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Acquiring and building Digital Outcrop Models (DOM) becomes an essential approach in geosciences. This study highlights the strong potential of Structure-from-motion (SfM) photogrammetry for full-3D mapping of inaccessible outcrops, combining pictures captured from field and from unmanned aerial vehicle-embedded digital cameras. We present a workflow for (i) acquiring and reconstructing a DOM of a geometrically complex natural cave site using digital photogrammetry in a lowlight environment, (ii) georeferencing the 3D model in underground environments, (iii) identifying and characterizing the geometry of inaccessible geological structures and their tectonic kinematics (e.g., faults, joints, sedimentary bedding planes, slickenlines) for structural geology purposes. We illustrate our method by modelling a challenging case study: the main chamber of the Lorette cave (Rochefort Cave Laboratory, Belgium). First, we produced a high resolution, highly realistic model made of 395 million points cloud. This allowed to draw a detailed lithostratigraphic log of the exposed sedimentary pile, alternating decimetric carbonate mudstones with minor centimetric clay-rich layers. Secondly, we extract the orientation of brittle structures from the cave DOM which consist of joints, calcite-filled veins, fault planes with observable slickenlines and their kinematic indicators. Calcitic veins consist of tension gashes structures. Two subsets of tension gashes are distinguished based on their orientation (WNW-striking with low- vs subhorizontal dips) and morphology (planar vs en-echelon sigmoidal veins). Two faults subsets are identified: (i) a first one comprises south-dipping fault planes with mean strike-dip of N069-S42 and consist of bedding surface slip; (ii) a second one which corresponds to neoformed north-dipping faults (mean strike-dip: N279-N60). We recognize and characterize tectonic markers on fault planes directly from the high-resolution DOM (slickenlines and asymmetrical microscarps) pointing to a reverse shearing movement for all investigated faults. Based on their geometrical relations and fault-slip data, paleostress reconstruction points to a NW-SE to NNW-SSE subhorizontal compressive regime. This one is interpreted as the record of early phases of Variscan tectonics during the fold-to-fault progression. This research paper also highlights future possibilities for rapid semi-automatic interpretation of such 3D dataset for structural geology purposes as well as advances in technology and perspectives in terms of risk assessments and mitigation.

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

Senkyo is an equatorial plain on Titan filled with dunes and surrounded by hummocky plateaus. During the Titan targeted flyby T61 on August 25, 2009, the Cassini Visual and Infrared Mapping Spectrometer (VIMS) onboard the Cassini spacecraft observed a circular feature, centered at 5.4° N and 341°W, that superimposes the dune fields and a bright plateau. This circular feature, which has been named Paxsi by the International Astronomical Union, is 120±10 km in diameter (measured from the outer edge of the crater rim) and exhibits a central bright area that can be interpreted as the central peak or pit of an impact crater. Although there are only a handful of certain impact craters on Titan, there are two other craters that are of similar size to this newly discovered feature and that have been studied by VIMS: Sinlap ( Le Mouelic et al., 2008 ) and Selk ( Soderblom et al., 2010 ). Sinlap is associated with a large downwind, fan-like feature that may have been formed from an impact plume that rapidly expanded and deposited icy particles onto the surface. Although much of the surrounding region is covered with dunes, the plume region is devoid of dunes. The formation process of Selk also appears to have removed (or covered up) dunes from parts of the adjacent dune-filled terrain. The circular feature on Senkyo is quite different: there is no evidence of an ejecta blanket and the crater itself appears to be infilled with dune material. The rim of the crater appears to be eroded by fluvial processes; at one point the rim is breached. The rim is unusually narrow, which may be due to mass wasting on its inside and subsequent infill by dunes. Based on these observations, we interpret this newly discovered feature to be a more eroded crater than both Sinlap and Selk. Paxsi may have formed during a period when Titan was warmer and more ductile than it is currently.

The mineralogical composition of the Martian surface is investigated by a Multiple-Endmember Linear Spectral Unmixing Model (MELSUM) of the Observatoire pour la Mineralogie, l’Eau, les Glaces et l’Activite (OMEGA) imaging spectrometer onboard Mars Express. OMEGA has fully covered the surface of the red planet at medium to low resolution (2–4 km per pixel). Several areas have been imaged at a resolution up to 300 m per pixel. One difficulty in the data processing is to extract the mineralogical composition, since rocks are mixtures of several components. MELSUM is an algorithm that selects the best linear combination of spectra among the families of minerals available in a reference library. The best fit of the observed spectrum on each pixel is calculated by the same unmixing equation used in the classical Spectral Mixture Analysis (SMA). This study shows the importance of the choice of the input library, which contains in our case 24 laboratory spectra (endmembers) of minerals that cover the diversity of the mineral families that may be found on the Martian surface. The analysis is restricted to the 1.0–2.5 μm wavelength range. Grain size variations and atmospheric scattering by aerosols induce changes in overall albedo level and continuum slopes. Synthetic flat and pure slope spectra have therefore been included in the input mineral spectral endmembers library in order to take these effects into account. The selection process for the endmembers is a systematic exploration of whole set of combinations of four components plus the straight line spectra. When negative coefficients occur, the results are discarded. This strategy is successfully tested on the terrestrial Cuprite site (Nevada, USA), for which extensive ground observations exist. It is then applied to different areas on Mars including Syrtis Major, Aram Chaos and Olympia Undae near the North Polar Cap. MELSUM on Syrtis Major reveals a region dominated by mafic minerals, with the oldest crustal regions composed of a mixing between low-calcium pyroxenes (LCPs) (orthopyroxenes (OPx)) and high-calcium pyroxenes (HCPs) (clinopyroxenes (CPx)). The Syrtis volcanic edifice appears depleted in LCP (OPx) and enriched in HCP (CPx), which is consistent with materials produced with a lower partial fusion degree at an age younger to the surrounding crust. Strong olivine signatures are found between the two calderas Nili Patera and Meroe Patera and in Nili Fossae. A strong signature of iron oxides is found within Aram Chaos, with a spatial distribution also consistent with thermal emission spectrometer (TES). Gypsum is unambiguously detected in the northern polar region, in agreement with the study of Langevin et al. [2005. Sulfates in the north polar region of Mars detected by OMEGA/Mars Express. Science 307(5715), 1584–1586]. Our results show that the linear spectral unmixing provides good first order results in a variety of mineralogical contexts, and can therefore confidently be used on a wider scale to analyze the complete archive of OMEGA data.

Visible and near-infrared spectroscopic properties have been measured on elemental iron experimentally weathered in simulated Martian atmosphere and correlated to mineralogical compositions determined by X-ray diffraction. Two main features are observed in the reflectance spectra, corresponding to two deep bands located at 0.9 and 3.1 μm, respectively the iron band and the hydration band. In early weathering stages both Fe2+ and Fe3+ bands are identified. In addition, whereas the water band position does not change with time, the Fe2+ band disappears, and the Fe3+ band shifts towards longer wavelength (from 0.88 to 0.92 μm) because of transition from Fe2+ phases (siderite) to Fe3+ phases (ferrihydrite and goethite). Apart from these spectral signatures, other bands more specific of each phase are not clearly evidenced, especially for siderite. This is due to relatively low abundance of siderite (

A method has been developed to combine multiband imaging data with independent spot spectra. The method requires in input approximate spot sizes and locations within the image field, and the degrees of freedom on the spectra positioning and spot sizes. The algorithm determines the optimized solution among the overall set of possibilities. The method can be applied to both reflectance and scaled spectra. Sets of well-documented telescopic spectra have been used to produce refined calibration of Clementine UVVIS-NIR images on several lunar regions: Aristarchus, Apollo 17 and Reiner Gamma. The implementation of this methodology permits to reach a very high consistency between independent datasets of reference spectra and multispectral images, with absolute residuals less than 0.01 for each wavelength. In a broader use, it provides a systematic tool for assessing the level of maximal consistency between two independent observational datasets, taking into consideration both the instrumental and positioning uncertainties. It has immediate applications for intercalibration purposes, with the prospect of deriving an advanced determination of planetary regolith optical properties.

We have reduced Clementine near infrared (NIR) data in order to investigate the distribution of olivine in the crater Copernicus. Copernicus represents a good test area for the detection of olivine-rich areas with Clementine data as olivine was unambiguously detected in the central peaks from previous earth-based spectroscopic observations. Clementine provides a better spatial resolution ( ∼200 m / pixel ) than earth-based telescopic observations, which are limited by the atmospheric turbulence. This resolution is helpful to clarify the relationship between different lithologies and to detect heterogeneities at small scales. NIR filters at 1100, 1250, 1500, and 2000 nm are particularly useful to discriminate between olivine and pyroxene within the mafic minerals identified with Clementine UVVIS channels. Band ratios are presented to show the distribution of olivine-rich areas, which are detected mostly in the central peak complex. Three small areas in the northern walls are also consistent with olivine-rich materials, in agreement with previous studies. Nine-channels Clementine spectra confirm the overall heterogeneity of the crater site. This demonstrates the great potential of Clementine NIR data when integrated with UVVIS data for the detection of olivine dominated areas on the Moon.