Planetary and Space Science 72 (2012) 18–30 Contents lists available at SciVerse ScienceDirect Planetary and Space Science journal homepage: www.elsevier.com/locate/pss Late Hesperian aqueous alteration at Majuro crater, Mars N. Mangold a,n, J. Carter b,c, F. Poulet b, E. Dehouck a, V. Ansan a, D. Loizeau b,d a Laboratoire Plane´tologie et Ge´odynamique de Nantes, CNRS/Universite´ de Nantes, 2 rue de la Houssiniere, Nantes, France b Institut d’Astrophysique Spatiale, CNRS/Universite´ Paris-Sud, Orsay 91405, France c European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile d ESA/ESTEC, Keplerlaan, Holland article info abstract Article history: Impact craters cover a large portion of the surface of Mars and could constitute a significant exobiology Received 6 January 2012 research target as their formation provided heat sources for aqueous processes. To date, only rare Received in revised form examples of hydrothermal alteration in craters have been reported on Mars while many studies have 14 March 2012 focused on modeling their effect. Using data from the Mars Reconnaissance Orbiter and Mars Express Accepted 22 March 2012 probes, we report the presence of hydrated minerals, mainly Fe/Mg phyllosilicates, with vermiculite as Available online 24 April 2012 best-fit, that are found in an alluvial fan. This fan is located inside a crater located in NE Hellas region Keywords: and dated to the Late Hesperian by crater counts and crosscutting relationships. The stratigraphic Mars position of the hydrated minerals and presence of small domes interpreted as hydrothermal vents Gale indicate that the alteration occurred in the lower level of the alluvial fan and was triggered by bottom- Impact crater up alteration. These observations are best explained by a combination of snow deposition and Fluvial landforms Alteration subsequent melting eroding crater rims and forming the fan, with impact warming, which triggered Phyllosilicates the alteration at the base of the fan. This example shows that phyllosilicates are able to form late in the Alluvial fan Martian history, especially in local niches of strong exobiological interest. It also suggests that a similar process was possible in alluvial fans of other large impact craters including those at Gale crater. & 2012 Elsevier Ltd. All rights reserved. 1. Introduction 2012), hydrothermal alteration from impact craters (Marzo et al., 2010), and accumulation of pre-existing hydrated minerals in Multiple instruments analyzing the surface composition of Mars sedimentary deposits (Ehlmann et al., 2008; Dehouck et al., have identified several classes of hydrous minerals (i.e., hydrated 2010; Ansan et al., 2011). In this paper, we present rare observa- silicates, sulfates, opaline silica, hydroxydes) related to past aqu- tions that imply a process of formation involving impact heats. eous activity. Among them, phyllosilicates, remotely identified by Impact craters cover a large portion of the surface of Mars. the OMEGA (Observatoire pour la Mine´ralogie, L’Eau, les Glaces et Impacts can provide heat and geological structures that generate l’Activite´, Bibring et al., 2004, 2005) and CRISM (Compact Recon- and focus convective hydrothermal systems (Newsom, 1980; naissance Imaging Spectrometer for Mars, Murchie et al., 2007) Newsom et al., 1996; Schwenzer and Kring, 2009). As such craters imaging spectrometers, are strong indicator of the interaction could thus constitute a significant exobiology research target. between water and rocks in the early history of the planet Several numerical studies have investigated the impact-induced (Poulet et al., 2005; Bibring et al., 2005, 2006; Loizeau et al., hydrothermal systems (Newsom et al., 1980, 1996; Abramov and 2007; Mangold et al., 2007; Mustard et al., 2008, 2009; Bishop Kring, 2005; Schwenzer and Kring, 2009), but to date, only rare et al., 2008). Depending on the geological context and mineralogi- and ambiguous examples have been reported as related to the cal composition, the phyllosilicate-bearing deposits suggest that hydrothermal activity of craters (Marzo et al., 2010). several settings could have occurred: weathering under a warmer Previous detections of hydrated minerals in craters ejecta and atmosphere (Poulet et al. 2005, 2008; Loizeau et al., 2007, 2010; central peaks have been reported (Mangold et al., 2007; Mustard Ehlmann et al., 2008; Gaudin et al., 2011), hydrothermal circulation et al., 2008; Carter et al., 2010), but these altered minerals are at depth from geothermal gradient or volcanic activity (Mangold often interpreted to be excavated material of the altered crust et al., 2007, 2010; Mustard et al., 2009; Carter et al., 2010; Skok rather than the product of impact-related hydrothermalism. et al., 2010, Ehlmann et al., 2011, Weitz et al., 2011; Thollotetal., Many craters exhibiting hydrated mineral signatures have been observed in the Tyrrhena Terra region (Mustard et al., 2008; Loizeau et al., 2009, 2012), with some of them being candidate for n Corresponding author. Tel.: þ33 251125340. ancient hydrothermalism, but the strength of the impact could E-mail address: [email protected] (N. Mangold). mix up excavated altered minerals with those formed by 0032-0633/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.pss.2012.03.014 N. Mangold et al. / Planetary and Space Science 72 (2012) 18–30 19 hydrothermalism, thus complicating interpretations. Toro crater, to provide a view of the surface properties (Christensen et al., a 42 km diameter crater located inside Hesperian plains, exhibits 2003). Thermal images may be indicative of the presence of hydrated minerals in its central peak and crater floor (Marzo indurated rock outcrops or coarse grains (as seen in bright for et al., 2010). The authors plead for hydrothermal origin based on high night time temperatures) compared to more dusty, less small patterns such as putative vents, but no other aqueous indurated particles (as seen in dark for low night time tempera- landforms such as fluvial valleys are associated to this crater to tures) (Putzig et al., 2005). strengthen the interpretation of crater-related aqueous processes, and the hydrated minerals in the peak could still correspond to 2.4. Altimetry altered material excavated from few kilometers beneath the crater. A digital elevation model (DEM) using 1/128 gridded Mars The example discussed in this paper is the crater named Majuro Orbiter Laser Altimeter (MOLA) (Smith et al., 1999) data was used located in North-East Hellas and exhibiting hydrated silicates to extract the regional topography. As the HRSC camera acquired inside its floor and peak. This crater presents a unique combination both nadir and stereo mode images, it was possible to generate of mineralogy (hydrated minerals) and morphology (alluvial fans) Digital Elevation Model (DEM) for each covered area, using providing strong evidence for the presence of past hydrothermal photogrammetric software developed both at the Deutsches activity. Of special interest is the Late Hesperian age of this crater, Zentrum fur¨ Luft- and Raumfahrt (DLR) and at the Technical an epoch postdating most outcrops of hydrated silicates. After a University of Berlin (Scholten et al., 2005; Gwinner et al., 2007). description of the datasets used (Section 2), we detail the mor- The height was calculated while taking into account the Martian phology, age, and mineralogy of this crater (Section 3)before geoid defined as the topographic reference for Martian heights, discussing the origin of the aqueous activity (Section 4). i.e., areoid (Smith et al., 1999). The quality of the HRSC DEMs strongly depends on the acquisition geometry of the triplet of images that controls their 2. Data spatial resolution, the image quality (compression ratio, aerosols, etc.), and the surface roughness. These parameters were good for 2.1. Visible imagery the studied impact crater in regard to spatial gridding (150 m pixelÀ1). A total of 2645256 homologous points were Geomorphic analyses were supported by two HRSC (High detected on the whole area covered by the image triplet, using Resolution Stereo Camera) panchromatic images in both nadir 3D coordinates that have a relatively good spatial accuracy and stereo mode by the HRSC camera during the N-S trending orbit (sx¼16.9 m, sy¼36.9 m and sz¼29.6 m), implying a statistical 2631 (Neukum et al., 2004), acquired January 23, 2006. The spatial height error of 29.3 m for each point. The distribution of homo- À1 À1 resolution of images is 37.8 m pixel , 70.0 m pixel and logous points inside the impact crater is quite homogeneous, À1 92.0 m pixel atthecenterofimageforthenadirandthetwo allowing the extraction of topographic data on morphologic stereo images, respectively. Two HiRISE images (High Resolution features. This method has shown its ability to determine the Imaging Science Experiment, McEwen et al., 2007)andaseriesof topography of fluvial landforms (Ansan et al., 2008). CTX images (Context camera, Malin et al., 2007)werealsousedto determine the detailed morphology of fluvial landforms and impact features within the study area. 3. Observations and interpretations 2.2. Near-infrared hyperspectral data 3.1. Geological context The surface composition of the region of interest was deter- Majuro crater is 45 km in diameter and is located on the mined from CRISM (Compact Reconnaissance Imaging Spectro- Northeastern Hellas Basin rim (331S, 841E). The area in North-East meter for Mars) data, which acquires reflectance spectra of the Hellas region displays a Noachian surface with flat floored impact Martian surface in the visible to near-infrared range ([0.3– craters, light-toned layered deposits, as well as periglacial and 3.9] nm, Murchie et al., 2009). The surface is sampled spatially glacial features, e.g., lobate aprons, debris aprons, ice-rich viscous down to 18 m pixelÀ1 and spectrally binned at 6.6 nm. Raw flow, and glacial tongues (Squyres, 1989; Crown et al., 1992; data were calibrated then processed to surface reflectance follow- Tanaka and Leonard, 1995; Baratoux et al., 2002; Mangold, 2003; ing the method described in Pelkey et al.