Characterization and mapping of surface physical properties of Mars from CRISM multi-angular data: application to Gusev Crater and Meridiani Planum

The analysis of the surface texture from the particle (grain size, shape and internal structure) to its organization (surface roughness) provides information on the geological processes. CRISM multi-angular observations (varied emission angles) allow to characterize the surface scattering behavior which depends on the composition but also the material physical properties (e.g., grain size, shape, internal structure, the surface roughness). After an atmospheric correction by the Multi-angle Approach for Retrieval of the Surface Reflectance from CRISM Observations, the surface reflectances at different geometries are analyzed by inverting the Hapke photometric model depending on the single scattering albedo, the 2-term phase function, the macroscopic roughness and the 2-term opposition effects. Surface photometric maps are created to observe the spatial variations of surface scattering properties as a function of geological units at the CRISM spatial resolution (200m/pixel). An application at the Mars Exploration Rover (MER) landing sites located at Gusev Crater and Meridiani Planum where orbital and in situ observations are available, is presented. Complementary orbital observations (e.g. CRISM spectra, THermal EMission Imaging System, High Resolution Imaging Science Experiment images) are used for interpreting the estimated Hapke photometric parameters in terms of physical properties. The in situ observations are used as ground truth to validate the interpretations. Varied scattering properties are observed inside a CRISM observation (5x10km) suggesting that the surfaces are controlled by local geological processes (e.g. volcanic resurfacing, aeolian and impact processes) rather than regional or global. Consistent results with the in situ observations are observed thus validating the approach and the use of photometry for the characterization of Martian surface physical properties

Surface reflectance of Mars observed by CRISM/MRO: 2. Estimation of surface photometric properties in Gusev Crater and Meridiani Planum

The present article proposes an approach to analyze the photometric properties of the surface materials from multi-angle observations acquired by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) on-board the Mars Reconnaissance Orbiter. We estimate photometric parameters using Hapke model in a Bayesian inversion framework. This work also represents a validation of the atmospheric correction provided by the Multi-angle Approach for Retrieval of Surface Reflectance from CRISM Observations (MARS-ReCO) proposed in the companion article.The latter algorithm retrieves photometric curves of surface materials in reflectance units after removing the aerosol contribution. This validation is done by comparing the estimated photometric parameters to those obtained from in situ measurements by Panoramic Camera instrument at the Mars Exploration Rover (MER)-Spirit and MER-Opportunity landing sites. Consistent photometric parameters with those from in situ measurements are found, demonstrating that MARS-ReCO gives access to accurate surface reflectance. Moreover the assumption of a non-Lambertian surface as included in MARS-ReCO is shown to be significantly more precise to estimate surface photometric properties from space in comparison to methods based on a Lambertian surface assumption. In the future, the presented method will allow us to map from orbit the surface bidirectional reflectance and the related photometric parameters in order to characterize the Martian surface

ICARE-VEG: A 3D physics-based atmospheric correction method for tree shadows in urban areas

International audienceMany applications dedicated to urban areas (e.g. land cover mapping and biophysical properties estimation) using high spatial resolution remote sensing images require the use of 3D atmospheric correction methods, able to model complex light interactions within urban topography such as buildings and trees. Currently, one major drawback of these methods is their lack in modelling the radiative signature of trees (e.g. the light transmitted through the tree crown), which leads to an over-estimation of ground reflectance at tree shadows. No study has been carried out to take into account both optical and structural properties of trees in the correction provided by these methods. The aim of this work is to improve an existing 3D atmospheric correction method, ICARE (Inversion Code for urban Areas Reflectance Extraction), to account for trees in its new version, ICARE-VEG (ICARE with VEGetation). After the execution of ICARE, the methodology of ICARE-VEG consists in tree crown delineation and tree shadow detection, and then the application of a physics-based correction factor in order to perform a tree-specific local correction for each pixel in tree shadow. A sensitivity analysis with a design of experiments performed with a 3D canopy radiative transfer code, DART (Discrete Anisotropic Radiative Transfer), results in fixing the two most critical variables contributing to the impact of an isolated tree crown on the radiative energy budget at tree shadow: the solar zenith angle and the tree leaf area index (LAI). Thus, the approach to determine the correction factor relies on an empirical statistical regression and the addition of a geometric scaling factor to account for the tree crown occultation from ground. ICARE-VEG and ICARE performance were compared and validated in the Visible-Near Infrared Region (V-NIR: 0.4-1.0µm) with hyperspectral airborne data at 0.8m resolution on three ground materials types, grass, asphalt and water. Results show that (i) ICARE-VEG improves the mean absolute error in retrieved reflectances compared to ICARE in tree shadows by a multiplicative factor ranging between 4.2 and 18.8, and (ii) reduces the spectral bias in reflectance from visible to NIR (due to light transmission through the tree crown) by a multiplicative factor between 1.0 and 1.4 in terms of spectral angle mapper performance. ICARE-VEG opens the way to a complete interpretation of remote sensing images (sunlit, shade cast by both buildings and trees) and the derivation of scientific value-added products over all the entire image without the preliminary step of shadow masking.De nombreuses applications dans les zones urbaines (par exemple la cartographie de la couverture terrestre et l'estimation des propriétés biophysiques) utilisant des images à haute résolution spatiale et à télédétection nécessitent l'utilisation de méthodes de correction atmosphérique 3D. Actuellement, un inconvénient majeur de ces méthodes est leur manque de modélisation de la signature rayonnante des arbres, ce qui conduit à une surestimation de la réflectance au sol à l'ombre des arbres. Aucune étude n'a été faite pour prendre en compte ces propriétés optiques et structurelles des arbres dans la correction apportée par ces méthodes. L'ICARE (code d'inversion pour l'extraction de réflectance des zones urbaines), ICARE-VEG (ICARE avec VEGetation). Après l'exécution d'ICARE, la méthodologie d'ICARE-VEG consiste en un arbre et un arbre, puis l'application d'un facteur de correction basé sur la physique dans une correction locale spécifique à l'arbre pour chaque pixel dans l'ombre de l'arbre. DART (Discrete Anisotropic Radiative Transfer), DART (Transfert Radiatif Anisotrope Discret), permet de déterminer les deux variables les plus importantes contribuant à l'impact d'un arbre isolé sur le bilan énergétique radiatif à l'ombre des arbres: l'angle zénithal solaire et la surface foliaire index (LAI). Ainsi, l'approche du facteur de correction est basée sur une régression statistique empirique et l'ajout d'un facteur d'échelle géométrique au compte de la dissimulation de l'arbre à partir du sol. Les performances ICARE-VEG et ICARE ont été comparées et validées dans la région infrarouge proche visible (V-NIR: 0.4-1.0μm) avec des données aéroportées hyperspectrales à une résolution de 0.8m sur trois types: gazon, asphalte et eau. Les résultats montrent que (i) ICARE-VEG améliore l'erreur absolue moyenne dans les réflectances récupérées par rapport à ICARE dans les ombres portées par un facteur multiplicatif compris entre 4,2 et 18,8, et (ii) réduit le biais spectral de réflectance du visible au NIR (dû à ICARE-VEG ouvre la voie à une interprétation complète des images de télédétection et à la dérivation de produits scientifiques à valeur ajoutée de partout dans le monde, grâce à un facteur multiplicatif compris entre 1,0 et 1,4 en termes de performance de la carte de l'angle spectral. ICARE-VEG ouvre la voie à une interprétation complète des images de télédétection et à la dérivation de produits scientifiques à valeur ajoutée de partout dans le monde