Boris Chauviré
email: boris.chauvire@gmail.com

Maxime Pineau

Eric Quirico

Pierre Beck

Near infrared signature of opaline silica at Mars-relevant pressure and temperature

Keywords: Opal, water, CRISM, low pressure, low temperature, ice

Opal is a mineral of great interest for tracing the aqueous Mars' history. Detection of opal on Mars is based on the near infrared (NIR) absorption bands related to the presence of water and hydroxyl. Because pressure and temperature can affect the amount and configuration of water in hydrated minerals, the associated absorption bands can be sensitive and vary according to the environmental conditions at the surface of Mars. In this study, the effects of Mars' relevant surface pressure and temperature on opal's NIR signature was investigated. By exposing opal samples to pressures varying between 1 and 8 mbar, and temperatures between -51.1 and -96.5°C, significant changes in opal samples' NIR features were observed. It was demonstrated that opal releases molecular water at low pressure, inducing changes on all NIR bands, as also observed in previous studies. However, such dehydration was not systematic, as it was noticed for only six out thirteen opal samples. When exposed to low temperature, water molecules in opal froze, inducing significant variation in shape and position of the bands at 5200 cm -1 and 7000 cm -1 (1.9 µm and 1.4 µm respectively). Low temperature experiments demonstrate that opal, and particularly opal-CT, can exhibit a water ice-like spectral signature. Such experimental data were compared with silica signature detected on Mars by CRISM and it was evident that martian opaline silica has a spectral signature specific of water ice, notably a shoulder near 5100 cm -1 (1.96 µm) and a wider 7000 cm -1 (1.4 µm) feature. We

Introduction

Tracing the presence of liquid water, and its association with potential habitable environments during Mars' history, is an outstanding science goal and a major driver for Mars exploration.

There is now clear evidence of the past presence of liquid water at the surface of Mars, based on geomorphological (e.g. [START_REF] Fassett | The timing of martian valley network activity: Constraints from buffered crater counting[END_REF][START_REF] Hauber | Asynchronous formation of Hesperian and Amazonian-aged deltas on Mars and implications for climate[END_REF] and mineralogical (e.g. [START_REF] Bibring | Mars surface diversity as revealed by the OMEGA/Mars express observations[END_REF]Carter et al., 2013a;[START_REF] Mustard | Hydrated silicate minerals on Mars observed by the Mars Reconnaissance Orbiter CRISM instrument[END_REF] studies. The interaction of rocks with liquid water triggers the hydrolysis of anhydrous minerals, releasing silica in the altering fluids, and subsequently the precipitation of a variety of secondary hydrated minerals.

Among all hydrated phases identified so far, hydrated silica has been detected through both orbit-and rover-based instruments [START_REF] Milliken | Opaline silica in young deposits on Mars[END_REF][START_REF] Squyres | Detection of Silica-Rich Deposits on Mars[END_REF] at many places across the planet (e.g. Carter et al., 2013a;[START_REF] Pan | Voluminous silica precipitated from martian waters during late-stage aqueous alteration[END_REF][START_REF] Pineau | Toward the geological significance of hydrated silica detected by near infrared spectroscopy on Mars based on terrestrial reference samples[END_REF][START_REF] Sun | Distinct Geologic Settings of Opal-A and More Crystalline Hydrated Silica on Mars[END_REF]. This phase represents a promising target for astrobiological purposes, as opaline silica hosts the most ancient biological signatures on Earth [START_REF] Djokic | Earliest signs of life on land preserved in ca. 3.5 Ga hot spring deposits[END_REF]. In this respect, if well-preserved hydrothermal deposits were identified on Mars, they would be high priority targets for investigating evidence of past life (Des Marais and Walter, 2019; [START_REF] Teece | Biomolecules from Fossilized Hot Spring Sinters: Implications for the Search for Life on Mars[END_REF]. Importantly, the recent finding of a nymphal fossil, along with wellpreserved organic material, in an opaline silica formed through recent continental weathering, has revealed that silica formed in weathering context may constitute a new source of ancient life fossils [START_REF] Chauviré | DSC of natural opal: insights into the incorporation of crystallisable water in the opal microstructure[END_REF][START_REF] Pan | Voluminous silica precipitated from martian waters during late-stage aqueous alteration[END_REF][START_REF] Pineau | Toward the geological significance of hydrated silica detected by near infrared spectroscopy on Mars based on terrestrial reference samples[END_REF].

The exact formation pathway of these opaline silica occurrences remains interpretative;

however, it appears in a wide range of geological contexts, from sedimentary environments to hydrothermal systems. For instance, rovers surveying Mars surface have detected the presence of opaline silica; at Gusev crater, the rover Spirit (Mars Exploration Rover-A) reported silicarich soils, interpreted as an evidence of hydrothermal activity, either formed by acid leaching or by direct precipitation [START_REF] Ruff | The Case for Ancient Hot Springs in Gusev Crater, Mars[END_REF][START_REF] Ruff | Characteristics, distribution, origin, and significance of opaline silica observed by the Spirit rover in Gusev crater, Mars[END_REF][START_REF] Ruff | Silica deposits on Mars with features resembling hot spring biosignatures at El Tatio in Chile[END_REF][START_REF] Squyres | Detection of Silica-Rich Deposits on Mars[END_REF]. Moreover, the Curiosity rover (Mars Science Laboratory, MSL) found silica-rich materials, identified as tridymite in a laminated mudstone [START_REF] Morris | Silicic volcanism on Mars evidenced by tridymite in high-SiO2 sedimentary rock at Gale crater[END_REF], as well as diagenetic amorphous to crystalline silica in the Gale crater's stratigraphic sequence [START_REF] Czarnecki | Identification and Description of a 24 Silicic Volcaniclastic Layer in Gale Crater, Mars, Using Active Neutron Interrogation[END_REF][START_REF] Frydenvang | Diagenetic silica enrichment and late-stage groundwater activity in Gale crater, Mars[END_REF][START_REF] Rampe | Mineralogy of an ancient lacustrine mudstone succession from the Murray formation, Gale crater[END_REF][START_REF] Rapin | In Situ Analysis of Opal in Gale Crater, Mars[END_REF].

Hydrated silica is also present on the landing site chosen for both the NASA rover Perseverance (Mars 2020 in Jezero crater; [START_REF] Pan | Voluminous silica precipitated from martian waters during late-stage aqueous alteration[END_REF][START_REF] Tarnas | Orbital Identification of Hydrated Silica in Jezero Crater, Mars[END_REF] and the ESA rover Rosalind Franklin (ExoMars2022 in Oxia Planum ; [START_REF] Carter | Composition of alluvial fans and deltas on Mars[END_REF][START_REF] Pan | Voluminous silica precipitated from martian waters during late-stage aqueous alteration[END_REF][START_REF] Pan | Hydrated silica in martian alluvial fans and deltas[END_REF].

The ongoing or future rovers could provide sufficient information to distinguish the exact silica phases. Importantly, among opaline silica, which refers to a large family of silica phases, opal (SiO2.nH2O) represents the natural hydrated non-crystalline to poorly crystalline silica phases. Two main types of opal are usually differentiated: (1) opal-A, consisting of amorphous silica; and (2) opal-CT composed by tridymite nano-domains with variable proportion of non-crystalline domains. Opal-CT can be interpreted as a solid solution depending on the proportion of various domains [START_REF] Elzea | Tem and X-Ray Diffraction Evidence for Cristobalite and Tridymite Stacking Sequences in Opal[END_REF][START_REF] Fröhlich | The opal-CT nanostructure[END_REF][START_REF] Jones | The nature of opal I. nomenclature and constituent phases[END_REF][START_REF] Wilson | The structure of opal-CT revisited[END_REF]. Opal types can easily be determined by either XRD or Raman spectroscopy [START_REF] Curtis | A Review of the Classification of Opal with Reference to Recent New Localities[END_REF][START_REF] Fritsch | Raman spectroscopy applied to Gemmology[END_REF][START_REF] Ostrooumov | Spectres Raman des opales: aspect diagnostique et aide à la classification[END_REF].

All opal varieties may contain water species (molecular water and OH groups; up to 15 % weight), entailing strong absorptions bands in the near infrared (NIR) region [START_REF] Boboň | State of water molecules and silanol groups in opal minerals: a near infrared spectroscopic study of opals from Slovakia[END_REF][START_REF] Chauviré | Near infrared signature of opal and chalcedony as a proxy for their structure and formation conditions[END_REF][START_REF] Langer | Near infrared absorption spectra (4000-9000 cm-1) of opals and the role of "water" in these SiO2-nH2O minerals[END_REF][START_REF] Rice | Reflectance spectra diversity of silica-rich materials: Sensitivity to environment and implications for detections on Mars[END_REF]. In the 4500 cm -1 (2.2 µm) region, the combination of OH stretching and siloxane bending indicates the presence of silanols groups. By contrast, the combination of OH and H2O modes in molecular water absorbs in the 5200 cm -1 (1.9 µm) region. Both bands are characterized by two main overlapping absorptions [at 4500 cm -1 (2.21 µm) and 4420 cm -1 (2.26 µm); 5250 cm -1 (1.91 µm) and 5100 cm -1 (1.96 µm)], with the higher energy component attributed to less H-bonded silanols or molecular water [START_REF] Langer | Near infrared absorption spectra (4000-9000 cm-1) of opals and the role of "water" in these SiO2-nH2O minerals[END_REF]. Similarly, the 7000 cm -1 (1.4 µm) region exhibits two main components, due to a combination of OH stretching and bending in water molecules near 6850 cm -1 (1.46 µm) and OH stretching overtones near 7100 cm -1 (1.41 µm). In commercial silica gel, [START_REF] Christy | New insights into the surface functionalities and adsorption evolution of water molecules on silica gel surface: A study by second derivative near infrared spectroscopy[END_REF] identified the presence of isolated silanols by the band at 4579 cm -1 and 7316 cm -1 (2.18 µm and 1.37 µm), never seen in natural opal except after heating or low-pressure exposure [START_REF] Boboň | State of water molecules and silanol groups in opal minerals: a near infrared spectroscopic study of opals from Slovakia[END_REF][START_REF] Milliken | Opaline silica in young deposits on Mars[END_REF][START_REF] Rice | Reflectance spectra diversity of silica-rich materials: Sensitivity to environment and implications for detections on Mars[END_REF].

Infrared signatures are the main criteria used to identify opaline silica by orbital instruments (e.g. Compact Reconnaissance Imaging Spectrometer for Mars -CRISM ; [START_REF] Milliken | Opaline silica in young deposits on Mars[END_REF][START_REF] Pineau | Toward the geological significance of hydrated silica detected by near infrared spectroscopy on Mars based on terrestrial reference samples[END_REF][START_REF] Sun | Distinct Geologic Settings of Opal-A and More Crystalline Hydrated Silica on Mars[END_REF][START_REF] Weitz | Mars Reconnaissance Orbiter observations of light-toned layered deposits and associated fluvial landforms on the plateaus adjacent to Valles Marineris[END_REF]. Previous studies had

shown that these features may vary depending on the analytical conditions, specifically in terms of atmospheric pressure and temperature [START_REF] Milliken | Opaline silica in young deposits on Mars[END_REF][START_REF] Sun | Clays and Opals on Mars : Implications for Water-Rock Interactions Through Time[END_REF][START_REF] Sun | Distinct Geologic Settings of Opal-A and More Crystalline Hydrated Silica on Mars[END_REF][START_REF] Swayze | Spectral evidence for hydrated volcanic and/or impact glass on Mars with MRO CRISM[END_REF][START_REF] Thomas | FT-NIR and DSC characterisation of water in opal[END_REF]. Consequently, the use of laboratory opal spectra acquired in ambient terrestrial conditions for detection on Mars' surface should be made cautiously.

The effect of Mars-relevant pressure and temperature on opaline silica has been investigated by few studies [START_REF] Poitras | Mars analog minerals' spectral reflectance characteristics under Martian surface conditions[END_REF][START_REF] Rice | Reflectance spectra diversity of silica-rich materials: Sensitivity to environment and implications for detections on Mars[END_REF][START_REF] Sun | Clays and Opals on Mars : Implications for Water-Rock Interactions Through Time[END_REF][START_REF] Sun | Distinct Geologic Settings of Opal-A and More Crystalline Hydrated Silica on Mars[END_REF]. At

Mars' relevant temperatures, the silica phase containing water molecules should exhibit infrared features typical of ice, as observed in low temperature experiments [START_REF] Poitras | Mars analog minerals' spectral reflectance characteristics under Martian surface conditions[END_REF][START_REF] Rice | Reflectance spectra diversity of silica-rich materials: Sensitivity to environment and implications for detections on Mars[END_REF][START_REF] Sun | Distinct Geologic Settings of Opal-A and More Crystalline Hydrated Silica on Mars[END_REF][START_REF] Thomas | FT-NIR and DSC characterisation of water in opal[END_REF]. In natural opal, the water freezing point is depressed (from -5°C to -47°C) due to the surficial interaction [START_REF] Chauviré | DSC of natural opal: insights into the incorporation of crystallisable water in the opal microstructure[END_REF][START_REF] Thomas | FT-NIR and DSC characterisation of water in opal[END_REF][START_REF] Thomas | Low temperature DSC characterisation of water in opal[END_REF]; this is a phase transition that occurs within the range of temperatures prevailing on Mars [START_REF] Smith | Thermal Structure and Composition[END_REF]. The effect of Marslike pressure was demonstrated to cause significant change in the opal's NIR strongest absorption bands, thus showing that molecular water is pumped out from the pores; on the other hand, effects of low temperature remain scarcely investigated.

In this study, we investigate the effect of typical martian pressures and temperatures on the NIR signature of a series of natural opal samples, and provide spectral proxies to facilitate their detection through infrared remote sensing. Transmission infrared spectra were collected on opaline silica chips, providing insights into compositional changes in bulk samples. We report on the water speciation and its evolution according to pressure and temperature conditions, and the associated spectral evolution in the near-infrared spectral range. The effects of different conditions are estimated in term of positions (apparent maximum, thus the local maximum independently of the position of the constitutive component) and ratio between components, i.e., parameters that can be easily compared with reflectance spectra.

The application of these new spectral tracers to Mars NIR observations suggest that water is mostly present as water ice in opaline silica.

Materials and Methods

Opal Samples

For this study, we selected 17 samples among the 38 samples studied in [START_REF] Chauviré | Near infrared signature of opal and chalcedony as a proxy for their structure and formation conditions[END_REF], based on the remaining materials from previous studies (Table A 1 in Appendix).

Samples were sliced in thin sections, 150 to 350 µm thick, and polished on both sides for transmission acquisition. Among the 17 samples, 4 were opal-A and 13 were opal-CT. The type of opal (opal-CT and opal-A) was deduced from Raman spectroscopy, since it has been previously used to reliably classify opal and extensively used for this purpose. Raman spectroscopy is a non-destructive (no grinding required), a significant advantage in our study that focus on bulk samples (opal chips). Opal-A has a low-frequency T-O-T regionvibrational modes associated to bridging tetrahedron-oxygen-tetrahedron Si-O-Si-with a maximum near 430 cm -1 , whereas opal-CT has a maximum between 330-370 cm-1 (e.g. [START_REF] Ostrooumov | Spectres Raman des opales: aspect diagnostique et aide à la classification[END_REF]. Homogeneity of each pieces used has been checked by measuring

Raman spectra on a minimum of five points (300 µm radius), and pieces that show variations in their structure have been discarded. Some opals are yellow to red (Table A 1), showing that they contain iron oxides and hydroxides as inclusions [START_REF] Fritsch | Mexican Gem Opals : nano-and micro-structure, origin of colour, comparison woth other common opals of gemological significance[END_REF], however

Raman and infrared spectra did not show any features characteristic of these components in our samples. Eight samples were analysed both at low pressure and low temperature (but independently), while other five samples were only exposed to low pressure, and four were exclusively used for low temperature experiments. To prevent potential irreversible changes, each piece has been used only for one type of experiment. Table 2 presents each sample and the respective exposure conditions.

Infrared spectra acquisition

Most of the near-infrared spectra were acquired using a Bruker Vertex 70 spectrometer coupled with a Bruker Hyperion 3000 microscope using a halogen lamp as source, an InGaAs detector and a CaF2 beam splitter. Samples were placed in a cell (designed at the Institut de Planétologie et d'Astrophysique de Grenoble) with sapphire windows enabling transmission spectra, where the temperature and the pressure were lowered using, respectively, an internal circuit for liquid nitrogen and an air pump (HiCube, Pfeiffer). The sample was placed on a sapphire window enclosed in a core metallic piece acting as thermal exchanger either for cooling (by circulating liquid nitrogen) or heating (by two heating resistances). Schema of the experimental apparatus is available in Figure A 1 .

During low-pressure experiments, pressure was stabilized to between 7.2 to 10 millibars. The low pressure was maintained for a minimum period of one hour (for samples with no reaction visible in the spectra) or until a variation lower than 1% per minute between the acquired spectra and the previous one is observed in the OH and H2O related absorptions (up to 21 hours).

Low-pressure experiments for samples 785, 1039, 1085, 1543d, 1552a, WT86 were performed using a Bruker Vertex 70V, which enables to have a vacuum in the sample chamber of about 1 mbar, and to keep all the optical paths under the same condition. A detector InGaAs, with a CaF2 beam splitter, and a tungsten halogen source equip this spectrometer.

Low-temperature experiments were all performed at ambient pressure; the gas in the cell was replaced by dry nitrogen, flushed during few minutes before maintaining ambient pressure.

The sample was isolated from the external metal which was heated to 45 °C (318K) to avoid condensation, which was checked periodically by acquiring a "blank" spectrum (i.e. without the sample). The temperature was measured by a thermocouple PT100 inserted in the core metallic piece, close to the sapphire windows holding the sample (Figure A 1). The temperature was lowered to a range between -88°C to -95°C (185K to 178K; temperature given by the cell sensor). The exact temperature of the sample was estimated using the ice NIR features and its dependency with temperature, considering the effect of porosity depressing the water freezing point (more details are available in Appendix 1). Table 2 presents the temperature measured for the presented spectra, as well as temperature of the sample estimated through the aforementioned approach.

Each spectrum was an accumulation of 100 scans with a spectral resolution of 4 cm -1 and a beam size of 300x300 µm. Infrared spectra were acquired from 4000 to 10 000 cm -1 , but the focus was on the 4000 to 8000 cm -1 (2.5 µm to 1.25 µm) near-infrared spectral range, due to an important spectral artefact around 8300 cm -1 .

Laboratory Data Processing

The spectra's baselines were corrected using five anchor points located at the minimum absorptions near the 4200, 4700, 5400, 5700 and 7400 cm -1 wavelengths. For each anchor point, a Virtual Basic for Applications (VBA) program was developed to find the position of the local minimum in a 100 cm -1 region surrounding the anchor points defined above. Once the minimum was found, a linear function was calculated between each anchor point. The functions were calculated and subtracted from the spectrum as shown in Appendix ( https://doi.org/10.26302/SSHADE/EXPERIMENT_LB_20200201_001).

Processing of CRISM hyperspectral images

The Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) on-board Mars Reconnaissance Orbiter (MRO) is a hyperspectral imaging spectrometer. It operates in the visible to NIR range from 0.36 µm to 3.92 µm through 544 channels with an average spectral resolution of about 6.55 nm [START_REF] Murchie | Compact Connaissance Imaging Spectrometer for Mars (CRISM) on Mars Reconnaissance Orbiter (MRO)[END_REF]. CRISM data were derived from the TRR3 calibration [START_REF] Seelos | CRISM data processing and analysis products update -Calibration, correction and visualization[END_REF], and converted into surface reflectance signal by processing photometric and atmospheric gas absorptions using the empirical volcano-scan method [START_REF] Mcguire | An improvement to the volcano-scan algorithm for atmospheric correction of CRISM and OMEGA spectral data[END_REF], as commonly used in previous work (e.g. [START_REF] Sun | Distinct Geologic Settings of Opal-A and More Crystalline Hydrated Silica on Mars[END_REF].

Opaline silica was detected on Mars by adapting spectral parameters related to its specific absorption features (e.g. the 2.2 µm absorption - [START_REF] Viviano-Beck | Revised CRISM spectral parameters and summary products based on the currently detected mineral diversity on Mars[END_REF]. Extraction of the spectra was then completed by using the methodology of Carter et al. (2013). Hydrated signatures were highlighted by subtracting a mean neutral spectral contribution (i.e. without the signature of hydrated phases) to hydrous mineral exposures over each observation. This method of extraction produces flat reflectance spectra close to a value of zero for spectrally neutral mineralogy. Arbitrary ratios of spectra of interest by spectra of selected neutral mineralogy were therefore not included, thus avoiding a process generally completed by the user. Spatial detection and localization of hydrated silica exposures were based on the spectral parameter BD2.25 introduced by Carter et al. (20013), which provides evidence of specific presence of hydrated silica within a CRISM scene. In a simplified way, this spectral parameter consists of the averaged band depth of the absorption band 2.25 µm, using moving anchor points in the ranges 2.05-2.15 and 2.35-2.40 µm. Averaged spectra of opaline silica in the CRISM images were calculated by encompassing as many pixels as possible (up to several tens) to increase the signal-to-noise ratio.

Selection of Mars sites

To compare our results with Martian signature, three sites were selected based on (i) the quality of the available CRISM spectral cubes (i.e., low dust content, satisfying atmospheric corrections, good signal-to-noise ratio) and (ii) the occurrence of spectrally "pure" opaline silica (i.e., which spectra do not show any additional spectral absorption, indicating the presence of other hydrated phases). These sites were: [START_REF] Morris | Silicic volcanism on Mars evidenced by tridymite in high-SiO2 sedimentary rock at Gale crater[END_REF] The Antoniadi crater region, (2) the Iani Chaos region, and (3) the Camichel crater.

In the Antoniadi crater region, opaline silica-rich materials formed aeolian accumulation (i.e.

dunes) of crystalline silica phase around the central peaks of the impact craters, where zeolites and Fe/Mg-smectites or chlorites were detected [START_REF] Ehlmann | Identification of hydrated silicate minerals on Mars using MRO-CRISM: Geologic context near Nili Fossae and implications for aqueous alteration[END_REF]. It is suggested that the Antoniadi crater silica represents the erosional product of silica-rich materials altered by either hydrothermal activity of the crust under alkaline conditions or diagenetic processes in the subsurface [START_REF] Ehlmann | Identification of hydrated silicate minerals on Mars using MRO-CRISM: Geologic context near Nili Fossae and implications for aqueous alteration[END_REF][START_REF] Smith | Geology of quartz and hydrated silica-bearing deposits near Antoniadi Crater, Mars[END_REF][START_REF] Pineau | Toward the geological significance of hydrated silica detected by near infrared spectroscopy on Mars based on terrestrial reference samples[END_REF].

The geomorphological context of Iani Chaos hydrated silica is reminiscent of the observations made for Antoniadi crater: i.e., in aeolian dunes around the central peak of an impact crater.

Fe/Mg-smectites (or chlorites) and hydrated sulfates are also detected on the crater rims and walls [START_REF] Thomas | Widespread exposure of Noachian phyllosilicates in the Margaritifer region of Mars: Implications for paleohydrology and astrobiological detection[END_REF]. Iani Chaos silica-rich materials have been suggested to have been formed though similar geological processes as those of the Antoniadi crater [START_REF] Pineau | Toward the geological significance of hydrated silica detected by near infrared spectroscopy on Mars based on terrestrial reference samples[END_REF].

In the Camichel crater, hydrated silica was detected within light-toned materials in the distal part of an intra-crater fan delta. These silica-rich deposits are less crystalline than the deposits detected at Antoniadi crater or Iani Chaos and are interpreted as being formed in-situ during delta formation or as product of continental weathering of deltaic silicic materials [START_REF] Carter | Composition of alluvial fans and deltas on Mars[END_REF][START_REF] Hauber | Asynchronous formation of Hesperian and Amazonian-aged deltas on Mars and implications for climate[END_REF][START_REF] Pan | Voluminous silica precipitated from martian waters during late-stage aqueous alteration[END_REF][START_REF] Pineau | Toward the geological significance of hydrated silica detected by near infrared spectroscopy on Mars based on terrestrial reference samples[END_REF].

Results

Low pressure effects

Among the thirteen opals exposed to low pressure (all spectra available in Figure 1, and the evolution of the parameters of the bands are displayed in Figure 2), six opals reacted. In Figure 1, a lower Signal-to-Noise Ratio (SNR) is observed near 5400 cm -1 and 7300 cm -1 , due to the variations in the uniformity of the purge during pumping. An example of the evolution of bands with time is shown in Figure 3. Only one opal-A (86.2) slightly dehydrated, and five opal-CT reacted significantly (from very strong for WT86 to weak for 1085). For reacting opals, the band area near 5200 cm -1 decreased strongly. Among the six reacting opals, two (i.e. 86.2 and 1085) displayed a weak reaction, with a decrease of the 5200 cm -1 absorption bands of less than 10% of the initial band area. The other four strongly reacting opals showed a decrease in this band between 50% and 70% of the initial band area. In addition, it was noticed that the lowfrequency wing reduced more than the high-frequency component, inducing an increase of the ratio between the two components (5250 cm -1 /5100 cm -1 ) from 2.4-2.9 to 3.3-4.0.

A strong reaction to the low pressure was associated to an increase of the band near 4500 cm -1

(from 35% to 67%), transforming the nearly flat band into a sharper asymmetrical band with a apparent maximum shifted toward 4570 cm -1 . However, the sample 86.2 showed a reduction of the 4500 cm -1 band, with no change in shape or apparent maximum. For sample 1085, this band was too weak to monitor its evolution. Similarly to the band at 5200 cm -1 , the 7000 cm -1 band waned, but with slighter changes in the proportion of its two components. Only the 86.2 sample showed a slight increase in such proportion, but this could be attributed to an increased noise in the spectra.

Interestingly, a band at 7315 cm -1 , present as a small shoulder in the spectra at ambient pressure, significantly increased the NIR spectra of samples VTB, YM12, WT86 and FT1111.

For VTB, the strengthening of such band was not sufficient to become the maximum.

Evolution of silica's NIR features at low pressure observed herein were consistent with previous studies [START_REF] Poitras | Mars analog minerals' spectral reflectance characteristics under Martian surface conditions[END_REF][START_REF] Rice | Reflectance spectra diversity of silica-rich materials: Sensitivity to environment and implications for detections on Mars[END_REF][START_REF] Sun | Clays and Opals on Mars : Implications for Water-Rock Interactions Through Time[END_REF][START_REF] Sun | Distinct Geologic Settings of Opal-A and More Crystalline Hydrated Silica on Mars[END_REF].

Low temperature effects

Spectra of all the 13 opals measured before and after exposure to low temperature are presented in Figure 4, and the evolution of the band parameters are displayed in Figure 5. All opals reacted strongly, with significant changes in the 5200 cm -1 and 7000 cm -1 bands. The 4500 cm -1 band showed no significant variations, except for those induced by the change in the 3300 cm -1 absorption band. The 4500 cm -1 band is actually located on the higher wavenumber edge of the main hydration band (3300 cm -1 ; 3 µm) which shifts to lower wavenumbers with lower temperature [START_REF] Cloutis | Spectral reflectance properties of minerals exposed to simulated Mars surface conditions[END_REF]. Opal-A and opal-CT exposed to low temperature in this study had different reactions. The 5200 cm -1 band in the opal-A samples showed no shift of the apparent maximum position, but displayed a rise of the low-frequency component, leading to a decrease of the ratio of the two components (5250/5100 cm -1 -1.91/1.96 µm). Simultaneously, the low frequency component in the 7000 cm -1 regions rose, causing a shift of the apparent maximum toward lower frequencies. This shift of the low frequency component increased the gap between the two components. This region thus exhibits two defined bands: one band near 6650 cm -1 , and a second band near 7100 cm -1 ( Figure 4), with a ratio 7100/6650 cm -1 (1.41/1.50 µm) greater than at ambient temperature.

Except for sample 521, which showed a behaviour similar to opal-A, the studied opal-CT show a different evolution. The 5200 cm -1 band substantially shifted toward lower frequency (approximately 200 cm -1 ), with a small shoulder persisting for the high-frequency component (i.e. near 5250 cm -1 , Figure 4). Additionally, the band became symmetrical. A similar reaction was observed in the 7000 cm -1 regions: an increase of the low-frequency component, an in turn of the ratio 7100/6850 cm -1 , and a shift of the position of the apparent maximum, with a small remaining shoulder near 7100 cm -1 (sometimes not visible).

Discussion

Low pressure effects

Six samples (among thirteen) reacted at Mars simulated pressure. By contrast, previous studies showed that most opals dehydrate at low pressure [START_REF] Poitras | Mars analog minerals' spectral reflectance characteristics under Martian surface conditions[END_REF][START_REF] Rice | Reflectance spectra diversity of silica-rich materials: Sensitivity to environment and implications for detections on Mars[END_REF][START_REF] Sun | Clays and Opals on Mars : Implications for Water-Rock Interactions Through Time[END_REF][START_REF] Sun | Distinct Geologic Settings of Opal-A and More Crystalline Hydrated Silica on Mars[END_REF]. One of the explanations could be the difference in the acquisition method (reflectance for previous studies, compared to transmission in this study); however, the band shape and position used here to characterize the effect of pressure, is not affected by the mode of acquisition [START_REF] Pineau | Toward the geological significance of hydrated silica detected by near infrared spectroscopy on Mars based on terrestrial reference samples[END_REF]. Samples preparation also differed from previous studies; opals are prepared as thin sections here, whereas opals were grinded in other studies. The grinding increases the surface available and opens the waterfilled porosity, facilitating the release of water; an effect demonstrated by thermal methods [START_REF] Smallwood | Characterisation of the dehydration of australian sedimentary and volcanic precious opal by thermal methods[END_REF]. It has been proposed to explain the difference of water content measured between the instrument SAM (after grinding) and ChemCam (directly on unprepared sample), aboard MSL rover [START_REF] Rapin | In Situ Analysis of Opal in Gale Crater, Mars[END_REF].

In the study of hydration state of minerals, the relative humidity (RH), referring to the ratio of real water content in an atmosphere compared to the water content at saturation of the same atmosphere, is a key parameter. [START_REF] Rice | Reflectance spectra diversity of silica-rich materials: Sensitivity to environment and implications for detections on Mars[END_REF] and [START_REF] Sun | Clays and Opals on Mars : Implications for Water-Rock Interactions Through Time[END_REF] have demonstrated that, at high RH, opal rehydrates, and Sun ( 2017) demonstrated that at low RH, opal dehydrates. In our experiments, we estimate the RH assuming a RH of the air-conditioned air (40%) at the pressure of the experiments. In the first setup (using a Bruker Vertex 70V under a pressure of approximately 1 mbar), we calculate a RH ≈ 0.04%, and a RH ≈ 0.32 % (at 8 mbar) for the second setup. According to the previous studies, the low RH in our experiments should involve a dehydration. However, seven samples did not lose water.

The maximum duration of our experiments was 21 hours, compared to previous studies that exposed samples for days or months. Our samples reacted in the first hours, but our results should be considered as a lower limit for duration. It appears that our observations are consistent with previous studies [START_REF] Poitras | Mars analog minerals' spectral reflectance characteristics under Martian surface conditions[END_REF][START_REF] Rice | Reflectance spectra diversity of silica-rich materials: Sensitivity to environment and implications for detections on Mars[END_REF][START_REF] Sun | Clays and Opals on Mars : Implications for Water-Rock Interactions Through Time[END_REF][START_REF] Sun | Distinct Geologic Settings of Opal-A and More Crystalline Hydrated Silica on Mars[END_REF], demonstrating that one day of exposure could be sufficient to observe spectral variations at Mars-relevant pressure if water loss is possible through opal's porosity.

In the literature, opaline silica exposed to low pressure showed a shift of the 7000 cm -1 band (1.4 µm) toward greater frequencies, a decrease of the 5200 cm -1 (1.9 µm) band, and an increase with a shift of the band at 4500 cm -1 /2.2µm [START_REF] Poitras | Mars analog minerals' spectral reflectance characteristics under Martian surface conditions[END_REF][START_REF] Rice | Reflectance spectra diversity of silica-rich materials: Sensitivity to environment and implications for detections on Mars[END_REF][START_REF] Sun | Clays and Opals on Mars : Implications for Water-Rock Interactions Through Time[END_REF][START_REF] Sun | Distinct Geologic Settings of Opal-A and More Crystalline Hydrated Silica on Mars[END_REF]. The band shapes changed through narrowing of the highfrequency component, and in combination with the decrease of the low frequency component on all bands, this leads to a change in the ratio between the two components [START_REF] Poitras | Mars analog minerals' spectral reflectance characteristics under Martian surface conditions[END_REF][START_REF] Rice | Reflectance spectra diversity of silica-rich materials: Sensitivity to environment and implications for detections on Mars[END_REF][START_REF] Sun | Distinct Geologic Settings of Opal-A and More Crystalline Hydrated Silica on Mars[END_REF]. All position shifts mainly resulted from the change in the proportion of each component of the band, the weakening of a component, and/or the strengthening of the other component, implying a shift of the apparent maximum.

Our results are consistent with previous studies. Quantitatively, the shift of the position of the apparent maximum of the 7000 cm -1 -band in reacting opal-CT is greater than observed by [START_REF] Sun | Distinct Geologic Settings of Opal-A and More Crystalline Hydrated Silica on Mars[END_REF], but consistent with [START_REF] Poitras | Mars analog minerals' spectral reflectance characteristics under Martian surface conditions[END_REF]. Our study shows that distinguishing both types of opals using the band near 7000 cm -1 (1.4 µm) is more challenging than previously thought.

In [START_REF] Chauviré | Near infrared signature of opal and chalcedony as a proxy for their structure and formation conditions[END_REF], it was proposed that opal-A could be distinguished from other varieties using the position of the apparent maximum of the 5200 cm -1 (limit between 5240 and 5245 cm -1 ). After exposure, the strongly reacting opal-CT has an apparent maximum overlapping with the opal-A's position; hence the limit is more blurred than previously defined, consistent with [START_REF] Pineau | Toward the geological significance of hydrated silica detected by near infrared spectroscopy on Mars based on terrestrial reference samples[END_REF].

Three of our sensitive opal-CT samples (WT86, FT1111 and YM12) showed an apparent maximum near 7300 cm -1 after dehydration, caused by the growth of the band at 7316 cm -1 , attributed to isolated silanols OH stretching, similar to silica gel after heating or low-pressure treatment [START_REF] Boboň | State of water molecules and silanol groups in opal minerals: a near infrared spectroscopic study of opals from Slovakia[END_REF][START_REF] Christy | Near infrared spectroscopic characterisation of surface hydroxyl groups on hydrothermally treated silica gel[END_REF][START_REF] Christy | New insights into the surface functionalities and adsorption evolution of water molecules on silica gel surface: A study by second derivative near infrared spectroscopy[END_REF][START_REF] Milliken | Opaline silica in young deposits on Mars[END_REF][START_REF] Rice | Reflectance spectra diversity of silica-rich materials: Sensitivity to environment and implications for detections on Mars[END_REF].

The emergence of the band near 7300 cm -1 is consistent with the change of shape of the band at 4500 cm -1 , attributed to a combination of siloxane and silanol groups. The four strongly reacting opals (VTB, WT86, FT1111 and YM12) showed a decreasing 4425/4510 cm -1

(2.26/2.21 µm) ratio and an apparent maximum close to isolated silanols bands at 4579 cm -1 [START_REF] Christy | Near infrared spectroscopic characterisation of surface hydroxyl groups on hydrothermally treated silica gel[END_REF][START_REF] Christy | New insights into the surface functionalities and adsorption evolution of water molecules on silica gel surface: A study by second derivative near infrared spectroscopy[END_REF]; this is in agreement with previous studies [START_REF] Poitras | Mars analog minerals' spectral reflectance characteristics under Martian surface conditions[END_REF][START_REF] Sun | Distinct Geologic Settings of Opal-A and More Crystalline Hydrated Silica on Mars[END_REF].

All these changes demonstrate that water was extracted from pores for almost half of the analysed samples. No correlation between colour, and thus presence of iron, and reaction at low pressure was observed. Similarly, both types of opal reacted, which is consistent with previous observations. The discrepancies between reacting and none-reacting samples could arise from their microstructure. Opal displays a variety of microstructures [START_REF] Fritsch | The nanostructure of fire opal[END_REF][START_REF] Fritsch | Mexican Gem Opals : nano-and micro-structure, origin of colour, comparison woth other common opals of gemological significance[END_REF][START_REF] Gaillou | Common gem opal: An investigation of micro-to nano-structure[END_REF] that changes according to the formation environments and/or aging (e.g. [START_REF] Herdianita | Mineralogical and textural changes accompanying ageing of silica sinter[END_REF]. Liquid-like water in opal is trapped inside nanometres to micrometres size pores [START_REF] Chauviré | DSC of natural opal: insights into the incorporation of crystallisable water in the opal microstructure[END_REF][START_REF] Thomas | Low temperature DSC characterisation of water in opal[END_REF]. The proportion between open and closed porosity (i.e., respectively, connected to atmosphere and isolated from it) could explain the varying behaviour observed. The role of the microstructure on the opal dehydration by low pressure should be a major consideration when collecting sample containing water as a possible return martian sample target. In any case, the presence of free H2O-molecules clusters in silica phases samples could be identifiable based on the 1.4 and 1.9 µm bands, which can be monitored by Supercam/IRS or Micromega [START_REF] Bibring | The MicrOmega Investigation Onboard ExoMars[END_REF]Maurice et al., 2021).

The loss of water induces changes in the silanol configuration, by increasing the number of isolated silanols as proposed in earlier studies [START_REF] Poitras | Mars analog minerals' spectral reflectance characteristics under Martian surface conditions[END_REF][START_REF] Sun | Clays and Opals on Mars : Implications for Water-Rock Interactions Through Time[END_REF][START_REF] Sun | Distinct Geologic Settings of Opal-A and More Crystalline Hydrated Silica on Mars[END_REF]. The emergence of the 7320 cm -1 and the rise of the band component near 4520 cm -1 demonstrate the increasing amount of isolated silanols. Water molecules, initially adsorbed into surficial silanols, are lost during low-pressure exposure and transform bonded silanols into isolated ones, inducing a rising of the 4520 cm -1 and 7320 cm -1 bands. The increase of the area of the 4500 cm -1 band could be associated to either the creation of silanol on opal surface or to a difference in the molar absorptivity between isolated and bonded silanols. Considering the condition of the experiments, i.e. at ambient temperature and without any chemical treatments, silanol genesis is deemed unlikely [START_REF] Christy | Near infrared spectroscopic characterisation of surface hydroxyl groups on hydrothermally treated silica gel[END_REF][START_REF] Kondo | The properties of silica surfaces treated at high pH[END_REF]. Therefore, water releases at low pressure affects all bands of NIR spectra of opal (A or CT) directly (by loss of water molecules) or indirectly (by reconfiguring silanols).

Low temperature effects

Our results show a significant shift of the apparent maximum for the bands at 7000 cm -1 and 5200 cm -1 , a decrease of the ratio 5250/5100 cm -1 (1.91/1.96 µm) and an increase of the ratio between components constitutive of the 7000 cm -1 (1.4 µm). The shift of the 5200 cm -1 is greater for opal-CT (> 100cm -1 , except for sample 521) than opal-A, which can be explained by the difference in water molecules enclosed. Opal-CT has more liquid-like water molecules, as demonstrated by NIR and thermal analysis [START_REF] Chauviré | DSC of natural opal: insights into the incorporation of crystallisable water in the opal microstructure[END_REF][START_REF] Langer | Near infrared absorption spectra (4000-9000 cm-1) of opals and the role of "water" in these SiO2-nH2O minerals[END_REF], whereas opal-A mostly contains water molecules in a single layer that do not freeze due to surficial interaction [START_REF] Ishikiriyama | Pore Size Distribution Measurements of Silica Gels by Means of Differential Scanning Calorimetry[END_REF][START_REF] Landry | Thermoporometry by differential scanning calorimetry: Experimental considerations and applications[END_REF]. Consequently, more water molecules freeze in opal-CT than in opal-A, involving greater changes in the 5200 cm -1 bands. The reaction of opal-CT 521, similar to opal-A, could be explained by a lower amount of water molecules as multilayers compared to the opal-CT samples. Sample 521 can be differentiated from other opal-A samples by the local minimum between 5100 cm -1 and 5250 cm -1 (deeper in opal-A).

Molecular water enclosed in the nanometric pores of opal freezes if temperature is low enough, i.e. usually < -30°C (<243K) for most opals, but up to -47°C (226K) in some cases [START_REF] Chauviré | DSC of natural opal: insights into the incorporation of crystallisable water in the opal microstructure[END_REF]Thomas et al., 2020[START_REF] Thomas | Low temperature DSC characterisation of water in opal[END_REF]. The change in shape and position of the apparent maxima of bands at 5200 cm -1 and 7000 cm -1 is similar to frozen water ( Figure 6). At 4500 cm -1 , no change that can be attributed to silanol reconfiguration was observed. A slight change was visible, but it is likely due to the proximity of the 3-µm water band, evolving with the crystallization of water. At the same time, we did not observe the isolated silanols band near 7320 cm -1 , but a shallow band remained near 7100 cm -1 . Therefore, during low temperature experiments, no isolated silanols, as seen for low-pressure experiments, were created. This can be explained by the non-freezable layer of water molecules on silica surface, due to surface interaction [START_REF] Ishikiriyama | Pore Size Distribution Measurements of Silica Gels by Means of Differential Scanning Calorimetry[END_REF][START_REF] Landry | Thermoporometry by differential scanning calorimetry: Experimental considerations and applications[END_REF]. It is thus consistent with the absence of silanol reconfiguration. The remaining shoulder near 5250 cm -1 (1.91 µm) and the shallow band near 7100 cm -1 , attributed to molecular water bonded to silanols [START_REF] Christy | Near infrared spectroscopic characterisation of surface hydroxyl groups on hydrothermally treated silica gel[END_REF][START_REF] Christy | New insights into the surface functionalities and adsorption evolution of water molecules on silica gel surface: A study by second derivative near infrared spectroscopy[END_REF], are evidence of such non-freezable layer. [START_REF] Rice | Reflectance spectra diversity of silica-rich materials: Sensitivity to environment and implications for detections on Mars[END_REF] exposed 11 opals to -13°C (260K), and they observed results consistent with our study; water phase changed in most of their samples, with low-temperature spectra dominated by water ice. Freezing point of water enclosed in nanometres-sized porous materials, including opals, shifts toward lower temperatures according to the size of waterfilled pores [START_REF] Chauviré | DSC of natural opal: insights into the incorporation of crystallisable water in the opal microstructure[END_REF][START_REF] Ishikiriyama | Pore Size Distribution Measurements of Silica Gels by Means of Differential Scanning Calorimetry[END_REF][START_REF] Landry | Thermoporometry by differential scanning calorimetry: Experimental considerations and applications[END_REF]Thomas et al., 2020[START_REF] Thomas | Low temperature DSC characterisation of water in opal[END_REF]. The presence of ice at -13°C demonstrates that their samples contained larger pores, but the temperature may not be sufficient to freeze water-clusters in smaller pores [START_REF] Chauviré | DSC of natural opal: insights into the incorporation of crystallisable water in the opal microstructure[END_REF][START_REF] Thomas | FT-NIR and DSC characterisation of water in opal[END_REF][START_REF] Thomas | Low temperature DSC characterisation of water in opal[END_REF]. In parallel, grinding the samples exposes them to water from the terrestrial atmosphere, which will inevitably adsorb at the surface of the grains, which is limited in our study by using silica chips. Also since samples were analysed through transmission, we are probing deeper in the sample compared to reflectance spectroscopy, which tends to analyse more surficial parts of the samples. [START_REF] Poitras | Mars analog minerals' spectral reflectance characteristics under Martian surface conditions[END_REF] have subjected opal to low temperature in Winnipeg's Mars Environment chamber [START_REF] Craig | ME ans mini-ME: two Mars environmental simulation chambers for reflectance spectroscopy[END_REF], but the specific temperature was not mentioned in their study.

Nevertheless, [START_REF] Poitras | Mars analog minerals' spectral reflectance characteristics under Martian surface conditions[END_REF] did not observe a crystallization of water. This can be explained by either the operating temperature being higher than the water phase changes (the operating temperature of ME is -25°C; [START_REF] Craig | ME ans mini-ME: two Mars environmental simulation chambers for reflectance spectroscopy[END_REF], or by the low pressure causing the removal of all molecular water that may freeze (phenomenon enhanced by the grinding effect). The final spectra after exposure [START_REF] Poitras | Mars analog minerals' spectral reflectance characteristics under Martian surface conditions[END_REF]) had a 3000 cm -1 band (3 µm), characteristic of H2O vibrations, and a low frequency wing of 5200 cm -1 band (1.9 µm), demonstrating that molecular water remained in the silica.

Our observations are consistent with the freezing of "liquid" water in opals, as observed by [START_REF] Rice | Reflectance spectra diversity of silica-rich materials: Sensitivity to environment and implications for detections on Mars[END_REF] and [START_REF] Sun | Clays and Opals on Mars : Implications for Water-Rock Interactions Through Time[END_REF]. Additionally, Sun (2017) also observed a rehydration of opal at RH > 90%. In our experiment, the low-temperature (circa -90°C) required a small amount of water to reach saturation, and even with the water impurity in nitrogen gas cylinder (maximum at 5 ppm), the RH was expected to be near 100%. High-RH prevented dehydration of our sample that could occur at lower RH, and it could explain why all samples contained ice, and at the same time the lack of rehydration. The main difference between [START_REF] Sun | Clays and Opals on Mars : Implications for Water-Rock Interactions Through Time[END_REF] and the present study could be the result of the grinding effect. It should also be noted that thermalization of samples is a key issue in reflectance studies under low temperature, in particular for powders which can be subjected to strong temperature gradients.

Implications for opal on Mars

We present in The temperature of the surface during the acquisition has been estimated from the location and local time using the last version of the Martian Climate Database developed from the General Circulation Model [START_REF] Forget | Improved general circulation models of the Martian atmosphere from the surface to above 80 km[END_REF]. Details on location, local time and the deduced atmospheric conditions are displayed in Appendix (Table A 2). These conditions, i.e. between -28 and -43°C, are favourable for the crystallization of water in opals [START_REF] Chauviré | DSC of natural opal: insights into the incorporation of crystallisable water in the opal microstructure[END_REF][START_REF] Thomas | Low temperature DSC characterisation of water in opal[END_REF].

All martian spectra exhibit an asymmetrical band at 4500 cm -1 (2.2 µm), with a greater contribution of the high frequency component. This may suggest, according to the spectra acquired at low pressure (here and by [START_REF] Poitras | Mars analog minerals' spectral reflectance characteristics under Martian surface conditions[END_REF][START_REF] Sun | Clays and Opals on Mars : Implications for Water-Rock Interactions Through Time[END_REF][START_REF] Sun | Distinct Geologic Settings of Opal-A and More Crystalline Hydrated Silica on Mars[END_REF], that opaline silica was dehydrated. In the Camichel crater, the significant band near 7250 cm -1 is an additional evidence of strong dehydration, and its absence in other spectra suggests that other deposits have experienced a dehydration to a lesser extent.

All spectra exhibited a band near 5200 cm -1 (1.9 µm) with an apparent maximum between 5100 and 5250 cm -1 . In our spectra, the crystallization of ice inside opal induced either a shift of the maximum (for opal-CT except 521) or the increase of the low-frequency shoulder (for opal-A and 521). No significant shift is visible in the martian spectra, suggesting that not enough water crystallizes to become the main contributor, which is consistent with the dehydration features observed in the 4500 cm -1 and 7000 cm -1 regions.

On the other hand, spectra from Antoniadi crater and Iani Chaos showed absorptions between 6500 and 6750 cm -1 , which is characteristic of the presence of OH in water molecules (Table 1). According to the atmospheric conditions (between 5.9 and 7.5 mbar and -28 and -43 °C), and from the thermal data on opal [START_REF] Chauviré | DSC of natural opal: insights into the incorporation of crystallisable water in the opal microstructure[END_REF][START_REF] Thomas | Low temperature DSC characterisation of water in opal[END_REF], water is likely in ice form, explaining the shoulder centred near 5100 cm -1

(1.96 µm, Figure 4) in Antoniadi crater and Iani Chaos spectra . The proximity of atmospheric carbon dioxide near 2 µm prevents the use of this shoulder only [START_REF] Gerakines | The Strengths of Near-Infrared Absorption Features Relevant to Interstellar and Planetary Ices[END_REF][START_REF] Khayat | Understanding the water cycle above the north polar cap on Mars using MRO CRISM retrievals of water vapor[END_REF]. Nevertheless, the large band near 6650 cm -1 associated with the shoulder near 5100 cm -1 suggest the presence of a small amount of water-ice.

As commonly discussed for mineral detections on Mars, a mixture of mineral phases is likely detected. Other hydrated phases, like sulphates and zeolites, possess absorption bands in the same regions. Sulphates, like gypsum or alunite, have a clear absorption at 5715 cm -1 (1,75 µm;Carter et al., 2013a;[START_REF] Ehlmann | Discovery of alunite in Cross crater, Terra Sirenum, Mars: Evidence for acidic, sulfurous waters[END_REF], not observed in our spectra. In addition, zeolites have a very shallow absorption near 4500 cm -1 (2,2 µm), as opposed to opaline silica.

In addition, Mg-sulfates such as hexahydrite, epsomite or kieserite, have complex 1.4 and 1.9 µm absorption features that overlap water ice absorptions (e.g., [START_REF] Dalton | Spectral comparison of heavily hydrated salts with disrupted terrains on Europa[END_REF]. Thus, the presence of small amounts of Mg-sulfates with opaline silica can be responsible for the spectral signatures that we observe. However, obvious spectral evidence for the presence of Mg-sulfates were not observed; this strengthens the hypothesis of opaline silica with water ice formation inside its structure. In any case, clear absorptions other than silica and water species

were not identified in this study.

Water-ice particle (that would be intimately mixed with silica) are not expected to be the source of the absorption feature observed. In the spectra presented here for the Antoniadi crater and Iani Chaos, ice is unlikely, considering their geographical position in the martian tropical zone (respectively at 21.3°N and 2.06°S), on flat surface inside craters during either spring or summer martian seasons. Detection of water frost is more common at low-and midlatitudes, but only on polar-faced slopes during fall or winter [START_REF] Carrozzo | Mapping of water frost and ice at low latitudes on Mars[END_REF][START_REF] Vincendon | Water ice at low to midlatitudes on Mars[END_REF]. Moreover, except for hydrated silica detections, none of the surroundings exhibit absorptions typical of ice. A contribution of water ice clouds could interfere with the detection [START_REF] Vincendon | New near-IR observations of mesospheric CO2 and H2O clouds on Mars[END_REF], but during the selection of the CRISM images, we prevented this interference by highlighting the presence of water ice through the application of the BD1500_2 from Viviano-Beck et al. ( 2014): only images that did not present any feature of ice, potentially originating from water-ice clouds or surface frost, were used here.

Therefore, in the Antoniadi regions and Iani Chaos, the detection of hydrated silica showed a slight but visible contribution of ice, suggesting that ice is present in opal structure at the time of the detection. This implies that for these regions, in contrast to the Camichel crater, opaline silica more efficiently retains molecular water in its structure. Opaline silica showing the presence of water in the porosity could be an interesting target for exploring the aqueous history of Mars. In addition to this, opaline silica could be used as a potential sample return target and in turn an efficient way to collect martian water (possibly ancient, preserved since the precipitation of silica).

Conclusion

By comparing data from available opal samples and from water ice at the same temperature (from [START_REF] Grundy | NIR optical constants spectrum of H2O Ih crystal and H2O liquid from 20 to 293 K. SSHADE/GhoSST[END_REF][START_REF] Grundy | The temperature-dependent near-infrared absorption spectrum of hexagonal H2O ice[END_REF], this study demonstrated that opal-CT and water/ice spectra are very close for bands near 5200 cm -1 and 7000 cm -1 (1.9 and 1.4 µm) ( 21Figure 6). The main band which allows to differentiate opal from water is the 4500 cm -1 (2.2 µm) band, and the contributions at 7100 cm -1 and 5250 cm -1 (1.405 and 1.905 µm).

For the detection of opal on Mars, the variety of band shapes according to the type of silica needs to be considered (as showed in [START_REF] Chauviré | Near infrared signature of opal and chalcedony as a proxy for their structure and formation conditions[END_REF]. Most of the studies aiming to improve silica detection focus on the most amorphous silica phases (i.e. opal-A or volcanic glasses), whereas opal-CT could be the most challenging to decipher. Ice features do not interfere with a potential identification of the opal-type, as opal-A appears to have an apparent maximum at higher frequency (shorter wavelength) than opal-CT, with two distinct bands whereas opal-CT exhibits only one broad band.

It was observed that in the Antoniadi crater and Iani Chaos martian opaline silica have a spectrum consistent with the presence of ice in their structure. Our results show that some opals may preserve molecular water in their structure, even when exposed to Mars atmosphere conditions, and that opaline silica could be a prime target for future sample return missions.

6.

Figures captions

Figure 1 (single column): Near infrared spectra of samples used in this study before (black) and after (green) exposure to low pressure. Spectra has been offset for clarity.

Figure 2 (1.5 column): Evolution of the parameters for each band (a: 4500cm -1 , b: 5200 cm -1 , c: 7000 cm-1) between ambient conditions (black filled) and after low-pressure exposition (empty green symbols). Each plot represents the evolution of the ratio of the two components of the band versus the apparent maximum. Reacting samples are shown with an arrow between the initial state and after exposition. Circles represent opal-A sample and triangles opal-CT. For the band at 4500 cm -1 , only reacting samples with a variation greater than 10% in area (implying a variation on other parameters) have been represented to improve readability.

Figure 3 (single column): evolution of bands with time during exposure to low pressure for opal-CT WT86. Shown spectra were acquired at room temperature every 30 minutes, demonstrating a strong evolution during the first hours. The last spectrum was acquired 16h45 after the beginning of the experiment.

Figure 4 (single column): Spectra before (grey) and after (blue) exposure to low temperature (applied temperatures are noted in Table 2). Spectra has been offset for clarity.

Figure 5 (1.5 column): Evolution of parameter of the band at 5200 cm -1 (a) and 7000 cm -1 (b); with the ratio of the two components versus the position of the apparent maximum. In black, parameters at room temperature and in blue at low temperature (temperature given in Table 2). Circles represent opal-A samples, and triangles opal-CT.

Figure 6 (single column): Absorption spectra of water (solid line) and an opal-CT (1543b; dotted line) at both room temperature (black line; 293.5 K) and low temperature (blue line; 180K measured and 220K from calibrated temperature for opal, 230 K for water). Data for water was extracted from [START_REF] Grundy | NIR optical constants spectrum of H2O Ih crystal and H2O liquid from 20 to 293 K. SSHADE/GhoSST[END_REF]. Y-axis was normalized and offset for clarity.

Figure 7 (single column): Continuum removed CRISM spectra from three locations where opaline silica was identified (Antoniadi crater, Iani Chaos region and Camichel crater) compared to the laboratory spectra acquired at similar temperature. To improve the comparison, the reflectance spectra of terrestrial opal has been simulated from transmission spectra according to Hapke model [START_REF] Hapke | Theory of Reflectance and Emittance Spectroscopy[END_REF]. Blue areas indicate water ice regions and vertical dotted line indicates absorptions bands specific to hydrated silica. The penultimate spectrum is a CRISM spectrum of water ice at the northern cap of Mars to make comparison with the 1551 opal-CT sample. The last one is a montmorillonite spectrum from the USGS dataset showing a 4500 cm -1 absorption band narrower than opaline silica. Table 1 : NIR absorptions (between 4000 and 8000 cm _1 ) of molecular water (H2Omol), silanol groups (SiOH) and OH in amorphous silica (after Anderson & Wickersheim 1964;Flörke et al., 1982;Kronenberg, 1994). Vibration mode: δSiOH = SiOH bending in plane; υOH = OH-stretching; υ2 = symmetric bending; υ3 = antisymmetric stretching. *The attribution of the band at 6580 cm -1 (1.46 µm) is debated.

Tables

Band positions

Figures captions

Figure A 3

 3 Figure A 3). Both corrected and raw data are available in the open access SSHADE database

Figure 7 a

 7 Figure 7 a comparison between two representative spectra of opaline silica from our

Figure A 1 :

 1 Figure A 1 : Schematics of the cell used to analyse at Mars-relevant conditions.

Figure A 2 :

 2 Figure A 2: Area of bands used for the temperature calibration. NIR spectrum of pure ice from Gundry & Schmidt (1998) at 180 K (-93 °C).

Figure A 3 :

 3 Figure A 3 : Process used to remove the baseline from the spectra (here shown for opal 86.2)

Competing interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The column "Measured" is the temperature measured by the sensor in the cell, the column "Sample" is the temperature calculated from the calibration based on the NIR ice bands. *indicates the sample used to recalculate the calibration from [START_REF] Fink | Temperature dependence of the water-ice spectrum between 1 and 4 microns: Application to Europa, Ganymede and Saturn's rings[END_REF] to take account of the onset temperature. The other sample's temperature has been calculated using the recalculated calibration by taking account of the onset temperature from the DSC measurements available [START_REF] Chauviré | DSC of natural opal: insights into the incorporation of crystallisable water in the opal microstructure[END_REF].

Sample Variety

Mars simulated

Appendix

Temperature of sample

As the sample is placed on a sapphire window (to acquire transmission spectra), and the temperature sensor is located on the surrounding freezing circuit, a difference between the temperature of the sample and the sensor exists. As water in opal could freeze at low temperature, we used the temperature dependency of the 5500-7000 cm -1 (1.82-1.43 µm) region of water ice [START_REF] Fink | Temperature dependence of the water-ice spectrum between 1 and 4 microns: Application to Europa, Ganymede and Saturn's rings[END_REF][START_REF] Grundy | The temperature-dependent near-infrared absorption spectrum of hexagonal H2O ice[END_REF] to estimate the exact temperature of the sample (consequently, this method is only applicable when ice is present in the sample). The area of the shoulder near 6050 cm -1 divided by all the area between 5500 cm -1 and 7000 cm -1 is dependent of the temperature of the ice (Appendix Figure A 2): the larger the ratio is, the lower the temperature is [START_REF] Fink | Temperature dependence of the water-ice spectrum between 1 and 4 microns: Application to Europa, Ganymede and Saturn's rings[END_REF]. Given that our spectra were acquired above 100K, it avoids the unreliability of this calibration at lower temperatures [START_REF] Grundy | The temperature-dependent near-infrared absorption spectrum of hexagonal H2O ice[END_REF]. In addition, the crystallization of water in opals demonstrates that this amount of water did not interact with the silica surface, or silanols linked to it [START_REF] Ishikiriyama | Pore Size Distribution Measurements of Silica Gels by Means of Differential Scanning Calorimetry[END_REF][START_REF] Landry | Thermoporometry by differential scanning calorimetry: Experimental considerations and applications[END_REF]. [START_REF] Fink | Temperature dependence of the water-ice spectrum between 1 and 4 microns: Application to Europa, Ganymede and Saturn's rings[END_REF] established the following linear equation:

Where T is the temperature in °C, and is the ratio between the area of the 6050 cm -1 band and the area of the 5500-7000 cm -1 region.

As the water trapped inside nanometric pores freezes at lower temperature due to surficial interaction, the temperature deducted from ice-related NIR bands needs to be corrected from the freezing point depression. In [START_REF] Chauviré | DSC of natural opal: insights into the incorporation of crystallisable water in the opal microstructure[END_REF], the six samples used to recalculate the calibration (1085, 1543a, 1543b, 1548, 1552a, 1552b) have been analyzed by low-temperature Differential Scanning Calorimetry (DSC), and the onset temperature (corresponding to the start of water freezing) has been added to the calibrated temperature. Based on the measured temperature, the temperature from the calibration and the onset temperature, we established a calibration equation as below:

= 5.518 * ( + ∆ ) + 429.14

Where Tm is the measured temperature, Tc is the temperature from the [START_REF] Fink | Temperature dependence of the water-ice spectrum between 1 and 4 microns: Application to Europa, Ganymede and Saturn's rings[END_REF] calibration and ΔTon is the onset temperature, with a R² = 0.81.