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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 for the first time 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 Image J software, the individualisation of figures within complex superpositions as well as 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.

Introduction: Louth Crater is a 36 km diameter located at 70 °N, 103.2 °E (Fig. 1) less than 1000 km from the Martian North Polar Cap. At the center of Louth crater a perennial water ice cap ~10 km in diameter, ~250 m in width (Fig. 1) that undergoes phase changes (condensation / sublimation cycles) during the Martian year [1 - 4]. Some periodic structure at the surface of its perennial water ice cap have been observed [2]. The presence of smaller ice undulations superimposed on this periodic structure, comparable to the sublimation waves on the Martian North Polar Cap [5] could be formed during sublimation and condensation period of the water ice. The characterization of this ice waves could make it possible to specify environmental conditions favorable to their formation. The first part of the study consists of identifying these ice waves using orbital topography and imaging data, and then associating these results with data from the Martian Database [6-7] and with the scaling laws inherent to the formation of sublimation and condensation waves. Fig. 1: Louth crater in summer. CTX product J02_045439_2504_XN_70N256W Ls = 133.2° Geomorphological analysis: Methods: Digital Elevation Model (DEM) at ~100 m/pixel and MOLA elevation data at ~128 m/pixel have been coupled with imagery data from HRSC ~10 m/pixel, CTX at ~6 m/pixel and HiRISE images for more precise areas for up to 25 cm/pixel. Data were georeferenced in ESRI’s ArcMap GIS software to produce geomorphological map and analyzed to evaluate ice waves shape and spatial organization. Observations: Louth presents two units (Fig. 2). (a) Lower unit with dark stucco texture and stratifications (Fig. 3a). (b) Fresh ice overlies this older, stratified structure (Fig 3.a). This fresh ice is distributed in a non-uniform manner as shown by the kilometrics waves of about 560 m wavelength on which are superimposed decametrics waves of about 55 m wavelength which are both perpendicular to the prevailing wind (Fig 3.b). The crests of both wave populations show similar NW-SE. The main wind direction was assessed from barkhane field on the sand mound. Fig. 2: Louth’s geomorphological map in summer. CTX product J02_045439_2504_XN_70N256W Ls = 133.2°. Fig. 3: Boxes from Fig. 2. (a) East side of the Louth crater ice cap. Green lines represents stratifications, HiRISE product EPS_045439_2505, Ls = 133.2°. (b) decametric waves on the ice waves. Black dashes represent leeside foot of each kilometric waves. Blue lines emphasize decametric waves, same HiRISE product from (a). Transport hypothesis: Preliminary experiments studied the threshold velocity necessary to initiate a transport of ice grains in Martian Simulation Wind Tunnel [8]. These experiments were carried out for grains from a few hundred micrometers to 2 mm in diameter under atmospheric pressures of 40 to 1000 mbar. To transport ice particles under near-Martian condition is hard because the threshold velocity is ten times higher than Earth. The wind speeds estimated from the Martian Database [6-7] in Louth crater are much lower than those needed to transport ice grains at Martian pressure. We support the hypothesis of diffusion by sublimation/condensation rather than the transport of icy particles by the wind. Mass transfer hypothesis: In a recent theoretical model [5] scaling laws explain the formation of sublimation waves and have been validated on terrestrial bedforms and on the Martian North Polar Cap. These waves are periodic and oriented in accordance with a turbulent boundary layer flow that diffuse the sublimated vapor. These laws, which are superimposed on the two ice waves studied, make these waves suitable geomorphological markers for climatic predictions. This model has been adapted to study similar waves created by condensation [9] and results indicate that condensation waves would be larger with one order gap than sublimation waves. Discussion: From the wind tunnel experimentation of ice particles transport in Martian-Like environment [8], wind necessary to initiate transport of ice particles is unlikely in Louth. Friction velocities of 3 m.s-1 would be required, compared to 0.6 m.s-1 deduced from scaling laws, to transport the most mobilizable 0.3 mm ice particles [8]. Wind speed in Louth is too weak for initiating particles transport. From the scaling laws, we extract values of friction velocity and flow velocity at a given altitude that can be compared with those extracted from the Martian Data Base over two Martian periods: a period of sublimation (during the summer in the northern hemisphere) at the origin of the formation of decametric waves and a period of condensation at the end of the summer, beginning of the autumn favorable for the kilometric waves formation. Scaling laws are in agreement with the predicted ratio velocity between sublimation and condensation period from the Martian Data Base values. From the Martian Data Base, independent velocity values are larger. This can be explained by the gridding of the database which takes place on a global scale larger than the crater. Conclusion: The observations indicate kilometric bedforms on the ice cap of Louth Crater, on which decametric bedforms appear with an order of magnitude gap. Comparison with numerical results suggests that the kilometric bedforms are formed by condensation and the smaller ones by sublimation. These condensation and sublimation waves are suitable markers to constrain mesoscale climate modelling in small, complex regions such as this type of crater, where topographic and/or seasonal effects can affect climate data. These waves can also be used as geomarkers on other planetary bodies where climatic conditions are not well constrained. Acknowledgments: We acknowledge PNP (Plan National de Planétologie) for founding the project “sublimation and condensation waves”. We used Planetary Data System (PDS) for observational data. References: [1] Conway et al. (2012) Icarus. [2] Brown et al. (2008) Icarus. [3] Hofstader & Murray (1990) Icarus. [4] Appéré et al. (2011) Journal of Geophysical Research. [5] Bordiec M. et al. (2020) Earth-Science Reviews, 211, 103350.[6] Forget et al. (1999). [7] Millour et al. (2018). [8] Herny et al. (2020). 7th Mars Polar Science Conf. Abstract #2099. [9] Carpy S. et al. (2022). EGU, Abstract #5998.

Sublimation combined with wind: Volatiles (N2, CH4, CO2, H20, NH3) are very common in the solar system (Fig 1). The average pressure and temperature conditions at the surface of many bodies suggest that volatile ices are stable and we thus expect that solid bedforms may form on these surfaces by either sublimation and/or condensation. These mass transfers between ice and overlying atmospheres are already known as landforms-shaping processes at different scales. These processes are very effective on the Martian North Polar Cap [2], whose spirals of the MNPC are known to be the result of sublimation on one side and condensation on the other [3-4], and as Pluto, where the Bladed Terrain Deposits are associated with sublimation of N2 and condensation of CH4 [5]. FIGURE 1 – Superimposed phase diagrams of the predominantly represented species (N2, CH4, CO2, NH3, H2O) in the Solar System in (p,T), using the Clausius-Clapeyron relationship. Scientific goal: To understand the process of material redistribution by the wind on icy planetary surfaces subject to phase changes, we propose a theoretical model coupling of flow dynamics and mass transfer and perform a linear stability analysis [6]. As done for loose bedforms, we use solid transverse bedforms as geomarkers to identify sublimation zones and to deduce erosion rate. We aim to physically and morphologically characterise these periodic waves perpendicular to the main flow over icy substrates and we are looking for analogues to assess the validity of our model before interpreting the observations on other bodies. On Earth, field analogues could be found in extreme cold and icy environment. Similar laboratory experiments are also suggested to complete the dataset. Sublimation waves analogues: We have identified two possible analogues for sublimation patterns from the literature: in the Blue Ice areas (Fig 2a) of the Antarctic ice sheets [1] and in ice caves such as the Eisriesenwelt cave (Fig 2b) in Austria [7]. We also use new measurements made in a laboratory experiment on the sublimation of CO2 ice in an atmospheric wind tunnel (Fig 2c). FIGURE 2 – Sublimation waves analogues [6]: (left: a) Blue Ice areas (middle: b) the Eisriesenwelt ice cave (right: c) CO2 ice in an atmospheric wind tunnel. In all these environments, linear and transverse bedforms appear. However, there are quite different in scales and involve different compositions of the icy substrate and the atmosphere. Despite those differences, the winds that flow on these surfaces are always turbulent and of infinite height and the environmental conditions are favorable to sublimation of ice: the surface temperatures are lower than the triple point and the partial pressure of the species is far from pressure at saturation. These sublimation waves have been classified as net ablation areas from surface energy balance. Theoretical model for redistribution processes: We consider a simple 2D case of a wavy surface of wavelength l and small amplitude with an overlying turbulent atmospheric boundary layer, of height H much larger that the wavelength. The mass transfer rate varies along the profile and has a maximum somewhere in the troughs, shifted from the crests by a phase lag. This effect allows the growth of a range of wavelengths and their migration, depending on the location of the maximum. The flow is perturbed by the topography and modify the mass transfer in turn, which may lead to the instability of such bedforms. From the stability analysis [6] we obtain 3 scaling laws: (1) the first law can predict either the friction velocity or the wind speed from a measure of the wavelength and if the viscosity is known, (2) the second law show that the migration velocity is found to scale linearly with the average value of the sublimation rates and thus depends on the kinetics of the phase transition, (3) the third law links the characteristic time of formation with the viscous length and the sublimation rate. FIGURE 3 – Model (black line), measurements (symbols) and prediction (MNPC) for sublimation waves [6]. Sublimation bedforms as geomorphic markers: The model prediction, black line (Fig 3), is superimposed on the natural terrestrial and the experimental model. This allows us to predict either the friction velocity or the wind speed from a measurement of the wavelength for the Martian Northern Polar Cap. The values predicted are in good agreement with those obtained by the martian climate database at the same place where these sublimation waves have been detected, and those both for the frictional velocities and the wind speeds. Conclusion: We propose a formation model for the sublimation waves by coupling mass waste with the hydrodynamic instability of the overlaying turbulent flow, in the case where the flow heigh is larger than the wavelength. The subjacent objective was to determine if terrestrial analogues exist or some experimental facilities could be used. We show it is possible to link the dimension of the sublimation waves to their environment and produce three scaling laws that links the geomorphological characteristics of these bedforms (wavelengths, migration, formation time) and the flow (velocity, viscosity, flow height). The adequacy between the observations/experiments and our model allows us to validate these environments as terrestrial/experimental analogues of sublimation waves. New experiments could thus be designed in controlled atmospheric wind tunnel like Aarhus wind tunnel, Denmark to explore controlling parameters. Acknowledgments: Plan National de Planétologie. References: [1] Bintanja R. (1999) Reviews of Geophysics, 37(3) :337–359 [2] Herny C. et al. (2014) EPSL, 4013, 56-66. [3] Howard A. D. (2000)Icarus, 144(2) :267–288. [4] Smith I. et al (2010) Nature, 465(7297). [5] Moore et al (2017) Icarus, 287 :320–333, 2017. [6] Bordiec M. et al. (2020) Earth & Sci. Reviews. Sci., 103350. [7] Obleitner and Spötl, (2011) Cryosphere, 5(1) :245–25.

Aeolian processes are at the origin of a large number of bedforms, which are topographic patterns that are spatially organised in a periodic manner and that can be observed both on Earth and on other planetary bodies. Two main categories of bedforms can be distinguished: (i) "loose" bedforms, generated on a bed of mobilisable grains by erosion, transport and deposition and (ii) "solid" bedforms, not induced by grain transport but by mass transfers such as ice sublimation or condensation under turbulent winds. Although the mechanisms involved in the growth of some solid bedforms have been studied (penitents, sublimation ripples, …), the subject remains largely less treated to date than loose bedforms, partly because of the lack of terrestrial environments favourable to sublimation. Comparison with other planetary environments has opened up new horizons for understanding these objects and the aeolian environments in which they develop.Among these bedforms, sublimation waves are transverse linear waveforms: regular and parallel ridges oriented perpendicular to the main direction of the turbulent flow interacting with the ice surface. The height of the flow is greater than their wavelength. The emergence of the bedforms is due to a hydrodynamic instability mechanism of the band-pass type which allows their growth. Our theoretical linear stability study shows that this instability appears in the laminar-turbulent transition regime, based on the near-wall Reynolds number, only if the modulation of the viscous sublayer by an effective longitudinal pressure gradient is taken into account in the turbulence model enabling to reproduce the feedback of the topography on the flow.These sublimation waves have been observed in different environments [Bordiec et al, 2020], by sublimation and diffusion of (a) water ice in air, in Antarctica or Ice caves, (b) water ice in CO2 atmosphere, on some areas of the northern polar cap of Mars, (c) and experimentally with CO2 ice in air. They are also observed on a Martian H2O glacier near the northern polar cap of Mars [Collet et al, in prep.], however, in the latter case, these sublimation waves are observed on larger icy waves. How can this difference in scale between two wavelengths be explained? What is their size selection process? To answer these questions, we investigate in our theoretical study the dependence on environmental conditions through (i) the fluid properties (wind speed, fluid viscosity) (ii) the direction of the transfer (sublimation or condensation) and (iii) the height of the flow in front of the wavelength (infinite or finite).

Gully systems on Mars were first reported by Malin and Edgett (Science, 2000) and because of their similarity to gullies on Earth were attributed to the action of liquid water. They are generally kilometre-scale systems where tributary alcoves lead into channel(s), which terminate in digitate deposits and/or fans. They are found on almost all steep slopes polewards of 30°N/S and are oriented towards the pole in the interval 30-40°, then occur on all slope-orientations >40° (e.g. Conway et al. 2019). Their latitudinal distribution and trends in orientation are strong indicators of a climatic factor playing a pivotal role in their formation. Repeat orbital observations have revealed changes in up to 20% of monitored gully systems (Dundas et al. 2019). When the timing of the changes can be constrained, they occur at the end of the seasonal defrosting period when carbon dioxide ice is present at the surface rendering the temperatures too cold for liquid water to be involved (Dundas et al. 2015, 2019; Pasquon et al., 2016, 2019a,b; Raack et al. 2015, 2020). Some changes involve resolvable quantities of sediment, including motion of metre-scale boulders and erosion of new channels (Dundas et al. 2015; de Haas et al. 2019; Pasquon et al., 2019a).Here, we exploit an exceptional time series to monitor the evolution of gullies and the seasonal frost deposits in Sisyphi Cavi (68-74°S, 345°-5°E). We use image data from HiRISE (High Resolution Imaging Science Experiment; 0.25-1 m/pixel), CaSSIS (Colour and Stereo Surface Imaging System; 4.5 m/pixel) and CTX (Context; 6 m/pixel). CaSSIS has four colour filters: BLU, PAN, RED and NIR (centred on 500, 675, 836 and 937 nm respectively); where the BLU filter is particularly useful for picking up surface frosts (Tornabene et al. 2019). We find that gullies and dunes are the last surfaces to defrost in the area. Independent of slope-orientation the alcoves of the gullies defrost first, followed by their channels then their fans. A surprising result considering that intuitively defrosting should progress from the equator-facing alcoves to the equator-facing fans, then from the pole-facing fans to the pole-facing alcoves. We infer that this is a consequence of a) fans and alcoves having contrasting thermal inertia and b) alcoves having slope-facets with a range of local orientations (with some proportion being equator-facing independent of overall orientation).We observe dark spots, dark flows and dark fans at the metre-to-ten-metre-scale. These features occur when a continuous solid slab of translucent CO2-ice is penetrated and warmed by sunlight at its base. The sublimation drives gas build-up under the slab, ruptures it, entraining dust and then depositing the dust on the surface (e.g. Kieffer et al. 2006) to form spots, flows and/or fans, depending on the context. We find that the recent activity of gullies promotes the formation of dark spots/flows/fans and are investigating the inverse relationship.Acknowledgement: CaSSIS is a project of the University of Bern, with instrument hardware development supported by INAF/Astronomical Observatory of Padova (ASI-INAF agreement n.2020-17-HH.0), and the Space Research Center (CBK) in Warsaw.

SummaryA successful immune response relies on a tightly regulated delivery of the right signals to the right cells at the right time. Here we show that innate and innate-like lymphocytes use two mechanisms to orchestrate in time and space the functions of conventional type 1 dendritic cells (cDC1) in spleen. Early after murine cytomegalovirus infection, XCL1 production by lymphocytes with innate functions attracts red pulp cDC1 near IFN-γ-producing NK cells, generating superclusters around infected cells in the marginal zone. There, cDC1 and NK cells physically interact reinforcing their reciprocal activation. Targeted IL-12 delivery and IL-15/IL-15Rα transpresentation by cDC1 trigger NK cell activation and expansion. In return, activated NK cells deliver GM-CSF to cDC1, triggering their CCR7-dependent relocalization into the T cell zone. This NK cell-dependent licensing of cDC1 accelerates the priming of virus-specific CD8+ T cells. Our findings reveal a novel mechanism through which cDC1 bridge innate and adaptive immunity.

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 LS ~ 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 CO2-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.

Successful immune responses rely on a regulated delivery of the right signals to the right cells at the right time. Here we show that natural killer (NK) and dendritic epidermal γδ T cells (DETCs) use similar mechanisms to spatiotemporally orchestrate conventional type 1 dendritic cell (cDC1) functions in the spleen, skin, and its draining lymph nodes (dLNs). Upon MCMV infection in the spleen, cDC1 clusterize with activated NK cells in marginal zones. This XCR1-dependent repositioning of cDC1 toward NK cells allows contact delivery of IL-12 and IL-15/IL-15Rα by cDC1, which is critical for NK cell responses. NK cells deliver granulocyte-macrophage colony-stimulating factor (GM-CSF) to cDC1, guiding their CCR7-dependent relocalization into the T cell zone. In MCMV-infected skin, XCL1-secreting DETCs promote cDC1 migration from the skin to the dLNs. This XCR1-dependent licensing of cDC1 both in the spleen and skin accelerates antiviral CD8

Summary: The monocyte-derived phagocytes termed LysoDCs are hallmarks of Peyer’s patches, where their main function is to sample intestinal microorganisms. Here, we study their differentiation pathways in relation with their sampling, migratory, and T cell-priming abilities. Among four identified LysoDC differentiation stages displaying similar phagocytic activity, one is located in follicles, and the others reside in subepithelial domes (SED), where they proliferate and mature as they get closer to the epithelium. Mature LysoDCs but not macrophages express a gene set in common with conventional dendritic cells and prime naive helper T cells in vitro. At steady state, they do not migrate into naive T cell-enriched interfollicular regions (IFRs), but upon stimulation, they express the chemokine receptor CCR7 and migrate from SED to the IFR periphery, where they strongly interact with proliferative immune cells. Finally, we show that LysoDCs populate human Peyer’s patches, strengthening their interest as targets for modulating intestinal immunity. : Wagner et al. dissect the differentiation pathways of the Peyer’s patch monocyte-derived dendritic cells termed LysoDCs. They show that LysoDCs mature as they get closer to the epithelium. Mature LysoDCs migrate to the periphery of the T cell zone, where proliferation of immune cells occurs only when stimulated. Keywords: intestinal immunity, Peyer’s patches, phagocytes, dendritic cells, monocyte-derived cells, LysoDC, cell differentiation, single-cell RNA sequencing, antigen sampling, naive T cell priming

Since 2012, the Mars Science Laboratory Curiosity rover has studied the sedimentary deposits within the Gale Crater, leading to the description of varying lacustrine to fluviatile and fluvio-deltaic environments. Here, we focus on the sedimentary record of the Kimberley outcrop traversed by Curiosity between sols 603 and 630. This section presents siliciclastic rocks with an unusually high potassic content (Le Deit et al., 2016, JGR-Planets). However, poorly constrained architecture and stratigraphic relations between the series of the Kimberley Formation and their local to regional surroundings still 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 the area to characterize the contacts observed on the outcrop itself and in its immediate vicinity, but also for a new assessment of the precise nature and morphology of the sedimentary structures and their spatio-temporal distribution throughout the outcrop and beyond.We therefore propose to use a true color highly resolved Digital Outcrop Model (DOM) of the Kimberley outcrop, obtained using Mars Science Laboratory imagery, integrated into a Virtual Reality (VR) environment (Caravaca et al., in press, PSS). Taking advantage of this “in situ” geological analysis of the DOM, we were able to observe and characterize such sedimentary structures and contacts, as well as their spatial extension throughout the reconstructed area of Kimberley with an unprecedented precision. We notably observe and describe both conformable and unconformable contacts over the entire outcrop, but also several sets of varying scale cross-stratifications (from cm- to pluri-meter scale). These results are in accordance with a fluviatile hydrodynamically active system. They tend to corroborate the idea of a complex yet diachroneous evolution of the area, with the possibility of laterally evolving depositional settings, spanning a significant amount of time.

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.

International audience; Sublimation waves are periodic, linear and transverse bedforms that grow by sublimation of icy substrates under turbulent winds. They occur in different environments of the Earth and other planets, where the climate is favorable to sublimation. Their morphological and kinematic characteristics (wavelength, migration velocity, characteristic time of formation) depend on the environmental conditions in which they develop (atmosphere viscosity, wind speed, sublimation rate). To highlight these relations, we designed a theoretical model for their formation, based on models previously designed for dissolution bedforms. From a linear stability analysis of the model, we derived a dispersion relation and three scaling laws. These are validated by comparison with measurements available for two natural terrestrial examples (Blue Ice Areas of the Antarctic ice sheets and ice caves) and with new measurements in a laboratory experiment. They are applied to linear bedforms, of hitherto unknown origin, that we identified on the Martian North Polar Cap. We thus demonstrate that sublimation waves are convenient geomorphic markers to constrain surface compositions, atmospheric properties, climates, and winds on terrestrial ice sheets and other icy planetary surfaces.

International audience

Macrophages possess intrinsic tumoricidal activity, yet tumor-associated macrophages (TAMs) rapidly adopt an alternative phenotype within the tumor microenvironment that is marked by tumor-promoting immunosuppressive and trophic functions. The mechanisms that promote such TAM polarization remain poorly understood, but once identified, they may represent important therapeutic targets to block the tumor-promoting functions of TAMs and restore their anti-tumor potential. Here, we have characterized TAMs in a mouse model of metastatic ovarian cancer. We show that ovarian cancer cells promote membrane-cholesterol efflux and depletion of lipid rafts from macrophages. Increased cholesterol efflux promoted IL-4-mediated reprogramming, including inhibition of IFNγ-induced gene expression. Genetic deletion of ABC transporters, which mediate cholesterol efflux, reverts the tumor-promoting functions of TAMs and reduces tumor progression. These studies reveal an unexpected role for membrane-cholesterol efflux in driving TAM-mediated tumor progression while pointing to a potentially novel anti-tumor therapeutic strategy. Goossens, Rodriguez-Vita et al. show that cancer cells scavenge membrane cholesterol from tumor-associated macrophages, resulting in their reprogramming toward an immune-suppressive and tumor-promoting phenotype. Targeting cholesterol efflux in macrophages counters this reprogramming and reduces tumor progression in a model of ovarian cancer.

Between 2004 and 2017, spectral observations have been gathered by the Visual and Infrared Mapping Spectrometer (VIMS) on-board Cassini (Brown et al., 2004) during 23 Enceladus close encounters, in addition to more distant surveys. The objective of the present study is to produce a global hyperspectral mosaic of the complete VIMS data set of Enceladus in order to highlight spectral variations among the different geological units. This requires the selection of the best observations in terms of spatial resolution and illumination conditions. We have carried out a detailed investigation of the photometric behavior at several key wavelengths (1.35, 1.5, 1.65, 1.8, 2.0, 2.25, 2.55 and 3.6 ${\mu}$m), characteristics of the infrared spectra of water ice. We propose a new photometric function, based on the model of Shkuratov et al. (2011). When combined, corrected mosaics at different wavelengths reveal heterogeneous areas, in particular in the terrains surrounding the Tiger Stripes on the South Pole and in the northern hemisphere around 30{\deg}N, 90{\deg}W. Those areas appear mainly correlated to tectonized units, indicating an endogenous origin, potentially driven by seafloor hotspots., Comment: Erratum: Fix south polar grid longitudes in Figures 9 and 11

The intense plume activity at the South Pole of Enceladus together with the recent detection of libration hints at an internal water ocean underneath the outer ice shell. However, the interpretation of gravity, shape, and libration data leads to contradicting results regarding the depth of ocean/ice interface and the total volume of the ocean. Here we develop an interior structure model consisting of a rocky core, an internal ocean, and an ice shell, which satisfies simultaneously the gravity, shape, and libration data. We show that the data can be reconciled by considering isostatic compensation including the effect of a few hundred meter thick elastic lithosphere. Our model predicts that the core radius is 180–185 km, the ocean density is at least 1030 kg/m3, and the ice shell is 18–22 km thick on average. The ice thicknesses are reduced at poles decreasing to less than 5 km in the south polar region.

Nature Geoscience http://dx.doi.org/10.1038/ngeo2546 (2015); published online 28 September 2015; corrected online 14 October 2015. In the version of this Letter originally published, the journal name in ref. 12 was incorrect, it should have been Nature Geoscience. This has been corrected in all versions of the Letter.

International audience; Many active and recently active surface processes on Mars have been controversially linked to the action of liquid water, yet the sediment transport capacity of water under martian environmental conditions is poorly understood. An understanding of the sediment transport capacity allows the amount of water required for any given landform to be back-calculated from its observed size/volume at the martian surface. Here we present a series of experiments where we explore the effects of grainsize, atmospheric pressure, humidity and temperature on the sediment transport capacity of seeping liquid water under martian conditions.

We describe and compare the morphology and activity of two types of gullies with different orientations collocated on the Kaiser dune field in the southern hemisphere of Mars: large apron gullies and linear dune gullies. The activity of large apron gullies follows an annual cycle: (i) material collapse into the alcove (mid-autumn/late winter) as CO2 condenses; (ii) remobilization by mass flows (late winter); and (iii) continuous appearance of hundreds of ‘digitate flows’ on the fan (autumn/winter). We find that large apron gullies could form in hundreds of Martian years. In contrast, linear dune gullies are active briefly in late winter, when the CO2 frost disappears. Their activity is characterized by the extension of channels, the creation of pits and the darkening of the surface. Linear dune gullies are likely to form within one to tens of Martian years. We infer that insolation, which influences the depth to ground ice and the amount of volatile deposited, may be the factor differentiating large apron gullies and linear dune gullies. Sediment transport by CO2 sublimation is a good candidate for the activity observed in all of these features. However, linear gullies could also be formed by brine release when the temperature rises abruptly after the removal of the CO2 ice.

International audience; In our previous laboratory work we reported a new sediment transport mechanism relevant for Mars: sediment saltation through boiling. We found seeping water was able to transport sediment downslope under martian conditions even though no sediment transport was observed under terrestrial conditions. This finding has important implications for inferring water reservoirs of recently active surface processes and hence for Mars' hydrosphere and habitability. However, in order to robustly transfer this finding to Mars we need to understand the physics behind this process. We can use this knowledge to define the limits of the process in terms of surface properties and/or environmental conditions found on Mars. In our previous work we inferred the physical mechanisms behind the saltation, yet we had little empirical data to test this inference. In this series of experiments we focused on the grain-scale processes involved in saltation via boiling with the objective of validating our physical model. Our experiments were performed in the large Mars chamber at the Open University. Our setup consists of two parallel confining walls 40 mm apart and 3.2 mm high adhered to an inclined roughened plane 0.4 m wide by 0.9 m long. Sediment was placed between the walls and levelled off. The water source was located at the top of the slope between the walls. We varied both initial slope angle (5 • , 15 • , 25 •) and grainsize (120, 250 and 500 µm). Each experiment was performed at least in triplicate. Once the water is released, it percolates downslope through the sediment between the walls and the grains are observed to saltate at the saturated-dry interface. We used a high-speed camera in order to capture videos of the grain scale processes. We analyse these data with an open source particle tracking code called tractrac developed by Joris Heyman (https://perso.univ-rennes1.fr/joris.heyman/trac.html). We extract particle trajectories and from those an estimate of the initial ejection velocity for tens to hundreds of particles for the videos we collected. We then compare our experimental trajectories and ejections speeds with those predicted by our physical model. In this contribution we will report on this comparison.

Tumor-associated macrophages (TAM) have been shown to have important roles in the malignant progression of various cancers. However, macrophages also posses intrinsic tumoricidal activity and can promote the activity of cytotoxic lymphocytes, but they rapidly adopt an alternative phenotype within tumors, associated with immune-suppression and trophic functions that support tumor growth. The mechanisms that promote TAM polarization in the tumor-microenvironment remain poorly understood, these mechanisms may represent important therapeutic targets to block the tumor-promoting functions of TAM and restore their anti-tumor potential. Here we have characterized TAM in a mouse model of metastatic ovarian cancer. We show that ovarian cancer cells promote membrane-cholesterol efflux and the depletion of lipid rafts from macrophages. Increased cholesterol efflux promoted IL-4 mediated reprogramming while inhibiting IFNγ-induced gene expression. These studies reveal an unexpected role for tumor-induced membrane-cholesterol efflux in driving the IL-4 signaling and the tumor-promoting functions of TAM, while rendering them refractory to pro-inflammatory stimuli. Thus, preventing cholesterol efflux in TAM could represent a novel therapeutic strategy to block pro-tumor functions and restore anti-tumor immunity.

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.

The initiation of the mucosal immune response in Peyer's patch (PP) relies on the sampling, processing, and efficient presentation of foreign antigens by dendritic cells (DCs). Among PP DCs, CD11b + conventional DCs (cDCs) and lysozyme-expressing DCs (LysoDCs) have distinct progenitors and functions but share many cell surface markers. This has previously led to confusion between these two subsets. In addition, another PP DC subset, termed double-negative (DN), remains poorly characterized. Here we show that both DN and CD11b + cDCs belong to a unique SIRPα + cDC subset. At steady state, cDCs and TIM-4 + macrophages are mainly located in T-cell zones, i.e., interfollicular regions, whereas a majority of subepithelial phagocytes are monocyte-derived cells, namely, LysoDCs and TIM-4 ' macrophages. Finally, oral administration of a Toll-like receptor 7 ligand induces at least three TNF-dependent events: (i) migration of dome-associated villus cDCs in interfollicular regions, (ii) increase of CD8α + interfollicular cDC number, and (iii) activation of both CD11b + and CD8α + interfollicular cDCs. The latter is marked by a genetic reprograming leading to the upregulation of type I interferon-stimulated and of both immuno-stimulatory and -inhibitory gene expression.

Transient streaks that appear seasonally on Martian slopes are consistent with brine flows, but evidence of water or salts has been lacking. Analysis of spectral data reveals hydrated salts associated with the streaks, confirming a briny origin.

As observed in most regions in the Arctic, the thawing of ice-rich permafrost (thermokarst) has been developing in Central Yakutia. However, the relationship between thermokarst development and climate variations is not well understood in this region, in particular the development rate of thaw slumps. The objective of this paper is to understand the current development of thermokarst by studying the evolution of the banks of thermokarst lakes. We studied retrogressive thaw slumps and highly degraded ice-wedge polygons (baydjarakhs), indicative of thermokarst, using high resolution satellite images taken in 2011–2013 and conducting field studies. The retrogressive thaw slump activity results in the formation of thermocirque with a minimum and maximum average headwall retreat of 0.5 and 3.16 m·yr − 1 respectively. The thermocirques and the baydjarakhs are statistically more concentrated on the south- to southwest-facing banks of thermokarst lakes. Moreover, the rate of headwall retreat of the thermocirques is the most important on the south-facing banks of the lakes. These observations indicate a control of the current permafrost thaw on the banks of thermokarst lakes by insolation. In the context of recent air temperature increase in Central Yakutia, the rate of thermocirque development may increase in the future.

International audience; Saturn’s moon Enceladus is an active world. In 2005, the Cassini spacecraft witnessed for the first time water-rich jets venting from four anomalously warm fractures (called sulci) near its south pole (1,2). Since then, several observations have provided evidence that the source of the material ejected from Enceladus is a large underground ocean, the depth of which is still debated (3,​4,​5,​6). Here, we report on the first and only opportunity that Cassini’s RADAR instrument (7,8) had to observe Enceladus’s south polar terrain closely, targeting an area a few tens of kilometres north of the active sulci. Detailed analysis of the microwave radiometry observations highlights the ongoing activity of the moon. The instrument recorded the microwave thermal emission, revealing a warm subsurface region with prominent thermal anomalies that had not been identified before. These anomalies coincide with large fractures, similar or structurally related to the sulci. The observations imply the presence of a broadly distributed heat production and transport system below the south polar terrain with ‘plate-like’ features and suggest that a liquid reservoir could exist at a depth of only a few kilometres under the ice shell at the south pole. The detection of a possible dormant sulcus further suggests episodic geological activity.

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

Recurring Slope Lineae (RSL) are active features on Mars that might require flowing water. Most examples observed through 2011 formed on steep, equator-facing slopes in the southern mid-latitudes. They form and grow during warm seasons and fade and often completely disappear during colder seasons, but recur over multiple Mars years. They are recognizable by their incremental growth, relatively low albedo and downhill orientation. We examined all images acquired by HiRISE during L s 250–10° (slightly longer than southern summer, L s 270–360°) of Mars years 30–31 (03/2011–10/2011), and supplemented our results with data from previous studies to better understand the geologic context and characteristics of RSL. We also confirmed candidate and likely sites from previous studies and discovered new RSL sites. We report 13 confirmed RSL sites, including the 7 in McEwen et al. (McEwen et al. [2011]. Science 333(6043), 740–743]. The observed seasonality, latitudinal and slope orientation preferences, and THEMIS brightness temperatures indicate that RSL require warm temperatures to form. We conclude that RSL are a unique phenomenon on Mars, clearly distinct from other slope processes that occur at high latitudes associated with seasonal CO 2 frost, and episodic mass wasting on equatorial slopes. However, only 41% (82 out of 200) of the sites that present apparently suitable conditions for RSL formation (steep, equator-facing rocky slopes with bedrock exposure) in the southern mid-latitudes (28–60°S) contain any candidate RSL, with confirmed RSL present only in 7% (13 sites) of those locations. Significant variability in abundance, size and exact location of RSL is also observed at most sites, indicating additional controls such as availability of water or salts that might be playing a crucial role.

International audience; 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 CO2 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.

International audience; Liquid water may exist on the Martian surface today, albeit transiently and in a metastable state under the low atmospheric surface pressure. However, the identification of liquid water on Mars from observed morphological changes is hampered by our limited understanding of how metastable liquids interact with sediments. Here, we present lab experiments in which a block of ice melts and seeps into underlying sediment, and the resulting downslope fluid propagation and sediment transport are tracked. In experiments at Martian surface pressure, we find that pure water boils as it percolates into the sediment, inducing grain saltation and leading to wholesale slope destabilization: a hybrid flow mechanism involving both wet and dry processes. For metastable brines, which are more stable under Martian conditions than pure water, saltation intensity and geomorphological impact are reduced; however, we observed channel formation in some briny flow experiments that may be analogous to morphologies observed on Mars. In contrast, under terrestrial-like experimental conditions, there is little morphological impact of seeping water or brine, which are both stable. We propose that the hybrid flow mechanism operating in our experiments under Martian surface pressure could explain observed Martian surface changes that were originally interpreted as the products of either dry or wet processes.

International audience; Peyer's patches (PPs) are primary inductive sites of mucosal immunity. The PP mononuclear phagocyte system, which encompasses both dendritic cells (DCs) and macrophages, is essential for the initiation of the mucosal immune response. We recently developed a method to isolate each mononuclear phagocyte subset of PP (Bonnardel et al., 2015). We performed a transcriptional analysis of three of these subsets: the CD11b + conventional DC, the lysozyme-expressing monocyte-derived DC termed LysoDC and the CD11c hi lysozyme-expressing macrophages. Here, we provide details of the gating strategy we used to isolate each phagocyte subset and show the quality controls and analysis associated with our gene array data deposited into Gene Expression Omnibus (GEO) under GSE65514.

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.

International audience; Complex interactions between katabatic winds and the cryosphere may lead to the formation of sedimentation waves at the surface of ice sheets. These have been first described and named snow megadunes in Antarctica. Here we use topographic data, optical images, subsurface radar soundings and spectroscopic data acquired by Mars orbiters, to show that the surface of the Martian North Polar Cap displays two superimposed sets of sedimentation waves with differing wavelengths. These sedimentation waves have similarities with Antarctic snow megadunes regarding their surface morphology, texture, grain size asymmetry, and internal stratigraphic architecture. Both sets of Martian sedimentation waves present young ice and occasional sastrugi fields, indicative of net accumulation, on their shallow-dipping upwind sides, their tops and the intervening troughs. Old layers of dusty ice, indicative of net ablation, are exhumed on the steep-dipping downwind sides of the larger waves. Smooth surfaces of coarse-grained ice, indicative of reduced accumulation associated with sublimation metamorphism, cover the steep-dipping downwind sides of the smaller waves. These surface characteristics and the internal stratigraphy revealed by radar soundings are consistent with the interpretation that both sets of Martian sedimentation waves grow and migrate upwind in response to the development of periodic accumulation/ablation patterns controlled by katabatic winds. The recognition of these sedimentation waves provides the basis for the development of a common model of ice/wind interaction at the surface of Martian and terrestrial glaciers. Martian smaller waves, characterized by reduced net accumulation on their downwind sides, are analogous to Antarctic snow megadunes that have been described so far. A terrestrial equivalent remains to be discovered for the larger Martian waves, characterized by net ablation on their downwind sides. © 2014 Elsevier B.V.

International audience; The North Polar Cap of Mars is associated with different kinds of superficial sediments, including the Circumpolar Dune Field, interior dune fields and sedimentary veneers scattered over the ice cap. In order to resolve the mineralogical composition and the regional distribution of these sediments, we processed OMEGA and CRISM hyperspectral data with an original method based on spectral derivation. We find that gypsum is present in all areas where undefined hydrated minerals had been previously detected, including superficial sedimentary veneers covering the North Polar Cap, interior dune fields and the whole Circumpolar Dune Field. Morphological and structural analyses reveal that these gypsum crystals derive directly from the interior of the ice cap. The source of superficial sedimentary veneers is the dust that was previously contained in the upper part of the ice cap, the ice-rich North Polar Layered Deposits (NPLD). This gypsum-bearing dust was released, on south-facing slopes of spiral troughs and arcuate scarps, by ice ablation controlled by katabatic winds. By the analysis of all associations of erosional scarps and dune fields over the North Polar Cap, we also demonstrate that the polar dunes are composed of sand-sized particles that were previously contained in the sediment-rich Basal Unit (BU), corresponding to the lower part of the ice cap. These particles contain gypsum and were released from the BU, by regressive ablation of ice at marginal scarps that border the North Polar Cap and by vertical ablation of ice on Olympia Planum. From a reconstruction of wind streamlines over and around the ice cap, we infer that katabatic winds descending from the polar high and rotating around the North Polar Cap control the release of these gypsum-bearing particles by ice ablation and the redistribution of these particles in the Circumpolar Dune Field. © 2011 Elsevier B.V.

International audience; Previous spectroscopic studies have shown the presence of hydrated minerals in various kinds of sedimentary accumulations covering and encircling the Martian North Polar Cap. More specifically, gypsum, a hydrated calcium sulfate, has been detected on Olympia Planum, a restricted part of the Circumpolar Dune Field. To further constrain the geographical distribution and the process of formation and accumulation of these hydrated minerals, we performed an integrated morphological, structural and compositional analysis of a key area where hydrated minerals were detected and where the main polar landforms are present. By the development of a spectral processing method based on spectral derivation and by the acquisition of laboratory spectra of gypsum-ice mixtures we find that gypsum-bearing sediment is not restricted to the Olympia Planum dunes but is also present in all kinds of superficial sediment covering the surface of the North Polar Cap and the Circumpolar Dune Field. Spectral signatures consistent with perchlorates are also detected on these deposits. The interpretation of landforms reveals that this gypsum-bearing sediment was released from the ice cap by sublimation. We thus infer that gypsum crystals that are now present in the Circum-Polar Dune Field derive from the interior of the North Polar Cap. Gypsum crystals that were initially trapped in the ice cap have been released by sublimation of the ice and have accumulated in the form of ablation tills at the surface of the ice cap. These gypsum-bearing sublimation tills are reworked by winds and are transported towards the Circum-Polar Dune Field. Comparison with sulfates found in terrestrial glaciers suggests that gypsum crystals in the Martian North Polar Cap have formed by weathering of dust particles, either in the atmosphere prior to their deposition during the formation of the ice cap, and/or in the ice cap after their deposition.

We present an Iterative Linear Spectral Unmixing Model (ILSUM) which is aimed at finding the main surface components that contribute to the signal in visible and infrared hyperspectral images. We processed the global dataset of the OMEGA imaging spectrometer onboard Mars Express up to orbit 5300, covering two martian years. We also present a preliminary test on AVIRIS data on the Cuprite (Nevada) site. We use ILSUM to identify the contribution of each endmember of an input library containing laboratory spectra of ices and mineral powders that are representative of the main mineral families. Synthetic spectra (pure slope endmembers) are included to account at first order for aerosol and grain size variations. Applied to the global OMEGA data set, this algorithm provides a distribution map for the main minerals present on the martian surface, which appears to be mainly dominated by pyroxenes, olivine, ferric oxides, with localized exposures of sulfates and phyllosilicates.

[1] Aram Chaos is a crater 280 km in diameter centered at 2.5°N, 338.5°E. It is filled by chaotic terrains overlain by a dome-shaped, layered 900 m thick formation displaying spectral signatures of ferric oxides on Thermal Emission Spectrometer (TES) and Observatoire pour la Mineralogie, L'Eau, les Glaces et L'Activite (OMEGA) medium spatial resolution data. We describe in detail the mineralogical composition, structure, and morphology of this crater fill using high-resolution data (OMEGA, Mars Orbiter Laser Altimeter, Mars Orbiter Camera, TES, Thermal Emission Imaging System, and High-Resolution Imaging Science Experiment). We infer the following formation scenario: the crater was first filled by a geological formation, the composition of which remains unclear because it is covered by dust. Widespread fracturing of this formation led to the development of chaotic terrains. Later, a second layered formation, presently dome shaped, was emplaced unconformably on the chaotic terrains. This younger unit is composed of a bright, poorly consolidated material that contains both monohydrated sulfates and ferric oxides according to OMEGA data. The surface of this formation is partially covered by dust and displays landforms indicating that the bright material has been mobilized by wind during or after its deposition. After its emplacement, this formation has been grooved down to various depths by large eolian erosion corridors. In these corridors, eolian removal of the bright material with a sulfate-rich matrix has left debris fans, sand sheets, and dunes, which display some of the strongest spectral signatures of ferric oxides on Mars. Similar residual deposits enriched in ferric oxides, overlying a layered formation containing both ferric oxides and sulfates, have been observed by the Opportunity rover in Meridiani Planum, suggesting a common formation process.

International audience; We describe in detail an annual seasonal process that occurs on the surface of the Russell Crater megadune on Mars. We give these features the name ‘perennial rills’, because their surface topographical expression persists from year-to-year and they form a distinctive, downstream-branching network of small channels, or rills. We used time-series images, elevation data from stereophotogrammetry and spectral data to characterize the evolution of these features over 6 Mars years. Growth and modification of these networks occurs abruptly in spring (at a solar longitude of c. 200°) after most of the seasonal CO2 ice has sublimated. We find that the peculiar morphology of perennial rills seems to be the only aspect that sets them apart from active linear dune gullies. By comparison to terrestrial analogues, we identified two conditions favouring the production of such a network: (a) the presence of an impermeable layer; and (b) the repeated formation of obstacles in front of propagating channels. We find that the most plausible formation mechanisms that can explain the formation of both the perennial rills and the active linear dune gullies are levitating CO2 blocks or liquid debris flows of water/brine, but neither can completely satisfy all the observational evidence.

International audience; 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 CO2 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 CO2 gas, producing morphologies similar to those in hyper-concentrated flows on Earth and 2) contribution of material from alternating of the alcove source location.