Icarus 194 (2008) 519–543 www.elsevier.com/locate/icarus Spectral and geological study of the sulfate-rich region of West Candor Chasma, Mars Nicolas Mangold a,∗, Aline Gendrin b, Brigitte Gondet b, Stephane LeMouelic c, Cathy Quantin a, Véronique Ansan a, Jean-Pierre Bibring b, Yves Langevin b, Philippe Masson a, Gerhard Neukum d a IDES, UMR8148 CNRS, Université Paris Sud, Bat 509, F-91405 Orsay, France b IAS, UMR8617, Bat 121, F-91405, Orsay, France c LPG, CNRS/Université de Nantes, 2 rue de la Houssiniere, BP 92208, F-44322 Nantes cedex 3, France d Freie Universität, Institut für Geologische Wissenschaften, Malteserstrasse 74-100, D-12249 Berlin, Germany Received 8 June 2007; revised 13 September 2007 Available online 14 December 2007 Abstract Sulfates have been discovered by the OMEGA spectrometer in different locations of the planet Mars. They are strongly correlated to light toned layered deposits in the equatorial regions. West Candor Chasma is the canyon with the thickest stack of layers and one with the largest area covered by sulfates. A detailed study coupling mineralogy derived from OMEGA spectral data and geology derived from HRSC imager and other datasets leads to some straightforward issues. The monohydrated sulfate kieserite is found mainly over heavily eroded scarps of light toned material. It likely corresponds to a mineral present in the initial rock formed either during formation and diagenesis of sediments, or during hydrothermal alteration at depth, because it is typically found on outcrops that are eroded and steep. Polyhydrated sulfates, that match any Ca-, Na-, Fe-, or Mg-sulfates with more than one water molecule, are preferentially present on less eroded and darker outcrops than outcrops of kieserite. These variations can be the result of a diversity in the composition and/or of the rehydration of kieserite on surfaces with longer exposure. The latter possibility of rehydration in the current, or recent, atmosphere suggests the low surface temperatures preserve sulfates from desiccation, and, also can rehydrate part of them. Strong signatures of iron oxides are present on sulfate-rich scarps and at the base of layered deposits scarps. They are correlated with TES gray hematite signature and might correspond to iron oxides present in the rock as sand-size grains, or possibly larger concretions, that are eroded and transported down by gravity at the base of the scarp. Pyroxenes are present mainly on sand dunes in the low lying terrains. Pyroxene is strongly depleted or absent in the layered deposits. When mixed with kieserite, local observations favor a spatial mixing with dunes over layered deposits. Sulfates such as those detected in the studied area require the presence of liquid water to form by precipitation, either in an intermittent lacustrine environment or by hydrothermal fluid circulation. Both possibilities require the presence of sulfur-rich groundwater to explain fluid circulation. The elevation of the uppermost sulfate signatures suggests the presence of aquifers up to 2.5 km above datum, only 1 km below the plateau surface. © 2007 Elsevier Inc. All rights reserved. Keywords: Mars; Mineralogy 1. Introduction glacial volcanic material (Lucchitta, 1982; Nedell et al., 1987; Beyer et al., 2000; Chapman and Tanaka, 2001; Komatsu et Interior Layered Deposits (ILD) in the Valles Marineris re- al., 2004). Now, the OMEGA (Observatoire pour la Minéralo- gion are the subject of many studies since the Viking missions gie, l’Eau, les Glaces et l’Activité) spectrometer on the Mars in the 70s. Many interpretations were proposed including lacus- Express orbiter detects sulfates in many layered deposits of trine deposits, volcanic ash deposits, aeolian deposits or sub- Valles Marineris canyons, and also in Margaritifer Terra chaos region and Meridiani Planum region (Gendrin et al., 2005; * Corresponding author. Arvidson et al., 2005). By landing on the Meridiani Planum, E-mail address: [email protected] (N. Mangold). the rover Opportunity of the MER (Mars Exploration Rover) 0019-1035/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2007.10.021 520 N. Mangold et al. / Icarus 194 (2008) 519–543 Fig. 1. (a) MOC wide angle image of central Valles Marineris region with West Candor Chasma inside the white box. (b) HRSC image mosaic (orbits 360, 1235, 2216, 2138, 2149), with areas 1 to 4 being region of interests. (c) Simplified context map of West Candor Chasma. mission accessed outcrops of layered deposits with sulfur- how to explain the thickness of material present? Sulfates are rich composition below a residual lag of hematite concretions not the unique constituent of layered deposits since iron oxides (Squyres et al., 2004). These terrains lay at the top of several are locally found (Gendrin et al., 2006). Are the relationships hundreds of meters of layered deposits that appear to be en- with iron oxides similar to those in the Meridiani Planum re- riched in sulfates (Arvidson et al., 2005). The presence of thick gion? All these questions require a detailed study using other sulfate-rich deposits in Valles Marineris canyons shows a much geologic or physical data available. larger extent than in Meridiani Planum alone. The strong cor- To answer some of these questions, we undertook a de- relation of all sulfates signatures with layered deposits through tailed study of West Candor Chasma (Fig. 1), a canyon of this broad area show that the type of outcrops found by the rover Valles Marineris canyon system, one of the regions with the Opportunity is developed across the surface of Mars and might most widespread sulfate signatures on Mars (Gendrin et al., signify a specific period of sulfate-rich material formation at 2005, 2006; Mangold et al., 2006). We used imagery and al- the end of the early Mars period, from Late Noachian to Late timetry data for geologic and morphologic interpretations. After Hesperian (Bibring et al., 2006). describing spectral detections and geology in the West Candor Sulfates on Mars have been interpreted either as chemical Chasma canyon, we present the investigation of sulfates using precipitation and deposition through evaporitic processes or al- two complementary approaches. We first examine the overall teration through groundwater circulation (Gendrin et al., 2005). distribution of minerals derived from spectroscopic measure- Nevertheless, many issues remain open concerning their forma- ments and present a statistical comparison of sulfate detections tion. Are sulfates primary minerals formed during deposition of with other characteristics such as slopes, albedo and thermal layers or secondary minerals formed well after? Coatings and inertia throughout the whole canyon. In a second part of the duricrust formation are locally able to explain sulfates on Earth study, we investigate the detailed geologic context of four spe- (e.g., Warren, 1999). Is this process possible on Mars? Could cific locations by mapping precisely all minerals detected by sulfates be present through the full stack of layers? In that case, OMEGA and comparing them with geology, especially using Sulfates in West Candor, Mars 521 the HRSC (High Resolution Stereo Camera) images and Digital absorption bands (Fig. 2c). Spectra show minerals such as sul- Elevations Models (DEMs) calculated from stereoscopic im- fates, iron oxides and pyroxene. ages. Implications for the mineral origins and layered deposit formation are then proposed from these two approaches. Be- 2.1. Sulfates cause the first aim of this article is the comparison of spectral data to other physical properties and geology, no detailed geo- Early investigations of the OMEGA dataset led to the logical maps or cross-sections will be provided here; these will identification of three different sulfate types (Gendrin et al., be the goal of future studies. 2005): monohydrated sulfates (where kieserite (MgSO4·H2O) is the best spectral match for strong absorptions), gypsum · 2. OMEGA spectral data (CaSO4 2H2O), and polyhydrated sulfates (with more than one molecule of water in the sulfate formula) which can correspond to Mg-, Ca-, Na- or Fe-sulfates. Fe-sulfates might have been OMEGA is a visible and Near InfraRed (NIR) mapping detected specifically in Meridiani Planum (Poulet et al., 2008). spectrometer, operating in the spectral range 0.38–5.1 µm. The First results of the spectrometer onboard Mars Reconnaissance spectrometer is divided in three detectors, which cover respec- Orbiter (MRO) confirm these detections (Murchie et al., 2007). tively the 0.38–1.05 µm range with a spectral sampling of 7 nm Kieserite has been identified thanks to three absorption (or 4.5 nm), the 0.93–2.73 µm range with 14 nm spectral sam- bands at 1.6, 2.1 and 2.4 µm (Gendrin et al., 2005)(Fig. 2d). pling, and the last covering 2.55–5.1 µm range with 20 nm The broad absorption for kieserite at 1.6 µm results from a sampling. In this domain, most of the signal of the two first de- combination of bands at 1.5 and 1.75 µm (Cloutis et al., 2006). tectors is due to the solar reflected light and the last one is partly Polyhydrated sulfates present a band at 1.4, 1.9 µm and a drop due to thermal emission. Six OMEGA orbits (360, 581, 1224, at 2.4 µm followed by a plateau (Gendrin et al., 2005). Gyp- 1235, 1462, 2116) have been used in the West Candor Chasma sum has a spectrum similar to polyhydrated sulfates with two area to provide a full coverage at low resolution (2 km/pixel), additional features, a band at 1.75 µm and a doublet at 2.21– locally reaching 300 m/pixel for orbit 2116. Other orbits (e.g., 2.27 µm. These absorption bands result from the combination number 1213) cover the same area but are not used because they of OH– or H2O bending, stretching and rotational fundamen- display ice clouds signature which hide the surface.