Spectral and Geological Study of the Sulfate-Rich Region of West Candor Chasma, Mars

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)

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. tals, or S–O bending overtones (e.g., Cloutis et al., 2006). The standard processing pipeline for the OMEGA data cal- The spectra of ILD (Fig. 2a) have different absorption bands ibration is described online in the Planetary Science Archive of different sulfates. We observe on the spectra Figs. 2b and 2c, (PSA) of the European Space Agency (ftp://psa.esac.esa.int/ that the spectra k1 and k2 exhibit a 2.1 µm band associated with pub/mirror/MARS-EXPRESS/OMEGA/MEX-M-OMEGA-2- a narrow 2.4 µm band, and a subtle broad band at 1.6 µm, espe- EDR-FLIGHT-V1.0/SOFTWARE/). OMEGA reflectance spec- cially for k2. The 2.1 µm band is not frequent in other minerals tra contain both atmospheric and surface components. Dust, than monohydrated sulfates, thus being a good parameter for CO2,CO,H2O (vapor, ice) signatures are often present in the their detection, with kieserite being the best fit. The 2.4 µm spectra. It is therefore fundamental to remove the spectral ef- absorption band of kieserite is very small, even for laboratory fects of the atmospheric constituents in order to retrieve the spectra, so that it often does not appear in all OMEGA spectra. surface properties. Assuming a constant surface contribution It shows up when the 2.1 µm band is strong enough only. The over the flanks of Olympus Mons, the ratio of a spectrum ac- 2.4 µm will not be used for mapping for this reason, but it con- quired at the base of the volcano to one acquired over the firms the detection when we look systematically at spectra, as summit provides the atmospheric transmission as a power func- shown in Fig. 2b. The 1.6 µm band is more subtle than in the li- tion of their difference in altitude. The atmospheric contribution brary spectra of pure samples. One explanation for this smaller of each spectrum is then removed by dividing the observa- band is that the presence of Fe3+ (see next section) might mod- tion by the derived atmospheric spectrum, scaled by using the ify the spectrum at wavelengths <1.6 µm. depth of the 2 µm CO2 atmospheric absorption band measured Alternatively, other minerals than kieserite might be consid- 2+ in the observation. Using this standard processing technique ered. The monohydrated Fe-sulfates szomolnokite (Fe (SO4)- for removing the atmospheric gas absorptions for each pixel (H2O)) has similar features as kieserite in the 1.5–2.5 µm (e.g., Mustard et al., 2007), it has been shown that unam- range because the two minerals differ only in terms of ma- biguous signatures of mafic minerals (pyroxene and olivine) jor cation, Fe2+ versus Mg2+. Nevertheless, szomolnokite has and hydrated minerals (sulfates, phyllosilicates) can be identi- a broad ferrous band minimum from 1.2 to 1.6 µm which is fied in the 1–2.5 µm wavelength range (Mustard et al., 2005; not observed. Furthermore, the band minimum of kieserite is Gendrin et al., 2005; Poulet et al., 2005; Mustard et al., 2007; located at 2.13 µm (Cloutis et al., 2006) which fits with the Mangold et al., 2007). In addition, the random noise has been value we found for the maximum for k1 and k2 (2.14 µm on studied and can produce artificial band depths which are always the spectra of Fig. 2b) while the minimum is at 2.10 µm for smaller than 2%, which is the minimum threshold of detec- szomolnokite. Other monohydrated sulfates than kieserite such tion for all the minerals discussed in this paper (e.g., Poulet as szmikite (MnSO4·H2O), and gunningite (ZnSO4·H2O) can et al., 2007). Figs. 2a and 2b represent 5 spectra of different be good spectral candidates too (Gendrin et al., 2005), but they units in West Candor Chasma. Ratios of interesting spectra are not major minerals in nature. As exceptions to monohy- over featureless spectra are also used to emphasize the detected drated sulfates, Cloutis et al. (2006) notice that the presence of a 522 N. Mangold et al. / Icarus 194 (2008) 519–543

Fig. 2. (a) Visible and NIR channels of five spectra corresponding to five different terrains of West Candor Chasma taken from orbit 360. ILD are layered deposits of bright tone (top) and dark tone (lower). Dark plains correspond to canyon floor sand dunes. ILD foothill is a pixel at foot of ILD scarps. Bright plains correspond to plateau material. (b) NIR spectra and (c) corresponding ratio taken from orbit 360 (of area 1 of Fig. 1, numbered k1, p1, etc.) and 1235 (of area 4 in Fig. 1, numbered k2, p2); k1 and k2 are ILD material with 2.1 and 2.4 µm absorption bands typical of kieserite; p1 and p2 are ILD material with a 1.9 µm band and a 2.4 µm drop typical of polyhydrated sulfates; f1 is a ILD foothill material with a strong 1 to 1.3 µm slope typical of strong iron oxides enrichment; d1 has broad bands at 1 and 2.2 µm typical of clinopyroxenes; (d) library spectra of different sulfates; from USGS (Clark et al., 1993) except kieserite (courtesy from the RELAB Reflectance Experiment Laboratory, Brown, Providence).

2.1 µm band for romerite, a complex polyhydrated Fe-sulfates observed for polyhydrated sulfates. In this part of the spec- 3+ 2+ with composition [Fe (SO4)2(H2O)4]2[Fe (H2O)6)]. How- trum, minerals such as epsomite (MgSO4·7H2O) or hexahydrite ever, its does not well fit the observed spectra because romerite (MgSO4·6H2O), bloedite (Na2SO4·MgSO4·4H2O), copiapite 2+ 3+ 3+ has a gentle 2.4 µm drop rather than a sharp band. Thus, mono- (Fe Fe4 (SO4)6(OH)2·20H2O), coquimbite (Fe 2(SO4)3· hydrated sulfates explain the observation well, with the Mg- 9H2O) or kainite (KMgClSO4·3H2O) are equally good spec- sulfates kieserite being the best fit. tral analogues that fit these two bands. These minerals also Figs. 2b and 2c show spectra p1 and p2 with a strong 1.9 µm present a 1.4 µm band in the library spectra that is not ap- absorption band (exactly 1.94 µm for the minimum) associ- parent in OMEGA spectra. The lack of this feature might be ated with a drop at 2.4 µm followed by a plateau as typically due to the presence of Fe3+ that decreases the amplitude of Sulfates in West Candor, Mars 523

Table 1 Criteria for spectral band depths 2(2R(2.12) + R(2.15)) 2.1 µm band depth d = 1 − 2.1 (R(1.93) + R(1.94) + R(1.96) + R(2.25) + R(2.26) + R(2.27)) 2(R(1.93) + R(1.94) + R(1.96)) 1.9 µm band depth d = 1 − 1.9 (R(1.79) + R(1.80) + R(1.81) + R(2.23) + R(2.25) + R(2.26)) 2(2R(2.41) + R(2.42)) Drop at 2.4 µm d = 1 − 2.4 (R(2.30) + 2R(2.31)) R (wavelength) is the reflectance. the band in this part of the spectrum (see next section). Al- Mineral abundances are not reported here because the ternatively, this might be due to minerals with 1.4 µm band strength of each spectral index depends on a variety of parame- lower than the two bands at 1.9 and 2.4 µm, as for poly- ters such as the mineral mixture, the relative concentration, the · 3+ halite (K2MgCa2(SO4)4 2H2O) and schwertmannite (Fe16 O16- optical constants and the grain size distribution of each min- (OH)12(SO4)2). Thus, the studied OMEGA spectra at the spa- eral, the surface texture and the atmospheric condition. As a tial sampling (300 m to 2 km/pixel) and spectral sampling first approximation, we assume that the spectral detection can (14 nm) considered are not diagnostic enough to identify a be related to either the abundance or the grain size, or both. given mineral inside the family of sulfates, but they still enable The mapping of the 1.9 and the 2.4 µm spectral parameters us to identify the sulfates family. Polyhydrated sulfates can cor- shows that most locations with the 1.9 µm band contain a sig- respond to Mg-sulfates more hydrated than kieserite, or Ca-, nature at 2.4 µm too (Figs. 3a and 3b), indicating the presence Fe-, Na-sulfates different from the Mg-sulfates. of polyhydrated sulfates. The 1.9 µm band extends larger than Spectra of many polyhydrated sulfates are sufficiently simi- the 2.4 µm one. This is not surprising since the 1.9 µm band is lar to prevent the identification of a unique mineral, except for better expressed in the spectra than the 2.4 µm drop. This does the specific case of gypsum. Gypsum could be identified if the not exclude that some of the 1.9 µm signature, when not asso- bands of polyhydrated sulfates are associated with a 1.75 µm ciated with a 2.4 drop, is related to hydrous minerals different band and a shallow doublet at 2.22–2.28 µm. These two lat- than sulfates, such as, for example, phyllosilicates (e.g., Poulet ter bands being more subtle signatures, gypsum might be a et al., 2005). However, no 2.2 or 2.3 µm band is detected to- candidate for some of the polyhydrated sulfates too, as well gether with the 1.9 µm band in West Candor Chasma as well as bassanite (2CaSO4·H2O), which has similar band positions. as in all Valles Marineris canyons (Gendrin et al., 2005, 2006). Notice that the p1 spectrum displays a shallow band at 1.7 µm As the 2.4 µm band is spatially well correlated to the strongest and small features between 2.2 and 2.3 that might correspond to signatures at 1.9 µm, we interpret the overall 1.9 µm signature gypsum. Nevertheless, these features are at the limit of the de- as being due to the presence of polyhydrated sulfates too. As a tection, and no widespread area with obvious gypsum spectra consequence, the mapping of the 1.9 µm band and the 2.1 µm has been found in the studied area. In summary, the simultane- band gives a simple way to map the presence of the two main ous presence of 1.9 and 2.4 µm absorption bands is diagnostic types of sulfates detected, polyhydrated sulfates for the 1.9 µm of polyhydrated sulfates, whereas the 2.1 µm band is diagnostic band, and monohydrated sulfates, as being very likely kieserite, of kieserite. for the 2.1 µm band. Spectral index are calculated using the band depths at 1.9 and 2.1 µm (Table 1). The usual formula used to derive band 2.2. Iron oxides depths is 1 − R1/R2, where R1 is the reflectance at the wavelength W1 (corresponding to the position of the band center), In this section, we focus on the 0.9–1.3 µm region which is and R2 is the reflectance of the interpolated level of the con- very diagnostic of the Fe2+/Fe3+ occurrence, present in iron tinuum at the same wavelength W1. For a symmetric band, R2 oxides or any iron rich minerals. Strong ferric signatures have can be easily derived by using the mean of the reflectance taken been identified inside Valles Marineris and Margaritifer Terra, both at the left and right wings of the band. In order to increase closely associated with sulfate deposits, either in the sulfate rich the signal to noise ratio, we decided to average two or three ILDs or at their base (Gendrin et al., 2006). The presence of a contiguous wavelengths (such as 1.93, 1.94 and 1.96 µm for the 0.9 µm band in the spectra of different components (Fig. 2a) 1.9 µm band center). The spectral index of the 1.9 and 2.1 µm shows the presence of iron oxides in a ferric phase such as in bands are mapped only when they are larger than 2% (Figs. 3a nanophase hematite. A specifically strong rise of the reflectance and 3b). The spectral index for the 2.4 µm spectral region has between 1 and 1.3 µm is observed for spectra of the ILD foothill been designed to detect the drop of polyhydrated sulfates. It is (Fig. 2a). This is also visible by the ratio of f1 over a featureless calculated as for a band depth despite it detects a slope. Usual spectrum as in Fig. 2c. Such a strong rise is best explained by minimum thresholds at 2% cannot be used for this reason. Nev- the presence of a crystalline ferric component creating a strong ertheless, the empirical use of this index shows that a value of 0.9 µm feature (Poulet et al., 2008; Bibring et al., 2007). 2 or 3%, depending on the orbits, guaranties a positive detec- In order to map the ferric oxides, Gendrin et al. (2006), tion. This index is not designed to detect the small signature of Bibring et al. (2007) and Ledeit et al. (2007) use 3 different monohydrated sulfates at the same wavelength. methods: the Modified Gaussian Model (where the detection is 524 N. Mangold et al. / Icarus 194 (2008) 519–543

Fig. 3. (a) 2.1 µm band (red to yellow) and 1.9 µm band (green to light green) mapped over HRSC mosaic. The 2.1 µm band is diagnostic of kieserite. (b) 2.4 µm drop mapped over HRSC mosaic. The correlation between the 2.4 µm drop and the 1.9 µm band is diagnostic of polyhydrated sulfates. (c) Map of high calcium pyroxenes (blue) with band strengths in excess of 5%. (d) Iron oxides mapped with the MGM method. Signatures in yellow-orange color are iron-oxide rich. (e) Iron oxides mapped with the linear unmixing method with band strength in excess of 10%. (f) Iron oxides mapped with the 1 to 1.3 µm slope method. (g) MOC wide angle image plotted in color scale for albedo variations. (h) HRSC DEM with contours every 1 km. (i) TES thermal inertia map (Putzig et al., 2005). (j) MOLA ◦ slope map in color scale. (k) Sulfates as in Fig. 3a plotted over MOLA slope map in grayscale. (l) Sulfates occurrence on slopes <5 mapped on the MOLA slope map in grayscale. positive for strong signatures, orange-red color in Fig. 3d); lin- plays a shallow 2.1 µm band revealing monohydrated sulfates. ear unmixing (with band depth exceeding 10% as in Fig. 3e) A question is whether this iron oxide signature corresponds to and the slope between 1.0 and 1.3 µm (using the band ratio be- a mixing of sulfates and oxides, or to ferric sulfates, such as tween reﬂectance values at 1.0 and 1.3 µm and selecting the schwertmannite, which display a strong slope between 1 and highest values, as in Fig. 3f). These methods have concordant 1.3 µm. In fact, the presence of iron oxides is necessary for two results with slight differences. In the following, we will study reasons. Firstly, the iron oxides signature extends out of the area iron oxides in detail only in locations where the three methods where sulfates are mapped (Fig. 3), thus requiring the presence identify iron oxides independently. of oxides without sulfates. Secondly, no monohydrated sulfates The presence of iron oxides signature is often accompanied with signature at 2.1 µm contain Fe3+ that would explain both by the signature of sulfates. For example, the spectrum f1 dis- signatures in only one mineral. Thus, signatures of iron ox- Sulfates in West Candor, Mars 525

Fig. 3. (continued) ides together with a 1.9 µm absorption band can be discussed on the relative proportion of iron, magnesium, and calcium in in terms of mixing versus single ferric sulfates but mixtures the crystal. In this study, we concentrate on the analysis of the together the 2.1 µm require the presence of two phases at a ∼2 µm absorption of pyroxenes. The spectrum of the dark floor minimum. In summary, we interpret these strong iron oxides of Candor Chasma, d1 in Figs. 2b and 2c, clearly exhibits a signatures as due to the presence of iron oxides, out of sulfates, broad band centered at 2.2 µm that is typical of HCP (High or mixed with kieserite, not excluding the presence of ferric Calcium Pyroxene) such as diopside [see Mustard et al. (2005) sulfates to explain these signatures in areas mapped as polyhy- for comparisons to library spectra]. drated sulfates. The strength of the slope between 1 and 1.3 µm We use the MGM (Modified Gaussian Model, Sunshine et al. indicates the abundance of iron oxides, which, for example, differs between the spectra k1 (strong) and k2 (weak) (Fig. 2c). 1990) to separate the HCP and LCP (Low Calcium Pyroxene) A more detailed view of iron oxides mapping is given in Bibring contributions and to retrieve the band depths of each of these et al. (2007), Poulet et al. (2008) and Ledeit et al. (2007). components. This method was used and described by Mustard et al. (2005, 2007). In West Candor Chasma, HCP is the major 2.3. Mafic minerals pyroxene found while LCP is not predominant. We will only map the HCP detection. Band strengths in excess of 5–6% is Pyroxenes are characterized by the presence of two broad generally used as a minimum threshold value for HCP detection absorption bands at 1 and 2 µm whose exact location depends (Mustard et al., 2005). In this study, we plot HCP when the 526 N. Mangold et al. / Icarus 194 (2008) 519–543 criteria exceeds 5% (Fig. 3c). It can be noted that no signature Table 2 typical of olivine has been detected in the studied area. Classification of main West Candor Chasma components from HRSC images and OMEGA mineralogy with respective total area 3. HRSC data West Candor Chasma interior (HRSC) 35,000 km2 Interior Layered Deposits (HRSC) 17,000 km2 Kieserite (OMEGA) 3700 km2 The High Resolution Stereo Camera (HRSC) is a multi- 2 sensor pushbroom instrument, with nine CCD line sensors Polyhydrated sulfates (OMEGA) 1800 km Pyroxene (OMEGA) 10,700 km2 mounted in parallel delivering nine superimposed image swaths. Pyroxene and sulfates (OMEGA) 680 km2 HRSC data used consist of nadir images taken at resolution Iron oxides (OMEGA) 300 km2 from 12 to >35 m/pixel (depending on the pericenter position of the elliptical orbit) and the two stereo images which allows the calculation of the Digital Elevation Model (DEM). 5. OMEGA data at the regional scale Five Mars Express orbits (360, 1235, 2116, 2138, 2149) are used in this study with nadir image spatial sampling between 5.1. Overview of regional geology and minerals distribution 12 and 30 m. A mosaic of these images has been completed at 30 m/pixel in the region of interest for regional mapping West Candor Chasma lies in the central area of the Valles (Fig. 1) whereas local mapping has been done at full resolu- Marineris canyon system (Fig. 1). The canyon is closed on three tion. A simplified map gives the main units observed in the sides measuring about 200 km long from West to East and over canyon (Fig. 1). A DEM mosaic using these orbits has been 150 km long from North to South. The HRSC mosaic and con- computed using DLR/Berlin stereoscopic programs and tools text map shows the main units found in the canyon (Figs. 1 [see Gwinner et al. (2005) and Scholten et al. (2005) for more and 3h). Layered deposits are widespread in the West Candor details on the processing]. The grid sampling of this DEM is of Chasma canyon covering about half of the canyon interior (Ta- 50 m with a statistical vertical accuracy of about 30 m (Fig. 3h). ble 2). Layered deposits are mainly composed of two parts, a narrow steep hill to the east, named Candor Mensa, and a more 4. Other datasets developed area in the west of the canyon with a second hill, Ceti Mensa. Locally, the top of these ILD hills is covered by a dark TES (Thermal Emission Spectrometer) of the MGS (Mars cap unit. The floor of the canyon is also darker and smoother Global Surveyor) mission data have been used for two goals than layered deposits. A couple of landslides cut the northern (Christensen et al., 2001a). Firstly, the thermal inertia map wallslopes. The difference in elevation from the floor to the has been used for quantitative comparisons with the OMEGA plateau is >9 km, from −5 km for the lowermost floor at the dataset. TES sampling is not as precise as OMEGA, but the SE edge of the canyon to 4.3 km on the plateau (Fig. 3h). The pixel size enables us to use it as a comparison for the surface top of the western mesa lies at about 3.6 km above datum, thus properties discussion. Indeed, thermal inertia gives information only 700 m below plateau level. Assuming a subhorizontal lay- about the type of material present at surface, from fine dust to ering for the overall ILD, the difference of elevation found in strongly indurated rocks (Mellon et al., 2000). The TES map West Candor Chasma would correspond to a thickness of ILD used is the 2003 derivation by Putzig et al. (2005). Secondly, of about 7 to 8 km; a thickness consistent with estimations made TES most important finding is probably the gray hematite de- using Viking data (Lucchitta et al., 1994). tected on Meridiani Planum and some other areas (Christensen Sulfates are found only on ILD as mapped from the HRSC et al., 2001a). We have used TES maps of Christensen et images (Fig. 3). They are found from −3.6 km to +3.1 km, thus al. (2001b) for hematite detection corresponding to the Valles over much of the layered deposits sequence. Sulfates signatures Marineris area to compare with the OMEGA iron oxides detec- are mostly developed on the scarp of the two hills, and some tion. other escarpments as seen from the superimposition with the For views more detailed than HRSC, we have used the MOLA slope map (Figs. 3j–3l). When we exclude all sulfates ◦ MGS Mars Observer Camera (MOC) images (Malin and Ed- detected on slopes of more than 5 , about 90% of sulfates do not gett, 2001), which provided 150 narrow angle images in the appear on the map. Only a few outcrops remain visible suggest- ◦ studied area. The 1/128◦ interpolated MOLA (Mars Observer ing that the sulfates are found on hillslopes >5 preferentially. Laser Altimeter) map has been used for most of the eleva- In addition, kieserite and polyhydrated sulfates are found on tion statistics and slope maps presented in this study. For lo- separate outcrops, i.e., outcrops containing the two signatures cal studies and 3D sketches, HRSC DEM were preferred be- are not frequent. This observation is interesting: either it shows cause of the improved spatial resolution. OMEGA and HRSC a strong difference of composition between some of the layers, have been introduced in a GIS in order to be able to super- or it shows that minerals, when mixed together, cannot produce imposed any type of data such as MOC, TES and MOLA us- a signature as high as when they are found alone. ing the IAU2000 system as reference (Duxbury et al., 1999; Sulfates and HCP do not follow the same overall distribu- Seidelmann et al., 2002). The projection used for mapping is tion, but, in a few locations, they are found on the same pixels geodetic; distortion from projection are nonetheless very small (Figs. 3a and 3c). HCP are found mainly in the low lying ar- thanks to the proximity of the equator and the scale of the stud- eas of dark tone and a few areas at higher elevations such as ied area. the top of Candor Mensa. Iron oxides are found mainly in ge- Sulfates in West Candor, Mars 527 ographic association with sulfates, but they extend out of the images. This is done in Fig. 4d which represents for clarity only sulfates areas. Candor Mensa exhibits apparently the location the two types of sulfates normalized at the same surface. with the clearest signatures of iron oxides because the three We used the slopes extracted from the MOLA map at 1/128◦. methods can detect them. This scale fits well with the scale of our data. Results are plot The TES thermal inertia map (Fig. 3i) shows a strong vari- as for those of the albedo: The first plot (Fig. 4e) compares the ation of thermal inertia in the canyon, with only few locations proportion of terrain from each type relative to each type of ter- being covered by dust, i.e., dark blue color with inertia <200 SI rain. The second plot (Fig. 4f) displays the area of each mineral unit on the map. The lack of dust is probably a determinant pa- group divided by the total area of ILD in order to extract the rameter in the detection of broad areas with spectral signatures behavior of minerals relative to the total proportion of layered in this canyon relative to other canyons in the vicinity (i.e., East deposits. Candor Chasma, Ledeit et al., 2007).

5.2. Statistical approach 5.3. Mineralogy versus albedo

We use in the following a systematic approach to compare ILD have an albedo of 0.12 to 0.23 and a modal peak at the spectral data with (a) albedo, (b) thermal inertia, (c) slopes 0.17, brighter than pyroxene-rich region of typically less than (Figs. 3g, 3i and 3j). These parameters are compared to 6 dif- 0.15 (Fig. 4a). There is a difference in albedo between kieserite ferent components (Table 2): (1) ILD mapped from their vi- and polyhydrated sulfates with polyhydrated modal peak at 0.16 sual identification on HRSC images, and minerals mapped by whereas kieserite peaks at 0.19. Nevertheless, polyhydrated sul- OMEGA such as (2) kieserite mapped by the 2.1 µm band, fates have a secondary peak at 0.20 showing more diversity of (3) polyhydrated sulfates mapped with the 1.9 µm band as ex- albedo. Iron oxides has two peaks, one at 0.12 and an other at plained in Section 2, (4) HCP using the MGM method, (5) re- 0.17. This likely indicates the presence of oxides on two types gions with simultaneous presence of sulfates and HCP and of material, first the light toned albedo on ILD (peak at 0.17) (6) iron oxides mapped by the method of slopes. and second the dark area outside ILD (peak at 0.12). The cell size of the classified regions has been taken at ◦ The relative proportion between each component requires 1/128 . For the albedo map, the best data available is the bolo- a comparison at different ranges of albedo (Fig. 4b). Firstly, metric TES albedo map (Christensen et al., 2001a). However, ◦ below 0.15, the mixing of “pyroxenes and sulfates” in pink its low spatial resolution of 1/8 is too low to be compared to has almost the same trend as kieserite in red, suggesting a ◦. The albedo map taken from our regions classified at 1/128 strong contribution of the pyroxene in the albedo of kieserite the MOC wide angle mosaic at 1/256◦ can be used to improve when <0.15. This is easy to explain if the low albedo values this resolution (Fig. 3g). The MOC albedo is taken at about of the kieserite rich layered deposits here are due to the spa- 0.600 µm (±0.025 µm) with resolution 1/128◦ (the initial reso- tial mixing of pyroxene-rich material over layered deposits. For lution of 1/256◦ has been degraded by a factor of two in order iron oxides, the peak at 0.12 surpasses 1.00 because iron ox- to fit the cell size taken) (Malin et al., 1992). However, this ides are found out of ILD. Secondly, between 0.15 and 0.20 instrument is not absolutely calibrated to be used for a quanti- the trend between kieserite and polyhydrated sulfates is simi- tative albedo study. In order to address this issue, we used TES lar more than for the absolute proportion. Thirdly, above 0.20, albedo values (Christensen et al., 2001a) in three different locations of homogeneous MOC albedo such as on canyon floor the proportion of both types of sulfates increases: this sug- and hills to locally calibrate the MOC wide angle albedo. This gests that the sulfates are more often found on high albedo method permits a better spatial sampling with a good accuracy terrains. on albedo values, despite the fact that it will not completely In summary, the variations of albedo of the layered de- eliminate the effects of different insolation on hillslopes. We posits might follow some differences in the composition. In present the result with two curves. The first plot (Fig. 4a) com- general, sulfates are found more easily on brighter layered de- pares the proportion of terrain from each type relative to each posits whereas the presence of pyroxene decreases the albedo type of terrain. The second plot (Fig. 4b) displays the area of of sulfate rich layered deposits when they are found together. each mineral group divided by the total area of ILD in order to There is a slight difference in albedo between kieserite and extract the behavior of minerals relative to the total proportion polyhydrated, with polyhydrated sulfates generally darker than of layered deposits. kieserite. Nevertheless, this observation is just a trend. Detailed We used the TES thermal inertia (Putzig et al., 2005) with observations on visible images are required to evaluate if the a cell size of 1/20◦ that is not optimum. Results are shown in presence of dark sand can explain the lower values of polyhy- Fig. 4c. To improve this resolution we also used the THEMIS drated sulfates, especially when the presence of pyroxene-rich night time mosaic. These images give the relative differences sand is too low in proportion to be detected by OMEGA, despite of temperatures rather than direct estimations of thermal iner- in enough high proportion to decrease the albedo. Iron oxides tia, despite these differences are directly related to the thermal are partially found on albedo as low as 0.12 showing that they properties. Nevertheless, we can do an approximation of the do not correspond to iron oxides present in a veneer of eolian thermal inertia of these images by pointing values of TES ther- dust because dust would have much higher albedo, typically mal inertia for the radiometry of some chosen areas in THEMIS higher than 0.25 (Putzig et al., 2005). 528 N. Mangold et al. / Icarus 194 (2008) 519–543

Fig. 4. Statistics of mineral distribution relative to albedo (a, b), thermal inertia (c, d) and slopes (e, f). Plot (a), (c) and (e) are histograms using the area in ◦ Y -coordinate (in number of cells of the grid) respectively versus albedo (in %), thermal inertia and slopes. (a) and (e) are plotted using 1/128 grid, (c) is plotted ◦ using 1/20 grid. (b) and (f) are the same plot ratioed by the ILD abundance (black curve on the left column). For example, in (b) a proportion of 1 for kieserite would indicate that all ILD of a given albedo contains kieserite. The proportion of more than 1 of the iron oxides is explained because they extend outside ILD. (d) is similar to (c) with the X-coordinate being the radiometry (gray level) of the THEMIS mosaic. This plots thermal variations in which the polyhydrated sulfates area has been normalized to that of kieserite for clarity. The plot shows that the difference on modal thermal inertia existing in TES (c) also exists using better resolution data (THEMIS). See text for more explanations.

5.4. Mineralogy versus thermal inertia or more indurated when thermal inertia increases (Mellon et al., 2000). The overall ILD unit has values of thermal inertia from Thermal inertia helps to understand the material properties 200 to 500 SI whereas pyroxenes have slightly lower values at the surface. Values lower than 200 SI usually corresponds to between 200 and 400 SI (Fig. 4c). Kieserite and polyhydrated mobile dust (Mellon et al., 2000; Putzig et al., 2005). Thermal sulfates have slightly different thermal inertia with values from inertia higher from 200 to 500 SI characterizes coarse sand or 250 to 400 SI for kieserite and 300 to 500 SI for polyhydrated partially indurated surface. This material becomes more coarse sulfates, both ranges of values corresponding to relatively in- Sulfates in West Candor, Mars 529

Fig. 5. Area 1 in detail. (a) Kieserite (red) mapped over HRSC. (b) Polyhydrated sulfates (green). (c) OMEGA 1 µm reflectance (grayscale). (d) HCP (blue). (e) Iron oxides (yellow). (f) Sulfates plotted with slope map contours. (g) 3D view of sulfates and iron oxides over the eastern mesa. Kieserite (red) occurs on flanks of the Candor Mensa over about 3 km of elevation difference. Iron oxides (yellow) is present more on the foothill of the Mensa. The orange color is the mixing zone between the two components. Polyhydrated sulfates are present on flatter area out of the hill. durated or coarse material. Iron oxides have similar values from is obviously a difference in mineralogy between kieserite and 250 to 460 SI. polyhydrated sulfates that can explain part of the difference. TES having a poor spatial resolution, we have also done the The role of slopes in providing more granular particles (see be- same calculations using THEMIS night time images. Results low) and lowering the inertia might also be important. show that the two types of sulfates do have a difference in their All materials are clearly above the typical values for dust modal distribution. This difference is slight, with the variations (<200 SI). Thermal inertia of iron oxides is important to com- in the peak distribution of kieserite and polyhydrated sulfates pare to usual values of dust because iron oxides are known to corresponding to about 40–50 SI, thus less than expected from exist inside dust (Gendrin et al., 2006). The values of more than TES data. The difference between TES and THEMIS might 270 SI confirms that the high level of iron oxides identified by come from the southwestern edge of the canyon where TES OMEGA corresponds to outcrops of sand size, coarse grains or detects an anomalous high inertia compared to THEMIS. De- indurated material, and not to dust. Most of the ILD without spite this lower difference, the distribution of the two sulfate sulfates have also thermal inertia higher than 200 SI showing types is different suggesting the material has slightly differ- that the lack of mineral signature in 70% of the ILD surface is ent thermal properties. Differences in thermal inertia are due to not related to the presence of dust. There might be an excep- thermal conductivity, density or caloric capacity of the material, tion if we would have a very fine dust layer at the surface which with thermal conductivity being the most important parameters might not affect the thermal inertia primarily, since OMEGA (Mellon et al., 2000). Thus, the difference can come from the spectra resolve the first tens of microns, but the dust cover in- difference (1) in mineral composition, (2) in the particle size, dex calculated using TES data shows very low dust proportion (3) in the induration state, or (4) in the hydration state. There in this region (Ruff and Christensen, 2002). However, no obser- 530 N. Mangold et al. / Icarus 194 (2008) 519–543

Fig. 6. (a) Band depths contours of kieserite (red to yellow) and polyhydrated sulfates (green to black). Both minerals contours begin at 2% band depth with 1% step. (b) MOC close-up image E1801363. (c) MOC close-up of E0301293. (d) Close-up of the northern scarp of the eastern mesa. Kieserite in red pixel, HCP in blue and slopes in contours. (e) MOC close-up of M1700467. (f) MOC close-up of M0003125. vation favors such effect. In absence of surface dust, the lack mean 20% for kieserite, and 10% for polyhydrated sulfates, as of spectral signature on ILD can be due to different reasons: measured in overall (Table 2). On the contrary, the proportion (1) spectral signatures too small because of spatial mixing of of kieserite increases gradually, from a few percent at a few de- the different components, (2) modification of surface material grees of slopes, to almost 80% on the steepest slopes at 25◦. by desiccation, coating, grain size effect, etc., (3) the predomi- This gradual increase shows that kieserite becomes more and nance of minerals that are transparent in NIR. more common on layered deposits for slopes more and more steep. Notice the strong variations for slopes >25◦ are due to 5.5. Mineralogy versus slopes the small area covered by such steep slopes. Unlike kieserite, the polyhydrated sulfates do not show such increase in abun- ◦ ◦ The different components have different behavior with dance and decrease after a peak at 6 –7 of slope. Iron oxides slopes (Fig. 4e). Modal peak slopes are about 2◦–3◦ for HCP, also show an increase for steep slopes, like kieserite, but not as 3◦–4◦ for ILD, 6◦–7◦ for polyhydrated sulfates and 8◦–9◦ for strong. kieserite and iron oxides. The large variation in the slope of The variations in the kieserite proportion requires some com- each component, especially ILD and sulfates, is a consequence ments. An increase in the slope can result from an observational of the hilly topography of West Candor Chasma. Pyroxenes are bias. We do not see in the regional map any predominance of found on the flattest area as seen on the regional maps (Fig. 1b). one slope direction to another, showing that there is no arti- The distribution of slopes for polyhydrated sulfates is slightly fact due to this effect. In addition, the polyhydrated sulfates shifted from the ILD distribution, but this effect is much more do not follow the same trend as kieserite, because they show important for kieserite. Thus, sulfates exhibit a preference for a drop of proportion on steeper slopes. If the increase in abun- slopes as seen in regional maps (Fig. 3), but this effect is not just dance of kieserite on steep slopes was an instrumental bias, this a consequence of the presence of hillslopes in ILD, or sulfates would be the case for all minerals, not only kieserite. The ef- would have a peak at 3◦–4◦ as ILD. fect of sand could explain such a trend, because it would cover The plot of the proportion of kieserite relative to ILD con- layers mainly in the areas of low slopes. However, here too, firms this result (Fig. 4f). On this figure, kieserite and poly- the fact that polyhydrated sulfates follow a trend opposite to hydrated sulfates would have the same behavior as ILD if the kieserite is not consistent with this argument, or they would not area of each sulfate was the same at any slope value: this would peak at low slopes. Sand seems to cover mainly the more gentle Sulfates in West Candor, Mars 531

Fig. 7. (a) Comparison between HCP (blue) and kieserite (contours as in Fig. 6). (b, c) MOC close-up M0003125. slopes (<10◦) as visible by the HCP + sulfates curve in pink. sand. The MOC images also show that the kieserite outcrops This cannot explain the difference seen in sulfates distribution are very eroded and fresh with almost no small craters visible for slopes higher than 10◦. Notice that the pyroxene found on at that scale (Fig. 6). The polyhydrated sulfate outcrops appear slopes >10◦ in Fig. 4e can correspond to landslides and begin- less eroded and slightly less fresh than those of kieserite, with ning of wallslopes rather than part of the ILD that would be the presence of a few small impacts (Fig. 6), despite this obser- blanketed by sand. Actually, the difference between the distri- vation does not give a quantitative difference. bution of the two sulfate types might sign a process specific to The band depths of kieserite are found to be maximum sulfates. Slopes can present a difference in grain size because (about 6% here, corresponding to k1 in Fig. 2) for the most of erosion effects and then have a stronger spectral signature eroded outcrops (Fig. 6a). We can see on the slope map (Figs. 5f because NIR is known to improve the detection of sand grains and 6d) that these outcrops also correspond mainly to the steep- size (e.g., Poulet et al., 2007). In that case, this would mean that est slopes, up to 25◦. This observation is valid on the three the surface state of most polyhydrated sulfates is different from flanks of the hill, therefore it is not the result of any artefact due that of kieserite. to the observation angle. In contrast, the deepest band depths In summary, the general trend of sulfates with regards to of polyhydrated sulfates correspond to relative flat surface with slope shows that kieserite is much more common on steep less than 10◦ of slope. slopes than polyhydrated sulfates. We propose to interpret this Pyroxene is observed mainly on the floor of the canyon, but effect as a result of the erosion of the scarp on which the also on part of the hill and especially at its top. All pyroxene rich kieserite is detected: as the slope increases, the erosion becomes areas display dark sand dunes, including the pyroxene detected stronger, improving the freshness of the outcrops and the clarity at the top of Candor Mensa (Fig. 7). In some locations, pyrox- of the signature. ene and kieserite are observed together. These areas never correspond to areas where kieserite band depth is large (Fig. 7a). 6. Correlation between OMEGA data and local geology It corresponds to areas at the edge of the detection as well as for pyroxene. Close-ups on MOC images of these areas show In this section, we detail the observations in 4 regions as the presence of dark dunes over bright layered deposits. At the sketched in Fig. 1. The four regions are those with the deepest spatial resolution of 1 km, the MOC image close-up Fig. 7b cor- sulfates band depths. Only the first one will detail the relation- responds to a single OMEGA pixel. This favors a spatial mixing ships between iron oxides and sulfates. to explain this effect; i.e., pyroxene and kieserite are observed in the same pixel but they correspond to two different units, eo- 6.1. Candor Mensa (the eastern hill) lian dunes and bedrock. Iron oxides are found in this area using the three methods Spectral data used here correspond to the single orbit 360 of detection, especially on the sides and foothill (Fig. 3). When with spatial sampling of about 1 km (Fig. 5). This area has mapped together with sulfates, we observe that iron oxides are the outstanding aspect of displaying the four types of minerals present together with the sulfates on the bright eroded outcrops detected in the region. We detail hereafter three observations of the mesa scarp (Figs. 8a and 8b). They are also present at the corresponding to the difference between the sulfate types from footwall of this scarp in a much darker material displaying local their albedo and band depths, the presence of pyroxene and the layering and small dark dunes (Fig. 8c). No pyroxene is de- iron oxides distribution. tected here suggesting these dunes are different from the usual Kieserite is detected mainly on the slopes of Candor Mensa pyroxene rich sand dunes, or that pyroxene becomes too much and in scattered outcrops. Polyhydrated sulfates are found at mixed with iron oxides to be detected. On Fig. 8d, the iron ox- the foot of wallslopes mainly. The comparison of sulfates ides are found together with the bright outcrops which display detection (Figs. 5a and 5b) with OMEGA 1 µm reflectance a strong band depth here too. Iron oxides continue downslope (Fig. 5c) shows the predominance of kieserite on bright out- after sulfates are no longer detected. This effect is clearly vis- crops whereas polyhydrated sulfates are found on darker ter- ible on the 3D sketch in Fig. 5g where iron oxides (yellow) rains. We observe that these terrains lack any sand dunes so and kieserite (red) mixed in the orange colored pixel. On the that the lower albedo is not a consequence of a mixing with MOC image (Fig. 8e), we see the presence of eroded bright 532 N. Mangold et al. / Icarus 194 (2008) 519–543

Fig. 8. (a, d) Comparison between iron oxides (yellow), HCP (blue), and kieserite (in contours as in Fig. 6). (b, c) MOC close-ups of E0301293. On (c), small ripple dunes cover the bottom of the image. (e) MOC image close-up M0003125 with kieserite band depth decreasing from top to bottom. outcrops, as usual for kieserite over this hill, with an increasing 6.3. The southern complex area proportion of dark mantling to the bottom of the image. Thus, we interpret this image as being a sulfate bedrock partially cov- Area 3 (see Fig. 1) is located to the southeast of the ILD ered at its surface by a dark material corresponding to the iron western section. Region 3 is mapped using three orbits at dif- oxides. Iron oxides can also be compared to the TES hematite ferent resolution, 360 for the first part of the area, and 1224 and detection of coarse-grained hematite (Fig. 9). This comparison 2316 for the second part (Fig. 12). shows that TES finds gray hematite exactly where OMEGA has The strongest signature of kieserite is a large outcrop of lay- strong signatures of iron oxides, with OMEGA detection ex- ered deposits that are bright and heavily eroded (Fig. 13). The tending over a larger area. strongest band depths are found in the area of greatest erosion or slopes of more than 10◦ steep (Fig. 13a). The layered deposits show here fine layering well exposed in the western part of the 6.2. Ceti Mensa center (the western hill) elongated outcrops (Figs. 13c and 13d). Layers are organized in two circular shapes here that are present on the northward Theregion2(seeFig. 1 for context) is the only one to dis- slope. Using the HRSC DEM, these two round-shaped features appear to correspond to topographic effects of layers that are play a large sulfate rich area over a relatively flat surface of ◦ ◦ tilted toward the north by about 15 (Fueten et al., 2006), a less than 5 (Fig. 3l). Sulfates are found on an intermediary value close to the slope (of 15◦–20◦ here). This geometry has plateau located about 1 km below the top of the mesa. Kieserite tentatively been interpreted as possible volcanoes (Lucchitta, is found on bright outcrops only, with a strong erosion visible at 2004) or deposition of layers draping over a faulted bedrock the surface from the presence of yardangs, i.e., elongated pat- (Fueten et al., 2006). From our DEM, the visible geometry does terns due to eolian erosion, and lack of small craters (Fig. 10d). not fit the volcano hypothesis, since layers of volcanoes should Thus, kieserite is present over a less steep area than region 1, be tilted in different directions concentrically, which is not the but the region appears to be strongly eroded as well. A few py- case here, since layers are tilted to the northeast only. Draping roxene rich pixel are found in this area sometimes associated of a preexisting topography is possible, but such a dip (15◦) rep- with kieserite (Figs. 10a and 10b). MOC images of these mix- resents a strong slope of deposition. This does not explain the ing zones show that we have a dark material organizing in dunes presence of an angular unconformity at the top of the hill (indi- blanketing part of the layered deposits (Fig. 10f), therefore sug- cated by arrows in Fig. 13c). This angular unconformity shows gesting same conclusion of spatial mixing as for the region 1. that layers might have been tilted before having been subjected Polyhydrated are found in between two zones of kieserite to erosion and deposition again. This indicates the presence of (Fig. 10a). In contrast to region 1, this signature does not lie on another structural unit at the hill top. The presence of pyroxene- terrains darker or less eroded than kieserite (Fig. 10e). We see rich sand at the hill top hides the nature of this uppermost unit that the surface is almost similar, strongly eroded and relatively but its eroded slopes look likes another stack of light toned lay- bright (Fig. 11). The coexistence of polyhydrated and sulfates ered deposit. here is questionable. Indeed, layers can be followed from left To the west, the second region of interest in this area 3 dis- to right as being of the same unit for both minerals. Thus, ei- plays a much larger variety of outcrops (Fig. 14). Polyhydrated ther the difference of mineralogy shows a lateral variation in the sulfates are found on relatively dark and flat areas with lay- composition of the rocks, or it shows a variation due to surficial ered deposits (Figs. 14a and 14b). The apparent circular shape processes, for example, by dehydration or rehydration. of layers is due to a nearly circular depression. Kieserite is ob- Sulfates in West Candor, Mars 533

with much darker tone and less eroded texture as it is the case in region 1. Polyhydrated sulfates on the eastern edge are found on the same texture as kieserite. It indicates the same layers or same erosion level. As for area 2, these polyhydrated sulfates might show a lateral variation in the composition of layers or a variation due to surface evolution. No sand dunes exist to explain any variations of albedo here. A closer look at the kieserite outcrops shows that the greatest band depths are found on an intermediary plateau that appears relatively flat and significantly darker (Fig. 16). This observation differs from the usual observation of kieserite on bright eroded outcrops. MOC images shows that this dark tongue with strong kieserite band depths does not correspond to an ancient preserved surface: it is eroded with same typical texture and layers visible through the texture. What differs here, is the dark- ness of the outcrop. This outcrop could correspond either to a specific strata with strong kieserite signature with unusual dark tone. Alternatively, this change might also be due to the presence of iron oxides, despite the signature seems less strong than Fig. 9. Comparison between OMEGA iron oxides from the band depth (a) and in the eastern mesa (Fig. 3d). the TES hematite (b) from Christensen et al. (2001b). served on a relatively dark outcrop too. The band depth is low 7. Discussion 1: Characteristics and formation process of here, with about 2% only, but the outcrop is geologically strik- the minerals detected ing. The area shows layers that are partially folded at the contact of apparent shear zones identified by their appearance of a fault Sulfate minerals are strongly associated with the presence of zone that is not as sharp as usual for faulted material. The corre- layered deposits but their exact relation to the layers must be lation with topography (Fig. 14e) shows that neither shear zones developed. First, this correlation does not mean that the whole nor layers follows a horizontal plane. This shows that this area rocks are composed of sulfates only, minor minerals and miner- exhibited a strong deformation and that this deformation was als that are transparent in NIR can also be present. Second, NIR not purely brittle with shear zones, but showed also soft folding spectrometry penetrates in the rocks no deeper than one mil- of layers. limeter. Thus, sulfates might have formed subsequently to the East of this kieserite outcrop is observed an area with a deposition by surface alteration through coating or duricrust, strong deformation of layers (Fig. 14c). Here layers are folded, or might be the result of transformation by processes of hy- sometimes upturned, never follow topography and display lo- dration or dehydration at the surface. The detection of sulfates cally breccia-like texture at the hundred meters scale. This area from spectral data should therefore take these limitations into has been subjected to a strong soft deformation from unknown account. In this section we summarize, interpret and discuss the origin: Gravity sliding or tectonic folding are the two most results of the correlation of minerals with geology and physical likely origins. No clear spectral signatures have been found on parameters as summarized in Table 3 to understand the overall these folded layers yet. The location at the west bottom of the mineralogical assemblage of the layered deposits. Implications small mesa visible in Fig. 13 shows that they might be part of for sulfate formation and the evolution of layered deposits are the same deformed unit as kieserite-rich layers on the other side proposed in the next section. of the mesa. 7.1. The behavior of sulfates 6.4. The northwestern edge The observations concerning the presence of kieserite give The area 4 is at the NW edge of the canyon. It displays the following information: strong kieserite signatures (up to 7% band depth, as on k2 of Fig. 2) and polyhydrated sulfates are found to the west and to (a) Kieserite is always found on outcrops of layered deposits the east of this kieserite-rich region. The sulfate detection is with fresh eroded texture, either on scarps (Figs. 6 and 13), correlated with a depression mostly composed of bright ma- or on plateaus presenting yardangs-like ridges typical of terial surrounded by scarps of a few hundreds of meters high eolian erosion (Figs. 10 and 16). Band depths are often the (Fig. 15). Most of the kieserite is detected on a bright surface. deepest for the most eroded or steepest outcrops. This surface is not on a steep slopes but we can see from the (b) Kieserite is present on bright outcrops (typical albedo of MOC image the eroded pattern and strong erosion of the surface about 0.2) of layered deposits. Most of the relatively dark material (Figs. 15 and 16). Polyhydrated sulfates at the west- kieserite-rich terrains (albedo 0.15) consists of areas mixed ern edge of the kieserite outcrop occurs on layered deposits but with pyroxenes (Fig. 7) or iron oxides (Figs. 8d and 16). 534 N. Mangold et al. / Icarus 194 (2008) 519–543

Fig. 10. Area 2 in detail: (a) kieserite (red) and polyhydrated sulfates (green); (b) pyroxene; (c) elevation contours; (d) MOC close-up M0906480; (e)MOC M1901827; (f) MOC R1301043; (g) 3D sketch of this area. Polyhydrated sulfates are found at the same elevation as kieserite on a plateau below the uppermost scarp of Ceti Mensa.

(c) Kieserite is more frequent on slopes than on ﬂat areas we should better ﬁnd chemical alteration on less eroded out- (Figs. 3, 5 and 12). Its detection increases in frequency with crops. The detection on the most eroded outcrops also suggests the slope increase, therefore likely in relation with mechan- that this kieserite was not a dehydration product of more hy- ical erosion. Burial by sand deposited on gentle slopes im- drated Mg-sulfates because the rock is freshly exhumed, other- proves this effect but cannot explain it uniquely for slopes wise kieserite should be more frequently detected on less fresh >10◦. and less steep location. The result of these observations is that (d) A few observations indicate the presence of soft deforma- kieserite is likely a mineral primarily present inside the layered tion (Fig. 14). Kieserite-rich rocks, as most sulfates and rocks and not only at the surface. salts, have a mechanical behavior of soft material under Kieserite can form by different processes: (1) by precipi- ambient or relatively low temperatures (<200 ◦C) (e.g., tation in an aqueous system at the surface, (2) by dehydra- Warren, 1999). tion of more hydrated minerals during diagenesis at depth, or (3) by alteration of minerals during subsurface circulation (e.g., These observations, especially the correlation with slopes, Warren, 1999). Kieserite does not form at terrestrial ambient disfavor any post-exhumation chemical alteration to explain temperatures except in hot desertic areas. It requires tempera- sulfates. Indeed, mechanical and chemical alteration have op- tures of about 30 to 50 ◦C to form (e.g., Warren, 1999). Sur- posite trends: when mechanical alteration increases, chemical face temperatures as high on Mars surface would also be re- alteration decreases because it has less time to develop. Thus, quired. These temperatures can more easily be found in the sub- Sulfates in West Candor, Mars 535

Fig. 11. (a) Close-up in Fig. 10. (b) Close-up on HRSC image 1235. surface. Kieserite is observed over layered deposits that have Mg-sulfates, Na-sulfates or Fe-sulfates with different hydra- been buried and only later exhumed. Diagenesis at tempera- tion levels. Nevertheless, the characteristics of layered deposits ◦ tures >30 C is possible at shallow depth currently on Earth, with polyhydrated sulfates are often different from those with and probably as well on early Mars when the thermal gradi- kieserite. Some characteristics, such as the difference in their ent was higher (e.g., Schubert et al., 1992). As a consequence, frequency with slopes, suggest that polyhydrated sulfates, or at kieserite might have formed by dewatering of more hydrated least some of them, could have been formed by surface mod- sulfates during burial and diagenesis. Another possibility is that ifications. We therefore do not exclude surface effects such as it formed directly at the required temperatures in the subsur- coating or rehydration from less hydrated phases to be possible face by the alteration of preexisting material submitted to rather for this mineral type. Since good candidates for polyhydrated long subsurface water circulation at depth with presence of sul- sulfates include epsomite (MgSO4·7H2O), or its less hydrated fur rich fluids. molecules such as hexahydrate (MgSO4·6H2O), a rehydration Our observations of the distribution of polyhydrated sulfates of kieserite is an interesting possibility. gives more complex characteristics than for kieserite: On the basis of these results, the stability of these minerals can be questioned: Can polyhydrated sulfates correspond (a) Polyhydrated sulfates are always found on outcrops of lay- to rehydration of kieserite? Is kieserite currently stable at mar- ered deposits (Fig. 3) but with less eroded texture and lo- tian surface? Mg-sulfates have many levels of hydration from cally small impact craters compared to kieserite outcrops kieserite to hexahydrite or epsomite. According to Vaniman et (Figs. 6c and 15e). al. (2004), the current cold temperatures might favor the stabil- (b) Polyhydrated sulfates have frequently a lower albedo ity of polyhydrated phases, disfavoring the long term stability (Figs. 5 and 15) than kieserite but light toned layers are also of less hydrated phases such as kieserite. Experiments in en- observed. Those with lower albedo are likely not a conse- vironmental cells show that kieserite is easily hydrated on ex- quence of spatial mixing with pyroxene-rich sand dunes posure to elevated humidity where it converts to hexahydrite since this trend is observed on outcrops apparently devoid of dunes on high resolution images. and epsomite (Vaniman et al., 2004). When desiccated at low (c) The thermal inertia of polyhydrated sulfates is slightly temperatures, the latter minerals become amorphous rather than higher than that of kieserite possibly suggesting slightly transforming back to kieserite. As amorphous sulfates have the more coarse or indurated material, or different composition hydration band depth closer to 1.9 than 2.1 µm (Freeman et al., (Fig. 4). 2007), this also indicates that the detected kieserite is not the (d) This sulfate type is sometimes found together kieserite over product of desiccation of polyhydrated sulfates, in agreement the same stack of layers (Fig. 10). with our geologic interpretation. In addition, recent experiments (e) Polyhydrated sulfates are not so frequently found on slopes show that hexahydrite has an extended stability at martian sur- than kieserite and their occurrence seems less dependent on face temperature conditions when they are mixed with anhy- mechanical erosion (Fig. 4). drous Ca-sulfates (Freeman et al., 2007), a possibility that we cannot confirm due to the lack of signature of anhydrite in NIR. The variability in properties of this spectral type might cor- In summary, it is strongly possible that a part of the polyhy- respond to a difference in the composition of rocks. This was drated sulfates detected comes from the rehydration of kieserite expected from the spectral differences because polyhydrated when exposed at the surface for longer time than on the most sulfates can include many diverse minerals such as Ca-sulfates, fresh outcrops. 536 N. Mangold et al. / Icarus 194 (2008) 519–543

Chasma as well in the surrounding canyons. The pyroxene rich material is initially of magmatic origin and could come from the grinding of debris aprons and landslides of the canyon wallslopes. However, it is then difficult to explain the presence of sand dunes on the top of the Candor Mensa (Fig. 7c). Indeed, dunes are widespread on the flat top and it is unlikely that sand grains climbed along the 3 km high hill surrounded by >20◦ slopes. This implies either: (1) sand dunes accumulated at the top of the hill prior to the erosion of the canyon interior material; (2) pyroxene grains were originally particles of smaller size (dust-silt) having mantled the topography as seen in other locations on Mars (e.g., Edgett and Malin, 2000) and then having aggregated to form sand size particles; (3) sand grains come from the erosion of layered deposits which once contained sand grains. In the third possibility, pyroxene grains would have been altered in a sulfate-rich matrix. They might be present either at the uppermost part of the layers, devoid of alteration, or after mechanical erosion of sulfate rinds around grains. Neverthe- less, this last case is not consistent with the lack of any residual sulfates signatures in the sand dunes area. Fig. 12. (a) Area 3 with kieserite (red) and polyhydrated sulfates (green). (b) 3D The overall results favor a lack of correlation between the sketch of this area. presence of dark sand dunes rich in pyroxenes and the pres- Another question raised by the stability of Mg-sulfates is that ence of layered deposits devoid of any significant pyroxene these minerals are also highly soluble. Nevertheless, this kind signature. Nevertheless, the possibility of pyroxene grains very of material usually recrystallizes immediately after being dis- strongly altered into sulfates should not be excluded, especially solved if the subsurface fluids remains saturated in ions (e.g., because of the uncertainty about the origin of dark sand dunes Warren, 1999). At least, this ability to be soluble suggests that present at the top of hills and the presence of spectrally feature- the observed erosion of layered deposits was made mainly in less layered deposits. a modern Mars environment with liquid water not involved as the major eroding process, thus after that aquifers vanished or 7.3. Iron oxide grains inside sulfate-rich rocks froze. In summary, we interpret kieserite as a primary bulk com- The strong iron oxides detection is clearly connected to the ponent of the most freshly exposed layered deposits formed location where sulfates are detected, despite not everywhere in during either deposition, diagenesis or hydrothermal alteration. the canyon. The distribution of iron oxides has the following Polyhydrated sulfates are interpreted as two possible subtypes, properties: (1) polyhydrated Mg-sulfates formed by the rehydration of kieserite, and (2) sulfates with other cations such as Ca-, Fe- or (a) Iron oxides are observed on two types of material, sulfate- Na-sulfates present in layers different from those with kieserite. rich scarps and dark dunes at the foothill (Fig. 8). (b) Iron oxides on dark dunes have albedo of about 0.12 and 7.2. Pyroxene rich sand dunes are coincident with a depletion of pyroxene in the spectra (Fig. 5). Pyroxenes are observed in dark and flat areas correspond- (c) Iron oxides on layered deposits can be observed with a high ing to interior canyon floor or plateau summits. Except canyon albedo (0.2) (Fig. 8) whereas the presence of oxides might wallslopes, pyroxenes are always observed on sand dunes and explain the lower albedo (0.15) of several sulfate-rich out- dark smooth areas corresponding likely to sand sheets. When crops (Fig. 16). the signature is found together sulfates, high resolution images (d) Gray hematite found by TES is located where the OMEGA show that these regions are spatially mixed, showing sand dunes signature of iron oxides is the most clear (Fig. 9). over layered deposits. No sulfate signature has been found in regions displaying only sand dunes. No pyroxene is found in These observations suggest that the iron oxides are spatially regions presenting clear sulfate signatures devoid of any dark connected with ILD, in contrast to pyroxenes. These observa- dunes in the vicinity. This remark is important as it likely indi- tions also suggest a connection with the gray hematite observed cates a process different than sulfates found at the northern pole by TES. A possible scenario of formation is that layered de- which are observed over sand dunes (Langevin et al., 2005). posits locally contain hematite concretions, such as the blue- The origin of pyroxene rich sand dunes can be questioned. berries at the Opportunity rover site, with a smaller size as Past studies have proposed the possibility to have lava flows on suggested by sand dunes that enable us to detect them in NIR. the bottom of Valles Marineris (e.g., Geissler et al., 1993), but Due to erosion and grinding of the scarps, these grains fall we did not find any evidence for obvious lava flows in Candor down and are stored into sand dunes at the foothill of layered Sulfates in West Candor, Mars 537

Fig. 13. (a) Kieserite in red with MOLA slope contours, (b) kieserite 2.1 µm band depth contours from 2 by 1% step, (c) HRSC image close-up, arrows indicate the unconformity present below the uppermost layer, (d) MOC close-up M0261878, (e) MOC close-up M0902146. deposits. This explanation fits most observations including the 8. Discussion 2: Implications for the mineralogic difference of albedo: when iron oxides concretions concentrate assemblages and the layered deposits formation at the foothill, they display a darker tone than when they are scattered inside the scarps. Nevertheless, the fact that iron ox- Our observations demonstrate that layered deposits are com- ides are found with strong signatures on scarps too could also posed of sulfates, and locally iron oxides, but the total com- show that these oxides are not only present as concretions but position of the rock is still unknown: the presence of sulfates as smaller particles inside the rock matrix. Note the lack of sul- and oxides does not rule out the presence of other minerals, fates in erosional deposits favors their presence as fine particles especially those not detected in NIR, such as amorphous sil- rather than sand-size particles or we would expected for sulfates ica, quartz, anhydrite (CaSO4), halite or sylvite (NaCl or KCl), present at foothill too. as well as minerals in proportion too low to be detected. The Using the ISM (Imaging Spectrometer of Mars) onboard band depths of the spectra are much lower than those measured for library spectra suggesting some mixing with other miner- Mars Phobos, Geissler et al. (1993) proposed the presence of als. However, it can also be due to the size of grains: a small crystalline hematite to be present on the “red mesa,” which cor- grain size (<10 µm) would have a low absorption despite the responds to Ceti Mensa. This observation is based on a strong proportion of minerals being large. These limitations do not al- absorption band at 0.9 µm, similar to what is observed with low us to end up with a total rock composition yet. In addition, OMEGA, and on the red color in visible wavelengths. The phyllosilicates seem absent, or minor, in West Candor Chasma signature is not found at the exact location where iron ox- because no 2.2 or 2.3 µm features typical of metal–OH bond are ides are observed with the most clear signatures in OMEGA, present. From these interpretations we can discuss some impli- but the MGM method shows significant enrichment in iron cations for the layered deposits formation. oxides in Ceti Mensa (Fig. 2). Due to the spatial sampling Layered deposits formed by direct alteration of volcanic of ISM of 22 km/pixel, it is not possible to correlate ex- lava flows is unlikely given our observations. First, pyroxene actly the locations found by Geissler et al. (1993), but it is is strongly depleted in all sulfate rich deposits (Fig. 3). Sec- likely that their observation corresponds to some of the high ond, lava flows, even strongly altered likely would not show iron oxides signatures found by OMEGA. These signatures the ductile deformation visible in some parts of the layering were interpreted as evidence for an aqueous or hydrothermal (Fig. 14). The size, erosional aspect and the thickness of the alteration inside the interior layered deposits. A similar con- layers themselves do not appear to have been produced by lava clusion can be drawn after more detailed OMEGA observa- flows; or even lava rocks modified by mass wasting of canyons tions. wallslopes. Duricrust and coating at the current surface are not 538 N. Mangold et al. / Icarus 194 (2008) 519–543

Fig. 14. (a) OMEGA map of sulfates and HCP with colors as in Fig. 2. (b) MOC close-up on R0901646. (c) Ibid. MOC E0201080. (d) Ibid. MOC R0200633. (e) HRSC image of same location as (d) with HRSC DEM topographic contours. (f) Structural sketch of (d) showing shear zones and deformation of layers at the shear zone contact. favored because of the predilection of detection of sulfates for et al., 1993) also suggests the presence of iron oxides in the slopes and very eroded outcrops. Nevertheless, coating can oc- matrix. cur at the surface of grains in the rock when, for example, The third and fourth cases differ from the previous ones by submitted to fluid circulation at depth. Subsequent transport and the presence of a third phase. The third assemblage is similar remobilization of sulfate-rich material into more recent mater- to the second one, with the addition of grains of undetermined ial might also exist locally. composition. This case is difficult to explain if the sulfates We can try to improve the understanding of the whole formed by alteration; grains should also be altered. Thus, the rock composition by proposing different mineral assemblages fourth case, might be more realistic, with the sand grains being (Fig. 17): (1) sulfate grains in iron oxide rich matrix; (2) a fine- pyroxene grains altered into sulfates. This last case is also one grained sulfate-rich matrix with hematite coarse sand-size con- possibility to explain the presence of pyroxene grains unmod- cretions; (3) a sulfate rich matrix with iron oxides concretions ified at the top of mesas. Here, the matrix can be sulfate-rich and grains of unknown composition; (4) sand size pyroxene as in the third case, but without another composition not iden- grains altered into sulfates with hematite in sand-size concre- tified by NIR spectrometry. Notice that in some of the previous tions and a matrix of unknown composition. examples, sulfates might not be the main abundant constituent The first assemblage could explain the two phase mixture, in terms of volume proportion. This is also something to take in but it would not explain the presence of iron oxides in sand account when suggesting that sulfates are easily soluble: even in dunes, as due to larger grains iron oxides. Several observa- presence of water, the sulfate-rich layers will not dissolve fully tions favor the second possible assemblage: The presence of if they are not composed of sulfates only. In summary, many iron oxides in dunes at the foothill suggests they contain coarse observations favor the second mineral assemblage, but they are material such as sand-size grains or larger grains. The lack of not enough constrained to end this discussion with a single so- sulfates in foothill deposits suggests that they consist of weak lution, mainly due to the lack of constraints on other possible material removed by wind because they might not accumulate components. by scarp erosion. Sulfates might therefore correspond to fine The presence of sulfates, in concert with iron oxides, re- particles blown away by wind during the erosion of scarps. quires, at least, liquid water to have been present in the subsur- Nevertheless, the reddish color of layered deposits (Geissler face, and probably at the surface in local ponds. In agreement Sulfates in West Candor, Mars 539

Fig. 15. (a) HRSC image of area 4 from Fig. 1. (b) HRSC image with kieserite (red to yellow) and polyhydrated sulfates (green to light green). Superposition of two OMEGA orbits. (c) MOC close-up E03-00477. (d) Ibid. MOC M1400631. (e) 3D sketch of area 4. with Gendrin et al. (2005), our observations in West Candor tween ILD and wallslopes is visible in MOC images. Con- Chasma suggest layered deposits formed either by a subaque- tacts in the East Candor Chasma shows that ILD are clearly ous deposition and subsequent modiﬁcation at depth (diage- over the wallslopes (Ledeit et al., 2007) and in the Melas nesis), or an alteration at depth; i.e., hydrothermal alteration Chasma region (Quantin et al., 2006). A similar interpreta- of previously existing deposits such as ash, lacustrine or wind tion may apply in West Candor Chasma where patches of blown material. Discussions about the chemical implications of layered deposits on the wallslopes appear to dip toward the these minerals are proposed in Arvidson et al. (2005) and Poulet canyon unconformably over the wallslopes rocks. In addition, et al. (2008). the difference between ILD and wallslope rock also shows A question comes from the contact between ILD and walls- that the ILD hills are not residue of altered wallslopes. We lope rocks, as shown with a question mark in the cross-section do not discuss further this question that will be the aim of fu- of Fig. 18. This contact is very difﬁcult to interpret in West ture papers; as it is an important point for the timing of the Candor Chasma since no true stratigraphic cross-section be- events. 540 N. Mangold et al. / Icarus 194 (2008) 519–543

Fig. 16. Close-up on the kieserite rich area: (a) kieserite map in pseudocolor mode with 2.1 µm band depth from 2% (blue) to 7% (red). (b) HRSC view of the dark ﬂat area. (c, d) Close-up of MOC images E0901673 and E1203404 showing the central part of this high band depth which is heavily eroded and darker than the surroundings.

Table 3 Summary of surface properties for each mineral component detected Mineral Albedo Th. inertia Slopes Interpretation proposed Kieserite 0.19 of median peak, 250–400 SI, More frequent on steep Strongly eroded bedrock with kieserite often brighter than lower than polyhydrated slopes present in the rock polyhydrated

Polyhydrated 0.16 of median peak, 320–480 SI, More frequent on gentle Consistent with a different sulfates darker than kieserite slightly higher thermal slopes composition (ex: Ca-, Fe-, Na-sulfates) but a second peak inertia than kieserite or with a rehydration of kieserite into higher at 0.20 more hydrated Mg-sulfates

HCP 0.16 of median peak 200–420 SI, On the lowermost slopes Mostly on sand dunes and sand sheets. consistent with coarse and ﬂat areas No signatures on ILD except by spatial sand size grains mixing of dunes on layers Iron oxides Bimodal with two 280–400 SI, Same as kieserite except Present together sulfates, as peaks at 0.12 and 0.17 Higher than mobile dust some on foothills sand/gravel size grains eroded and transported downhill

The involvement of liquid water in West Candor Chasma (level 1) and iron oxides formation (level 2). The ﬁrst one is lo- is especially important in terms of elevation and thickness of cated at 2.5 km high, 500 m below the hill top. It is also about material. Fig. 18 shows a cross-section of the Candor Mensa. 7 km higher than the lowermost place at the canyons eastern Sulfates and iron oxides are present at about the same maxi- border and only 1.5 km below the plateau level. It is possible mum elevation on both sides of the hill. The two levels there- that iron oxides only formed at deeper levels (below 1 km), as fore indicate the uppermost limit required for sulfate formation they are not found at such high elevations. These levels indicate Sulfates in West Candor, Mars 541

terial in ILD would be at the uppermost part of ILD hills which are levels that groundwater might not have reached, therefore preserving the original unaltered material, if it ever was anhy- drous.

9. Conclusion

The detailed correlation between the different spectral types found in West Candor Chasma and local geology and physical properties end up with some important conclusions for the formation of these minerals and the overall layered deposits:

(i) Kieserite is mainly present over strongly eroded scarps of light toned material (albedo about 0.2) with intermediate Fig. 17. Different possible mineralogical assemblages of the sulfate-rich layered deposits according to the observations reported. Grains are typically of thermal inertia. It corresponds likely to a mineral present sand grains size (around 100 µm). primary in the rock formed either during formation and diagenesis of sediments, or during hydrothermal alteration that aquifers were close to the surface and were present over a at depth. significant thickness in the West Candor Chasma. Then, the eas- (ii) Polyhydrated sulfates are preferentially present on less iest way to explain the highest groundwater level at 2.5 km of eroded and darker outcrops than outcrops of kieserite; with Fig. 18 is that the canyon was once completely filled by layered a strong variation in these characteristics. These variations deposits above the 2.5 km level, enabling the water to reach this might be the result of a diversity in their composition (Ca-, level, followed by a substantial erosion that removed materials Na-, Mg-sulfates) and different hydration levels. later. (iii) Pyroxenes are present mainly on sand dunes and sand This observation is also important to constrain the strong sheets in low lying terrains. Pyroxene is strongly depleted hydraulic head that might have existed at that period and may or absent in layered deposits. When mixed with kieserite, have been involved in the formation of outflow channels (Hanna local observations favor a spatial mixing with mobile sand and Phillips, 2006). For example, Coleman et al. (2007) notice over layered deposits. The presence of pyroxene-rich sand the presence of two outflow channels formed from sudden sub- dunes on top of the Candor Mensa remains poorly ex- surface water release, Allegheny Vallis and Walla Walla Vallis, plained. that are located east of the East Candor Chasma canyon. These (iv) Strong signatures of iron oxides are present on sulfate-rich two outflow channels have their sources at 2500 m of eleva- scarps and at the base of layered deposits foothill. They tion, a value similar to the elevation of sulfates in West Candor are correlated with TES gray hematite signature and might Chasma, therefore suggesting the high elevation of aquifers was correspond to iron oxides present in the rock as sand-size not limited to West Candor Chasma. grains, or possibly larger concretions, that are eroded away Finally, in order to distinguish in between the two main hy- scarps and deposited downhill as a residual lag. potheses of formation, chemical precipitation at the surface in (v) Sulfates require liquid water to form, at least as aquifers. lakes or in subsurface fluids, one would have to understand the The elevation of the uppermost sulfate signature suggests total composition of ILD and to understand spectrally feature- the presence of aquifers up to 2.5 km above datum, only less parts of ILD. A specific location to look for nonaltered ma- 1 km below the plateau surface.

Fig. 18. North–south cross-section of the Candor Mensa. The layering is postulated as subhorizontal. Colors indicate the presence of minerals detected, see text for more explanations. 542 N. Mangold et al. / Icarus 194 (2008) 519–543

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