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This paper presents a methodological contribution to rock art archaeology by demonstrating the benefits of hyperspectral imaging, a relatively new method, for the understanding of rock art sites. It illustrates the complementarity of VNIR hyperspectral imaging, applied in rare cases to rock archaeology, and SWIR hyperspectral imaging, implemented here in a unprecedented way to a rock art panel. Applied to a schematic rock art site in southern France, the Otello rock shelter (Saint-Rémy-de-Provence, France), this method allowed the discovery of numerous new figures invisible to the naked eye or unsuspected after image enhancement with the DStretch plug-in of the ImageJ software, the individualisation of figures within complex superpositions, and the discovery of figures covered by weathering products. Moreover, by conferring a spatial dimension to the analysis of pictorial matter, thus allowing a classification of pigments at the scale of the wall, hyperspectral imaging makes it possible to automatically isolate different paintings and to carry out objective groupings of figures on the basis of their composition. Finally, hyperspectral imaging allows us to precisely document, distinguish and characterise weathering products interacting with painted figures. For all of these reasons, this method appears essential to highlight the relative chronology and syntax of iconography, and consequently to understand its cognitive nature. [ABSTRACT FROM AUTHOR]

Chemical erosion, one of the two major erosion processes along with mechanical erosion, occurs when a soluble rock-like salt, gypsum, or limestone is dissolved in contact with a water flow. The coupling between the geometry of the rocks, the mass transfer, and the flow leads to the formation of remarkable patterns, like scallop patterns in caves. We emphasize the common presence of very sharp shapes and spikes, despite the diversity of hydrodynamic conditions and the nature of the soluble materials. We explain the generic emergence of such spikes in dissolution processes by a geometrical approach. Singularities at the interface emerge as a consequence of the erosion directed in the normal direction, when the surface displays curvature variations, like those associated with a dissolution pattern. First, we demonstrate the presence of singular structures in natural interfaces shaped by dissolution. Then, we propose simple surface evolution models of increasing complexity demonstrating the emergence of spikes and allowing us to explain at long term by coarsening the formation of cellular structures. Finally, we perform a dissolution pattern experiment driven by solutal convection, and we report the emergence of a cellular pattern following well the model predictions. Although the precise prediction of dissolution shapes necessitates performing a complete hydrodynamic study, we show that the characteristic spikes which are reported ultimately for dissolution shapes are explained generically by geometrical arguments due to the surface evolution. These findings can be applied to other ablation patterns, reported for example in melting ice. [ABSTRACT FROM AUTHOR]

Martian gullies have been widely studied, but their formation mechanism is still under debate. Their channels generally trend straight downslope, but some display sinuosity. Seasonally active gullies are common on sand dunes and their channels have been reported to develop sinuosity. Here, we perform a detailed analysis of a gully on a dune within Matara Crater (49.5°S; 34.9°E) where development of channel sinuosity has taken place over 5 martian years (MY29-33) of observation. This study was performed using HiRISE images, HiRISE elevation data, spectroscopic CRISM data and a 1D GCM for surface temperature modelling. The morphological evolution of the gully suggests a significant seasonal contribution of fluid. Each year we observed material collapse and accumulation in the alcove, followed by transport events during which lateral migration and extension of the channel occur together with growth of the debris apron. Over one martian year, the debris apron propagated by almost 140 m from an initial length of 800 m. These transport events occur in the middle of winter when CO 2 frost is still present and are contemporaneous with the beginning of the defrosting. We propose that the activity and the sinuosity development in the gully could be explained by: 1) a flow composed of sand and CO 2 gas, producing morphologies similar to those in hyper-concentrated flows on Earth and 2) contribution of material from alternating of the alcove source location. • Detailed analysis of a dune-gully where development of channel sinuosity has taken place over 5 martian years. • The morphological evolution of the gully suggests a significant seasonal contribution of fluid. • Transport events on the gully occur in the middle of winter when CO 2 frost is still present. • The activity and development of sinuosity in the gully could be explained by a flow composed of sand and CO 2 gas. • Material from alternating source locations contributes to the gully activity. [ABSTRACT FROM AUTHOR]

Linear dune gullies are a sub-type of martian gullies. As their name suggests they only occur on sandy substrates and comprise very long (compared to their width) straight or sinuous channels, with relatively small source areas and almost non-existent visible deposits. Linear dune gullies have never been observed on terrestrial dunes and their formation process on Mars is unclear. Here, we present the results of the first systematic survey of these features in Mars’ southern hemisphere and an in-depth study of six dunefields where repeat-imaging allows us to monitor the changes in these gullies over time. This study was undertaken with HiRISE images at 25–30 cm/pix and 1 m/pix elevation data derived from HiRISE stereo images. We find the latitudinal distribution and orientation of linear dune gullies broadly consistent with the general population of martian gullies. They occur predominantly between 36.3°S and 54.3°S, and occasionally between 64.6°S and 70.4°S. They are generally oriented toward SSW (at bearings between 150° and 260°). We find that these gullies are extremely active over the most recent 5 Martian years of images. Activity comprises: (1) appearance of new channels, (2) lengthening of existing channels, (3) complete or partial reactivation, and (4) disappearance of gullies. We find that gully channels lengthen by ∼100 m per year. The intense activity and the progressive disappearance of linear dune gullies argues against the hypothesis that these are remnant morphologies left over from previous periods of high obliquity millions of years ago. The activity of linear dune gullies reoccurs every year between the end of winter and the beginning of spring (Ls 167.4°–216.6°), and coincides with the final stages of the sublimation of annual CO₂ ice deposit. This activity often coincides spatially and temporally with the appearance of recurrent diffusing flows (RDFs)—digitate-shaped, dark patches with low relative albedo (up to 48% lower than the adjacent dune) that encompass the active site. South- and SSW-facing dune slopes are those which preferentially host CO 2 frost deposits, however, it is only those with angles of ∼20° just below the crest which possess linear dune gullies, suggesting a slope-limited formation process. These observations provide a wealth of temporal and morphometric data that can be used to undertake numerical modeling, to direct future image monitoring and guide laboratory experiments that can be used to better constrain the formation process of these features. [ABSTRACT FROM AUTHOR]

Jezero crater has been selected as the landing site for the Mars 2020 Perseverance rover, because it contains a paleolake with two fan-deltas, inlet and outlet valleys. Using the data from the High Resolution Stereo Camera (HRSC) and the High Resolution Imaging Science Experiment (HiRISE), we conducted a quantitative geomorphological study of the inlet valleys of the Jezero paleolake. Results show that the strongest erosion is related to a network of deep valleys that cut into the highland bedrock well upstream of the Jezero crater and likely formed before the formation of the regional olivine-rich unit. In contrast, the lower sections of valleys display poor bedrock erosion and a lack of tributaries but are characterized by the presence of pristine landforms interpreted as fluvial bars from preserved channels, the discharge rates of which have been estimated at 103–104 m3s−1. The valleys' lower sections postdate the olivine-rich unit, are linked directly to the fan-deltas, and are thus formed in an energetic, late stage of activity. Although a Late Noachian age for the fan-deltas' formation is not excluded based on crosscutting relationships and crater counts, this indicates evidence of a Hesperian age with significant implications for exobiology. [ABSTRACT FROM AUTHOR]

Abstract The surface of Ganymede is characterized by intense tectonic deformation. The two terrains, light and dark, show morphotectonic features, namely, grooves and furrows. These represent kilometric systems at the regional scale, interpreted as extensional structures (e.g., horst and graben systems). Currently, evidence of significant compression has not yet been recognized, leaving open the opportunity to clarify the tectonic balance of Ganymede's surface. Debate still exists on the geodynamics responsible for surface deformation, and the investigation of these features contributes to understanding the internal processes. To understand the deformation that shapes the surface of the satellite, we explore the tectonic setting of the light terrain region of Uruk Sulcus. Using structural geology methodologies by remote sensing and systematic analysis of grooves, we recognize three groove systems within the sulcus and one furrow system in the neighboring dark terrain. Uruk Sulcus is also characterized by the presence of two domains of lineaments, subtle linear texture anisotropies related to the influence of tectonic processes on morphology. Comparison of the combined results from the analyses of the recognized groove systems with the lineament domains allowed us to infer the stress field responsible for their development in the studied area. According to these results, an evolutionary tectonic scenario for Uruk Sulcus is proposed: a tectonic corridor characterized by dextral transpression, with 58% of the internal compressional component approximately N-S oriented and responsible for localized, E-W extensional features among crustal blocks of the shear zone. Highlights • A new high-resolution Voyager and Galileo image mosaic of Uruk Sulcus is produced. • Automatic lineament analysis allowed inferring the stress field at Uruk Sulcus. • Structural analysis allows identifying groove and furrow systems. • Variation of lithospheric thickness/brittle deformation is recognized. • Uruk Sulcus is a transpressive right-lateral shear corridor. [ABSTRACT FROM AUTHOR]

Determining whether liquid water exists on the Martian surface is central to understanding the hydrologic cycle and potential for extant life on Mars. Recurring slope lineae, narrow streaks of low reflectance compared to the surrounding terrain, appear and grow incrementally in the downslope direction during warm seasons when temperatures reach about 250-300?K, a pattern consistent with the transient flow of a volatile species. Brine flows (or seeps) have been proposed to explain the formation of recurring slope lineae, yet no direct evidence for either liquid water or hydrated salts has been found. Here we analyse spectral data from the Compact Reconnaissance Imaging Spectrometer for Mars instrument onboard the Mars Reconnaissance Orbiter from four different locations where recurring slope lineae are present. We find evidence for hydrated salts at all four locations in the seasons when recurring slope lineae are most extensive, which suggests that the source of hydration is recurring slope lineae activity. The hydrated salts most consistent with the spectral absorption features we detect are magnesium perchlorate, magnesium chlorate and sodium perchlorate. Our findings strongly support the hypothesis that recurring slope lineae form as a result of contemporary water activity on Mars. [ABSTRACT FROM AUTHOR]

Gully morphology (often summarized as comprising an alcove, channel and debris apron) is one of the key elements used to support the argument for liquid water in the recent past on Mars. Nevertheless, the processes that create different gully morphologies, on both Mars and Earth, are not fully understood. One of the puzzling morphologic attributes of Martian dune gullies is their apparent lack of an apron, or terminal deposit, which has caused debate about their formation process. Several physical processes such as runoff, debris flows, granular flows, and sliding blocks falling downslope could explain the formation of these gullies. In this work, we focus on the role of liquid in the substrate as well as in the flow and choose to experimentally test the plausibility of this hypothesis. We performed a series of analogue experiments to investigate the formation of gullies on sand dune-like substrates. We used controlled flows of water over an inclined sand-box to produce gully-like forms. Ice-rich sedimentary substrates were used, including substrates that included a thin liquid water-saturated thawed layer (an ‘active layer’) above the ice-saturated zone to give an analogue for a ‘periglacial’ environment. We quantitatively demonstrate that debris flow processes in ‘periglacial’ experiments are conducive to the formation of narrow and long channels with small terminal deposits with perched channels. By re-analysis of Martian elevation data for dune-gullies on Mars, we have found good evidence that such terminal deposits could exist. Our experiments revealed that increased water content in the thawed layer above the frozen bed increases flow-length due to the subsequent reduction in infiltration capacity. Water is incorporated into the flow by erosion of the wet thawed layer (sand plus water) and by drainage of the thawed layer. Using a Mars environment simulation chamber, we found that atmospheric pressure conditions seem to have a limited influence on the morphology of the flows. Our experimental investigation allowed the reproduction of terrestrial debris flow and Martian gully morphologies, suggesting that a substrate that is resistant to infiltration could be present beneath the dune gullies on Mars. We suggest that, like in our laboratory experiments, the presence of ice at shallow depth is a possible explanation for the formation of these morphologies and that a wet thawed layer is a possible explanation for the long flow-length. [ABSTRACT FROM AUTHOR]

Abstract: Self-consistent landform assemblages suggest that Valles Marineris, the giant valley system that stretches along the Martian equator, was entirely glaciated during Late Noachian to Early Hesperian times and still contains huge volumes of fossil ice. Some of these glacial landform assemblages are illustrated here, with representative examples selected in three regions: Ius Chasma, Central Candor Chasma and the junction between Coprates Chasma and Capri Chasma. A morphological boundary separating an upper spur-and-gully morphology from a smooth basal escarpment has been spectacularly preserved along valley walls throughout Valles Marineris. The boundary winds around topographic obstacles and displays long-wavelength variations in elevation. It is associated with lateral benches, hanging valleys and truncated spurs. Comparisons with terrestrial analogs indicate that it is most reasonably interpreted as a glacial trimline. Chasma floors are covered by various kinds of terrains, including hummocky terrains, platy terrains, lateral banks, layered benches and a draping mantle. Landforms in these terrains and their spatial relationship with the interpreted trimline suggest that they correspond to various disintegration stages of an ancient glacial fill, currently protected by a superficial cover of ablation till. Altogether, these landforms and terrains compose a full glacial landsystem with wet-based glaciers that were able to flow and slide over their beds. It was most probably fed by ice accumulating at low elevations directly from the atmosphere onto valley floors and walls, with only minor contributions from tributary glaciers flowing down from higher elevations. Similar fossil glacial landsystems dating back from the early Martian history are to be expected in many other low-latitude troughs such as chasmata, chaos, valleys, impact craters and other basins. [Copyright &y& Elsevier]

The continuously increasing number of multi-temporal high-resolution images from the surface of Mars offers the possibility for detailed studies of present-day surface activity. In this study we investigated all gullies in the Sisyphi Cavi region (355° E, 71° S) of the south polar region of Mars. This region is influenced by the seasonal deposition of a decimeters-thick translucent slab ice in late autumn/winter and its subsequent sublimation in spring. We mapped all gullies (n = 17.760) and measured their orientations. We also identified gullies with contemporary activity (n = 35) using multi-temporal HiRISE images for martian years (MY) 28 to 34. We observed two different kinds of activity: (1) dark flow-like features, and (2) movement of blocks. For both, sediment was transported from the source region (alcove and/or flanks of channels) down the gully. Using image data from HRSC, CTX, and HiRISE, we monitored the general defrosting of the study region. We also analyzed the maximum daytime surface temperatures of the complete study region based on TES data from MYs 24 to 26. To identify the origin and triggering mechanism of the observed activity, we used: (1) detailed topographic investigations (e.g., slope angles) of two extensively gullied slopes based on two HiRISE-DTMs, (2) identification of small scale displacements with Digital Image Correlation (DIC), and (3) orientation measurements of active gullies and comparison to non-active gullies. We found that for the active gullies studied, activity happens at the end of spring between L S ~ 225° and ~250°. This is consistent with the timing of final stages of defrosting in the region. At this time, some surfaces are already defrosted while others still host the seasonal slab ice cover. For the surfaces with slab ice, dark defrosting spots (and flows, if the surface is inclined) are observed on dark dunes as well as on gully aprons and in gully channels. These spots form when, triggered by basal sublimation generated overpressure, sediment entrained in CO 2 -gas is transported through cracks within the ice and redeposited onto the frosted surface. We compared and linked both morphologic features (dark dune spots and dark flow-like features) and concluded that these features have comparable or even the same triggering mechanisms. Based on this extensive study, the most plausible mechanism for ongoing gully activity can be divided into two steps: 1) accumulation of material within gully channels via small dry flows on top of the slab ice (comparable to dark dune spots/flows), 2) when a critical mass is reached, the sediment flows down the still frosted gully on top of the sublimating ice or as a mixture of dry material and ice in a catastrophic flow. The triggering factor for the movement of blocks remains unclear, as their timing could not be constrained with the available data. We identified headwall erosion in one gully in the study region, whereas a discrete source could not be identified for the other sites, suggesting multiple failure mechanisms could be active in such gullies. Finally, through volume balance calculations we show that active gullies in Sisyphi Cavi could have been formed within decades to several tens of thousands of MY, but gully-morphology indicates a much-longer period for formation of the entire gully-landform. • Contemporary gully activity was identified on Mars during a multi-temporal study. • All activity took place during the end of spring, when surface is partly frosted. • Activity is flow-like features and block movements; sediment sources were detected. • Flow-like features are most likely flows of dry material over sublimating CO 2 -ice. • Link between the observed activity and dark spots/flows on dunes is likely. [ABSTRACT FROM AUTHOR]

Structure-from-Motion photogrammetry has recently become a cheap and efficient method to reconstruct accurate and highly-resolved 3D Digital Outcrop Model (DOM) from a single set of images. This enables the 3D visualization of hardly accessible and/or remote geological scenes, which is of strong interest for planetary bodies. This paper focuses on the reconstruction of the DOM of the Kimberley outcrop (Gale crater, Mars) using the Agisoft PhotoScan Professional software. This software is used to compute an accurate, scaled and georeferenced 3D mesh of this outcrop from set of multi-scale images taken by the Mars Science Laboratory rover Curiosity. This model was merged with a 3D model computed from orbital images from the High Resolution Imaging Science Experiment camera (HiRISE) to provide the context. One of the challenges is to integrate data coming from different cameras (with varying optical parameters) not specifically designed for 3D rendering, and with limited points of views. While the obtained DOM allows to observe and characterize geological features of Kimberley's sedimentary series, classic viewing methods through a 2D screen limits the understanding of the real 3D geometry and scale of the outcrop, as there is no feature such as trees or roads on Mars to provide size references to the user. To overcome this issue and facilitate the interpretation of the DOM, the latter is integrated into a Virtual Reality (VR) environment that enables one or several users working in a collaborative mode to experience a real scale, reliable and realistic depiction of the actual geometries of the geological features reconstructed on the mesh. Precise and accurate description, contextualization of the samplings and mapping of the Kimberley outcrop can therefore be achieved in VR allowing for more precise characterization and interpretations, the same way one would do on a real geological field trip. • Digital Outcrop Modeling of a Curiosity rover field site on Mars. • Integration into Virtual Reality for real scale visualization of the features. • VR-enabled geological analysis of the accurately reconstructed Martian outcrop. • VR allows for characterization of previously underrated sedimentological features. [ABSTRACT FROM AUTHOR]

Reconstruction of highly-resolved Digital Outcrop Model (DOM) using Structure-from-Motion photogrammetry is a low-cost but very effective method to explore and study remote planetary outcrops. Extensive data gathered by Mars Science Laboratory (MSL) rover Curiosity can be used to produce models of specific outcrops visited by the rover in the Gale Crater, on Mars. Integration into a Virtual Reality (VR) environment of these DOM enables one or several users to experience a reliable and realistic depiction of the actual geometries of the geological features, which is usually prevented by classic viewing methods on computer screens. Also, use of a VR environment authorizes the observation at real scale of various sedimentary series, structures and objects present at an outcrop, the same way they would on a real field. Moreover, this practice allows for a very precise and accurate characterization, description and therefore mapping of the features, as well as the contextualization of the sampling and remote analyses underwent by the Curiosity rover within their geological setting (e.g. ChemCam Laser-Induced Breakdown Spectrometer and Remote Micro-Imager data). Here, we focus on the Kimberley outcrop, traversed by Curiosity in 2014 between martian sols 603 and 630. This section presents a sedimentary succession with unusually high potassic content (Le Deit et al., JGR-Planets, 2016). However, poorly constrained stratigraphic relations between the series of the Kimberley Formation and its local to regional surroundings prevent further understanding of the exact extent of these accumulations and their significance within the broader Gale Crater paleoenvironmental scheme. Such questions highlight the need for a new finer mapping of, first, the outcrop itself to notably achieve precise characterization at the cm-scale of the Mount Remarkable butte; and then of the various structures present in the immediate vicinity of the outcrop. We therefore propose to use a custom true color highly resolved (up to the mm-scale) DOM of the Kimberley outcrop integrated into a VR environment to achieve precise and accurate mapping of the area and of the different geomorphological and sedimentological features (beds, structures and contacts alike). This represents a first step toward a better understanding of the intra- and inter-formational relations of the Kimberley series.We acknowledge the EU H2020 PlanMap project for supporting this work. [ABSTRACT FROM AUTHOR]

Structure-from-Motion (SfM) photogrammetry is an efficient, low-cost and powerful method to reconstruct Digital Outcrop Models (DOM) only from a set of overlapping photos. Apparent displacement of similar points across the set are used to determine the position of these points within a 3D space, and therefore recreate the tri-dimensional geometry (3D mesh) of the photographed objects. This method has recently been explored for remote planetary exploration such as that of the Gale Crater, Mars, thanks to the extensive imagery data from the Mars Science Laboratory (MSL) rover Curiosity.Here, we reconstructed a DOM of the Kimberley outcrop, traversed by Curiosity between sols 603 and 630, based on a comprehensive set of multi-scale photos gathered by 3 different imaging instruments aboard Curiosity, namely the navigational cameras (NavCam, 530 photos), mast cameras (MastCam, 1443 photos) and the Mars Hand Lens Imager (MAHLI, 32 photos).Despite the difference in resolution, colorization (greyscale vs full color), coverage and overlap parameters across these three different image data, we were able to compute a highly resolved full color DOM of the Kimberley outcrop using Agisoft PhotoScan software. Moreover, use of PhotoScan's embedded advanced geospatial features allowed us to obtain a geographically constrained DOM and a direct and proper scaling of the model (validated using the rover's tracks on the model).As the Kimberley outcrop presents a sedimentary succession with unusually high potassic content (Le Deit et al., JGR-Planets, 2016), the stratigraphic relations within this series and with its immediate to local surroundings are critical to understand the extent of these potassic accumulations and their signification from a paleoenvironmental point of view. We therefore integrated this high-resolution DOM into a collaborative Virtual Reality (VR) environment. VR lets one or several users observe at real scale the various sedimentary series and structures of the outcrop. This way, precise and accurate description, quantification and mapping of the outcrop is possible, allowing for more precise characterization and interpretation, as well as enabling contextualization within the local geological setting of the various data gathered by Curiosity (e.g. ChemCam Laser-Induced Breakdown Spectrometer and Remote Micro-Imager data).Collaborative VR exploration and characterization of photogrammetrically reconstructed reliable DOM is paving the way for remote geological exploration of Martian outcrops and other planetary bodies in near future.We acknowledge the EU H2020 PlanMap project for supporting this work. [ABSTRACT FROM AUTHOR]

A correction to the article "Spectral evidence for hydrated salts in recurring slope lineae on Mars" that was published in the September 28, 2015 issue is presented.