to have formed. The hydrothermal system, and Phyllosilicates Astrobiology associated lakes and springs, likely caused mineral alteration and precipitation. In the presence of S-rich host rocks, the alteration phases are modelled to contain sheet silicates, quartz, sulphates, and sulphides. Modelled alteration assemblages may be more complex if groundwater interaction persisted after initial alteration. The warm-water environment might have provided conditions supportive of life. Deep fractures would have allowed for hydraulic connectivity into the deep subsurface, where biotic chemistry (and possibly other evidence of life) may be preserved. & 2012 Elsevier Ltd. All rights reserved. 1. Introduction n Corresponding author at: Lunar and Planetary Institute, USRA, 3600 Bay Area Blvd., Houston, TX 77058, USA. Tel.: þ1 44 1908 659987; Gale Crater, target of the Mars Science Laboratory (MSL) fax: þ1 44 1908 655151. mission, has significant potential for elucidating impact processes E-mail addresses: [email protected], [email protected] (S.P. Schwenzer), [email protected] (C.C. Allen), and products on Mars, including hydrothermal systems that [email protected] (J.C. Bridges), [email protected] (S.M. Clifford), might have supported life. Gale is a 150-km diameter impact [email protected] (J. Filiberto), [email protected] (D.A. Kring), structure, formed during the late Noachian, at the border of the [email protected] (J. Lasue), [email protected] (P.J. McGovern), southern highlands and Elysium Planitia (4.491S, 137.421E). Some [email protected] (H.E. Newsom), [email protected] (A.H. Treiman), [email protected] (D.T. Vaniman), [email protected] (R.C. Wiens), aspects of Gale’s geology have been studied extensively, but not [email protected] (A. Wittmann). its impact formation and the era shortly after its formation, which 1 Tel.: þ1 281 486 2146. we will investigate here. 2 Tel.: þ1 618 453 4849. From morphology and geology, Gale is known to include four 3 Tel.: þ1 281 486 2119. major geologic units: the ejecta blanket, a raised rim, a flat floor, 4 Tel.: þ1 281 486 2187. 5 Tel.: þ1 281 486 2117; fax: þ1 281 486 2162. and a central mound, which consists of sediments and potentially 6 Tel.: þ314 9356151. an underlying central peak (Greeley and Guest, 1987; Pelkey and 0032-0633/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.pss.2012.05.014 S.P. Schwenzer et al. / Planetary and Space Science 70 (2012) 84–95 85 Jakosky, 2002; Cabrol et al., 1999; Malin and Edgett, 2000; Edgett Gale’s central mound, recently named Mount Sharp (NASA, and Malin, 2001; Anderson and Bell, 2010; Milliken et al., 2010; 2012), is a particular target of interest—a 6 km-thick pile of Thomson et al., 2011; Le Deit et al., 2011). These units have sediments of various morphologies and compositions. The mound suffered erosion and are locally buried in aeolian ‘dust’. However, sediments have been interpreted as late Noachian to early the erosion has not been uniform across the crater; the northern Hesperian in age, and are divided by an angular unconformity rim (adjacent to Elysium) is both lower and less steep than the into two groups (Thomson et al., 2011). The upper group forms rest of the crater rim (Fig. 1). Some parts of the ejecta blanket rounded-hillslopes and its IR reflectance spectra (from CRISM, on have been preserved, especially in the highlands to the south the MRO spacecraft) are dominated by sulphate signatures (Fig. 1, Greeley and Guest, 1987). (Thomson et al., 2008a; Milliken et al., 2010). Channels, which The MSL landing site is located inside the crater (Golombek et al., appear to be sourced from the upper section of the layered mound 2011), on the NNW quadrangle part of the flat floor between the material, cut into the underlying sediments and form depositional central mound and the rim. The floor is covered by relatively little fans on lower slopes (Thomson et al., 2008b). The lower group is dust, and can be divided into three distinct units based on thermal strongly layered, and many layers have IR reflectance spectra inertia, which are interpreted as impact melt, volcanic rocks, and characteristic of hydrous phyllosilicates (Milliken et al., 2010). indurated sediment (Pelkey and Jakosky, 2002; Pelkey et al., 2004). Therefore, the lower phyllosilicate bearing strata appear to be Numerous small craters ranging from 0.4orlessto3.5kmin formed by a different process than the overlying sediments diameter have excavated the local subsurface (Pelkey et al., 2004). (Milliken et al., 2010; Thomson et al., 2011). Fig. 1. Maps of Gale Crater. (a) topography, highest point of the central mound (triangle on mound) at 788 m; lowest point on the crater floor (circle to the NW of mound) at -4649 m, but probably located in a small crater; highest point on the crater rim at 1413 m (star to S of central mound); lowest point on the crater rim at À3300 m (square to NW of mound) (b) slope steepness (c) map of the directly impact-related geologic units (blue¼country rock and ejecta, orange¼crater wall and slump deposits, grey¼moat zone deposits, yellow¼central mound; not shown are any younger sediments), and (d) small craters and their potential target lithologies. Black, thick outline in panel c is 150 km diameter Gale Crater (thick black line is the crater rim). Smaller craters above 3/4 km diameter are mapped on the basis of visible imagery. Dark green line is outline of the central mound. Light green cross is the location and approximate size of the proposed MSL landing ellipse as per March 2011 (Caltech 2011). The small craters are classified for the rocks they potentially expose using the map by Greeley and Guest (1987) and the investigations by Pelkey and Jakosky (2002) and Pelkey et al. (2004). Colour legend: black thin lines: unclassified. blue: ejecta blanket, red: rim area, yellow: sediments of the crater floor, grey: impact-melt sheet, violet: crater mound. Note the variety of rocks that might be found in their walls and ejecta. 86 S.P. Schwenzer et al. / Planetary and Space Science 70 (2012) 84–95 The existence of mid-mound channels and fan deposits on the to the currently favoured view of sedimentary deposition of these lower slopes has been interpreted as evidence for a lake in the minerals as result of the overall Martian climatic environment youngest preserved history of Gale (Cabrol et al., 1999). Therefore (Milliken et al., 2010; Le Deit et al., 2011; Ehlmann et al., 2011). the early sedimentary environment may have been followed by later episodes of inundation related to Late Amazonian flooding of the Elysium basin (e.g., Cabrol et al., 1999; Clifford and Parker, 3. Topography and impact formation of Gale Crater 2001)).