Spectroscopy and detectability of liquid brines on mars

Planetary and Space Science 92 (2014) 136–149

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Spectroscopy and detectability of liquid brines on mars M. Massé a,b,n, P. Beck c, B. Schmitt c, A. Pommerol d, A. McEwen e, V. Chevrier f, O. Brissaud c, A. Séjourné b a

Institut d’Astrophysique Spatiale, Bat. 121, Université Paris-Sud, 91405 Orsay cedex, France Institute of Geological Sciences, Polish Academy of Sciences, Research Centre in Wroclaw, ul. Podwale 75, 50-449 Wroclaw, Poland c Institut de Planétologie et Astrophysique de Grenoble, Université de Grenoble, CNRS/INSU, 414 Rue de la Piscine, Domaine universitaire, 38400 St-Martin d’Hères, France d Physikalisches Institut, Universität Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland e Lunar and Planetary Laboratory, University of Arizona, 1541 E. University Bvd, Tucson, AZ-85721, USA f Keck Laboratory for Space and Planetary Simulation, Arkansas Center for Space and Planetary Science, FELD 202, University of Arkansas, Fayetteville, AR 72701, USA b

art ic l e i nf o

a b s t r a c t

Article history: Received 24 October 2013 Received in revised form 26 December 2013 Accepted 20 January 2014 Available online 31 January 2014

Recent geomorphological observations as well as chemical and thermodynamic studies demonstrate that liquid water should be stable today on the Martian surface at some times of the day. In Martian conditions, brines would be particularly more stable than pure water because salts can depress the freezing point and lower the evaporation rate of water. Despite this evidence, no clear spectral signature of liquid has been observed so far by the hyperspectral imaging spectrometers OMEGA and CRISM. However, past spectral analysis lacks a good characterization of brines' spectral signatures. This study thus aims to determine how liquid brines can be detected on Mars by spectroscopy. In this way, laboratory experiments were performed for reproducing hydration and dehydration cycles of various brines while measuring their spectral signatures. The resulting spectra first reveal a very similar spectral evolution for the various brine types and pure water, with the main difference observed at the end of the dehydration with the crystallization of various hydrated minerals from brines. The main characteristic of this spectral behavior is an important decoupling between the evolution of albedo and hydration bands depths. During most of the wetting/drying processes, spectra usually display a low albedo associated with shallow water absorption band depths. Strong water absorption band depth and high albedo are respectively only observed when the surface is very wet and when the surface is very dry. These experiments can thus explain why the currently active Martian features attributed to the action of a liquid are only associated with low albedo and very weak spectral signatures. Hydration experiments also reveal that deliquescence occurs easily even at low temperature and moderate soil water vapor pressure and could thus cause seasonal darkening on Mars. These experiments demonstrate that the absence of water absorptions in CRISM in the middle afternoon does not rule out water activity and suggest future spectral investigations to identify water on the Martian surface. & 2014 Elsevier Ltd. All rights reserved.

Keywords: Mars Composition Laboratory experiments Spectroscopy Brines

1. Introduction From the beginning of Martian exploration, one of the main research interest concerns the investigation of possible liquid

n Corresponding author at: Institut d'Astrophysique Spatiale, Bat. 121, Université Paris-Sud, 91405 Orsay cedex, France. Tel.: +33 169858732. E-mail addresses: [email protected] (M. Massé), [email protected] (P. Beck), [email protected] (B. Schmitt), [email protected] (A. Pommerol), [email protected] (A. McEwen), [email protected] (V. Chevrier), [email protected] (O. Brissaud), [email protected] (A. Séjourné).

0032-0633/$ - see front matter & 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.pss.2014.01.018

water on the Martian surface. This possible presence of liquid water is a key element for understanding the climatic history and for deciphering if some biological processes could have developed on Mars. There is much evidence for water on ancient Mars, particularly during the Noachian period, but the recent Amazonian period is generally considered as a time of low geological activity with a surface evolution dominated by the action of ice and wind (e.g. Carr and Head, 2010). However, various currently active Martian geomorphological features have been attributed to the action of a liquid such as the “Recurring Slope Lineae” (or RSL) (McEwen et al., 2011), the perennial rills observed on the Russell Crater dunes (Jouannic et al., submitted for publication) or the putative droplets observed on the Phoenix lander struts (Renno et al., 2009). The possible presence of a liquid activity on the current

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Martian surface is also enhanced by several laboratory studies and thermodynamic modeling, which demonstrate that liquid should be at least transiently present today (Brass, 1980; Haberle et al., 2001; Knauth and Burt, 2002; Chevrier and Altheide, 2008; Chevrier et al., 2009a,b; Möhlmann, 2011; Gough et al., 2011). Because the present-day surface conditions of Mars are close to the triple point of water, pure liquid water is generally unstable (Haberle et al., 2001). However, the addition of salts can depress the freezing point and reduce the evaporation rate of water making liquid brines much more stable and more likely to occur on Mars than pure water (e.g. Brass, 1980; Altheide et al., 2009; Zorzano et al., 2009; Möhlmann and Thomsen, 2010). Moreover, the possible formation of liquid brines is supported by the identification of large amounts of various salts (sulfate, chloride or perchlorate), spread in various forms on the whole Martian surface (e.g. Gendrin et al., 2005; Squyres et al., 2006; Wang et al., 2006; Hecht et al., 2009; Massé et al., 2010; McClennan, 2012). The thermodynamic stability of liquid brines, the extensive presence of salts, and the observation of various features interpreted as due to a liquid, suggest that stable or metastable liquid brines occur today on the Martian surface. Despite of the evidence, no clear spectral signature of liquid has been observed so far by the hyperspectral imaging spectrometers OMEGA and CRISM (Bibring et al., 2004; Murchie et al., 2007). However, past spectral analysis lacks a good characterization of brines' spectral signatures. Therefore, our study aims to determine how we can detect liquid brines on Mars by spectroscopy and more particularly to answer the following questions: (1) What are the diagnostic absorption features of liquid brines? (2) What is the spectral behavior of brines during their formation and their disappearance? In order to answer these questions we have performed some laboratory experiments that aim at reproducing hydration and dehydration cycles of various brines while measuring their spectral signatures. In Section 2, we briefly introduce the different hypotheses suggested so far for the brines formation. Then, we describe in Section 3 the experimental setup used in this study. The results obtained from these laboratory experiments are presented in Section 4 for dehydration experiments and in Section 5 for hydration experiments. Finally, in Section 6 we discuss their implications for the detection of liquid on current Mars.

2. Possibility of current liquid brine on Mars 2.1. Current liquid activity on Mars The recent presence of liquid water on the Martian surface has been suggested for the formation of gullies (Malin and Edgett, 2000; Malin et al., 2006), but the involvement of water in gully activity seen today remains debatable (Dundas et al., 2010; Dundas et al., 2012). However, new data provided by HiRISE images and ground observations provided by the Phoenix lander reveal the presence of active features interpreted as due to a liquid activity. 2.1.1. Seasonal orbital observations Two current seasonal features, perhaps driven by a liquid activity, have been observed so far on the Martian surface:

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found in the southern mid-latitudes but some RSL have also been recently discovered in equatorial regions, especially Valles Marineris (McEwen et al., 2014). They develop on steep slopes (241–401), favoring equator-facing slopes in the middle latitudes, times and places with peak temperatures of 4250– 300 K. Repeat MRO/HiRISE images reveal that they appear and grow during warm seasons (from the late southern spring to early fall in Southern mid-latitudes) and fade and disappear during cold seasons. In Valles Marineris they are active over the full year. The presence of several unique characteristics such as a low albedo, the association with warm places and times, the seasonal occurrence and their incremental growth strongly suggest the involvement of a liquid in RSL formation. (2) Perennial rills. Perennial rills (Reiss et al., 2010; Jouannic et al., submitted for publication) are located on a specific portion of the southwest-facing slip face of the Russell crater megadune and are superimposed to the Russell gullies. Their morphology (Fig. 1b) comprises a distributary system formed by numerous small branching and sinuous rills (sinuosity index
1.1). They can extend up to hundreds meters in length and their average width is
1 m. They are located on gentle slopes with an average value of
121. The rills are surrounded by dark areas, which appear at the same time as the generation of the rills. Like the RSL, the perennial rills correspond to a seasonal process with an activation period ranging from the early spring to the beginning of summer. They also develop during the warmest times, when the surface temperatures exceed 273 K. However, on the contrary to the RSL, the perennial rills and their associated dark zone persist after their first appearance and do not disappear during cold seasons. Therefore, each spring reveals the creation of new rills either overprinting an older network or onto virgin dunes. The presence of several unique characteristics like their seasonal formation during spring, their development on gentle slopes, and the creation of sinuosity ruled out the hypothesis of a dry granular flow. This conclusion is enhanced by the development of numerical experiments and the comparison with terrestrial analogs in Iceland (Jouannic et al., submitted for publication). Diniega et al. (2013) presented the alternative hypothesis that these rills (or linear gullies) form by sliding blocks of CO2 ice.

2.1.2. Phoenix lander observations In addition to the orbital observations, images showing possible droplets on the legs of the Phoenix lander have been interpreted as evidence of a liquid phase (Renno et al., 2009). These spheroids first display a darkening, next they appear to grow and merge, and then they disappear. Considering the thermodynamical models and the sublimation rates of ice excavated by Phoenix, it seems unrealistic that these liquid droplets might be composed of pure water. As Phoenix has discovered the presence of various salts such as perchlorates, Renno et al. (2009) rather suggest that the droplets are formed by liquid brines. These brines could have formed by the deliquescence of salts (i.e. the transition to aqueous phase of salts by sorption of atmospheric water vapor) directly on the Phoenix lander. In addition, Cull et al. (2010) interpreted spectral data in the Phoenix landing site soils as localized patched of perchlorate salts transported by thin films of water. These observations provide direct evidence that a salt-rich liquid can exist on the current Martian surface. 2.2. Stability and formation of liquid on Mars

(1) Recurring Slope Lineae (RSL). RSL (McEwen et al., 2011; Ojha et al., 2013) are dark (up to 40% darker than the surrounding areas) and narrow (0.5–5 m width) (Fig. 1a). They are often associated with small channels or gullies. They were initially

2.2.1. Stability of liquid on Mars The present-day surface conditions on Mars are very close to the triple point of water. Pure liquid water is thus generally

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N

N

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RSL

50 m

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Fig. 1. Images of current active seasonal features probably driven by a liquid. (a) Recurrent Slope Lineae (RSL) on the wall of the Newton crater, HiRISE image: ESP_022689_1830. (b) Perennial rills observed on the Russell crater megadune, HiRISE image: ESP_021562_1255.

unstable on Mars because of a combination of low temperatures and pressures dominating the surface. These conditions keep pure water frozen and sublimating, or evaporating and boiling (Ingersoll, 1970; Chevrier and Altheide, 2008). However, favorable temperature and pressure conditions can appear at some specific places and times and lead to the formation of stable pure liquid water (Haberle et al., 2001; Hecht, 2002; Richardson and Mischna, 2005). In this case, pressure and temperature have to be above the triple point of pure water and below the boiling point. Haberle et al. (2001) determine that these conditions can occur only in the northern lowland (between 01 and 301N) and in the Southern Hemisphere impact basins of Hellas and Argyre. If pure liquid water seems thus to be transiently stable on the Martian surface, the RSL and the perennial rills are not located in these stability areas. Moreover, some RSL may have developed where surface temperatures were below 273 K (Ojha et al., 2013) and the rills formed at much colder temperatures. However, Haberle et al. (2001) did not consider the effect of slopes and the RSL form mostly on steep slopes. If pure liquid water seems to be not consistent with the formation of RSL and rills, dissolved salts can allow liquid water (at least temporarily) under the current Martian environment. Brines indeed display lower water activity, resulting in lower freezing temperatures and lower evaporation rates (Brass, 1980; Knauth and Burt, 2002; Chevrier and Altheide, 2008; Möhlmann and Thomsen, 2010). Therefore, the stability conditions of brines considerably extend the area where a liquid can form on the Martian surface (Haberle et al., 2001; Chevrier and Altheide, 2008; Möhlmann and Thomsen, 2010; Chevrier and Rivera-Valentin, 2012). In this way, Haberle et al. (2001) infered that the favorable regions could potentially include most of the planet and Chevrier and Rivera-Valentin (2012) demonstrated that the formation of brines is at least consistent with the mid-latitude RSL locations. Moreover, the plausibility of the Martian brine hypothesis is enhanced by the identification of large amounts of salts over the Martian surface (e.g. Gendrin et al., 2005; Squyres et al., 2006; Wang et al., 2006; Kounaves et al., 2010; Massé et al., 2010; Osterloo et al., 2010). These salts are detected both in sedimentary deposits and dispersed in the regolith. Various salts have been detected so far and can be at the origin of the brine formation. Sulfates have been detected in the dust covering the whole Martian surface (Clark and Baird, 1979), in some sedimentary deposits in the equatorial regions (e.g. Gendrin et al., 2005; Massé et al., 2008; Wray et al., 2009) and in the dune field surrounding the North Polar Cap (Langevin et al., 2005; Massé et al., 2010, 2012). These sulfates have been mainly identified as magnesium, calcium and ferric sulfates. Magnesium and calcium sulfates display

relatively high eutectic temperatures (around 270 K) and are thus nearly as unstable as pure liquid water (Kargel, 1991; Chevrier and Altheide, 2008). On the contrary, ferric sulfates present a very low eutectic temperature, perhaps as low as 205 K, and thus correspond to one of the most stable low-temperature brine (Chevrier and Altheide, 2008; Chevrier et al., 2009a). Chlorides bearing materials have been identified by Osterloo et al. (2010) thanks to data provided by the Thermal Emission Imaging System (THEMIS). These minerals have been found mostly in the southern highlands and particularly on areas broadly consistent with RSL locations. Data from the Phoenix lander also suggest high concentration of chlorides at the Phoenix landing site (Kounaves et al., 2010). Chlorides display a relatively low eutectic temperature: 252 K for magnesium chloride (Altheide et al., 2009), 236.5 K for ferrous chloride (Altheide et al., 2009) and 223.2 K for calcium chloride (Chevrier and Altheide, 2008). Chlorides are thus a good candidate for the formation of liquid brines on Mars. Perchlorates have been discovered in the Vastitas Borealis plains by Phoenix (Hecht et al., 2009; Kounaves et al., 2010) and are inferred to be present at the MSL landing site (Clegg et al., 2013). Perchlorates are part of a saline alkaline paragenesis dominated by chlorine, so we can expect to find them in other regions associated with chlorides (Chevrier et al., 2009b). They display very low eutectic temperatures such as 236 K for sodium perchlorates and 206 K for magnesium perchlorates (Chevrier et al., 2009b). The eutectic of calcium perchlorate has not yet been measured, but is below 223 K (Nuding et al., 2013). Magnesium and calcium perchlorates thus form much more stable brines than other minerals due to their lower eutectic temperature. Chevrier et al. (2009b) and Gough et al. (2011) have demonstrated that perchlorates can be aqueous during the summer for at least a few hours near sunrise and sunset.

2.2.2. Formation of liquid brines If liquid brines seem to be the most relevant ingredient in the origin of the RSL, perennial rills, or Phoenix lander droplets, the mechanism which leads to the formation of a sufficient amount of brines to trigger the gravitational instability and initiate the flow remains unclear. Preconditions for brines to evolve are the presence of water and salts. As already discussed in Section 2.2.1, salts are widely present on the Martian surface under various forms. Water on Mars is stored in four different ways: (1) atmospheric water vapor, (2) seasonal surface frost, either as pure water frost deposits, or as grains in CO2 frost or ice, (3) cryosphere, in the form of ground ice, perhaps with lenses of eutectic brines, or

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(4) polar ice caps. These four different water storages suggest two different processes for the formation of liquid brines. (1) Deliquescence. Deliquescence occurs by the sorption of atmospheric water vapor and produces transition to aqueous phase of salts. This process happens when the atmospheric humidity exceeds a critical threshold value different for each salt (Möhlmann, 2011). Möhlmann and Thomsen (2010) and Gough et al. (2011) have recently demonstrated that deliquescence is a very efficient mechanism to form liquid brines on Mars. Depending of the location, the season and the subsurface depth, the brines could be liquid during several hours per day. This process might be consistent with the observation of the Phoenix droplets (Renno et al., 2009). If deliquescence can also occur on the RSL and perennial rills areas, it can however hardly be at the origin of the amount of liquid required to trigger a flow. During spring and summer, the amount of water vapor can be enough for initiating the deliquescence (Smith, 2002), but remains relatively low and is probably far from enough for significant liquid features to form (Chevrier and Rivera-Valentin, 2012). Another method for increasing the quantity of water vapor could be sublimation of surface frost or of relict subsurface ice and diffusion of water vapor towards the surface (McEwen et al., 2011). (2) Ice melting. On the areas where RSL, perennial rills and Phoenix droplets are observed, ice can be present in the form of buried ice (Chevrier and Rivera-Valentin, 2012), niveo-aeolian deposits in dunes (Jouannic et al., submitted for publication), or seasonal condensation of frost in the shallow subsurface associated with bedrock (Stillman et al., 2013). Chevrier and RiveraValentin (2012) have demonstrated that RSL could result from episodic melting of frozen brine. Melting of pure water ice has been also suggested by Stillman et al. (2013) for the formation of RSL but such clean ice is highly unstable and the water rapidly evaporates (or boils) at the surface. Alternatively, pure water can melt and then dissolve salts present in the soil, resulting in an increased stability. The involvement of buried ice or niveo-aeolian deposits is relevant for the triggering of flows but implies an active recharge mechanism in order to maintain a source of brine over short geological timescales (Chevrier and Rivera-Valentin, 2012). The hypothesis of a seasonal water frost condensation in the bedrock during winter (Stillman et al., 2013) could lead to an appropriate recharge mechanism. Regarding all the observations described in part 2, the presence of brine seems highly plausible on the current Martian surface. However, if this hypothesis is enhanced by geomorphologic observations, thermodynamical models or experimental constrains, it suffers from the lack of an identification of liquid water by the OMEGA or CRISM near-infrared imaging spectrometers. The goal of this study is thus to simulate experimentally some hydration and dehydration cycles of brines or pure water and to record their reflectance spectra over the visible and near-infrared ranges. In this paper, the term “hydration” is used to describe any process that results in increasing the water content of the sample. These experiments aim to understand the spectroscopic behavior of liquids during these processes and to determine the diagnostic absorption bands that we can expect to find on Mars.

3. Methods 3.1. Experimental setup A picture of the experimental setup is shown in Fig. 2. 3.1.1. Reflectance spectrometer The spectra are measured with the IPAG spectro-gonio radiometer (Brissaud et al., 2004). This instrument is a bidirectional

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COLD ROOM Visible detector Illumination arm Infrared detector Observation SERAC arm

Source, Monochromator, Detection systems

Balloon containing water vapor

Sample in closed cell

Pressure sensors

Water bottle

Fig. 2. Experimental setup in the IPAG cold room.

visible and near-infrared reflectance spectrometer operating over the 0.4–4.8 mm spectral range. Instrument design is optimized to achieve a good photometric accuracy (o1%) under most of the measurement geometries. All spectra reported in this paper were measured using vertical incidence: θi ¼01 and an emergence angle of θe¼301. Corrections are applied to take into account minor absorptions in the Spectralons spectrum and non-Lambertian behaviors of the Spectralons and Infragolds reference surfaces (Bonnefoy, 2001). For all measurements presented in this paper, a spectral sampling interval of 0.02 mm was used. This value is consistent with the majority of hyperspectral datasets acquired on planetary surfaces. As we aim to use the spectra presented in this paper to interpret hyperspectral data of the Martian surface (OMEGA, CRISM), we have chosen this value of spectral sampling despite the fact that a higher spectral resolution could potentially help to better interpret the spectra. In those conditions, the measurement of a complete spectrum requires about 80 min. In order to focus on hydration features, we have restricted the measured spectral range to 1.1–4.2 mm.

3.1.2. Simulation chamber The reflectance spectrometer is combined with a simulation chamber named SERAC (Spectroscopy En Reflectance sous Atmosphere Contrôlée) (Fig. 2) designed by Pommerol et al. (2009), which is able to reproduce Martian surface conditions. The empty volume inside the chamber is 181 ml allowing a maximum sample size of 30 mm in diameter and 1–10 mm in thickness. A 3 mm thick sapphire window can close the chamber. In this case, a simple model of multiple reflections between the sample and the window is used to correct the measured spectra. Temperature of the sample is controlled using a heating resistance, a Pelletier cooling system and a PT100 Platinium resistance thermometer placed directly under the sample. This system can produce temperatures ranging from 40 1C to 130 1C. A turbo molecular pump allows us to pump the system to pressures lower than 10  6 mbar. Two MKS Baratrons high accuracy absolute pressure sensors are used to monitor pressure inside the chamber. These sensors are able to measure pressures between 10  5 and 130 mbar. Water vapor is injected or removed from the simulation chamber and pressure is measured every seconds. The source of water vapor is a volume of ultra-pure, demineralized and carefully outgassed liquid water maintained at a temperature of 293 K.

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3.2. Samples description As an analog of Martian soil we used a basalt sample (from “La Réunion”) grinded to obtain a homogeneous fine-grained powder (o30 μm). We have selected three salts for our experiments spanning a range of eutectic temperatures and ability to form hydrates and deliquesce (and thus their ability to be detected by hyperspectral data), and present in different environments on the Martian surface. Ferric sulfate (Fe2(SO4)3). Ferric sulfate displays a very low eutectic temperature of 205 K (Chevrier and Altheide, 2008; Chevrier et al., 2009a,b). Moreover, it is widely detected on the Martian surface both in the regolith (Lane et al., 2004) and in some sedimentary formations (Christensen et al., 2004). Magnesium chloride (MgCl2). Magnesium chloride displays an intermediate eutectic temperature of 252 K (Altheide et al., 2009). Chloride was identified in the broad region of RSL locations by THEMIS data (Osterloo et al., 2010) and on the Northern plains by Phoenix. Magnesium sulfate (MgSO4). Magnesium sulfate displays a high eutectic temperature of 270 K (Altheide et al., 2009), close to the one of pure water (273 K). Nevertheless, magnesium sulfate is widely spread on Mars both in sedimentary deposits (Christensen et al., 2004; Gendrin et al., 2005; Murchie et al., 2009) and in the regolith (Wang et al., 2006). We can thus infer that the formation of liquid brines with magnesium sulfate via melting of ice is likely. Magnesium sulfate forms also several hydrates with very diagnostic absorption bands. Even if perchlorates have been detected on Mars and display some very low eutectic temperatures, no perchlorates have been selected for these experiments because of laboratory safety concerns. 3.3. Experimental procedures 3.3.1. Dehydration experiment The dehydration experiment consists in manually depositing the basalt powder on the sample holder and wetting the sample with a syringe. The samples are wetted until soil saturation is achieved, i.e. until a mm-thick film of liquid is visible above the sample surface with the naked eye. Four experiments were performed: one “blank” with ultrapure water, and three experiments where MgCl2, Fe2(SO4)3 and MgSO4 salts have been dissolved in water. For the later three, commercial ultrapure salts were used and 8 g of salts were dissolved in 20 ml of ultrapure water (mass concentration: 125 g/L). Due to their high solubility in water, all salt solutions prepared were under saturated, and total dissolution was ensured by vigorously stirring the solutions for several minutes. The sample holder was subsequently placed in the simulation chamber under the spectro-gonio radiometer. Near-infrared reflectance spectra were acquired successively with a time interval of about 80 min between two consecutive spectra. For these experiments, the chamber was left open to air to permit evaporation of H2O. In order to force the evaporation, the samples were mildly heated to 30 1C, which ensured diffusion of H2O from the sample to the ambient air (18 1C). For the experiments with ionic solutions, total desiccation was not obtained even after a few days at 30 1C. Therefore, in order to achieve complete desiccation, the samples were placed at 100 1C in an oven for at least 3 h, and the reflectance spectra of the “dry” samples were measured afterwards. 3.3.2. Hydration experiment In order to perform hydration experiments of a soil by water vapor, we made use of the SERAC simulation chamber (Pommerol et al., 2009; Pommerol, 2009; Beck et al., 2010). This experiment is

designed to inject a controlled amount of water vapor in a closed environmental cell. In order to force deposition of water on the sample, the sample holder was cooled down to þ 4 1C, while the room was at a temperature of þ18 1C. Water vapor produced from outgassed liquid water is allowed to expand into a 10 L volume (Fig. 2). Both the liquid water and the 10 L volume being at a constant temperature of þ18 1C, water evaporates until the vapor pressure reaches the saturation pressure of water at this temperature. The connection between the 10 L volume and liquid water is then closed, and the 10 L volume filled with vapor is subsequently opened to the environmental cell. Supersaturated H2O gas, with regard to the sample maintained at 4 1C, enters the cell previously under secondary vacuum (10  4 mbar) and adsorbs or condenses on the sample. By successively opening and closing the connection between the 10 L volume and the environmental cell, it is possible to progressively hydrate the sample and calculate the total uptake of H2O (in moles or mass) by the sample. For each water addition step, the quantity of water adsorbed by the sample can simply be calculated by a mass balance considering the amount of water vapor introduced in the setup before adsorption, the amount of water vapor remaining in the simulation chamber from the previous hydration step and the amount of water vapor at equilibrium at the end of the current hydration step. We use the perfect gas law to convert absolute value of pressure in quantity of water. In the ranges of pressure and temperature of our measurements, considering water vapor as a perfect gas does not introduce severe bias in the mass balance (Burnett et al. 1996). The ratio between this mass and the mass of the sample give the relative mass (in wt%) of H2O, as given in the text and figures. Two hydration experiments were performed with SERAC, one with Fe2(SO4)3 and one with MgCl2. Because of their very time consuming nature, only one of the two experiments was performed with controlled H2O mass addition to the sample (Fig. 8a). This sample was a mixture of Fe2(SO4)3 and basalt (10 wt% and 90 wt% respectively). Because only a few mmol can be added one after the other, we had to stop the experiments before the soil was fully hydrated. As a counterpart we performed a second experiment (Fig. 8b), where the environmental cell was always connected with the 10 L volume, and during which we were able to fully wet the sample. In this case, we could not keep track of the exact mass of H2O added to the sample. This later experiment was done with a mixture of MgCl2 and basalt (10 wt% and 90 wt% respectively).

4. Dehydration experiment 4.1. Results The dehydration spectral series are presented in Fig. 3 for pure water, Fe2(SO4)3, MgSO4 and MgCl2 solutions. Except at the end of the drying, the spectral behavior observed during the dehydration is very similar for the four samples. 4.1.1. The spectra of saturated soils In all four experiments, the starting spectra (in black), corresponding to the wettest soil, display saturation in the 1.45 mm, 1.95 mm, 2.5 mm and 3 mm regions (reflectance below 3%). These wavelengths correspond to vibrational absorptions of the water molecules. Outside of these bands the “continuum” of the saturated soils shows a blue slope. Such a slope is observed in the optical constants of liquid and solid water, and is due to the overlapping of the wide and increasingly strong absorption bands of H2O (Milliken and Mustard, 2005). The reflectance values at 1.1 mm (considered in this paper as a proxy for the sample's visible albedo, outside of the bands) of our wet soil analogs are typically

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Fig. 3. Series of spectra during the dehydration experiments for: (a) pure water, (b) Fe2(SO4)3 brine, (c) MgSO4 brine, and (d) MgCl2 brine. Spectra are acquired every 80 min. For all the first spectra the sample is placed at ambient air and heated to 30 1C. For each last spectrum, the sample is placed in an oven at 100 1C for 3 h. All the spectra have been masked between 2.5 and 2.8 mm because this wavelength range is often perturbated by some variations in the water vapor content between the reference and the measurement.

about 40% of that of the dry basalt (41.5% for pure H2O, 39% for MgSO4 solution, 44.5% for MgCl2 and 31% for Fe2(SO4)3. 4.1.2. Spectral evolution through dehydration After the first spectrum is measured (which takes 80 min), as dehydration proceeds, the 1.45, 1.95 and 2.5 mm bands are no longer saturated and the blue slope disappears. In order to follow the spectral changes during dehydration, Fig. 4 represents the evolution in time of 3 spectral criteria that are the “albedo” (reflectance in the continuum at 1.1 mm) and the band depths at 1.45 mm and 1.95 mm (inside water bands), and Fig. 5 represents the evolution of the albedo as a function of the depth of the 1.95 mm band. The calculation of band depths at 1.45 mm and 1.95 mm is defined as follows (Massé et al., 2010): BDð1:45Þ ¼ 1 

Rð1:45Þ 0:45  Rð1:68Þ þ 0:55  Rð1:28Þ

ð1Þ

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Rð1:95Þ 0:38  Rð2:2Þ þ 0:63  Rð1:82Þ

ð2Þ

where R(x) is the value of reflectance corresponding to the wavelength at x mm. In all four experiments, a similar behavior is observed. In a first step, as dehydration progresses, the reflectance at 1.1 mm appears to remain nearly constant for several hours, while the band depths

at 1.45 and 1.95 mm decrease progressively. In other words, the sample remains dark, while the H2O spectral signatures become weaker. This plateau in term of continuum reflectance is followed by a second phase during which the 1.1 mm reflectance increases while the band depths at 1.45 and 1.95 mm are almost constant (Figs. 4 and 5). Although the 3-mm absorption is saturated, this band displays a constant narrowing all along the dehydration process. Also, in almost all spectra (with the exception of the pure water experiment after dehydration) a peculiar behavior is observed during all four experiments. There is a phase during each drying experiments when the reflectance within the 3-mm band increases (at 3.1 mm, see the gray arrows on Fig. 3). The timing of these “3-mm flickers” corresponds to the second or the third spectrum acquired (T1 for pure water, T2 for Fe2(SO4)3, T2 for MgSO4 and T3–T4 for MgCl2). As it will be discussed later, we attribute this “3-mm flicker” to a broad specular reflection and an effect of the real index of the optical constant of water.

4.1.3. Spectra after complete drying Although the spectral behavior of the four samples is relatively similar during the entire dehydration sequence, some differences are observed toward the end. Fig. 6a represents the spectra and the surface of the samples after a complete drying in an oven.

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Fig. 5. Evolution of the albedo as a function of the depth of the 1.95 mm absorption band during the dehydration experiment represented in Fig. 3 for: (a) pure water, (b) Fe2(SO4)3 brine, (c) MgSO4 brine, and (d) MgCl2 brine.

M. Massé et al. / Planetary and Space Science 92 (2014) 136–149

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Fig. 6. Final step of the dehydration experiments. (a) Spectrum of pure dry basalt and spectra obtained after final drying in an oven for pure water, Fe2(SO4)3 and MgCl2 experiments. All the spectra have been masked between 2.5 and 2.8 mm because this wavelength range is often perturbated by some variations in the water vapor content between the reference and the measurement. (b) Evolution of the 1.95 mm absorption band (area and barycenter) during the dehydration of the MgCl2 sample. (c) Surface of the samples after dehydration in an oven for pure Fe2(SO4)3, water, and MgCl2 experiments.

The sample containing MgSO4 is not represented here because of loss of this sample. The “pure water” sample displays a nearly flat spectrum whereas brines samples still exhibit deep hydration absorption bands. The sample containing the MgCl2 brine reveals an important re-increase of the 1.45 and 1.95 mm absorption band at the end of the experiment (Figs. 3 and 4d). The Fe2(SO4)3 sample only displays a still strong 3 mm absorption band and a residual band at 1.95 mm (Fig. 3b). A better observation of the MgCl2 sample evolution is represented on Fig. 6b with the evolution of the barycenter of the 1.95 mm absorption band as a function of its area. The exact positions of the absorption features can indeed provide information on the environment of the water molecules. Because the 3-mm band is saturated, and because the 1.45 mm band is too weak in the spectra of desiccated samples, we only focused on the 1.95 mm band. The barycenter is chosen rather than the wavelength of the maximum of absorption because it takes into account the full absorption band including its shape (rather than a single wavelength), i.e., the distribution of the position and energy of the bound, and because it does not require any hypothesis on the shape of the absorption band (i.e. functional expression for fitting). The evolution of the 1.95 mm absorption band during the dehydration reveals a blue shift of its barycenter position and a simultaneous decrease of the band area. The final albedo also varies depending on the sample (Fig. 6a and c). Compared to the initial reflectance of 0.36 at 1.1 mm for the pure basalt (Fig. 6a), the sample wetted with pure water displays a

small increase of albedo (0.43) once completely dry, the sample wetted with the Fe2(SO4)3 solution and then dried out displays a slightly lower reflectance (0.31), whereas the sample wetted with MgCl2 and dried out shows a much brighter surface (0.54). 4.2. Interpretation of the spectral evolutions Wet soils are typically darker than their dry counterpart (Angström, 1915; Gascoin et al., 2009). This effect is also observed for frozen soils (Barry and Gan, 2011) and is due to the fact that the wetting of interfaces diminishes the refractive indices contrasts, thus decreasing light scattering and favoring penetration and absorption of light (Twomey et al., 1986; Ishida et al., 1991). The spectral series measured can be separated in five steps represented on Fig. 7. (a) The first step (Fig. 7a) is the initial wetted soil, where a “thick” and flat film of water entirely covers the sample. At this step, the spectrum displays a low albedo associated to strong signatures of liquid water with saturated absorption bands. (b) In a second step (Fig. 7b), as the thickness of the water film decreases, it reveals the topography of the underlying grains. The albedo does not increase much in the continuum, but the band depths at 1.45 and 1.95 mm strongly decrease. In all four experiments, the “3-mm flicker” is associated to this step. We can compare this result with the results of Pommerol

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Fig. 7. Evolution of the sample during the dehydration (example for MgCl2 dehydration experiment). The thickness of the film at the beginning of the experiment is approximately 1 mm. (a) Thick and flat water film, (b) Rough water flim, (c) Intergrannular water, (d) Disappearance of intergranular water, (e) Formation of hydrated minerals.

et al. (2013) where the visible bidirectional reflectance of various Mars soil analogs, including wet soils, is characterized. These measurements reveal that a “broad specular lobe” as well as a broad forward scattering lobe (501–601 in width) can be observed at this specific stage of the dehydration process when thin menisci are present between the grains. We can see on the picture the strong contributions at local spots to this specular lobe. Because in our experiments the “3-mm flicker” occurs when the superficial liquid water is disappearing, and because it is associated with a general increase in reflectance at 3 mm, it is likely related to this “broad specular lobe”. Indeed, in the case of intense absorptions such as the 3-mm band (k
0.3 in the center), the real index displays a peak on the high wavelength side of the band (which peaks at 2.94 mm for liquid water, Bertie and Zhida (1996)) which impact the reflection coefficient and produce an increase of reflectivity where n is maximum (n
1.5), i.e. around 3.1 mm. (c) In the third step (Fig. 7c), the water menisci have disappeared from the surface and only intergranular water remains. The surface remains dark, the “3-mm flicker” disappears and the 1.45 and 1.95 mm absorption bands weaken. This stage likely corresponds to the presence of thin interfacial films of H2O between grains, which, without being present in significant amounts, still have a strong effect on the continuum absorption. (d) In the fourth step (Fig. 7d), there is a progressive disappearance of intergranular water. At this step, the albedo of the sample suddenly increases (increased scattering at grain-air-grain interfaces compared to grain-water-grain) and the bands at 1.45 and 1.95 mm are relatively shallow. This fourth step corresponds to the gradual increase in reflectance while the band depth at 1.45 mm and 1.95 mm remains constant, and is thus linked to the progressive disappearance of these interfacial water films. (e) In the fifth step (Fig. 7e), intergranular water fully disappears and the various spectral signatures observed are caused by the crystallization of hydrated minerals. The attribution to H2Obearing salts (i.e. salts with structural water) rather than liquid water is clear from the position of the 1.95 mm feature which is clearly shifted (peak and barycenter) toward higher wavelength (
1.99 mm) in the case of MgCl2 hydrates (Figs. 3d and 6b). MgSO4 also display a significant shift (
þ0.04 mm, Fig. 3c).

5. Hydration experiment 5.1. Results The two spectral series obtained during hydration and the surface evolution of the MgCl2 sample is presented in Fig. 8.

For both experiments, the evolution of the samples surface (Fig. 8c) reveals a progressive darkening, from rim to center, of the surface corresponding to its progressive moistening. At the end of the experiment realized in an atmosphere saturated in water vapor, liquid menisci appear on the surface. They are seen through local specular reflections (last picture on Fig. 8c). Spectroscopic observations with ferric sulfate in a controlled atmosphere (Fig. 8a) reveal a progressive growth of the water absorption bands at 1.45, 1.95 and 3 mm, as H2O is added to the sample. This increase of absorption is however very limited and the band depths at 1.45 mm and 1.95 mm remain relatively shallow (BD 1.45 o0.10, BD 1.95 o0.35) even for the highest hydration level reached (16.1 wt%). In the same time, an overall decrease of the albedo quickly occurs and the surface becomes 40% darker than the dry surface with only a moderate amount of H2O added (10 wt%). These observations reveal that hydration and dehydration experiments performed on Fe2(SO4)3 are not “symmetric” (Figs. 8a and 3b). Indeed, in the hydration experiment the starting material is almost anhydrous while the residue after dehydration contains water bands due to hydrated sulfates coating. The shape of the 3-mm band during the hydration experiment is also different than during the dehydration, with a very flat band rather than a “3 mm-flicker”. The hydration experiment performed in a saturated atmosphere with MgCl2 (Fig. 8b) also results in a progressive growth of the water absorption bands at 1.45, 1.95 and 3.0 mm. At the end of this experiment, a total wetting of the sample was achieved with a liquid film entirely covering the surface (Fig. 8c). In this final step, the absorption bands at 1.45, and 1.95 mm are very strong, the 3 mm band is saturated and the spectrum appear very similar to the starting spectrum of the dehydration experiments for MgCl2 (Figs. 8b and 3d). Between the initial dry and the final wet step, the albedo decreases by about 60%. 5.2. Interpretation of the spectral evolutions The phase diagrams of sulfates are complex and numerous hydrated solid phases can form depending on relative humidity and temperature. In the hydration experiments it appears that injection of water vapor saturated air over our samples leads to the deliquescence of some of the salts. This interpretation is based on the progressive decrease in the albedo of the sample that can be due to a wetting effect and could hardly be explained by the formation of solid hydrated sulfates. In addition, the shape of the 1.95 mm band with a maximum of absorption at 1.94 mm is in agreement with liquid water. Still, some polyhydrated ferric sulfates can show similar absorptions (Cloutis et al., 2006) but their formation will not decrease the albedo so much. The spectral

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Fig. 8. (a) Series of spectra during the hydration of a mixture of basalt and Fe2(SO4)3 (10% in mass) with controlled amounts of water. (b) Series of spectra of the hydration of a mixture of basalt and MgCl2 (10% in mass) under a water vapor saturated atmosphere. All the spectra have been masked between 2.5 and 2.8 mm because this wavelength range is often perturbated by some variations in the water vapor content between the reference and the measurement. (c) Surface evolution of the MgCl2 sample during hydration.

signature of deliquescence, with an important decreasing of the albedo, is observed very fast for a low total water content of the sample (4.3 wt%) (Fig. 8a). The fact that the decrease in albedo appears to be linear with the increase in the 1.95 mm band depth can be explained by the presence of a geographic mixture between hydrated parts of the sample where salts did deliquesce and dry areas. This is clearly seen in Fig. 8c. The water content we estimate for the sample using volumetry is an average value. It is possible that hydration gradient is present across the sample and that the surface is enriched in water with respect to the bulk samples. On Mars as in our experiment deliquescence is expected to occur on the very surface, and a deliquescence front might progress vertically in the soil as water vapor is diffusing.

6. Discussion The experimental results presented in this paper raise some interesting issue regarding the spectral detectability of liquid water and brines on the surface of Mars. First, these experiments provide some information on the spectral behavior of liquid water. Secondly, the absence of spectral signature on the RSL, perennial rills or Phoenix areas raises the question of the actual involvement of liquid in their formation. The analysis of the spectral behavior of liquid water during hydration and dehydration processes can help us concluding if the liquid hypothesis has to be ruled out because of the absence of spectral signatures or if another explanation can be suggested.

6.1. Spectroscopy of liquid water The three fundamental vibration modes of the water molecule (e.g. Bayly et al., 1963) are located (for liquid water at 25 1C) at 3.05 mm (symmetric stretching ν1; 3280 cm  1), 6.08 mm (bending ν2; 1645 cm  1), and 2.87 mm (asymmetric stretching ν3; 3490 cm  1). In the case of liquid water and for our spectral range, absorption features around 1.47 mm are explained by fundamental overtones and combination (2 ν1 and 2 ν3; ν1 þ ν3), features around 1.94 mm by combinations of bending and stretching (ν1 þ ν2 and ν3 þ ν2), while the broad 3-mm band is a composite of fundamental stretching (ν1 and ν3 near 2.85–3.05 mm) and bending overtone (2 ν2 near 3.1 mm). Fig. 9a represents the optical constants of liquid water and Fig. 9b and c represent the spectral signature of liquid water. Because the spectra of wet samples (acquired just after the “3 mm flicker”) are not perfectly flat, we decided to make a spectral division to remove any spectral signatures due to the basaltic “substrate” and focus on the brines/water signatures only. For the three solutions studied, the signatures and the typical absorptions of water described above are present. The presence of ionic salts dissolved in a water solution can modify the optical constants of pure water. Early studies on salty solutions (including sea water) revealed a general increase in the real index with salt concentration (e.g. Rhine et al., 1974; Hobson and Williams, 1971; Pinkley and Williams, 1976), and that band intensities and band positions are also influenced by the ionic content. It was also noticed that the negative ions tend to be responsible for these effects. More recent work, especially using ATR spectroscopy, has provided accurate estimates of the optical constants of H2O-salt solutions (Max and Chapados, 2001, 2009). In the case of various halides

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Fig. 9. (a) Optical constants of liquid water. Imaginary components in the 1.0–2.6 mm wavelength range is provided by GhoSST database. Imaginary components above 2.6 mm and real components come from Segelstein (1981). (b) Spectra of liquid extracted by dividing the spectra of wet samples acquired just after the “3 mm flicker” (for pure water, MgCl2 and Fe2(SO4)3) by the spectrum of pure basalt. All the spectra have been masked between 2.5 and 2.8 mm because this wavelength range is often perturbated by some variations in the water vapor content between the reference and the measurement. (c) Closer view of spectra presented on (b) centered on the 1.95 mm absorption band.

solutions studied by Max and Chapados (2001), a general decrease of vibrational frequency is observed with increasing salts concentrations. More specifically in the case of MgCl2, Max and Chapados (2001) observed a shifting of the 1.95 mm feature from 1.93 mm (pure H2O) to 1.97 mm (solvated MgCl2). On Fig. 9c, a slight shifting of the 1.95 mm band center toward higher wavelengths seems to appear on our data for salty solutions (shifting around 0.01 for Fe2(SO4)3 and 0.02 for MgCl2). However, this shifting cannot be conclusive and is not well defined because of the low spectral resolution (10 nm) of our spectra. Given our starting concentration with a H2O/MgCl2 molar ratio around 40, and according to Max and Chapados (2009), the salts should occur in the solution in the form of clusters of ions surrounded by water molecules. In the case of MgCl2, the clusters were found to involve 4 water molecules, suggesting that about 10% of the water molecules should “feel” the effect of the salts. This number is low, and explains the lack of strong effect on the shape on the 1.95 mm band. In the case of (Fe)2(SO4)3, some effects might also occur, but the existence of coupled acido–basic reactions with impact on the IR spectra (Max et al., 2000) prohibits further interpretation at this stage. Concerning the “3-mm flicker” observed at a specific step during the dehydration process (Fig. 3), as suggested earlier, this effect is likely related to the real index of the water optical constants. This hypothesis is enhanced by the presence of an increase of the value of n observed in the optical constants of liquid water with a maximum around 3.1–3.15 mm (Fig. 9a), similarly to the reflectance spectra obtained during the 3-mm flicker (Segelstein, 1981; Milliken and Mustard, 2005).

6.2. Comparison with Martian observations The experimental results presented in this paper can be compared with the observed characteristics of Martian RSL, perennial rills and Phoenix droplets. This comparison can allow to infer if these observations are consistent or not with the presence of a liquid.

6.2.1. Similarities Some similarities can be noticed between our experiments and the seasonal Martian features. As observed for the RSL and the perennial rills, a decrease of the albedo occurs during the hydration experiments. If this darkening is relatively low in the case of the perennial rills, the albedo decreases by up to 40–50% compared to the surrounded areas in the case of the RSL. In our hydration experiments, the complete moistening of the surface can lead to a maximum albedo decrease of 50%, and the formation of a liquid film produces a decrease of nearly 60%. The decrease of the albedo observed during the activation period of the RSL is consistent with the one observed during our hydration experiments and thus, with a wet soil. Moreover, we can notice that the spectra acquired during our experiments on a standing body of water are comparable to spectra of water flow, because the flow is very slow for RSL or perennial rills. In the same way, during cold seasons, a fading or a complete disappearance are observed for the RSL and the dark areas on perennial rills (McEwen et al., 2011; Jouannic et al., submitted for publication; Ojha et al., 2013). Some RSL even display a brighter surface at the end of the activation period. Regarding our dehydration experiments, various configurations have been seen at the end of the experiments: slightly darker surface with ferric sulfate, similar albedo with pure water, and brighter surface with magnesium chloride (Fig. 7). These various final surfaces are consistent with the various final albedo observed during cold season for the RSL. These differences could be explained by the crystallization of various types of minerals at the end of the dehydration. Concerning the formation of liquid by the deliquescence of salts, our experiments reveal that the decrease of albedo by deliquescence can be produced with relatively low water content (4–5 wt%, Fig. 8a). However, the formation of a liquid film that could trigger a flow takes around 160 min with an atmosphere saturated in water vapor during our experiments. As the good atmospheric conditions to initiate deliquescence only last few hours on the Martian surface (Gough et al., 2011) and as the atmosphere is then never saturated in water vapor, it seems unlikely that a quantity of liquid sufficient to form flows could

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be produced by deliquescence only. Nevertheless, deliquescence processes likely occur on Mars and can lead to seasonal darkening of the Martian surface. Melting of water ice (Chevrier and RiveraValentin, 2012) is thus more consistent with the formation of the observed flows but the deliquescence of salt can increase this liquid formation. Moreover, the addition of salt can improve the liquid stability.

6.2.2. Absence of spectral signatures Even if several observations are consistent with the presence of liquid on the RSL and the perennial rills and if some liquid droplets have been directly seen at the Phoenix landing site, spectral signatures of liquid have never been detected on the Martian surface. This absence of detection, however, doesnot rule out the possible presence of liquid on the current surface for the following reasons: (1) The spatial resolution of the OMEGA and CRISM imaging spectrometers is too low for detecting the spectral signature of very localized concentrations of liquids. If salts take a part in the formation of liquid on Mars, we can expect to find salt signatures with OMEGA or CRISM on RSL, perennial rills and Phoenix areas. However, the initial spectra acquired during our experiments with dry mixtures of 10 wt% of salt and basalt (T0 spectra on Fig. 8a and b) donot display any absorption features diagnostic of salts, since dry salts did not have strong near infrared features and did not have bands of structural water. This concentration of salts use in our experiments is nevertheless enough for triggering deliquescence and surface wetting. Moreover, spectra of the Phoenix landing site never showed any salt signature whereas the Phoenix lander has revealed the presence of various types of salts on the surface. Finally, with an average width of 1m, perennial rills and RSL are much smaller than the
18 m pixel scale of CRISM data and the resolution of their composition is thus challenging. These features are nevertheless sometime observed on an extensive area and some individual features can reach a width of 5 m. Therefore, it should be possible that some CRISM data have a few pixels which could display some diagnostic absorption features of liquid during the activation period. (2) The acquisition time of the imaging spectrometer data is also an essential parameter in the detection of possible liquid signatures on Mars. Gough et al. (2011) indeed demonstrate that liquid can be stable on the Martian surface during some specific times of the day. In summer, diurnal temperatures and relative humidity cycles at low latitudes on Mars could allow the surface salts to be aqueous during a few hours in the morning and in the late afternoon. CRISM data are never acquired during this stability time of liquid and are thus acquired during the hydration or the dehydration phase. The results of dehydration experiments have revealed a decoupling between the evolution of the albedo and the evolution of the hydration absorption band depth. The albedo remains very low whereas the absorption bands strongly weaken at the beginning of the dehydration (Fig. 4). Concerning the hydration experiments, we have observed a very fast decrease of the albedo at the very beginning of the hydration whereas the hydration absorption bands remain very shallow (Fig. 8a). Therefore, in both dehydration and hydration processes we can observe during a long period (several hours in all our dehydration experiments) a very low albedo coupled with some very shallow hydration absorption bands. This observation provides an explanation for the observation of low albedo features, which are not combined with any specific spectral signature. (3) We could have expected to detect the spectral signatures of crystallized salts at the end of the dehydration, but several salts such as Fe2(SO4)3 do not display strong absorption bands, which in

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mixture with other minerals makes very difficult to observe some diagnostic features for their identification (Fig. 8a and b). 6.3. Possible future detections This paper provides some new elements on the best way for identifying some liquid diagnostic absorption bands on Mars. First, as CRISM displays the highest spectral and spatial resolution, some additional and more suitable observations will be necessary for the identification of liquid with this instrument. The existing models of temperature and humidity conditions in the Martian atmosphere allow to infer the best time in a specific place on Mars where liquid can be the most stable (e.g. Gough et al., 2011; Chevrier and Rivera-Valentin, 2012). As the signature of liquid is probably detectable only at the end of the hydration process and disappears at the beginning of the dehydration, it will be necessary to acquire new data at the optimal time for the liquid stability and formation. If existing, the signature will probably be weak because of the limited resolution of the CRISM instrument and the resulting subpixel mixture between wet and dry areas. However, if no real water signature has been recognized, Ojha et al. (2013) identifies some hydration effect during RSL activity. The acquisition of more CRISM data in the near-infrared, over the same place and at different seasons would be very helpful for enhancing these results. Ojha et al. (2013) also compares spectra acquired during activation and fading periods, but no evidence for salt crystallization or 1.45 and 1.95 mm absorption bands was found. However, as the 1.45 and 1.95 mm absorption bands might be too shallow, the detection of these differences could focus on the change in the 3 mm absorption band shape. Even if this band is present on the whole Martian surface (Jouglet et al., 2007), the width of this absorption band displays some significant evolutions during the dehydration and the hydration processes (Figs. 3 and 8). In addition, some specific features due to specular effects and linked with some specific stages of the dehydration process could be identified. Finally, the detection of possible hydrated minerals on the RSL and perennial rills areas would be a key element for the understanding of their formation process. This study could focus on the brightest surfaces observed during cold seasons on the RSL.

7. Conclusion The experimental results presented in this paper give us some new elements on possible ways for detecting liquids on the Martian surface and on the possibility of liquid involvement for the formation of RSL and perennial rills. First, this study provides a basis for the future spectral analyses of the Martian surface and the search of evidences for liquid. In our experiments, the spectral signatures of liquid brines only slightly differ from the one of pure water, or these differences would be, at least, too small to be identified at CRISM resolution. The only important differences appear at the end of the dehydration with the crystallization of hydrated minerals. We also observed that the hydrated absorption bands at 1.45 and 1.95 mm with CRISM resolution on the Martian surface will be challenging, except during the very wet stage. The 3 mm absorption band is however very strong during all the process and the observed variations can be directly correlated to the different stages of the hydration/dehydration processes. Based on these observations we infer that the next spectral studies with CRISM will need to focus on the following points: (1) Data acquired during the time of the day when the liquid is the more stable. (2) Variations of the 3 mm absorption band.

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(3) Data acquired on surfaces where liquid activity seems to have occurred previously in order to identify the potential crystallized hydrated minerals. (4) Variations of spectral signatures at various seasons over a given area. These new spectral investigations could provide some new elements in favor or against the presence of liquid for RSL and perennial rills activity. Secondly, our laboratory experiments demonstrate that the absence of spectral identification of liquid on RSL and perennial rills areas does not rule out the liquid hypothesis, at least for the RSL. Albedo variations on RSL are indeed in agreement with our experiments and, thus, with a liquid formation and disappearance. The experiments also reveal an important decoupling between the evolution of albedo and hydration bands depths. During most of the wetting/drying processes we can thus observe a very low albedo associated to very shallow H2O absorption bands. Unless the data are acquired during the most favorable time of the day for the formation of liquid, CRISM data will display some low albedo features associated with very weak spectral signatures. Therefore, the absence of H2O spectral signatures over RSL sites is not necessarily contradictory with the involvement of liquid water in their process. Finally, if deliquescence will hardly provide enough water for the formation of current liquid flow, hydration experiments reveal that deliquescence of Fe2(SO4)3 can occur even at low temperature (4 1C) and for moderate water content (4–5 wt%). Regarding the extensive presence of salts on Mars and the surface temperature and humidity conditions (Gough et al., 2011), deliquescence likely occurs on Mars. We can thus suggest that deliquescence can, at least, lead to some seasonal darkening of the Martian surface. To conclude, if deliquescence is probably insufficient to trigger the activity of RSL and perennial rills, the deliquescence of salts in these areas can certainly increase the liquid formation and its stability.

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