The Zhurong rover conducted in-situ spectral investigations of southern Utopia Planitia, where orbital observations revealed the presence of spectrally featureless dust. However, in-situ reflectance spectra collected by the Short Wave Infrared (SWIR) spectrometer exhibit hydrated features for all observations along the traverse. These features have been interpreted as being associated with groundwater (Liu Y. et al., 2022) or ocean (Liu C. et al., 2022; Xiao et al., 2023) or atmospheric water (Zhao et al., 2023). Here, we combine the Multispectral Camera (MSCam) and SWIR data to characterize the spectra of landing site and provide some new insights into the surface composition diversity. Multispectral images suggest that most of surfaces are consistent with the presence of dust whereas a few of rock surfaces exhibiting dark tones are compositionally distinct. The co-observational SWIR data can be used to further constrain the surface compositions. With Principal Component Analysis (PCA) and unmixing analysis of the SWIR data, we found that these dusty surfaces are ubiquitously characterized with faint 1900 and 2200 nm absorptions and the dark rock surfaces exhibit strong blue slopes in the NIR. The hydrated dust features seem to contrast with previous knowledge, that the dust does not exhibit obvious NIR hydration features from orbital observations. Such discrepancies were also observed at Jezero crater, where the fine soils or dusty rocks exhibit a 1900 nm H2O absorption but without 2200 nm band (Mandon et al., 2023). Spectral variation may reflect distinct surface dust compositions between the Perseverance and Zhurong landing site, indicating different dust reservoirs or dust alteration processes. The surface dust of different sites may be mixtures of globally well-mixed fine materials and local/regional distinct hydrated phases. Another possibilities is that the dust underwent different post-deposition aqueous alteration. The dark rock surfaces may represent less dust-coated surfaces. The strong blue slope features have been previously attributed to coatings on a dark substrate. Furthermore, the morphological properties show that these surfaces exhibit relatively fragile surface context, consistent with surface coatings or rinds.

Introduction: Hydrated silica (SiO2,nH2O) occurs in various forms depending on the geological context, and as such are good tracers for paleoenvironmental reconstitutions on Earth and Mars (e.g. [1,2]). Their potential to sequester and preserver organic matter further makes them prime exobiological targets [3]. Observed on Mars since over a decade [4], hydrated (opaline) silica minerals have been used to describe aqueous geological processes in diverse regions. However, geological origins of some deposits are still misunderstood because no satisfactory terrestrial analogues were found (e.g. [5]). Likewise, the exobiological potential of hydrated silica as a prime host of Mars organic matter remains to be fully ascertained. The Makgadikgadi Salt Pans show a very high potential to be considered as a terrestrial analogue site for Mars ancient aqueous environments, especially in fluvio-lacustrine geological settings [6,7]. A field trip was conducted in 2023 to study more specifically the silica-rich environments which were reported in the area [8-10]. Samples and method: 15 locations in the Makgadikgadi Pans area (Nwetwe and Sua Pans), along the Nata, Old Boteti and Boteti rivers were visited (Fig.1). We sampled ≈80 silica-rich rocks (i.e., silcretes) in a (paleo) fluvio-lacustrine geological context. Some visible-near infrared spectra of the surface of the samples were acquired using an ASD FieldSpec portable spectrometer (0.35-2.5µm) to evaluate the mineralogy of the rocks. A detailed analysis of the mineralogy of the samples was conducted in the lab, so far including: optical microscopic observation on raw samples and extracted thin sections, VNIR hyperspectral imaging (402-2504 nm), Raman, electron microscopy and geochemical analysis (EDS). Results in the field: In-field analysis using the portable field VNIR spectrometer suggested the ubiquitous presence of hydrated silica and phyllosilicate minerals. Based on spectroscopic and macroscopic observations in the field, most of the samples are silicified/indurated clastic sedimentary rocks and consist of coarse-grained quartz-rich sandstones and conglomerates with a fluvio- lacustrine origin (Fig. 2). Clasts in the sedimentary rocks can be of various types, fragments of quartz pebbles or sometimes opaline silica to detrital fragments of other silica-rich clastic sedimentary rocks. Matrix between the clasts is of different nature from one sample to another; it can be either opaline or microcrystalline silica and sometimes consists of clay-rich cemented materials. Depending on the nature of the clasts and matrix, the samples exhibit different colours: from white to creamy-white shades, from dark-toned shades of green, brown, and black. Samples consisting of translucent opaline and/or microcrystalline silica plates were also taken. Field observations and spectral analyses confirm the large amount of amorphous to (micro-) crystalline silica in the samples, along with different clays and salts. This type of mineralogy, possibly indicating a formation in a fluvio-lacustrine context in semi-arid environments, is reminiscent of some silica-rich deposits on Mars in locations interpreted as potential paleo-lakes [e.g. 2,5]. Results in the lab: VNIR and Raman analysis permit us to identify several mineral phases that are present in our samples. Microcrystalline silica is present in most of the samples along with various phyllosilicatesand sulfates in mixtures with silica or as (clayey) salty crusts. The presence of the mineral glauconite, suggested in the literature for these silcretes, giving this pale green colour, is verified in our spectra and support the fluvio-lacustrine origin in a semi-desertic environment for these silica deposits. Microscopic and EDS analysis show that hydrated silica is found in several types of contexts, ranging from inherited in the sediments to later-stage reprecipitation in voids. Perspectives: Laboratory analyses have recently begun and a systematic comparison between samples is underway to find trends between geological setting, composition, and lithology. Of particular interest we are investigating how the whole-rock VNIR spectra relate to the fine distribution and abundances of the minerals comprising the rock. This has strong implications for better benchmarking the sensitivity level and potential detection biases of similar instrument at Mars, both in-situ (e.g. SuperCam/Mars 2020) or from orbit (OMEGA/Mars Express and CRISM/MRO imaging spectrometers). We will investigate how the carbonate/calcrete and phyllosilicate matrix may inhibit the hydrated silica signatures and vice-versa. These observations will be compared to the observations of silica-rich deposits at the surface of Mars, and possibly supported by remote sensing data from the Makgadikgadi area (e.g., EnMap data). Finally, we also aim to better understand the exobiological potential of this type of deposit by studying the sequestration of various organic compounds in these silicified rocks. Acknowledgments: We acknowledge support from the Agence Nationale de la Recherche in France (“PaleoSilica” project) under contract ANR-20-CE46-0013. We also acknowledge support from Europplanet Society Transnational Access Funding for site TA5.1 under program 22-EPN3-127. Our warmest gratitude for the logistical and scientific support provided on site by Prof. F. Franchi of BIUST. References: [1] Boudreau A., et al. (2012) J. Volc. & Geoth. Res., 247–248, 1-8. [2] Pineau M., et al. (2020) Icarus, 347, 113706. [3] McMahon S., et al. (2018) JGR, 123(5), 1012-1040. [4] Milliken R., et al. (2008) Geology, 36(11), 847–850. [5] Pan L., et al. (2021) Planet. Sci. J., 2, 65. [6] Franchi F., et al. (2020) PSS, 192, 105048. [7] Schmidt G., et al. (2023) Front. Astron. Space Sci., 10, 1108386. [8] Stephan K., et al. (2023) 54th LPSC, abstract #1507. [9] Nash D., et al. (2022) Quat. Sci. Rev., 297, 107811. [10] Ringrose S., et al. (2009), Sediment. Geol., 219, 262-279. Fig.1 Satellite view of the Makgadikgadi Salt Pans with sampling locations (blue circles). The yellow circles on the map represent silica-rich rocks (silcretes and duricrusts) locations from a publication by [9]. Fig.2 Example of a silcrete boulder outcrop at the Old Boteti River Paleo-Delta on the western margin of the Nwetwe Pan. Various examples of coloured silcretes are indicated, topped in places by a light-brown laminated crust.

Introduction: The Thumbprint Terrains (TT) are intriguing geological formations that can be observed in the northern plains (NP) of Mars. TT are commonly described as an alternation of parallel alignments of 10s of meters high high-albedo ridges and dome-like mounds, separated by shallow depressions with contrasting low albedo[1]. These smooth surface deposits are composed of fine-grained loose materials[2]. TT mounds have been tentatively proposed as being formed by: explosive volcanism[3], glacial processes[1,4], sedimentary volcanism[2,5,6], or tsunami-driven events formed after giant impacts[2,6,7]. TT formation may be recent (Late Hesperian-Early Amazonian), period overlapping with the outflow channel activity timing and a possible transient Hesperian Ocean on Mars[7]. However, our lack of knowledge about the nature of these deposits persists, not least because no mineralogical clue has been found[8]. This study proposes to provide constraints on the nature of these deposits and their possible subsurface layering, thanks to the recent observation of hydrated silica (HySi) and sulfates (PHS) on the TT of Acidalia Planitia[9]. HySi is systematically associated with TT’s mounds [Fig. 1A,B,C,D]. HySi is only observed in these mounds and seems to be directly linked to the activity of these. We calculated spectral criteria on HySi CRISM detection to decipher the type of silica and to infer its possible geological origins[10]. Silica possibly occurs as altered volcanic glasses, or as dehydrated opals. These criteria also suggest that the silica was formed by low-T fluid-rock interactions and/or late-stage water-limited alteration. We propose that HySi in the Acidalia’s TT cones represents weathered, Si-rich fine materials. Possibly in the form of low-density volcanic ashes, HySi was extruded to the surface by sedimentary volcanism involving low quantity of fluids/volatiles (no surface flow observed). Such type of mounds could be indicative of sand volcano-like mounds and/or hydrodynamic blowouts whose sources, constrained by the thickness of the TT (less than 100m). This low-lying origin is confirmed by the results obtained by calculating the estimated extraction depths of such HySi-rich sources by sedimentary volcanism using a buoyancy-driven model in subaerial environment[11]. HySi-rich materials seem to be sourced from several 10s of m underground (between -10 to -80 m), from subsurface layers that are internal to the TT. PHS are mainly associated with impact craters with diameters less than or close to ~1 km, and mostly located in their (sometimes lobate) ejecta [Fig.1A;B]. As such small impacts can excavate materials up to 100-150 m depth, PHS presence in these may indicate the presence of sulfate-bearing lithologies in the TT’ subsurface or in the underlying units. This is inferred from observations such as the one presented in Fig.1B where two impacts excavate PHS in two different settings. Further evidence of the presence of sulfates within the TT (or in deeper underlying units) is the observation of PHS in one large 165 m high mound that also display HySi at its base [Fig. 1C]. Applying the same buoyancy-driven model to this PHS mound, it returns that the PHS sources can be several hundred meters deeper (-200 up to -600 m), possibly out of TT’ stratigraphy. However, it is also possible that the sulfates present here have an origin directly within the TT. Indeed, when sedimentary volcanism occurs, it is common to have remobilization and incorporation of the crossed layers by the injected fluids or materials. This hypothesis of co-presence of PHS and HySi in the TT could explain the observation made of such high-mounds (Fig.1C). Implications for Subsurface Stratigraphy. The detection of hydrated minerals confirms the sedimentary volcanism hypothesis for the origin of TT mounds. HySi-rich mounds indicate that subsurface localized pockets of hydrated materials are present several 10s of m deep within the TT’ stratigraphy. The nature of surface materials suggests that the TT are partly composed of Si-rich volcanic ashes, that, mixed with moderate quantities of over-pressurized fluids, were able to migrate to the surface through (explosive?) sedimentary volcanism. Highest mounds, enriched with PHS, can have sources that lie several hundred meters below the TT. In such a configuration, these sulfates-rich sediments are older than the “mostly Amazonian” Si-rich lithologies making up the TT and may be an inherent part of the stratigraphy composing the underlying “mostly Hesperian” Vastitas Borealis Formation (VBF), possibly in the form of buried evaporites. Ongoing Perspectives. This study is currently being extended to numerous sites where sedimentary volcanism has been proposed on the basis of geomorphological observations[12] and where hydrated minerals have been detected[9]. Applied across all the NP, our study will enable us to better characterize the aqueous materials in the Martian subsurface. It will also provide constraints on the nature of the sedimentary volcanism-related edifices in various regions, but also on the origins of the VBF, for which numerous studies suggest a potentially “not so ancient” aqueous origin[13]. Acknowledgments. We acknowledge the support from the Agence Nationale de la Recherche (ANR, France) under the contract ANR-20-CE46-0013 entitled “PaleoSilica”, the Centre National d’Études Spatiales (CNES, France), the Centre National de la Recherche Scientifique (CNRS, France), and the French government under the France 2030 investment plan, as part of the Initiative d'Excellence d'Aix-Marseille Université - A*MIDEX AMX-21-RID-O47. References. [1]Lockwood et al. (1992) 23rdLPSC. [2]Di Pietro et al. (2021) Icarus. [3]Frey & Jarosewich (1982) JGR Solid Earth. [4]Souček et al. (2015) EPSL. [5]Salvatore & Christensen (2014) JGR Planets. [6]Costard et al. (2017) JGR Planets. [7]Costard et al. (2019) JGR Planets. [8]Oelher & Allen (2010) Icarus. [9]Carter et al. (2023) 54thLPSC. [10]Pineau et al. (2020) Icarus. [11]Hemmi & Miyamoto (2018) Geosciences. [12]Broz et al. (2023) Earth Surf. Dyn. [13]Kreslavsky & Head (2002) JGR Planets. Figure 1. A. to D. PHS (green) and HySi (cyan) CRISM detections in four sites within the Acidalia’s TT over CTX background, North is up. For each, coordinates of CRISM cubes are provided. Scale bars indicate 2000m.

Silica minerals constitute a main target to assess the origin of life or the possibility of its emergence. On Earth, ancient hydrothermal silica deposits have preserved the oldest forms of life. Beyond Earth, such silica-rich hydrothermal systems have been observed on Mars by orbital near-infrared (NIR) remote sensing and in situ rover exploration. This work investigates the variations of texture and NIR properties of opal with temperature, within a single geological context of hot springs. Silica sinters have been sampled in Icelandic hot-spring fields, in the Reykholt region, and at the Hveravellir site, with water temperature ranging from 14 to 101 ∘C. Variations in the NIR spectral features (concavity ratio criteria, CRC) vary with fluid temperature, lithofacies, and microtexture. Only high-temperature samples display high CRC values (CRC5200>0.85), but low CRC values (CRC5200 < 0.75) are measured for any temperature. Hence, temperature is not the only parameter controlling spectral properties of opal. Several other parameters such as the hydrodynamic context, the microbial activity, silica micro-textures, and porosity may also affect silica precipitation, the incorporation and speciation of water in it, and thus its NIR signature. The observations suggest a limitation in the use of NIR spectral features for the interpretation of the geological context of fossil opal on Earth or Mars: only opal with high CRC values can be inferred as being formed by hydrothermal activity. Low CRC values can be attributed to either low-temperature hydrothermal activity (< 50–60 ∘C) or to continental weathering.

Abstract Kaolinite is an Al-rich phyllosilicate commonly observed on Earth as a product of the chemical weathering of aluminosilicates. It has also been detected on the martian surface by orbital remote sensing observations. While the determination of the geological processes of formation of terrestrial kaolinite (i.e., hydrothermal activity, continental surface weathering, diagenesis) involves the coupling of field observation and multiple laboratory measurements, only geomorphology and associated minerals are generally available to determine their geological origin on Mars. Kaolinite crystallinity depends on many physicochemical parameters reflecting its conditions of crystallization. To determine if the near-infrared (NIR) spectral signature of kaolinite enables estimation of its crystallinity and furthermore if this method can be used to identify the geological processes involved in kaolinite formation, we carried out an in-depth analysis of NIR spectra of reference terrestrial kaolinites that formed in various geological contexts. We calculated second and third derivatives for each spectrum to highlight subtle variations in the spectral properties of kaolinite. This allowed the identification of 27 spectral contributions for the 4500 and 7000 cm−1 Al-OH-related regions of absorption bands. The position shifts and shape variations of these spectral contributions were intimately linked to variations of crystallinity, which was qualitatively estimated using Hinckley and Liétard XRD (dis)order indices. The results obtained show that the NIR signature of kaolinite is influenced by the stacking disorder of layers that has some influence on the vibrations of the interfoliar and inner Al-OH groups. Our study also confirms that: (1) well-ordered kaolinites are not restricted to hydrothermal deposits; (2) kaolinites from a similar sedimentary or pedogenetic context often display contrasting degrees of crystalline order; and (3) poorly ordered kaolinites are more likely to have a sedimentary or pedogenetic origin. Finally, this work highlights that obtaining spectra with sufficient spectral resolution could help to estimate the crystallinity of kaolinite and, in the best cases, its geological origin, both on Earth and Mars, especially with in situ NIR measurements.

Mars' transition from an early "warm and wet" to a "cold and dry" environment left fingerprints on the geological record of fluvial activity on Mars. The morphological and mineralogical observations of aqueous activity provided varying constraints on the condition and duration of liquid water on the Martian surface. In this study, we surveyed the mineralogy of Martian alluvial fans and deltas and investigated the hydrated silica-bearing deposits associated with some of these landforms. Using CRISM data, we identified 35 locations across Mars with hydrated silica in proximity to fans/deltas, where the spectral characteristics are consistent with immature or dehydrated opal-A. In a few stepped fans/deltas, we find hydrated silica occurs within the bulk fan deposits and form sedimentary layers correlated with elevation. Meanwhile, in the older fans/deltas, silica mostly occurs at distal locations, and the relation to primary sedimentary deposits is more complex. We propose that the hydrated silica-bearing deposits in stepped fans/deltas likely formed authigenically from Martian surface waters, mainly during the Late Hesperian and Early Amazonian. These silica-bearing deposits could be a tracer for the temperature or duration of water involved in the formation of these deposits, given more precise and detailed observations of the sedimentary context, accessory minerals, the concentration of hydrated silica, and sediment-to-water ratio. Therefore, we consider that silica-bearing deposits should be among the most critical samples to investigate for future Mars missions, which are accessible in the landing sites of Mars 2020 and ExoMars 2022 missions.

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 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 degrees 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 observed in previous studies. However, such dehydration was not systematic, as it was observed 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 mu m and 1.4 mu m 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 mu m) and a wider 7000 cm(-1) (1.4 mu m) feature. We show that opal can retain water under martian conditions, an interesting property for sample return missions. (C) 2021 Elsevier B.V. All rights reserved.

Introduction: Kaolinite, a 1:1 dioctahedral phyllosilicate of chemical formula Al2Si2O5(OH)4, has been detected at the martian surface by orbiters in: (i) Al-clay rich materials associated with opaline silica, overlaid by Fe/Mg-clays in inferred pedogenetic paleoprofiles [1,2,3,4], (ii) alone or associated with a great diversity of alteration phases in contexts suggested to be linked to hydrothermal activity [5,6,7]. On Earth, kaolinite forms through pedogenesis/surface weathering, hydrothermal activity, or diagenetic/sedimentary processes [8]. Thus, the occurrences of kaolinite may be of various origins on Mars as well. Kaolinite crystallinity has been suggested to reflect the physicochemical conditions where it forms, and thus its geological conditions of formation [8]. As spectroscopic sensors on Mars cover the Near-Infrared range, we investigated the NIR signatures of terrestrial kaolinite samples from various origins in order to reveal the degrees of crystalline disorder of this clay mineral.