¹C.Pilorget,²J.Fernando,³L.Riu,⁴K.Kitazato,^3,5T.Iwata
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.114126]
¹Institut d’Astrophysique Spatiale, CNRS/Université Paris-Saclay, UMR8617, Orsay 91405, France
²Independent scholar, Orsay 91400, France
³Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara 252-5210, Japan
⁴The University of Aizu, Fukushima, Japan
⁵Department of Space and Astronautical Science, The Graduate University for Advanced Studies, Sokendai, Hayama 240-0193, Japan
Copyright Elsevier

The type C asteroid Ryugu has been visited by the JAXA Hayabusa2 mission, a first for a primitive asteroid in the Solar System exploration. Samples have been collected at its surface by the spacecraft and are now on their way back to Earth. The surface of Ryugu has also been observed both in the visible and infrared wavelengths with the objective of characterizing its properties at a global scale. Here, we report on the albedo and spectro-photometric properties of Ryugu as derived from the NIRS3 IR spectrometer. Thanks to an innovative technique based on a Bayesian approach, we retrieved the Hapke photometric parameters from multi-geometric NIRS3 observations over the NIR spectral range and different areas of Ryugu, including the first collection site. Results reveal an overall spatial homogeneity of the photometric parameters, though some small variations can be highlighted. A new photometric correction was derived and applied to NIRS3 data. Reflectance spatial heterogeneity was quantitatively investigated, in particular by deriving and mapping the single scattering albedo (SSA) at various spatial scales. This parameter could be derived with confidence on about one fourth of Ryugu’s surface, especially around the equatorial region and the southern middle latitudes. Although Ryugu is to first order homogenous with a typical SSA of 0.045–0.050, we demonstrate with a local-scale photometric correction (1) the presence of a large “bright” area between E and E longitude around the equator and the southern middle latitudes, and (2) the presence of darker areas with a clear connection to geomorphological features. We show here that these darker regions tend to have a slightly deeper feature, at least compared to the surrounding areas, which can be explained by an enrichment of the top-surface in dark fines coupled to hydrated phases. Some spatial variability observed in the coupling between the SSA at and the feature also suggests that Ryugu exhibits some (slight) heterogeneity in its building blocks.

¹Arun M.Saranathan,¹Mario Parente
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.114107]
¹Department of Electrical and Computer Engineering, University of Massachusetts, Amherst, United States of America
Copyright Elsevier

The Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) has proven instrumental in the mineralogical analysis of the Martian surface. An essential tool for mineral identification for this dataset has been the CRISM summary parameters–which use simple mathematical functions to measure the presence/absence of specific spectral features. While the CRISM summary parameters and browse products (combinations of specific summary parameters) have proven valuable in guiding manual analysis, these hand-crafted representations are not well suited for automated analysis, as CRISM spectral artifacts and noise negatively affect their performance, making these parameters prone to false alarms. We propose an unsupervised technique based on Generative Adversarial Networks (GANs) to learn a more discriminative representation from CRISM data, such that simple metrics in the representation space are sufficient to discriminate between the various mineral spectra present in the data. We describe a simple pipeline using GAN based representations to map mineral signatures of interest across the CRISM image database. We show that the features learned by the GAN are better suited to discriminate mineral signatures in the CRISM database compared to the summary parameters and classical similarity metrics. Finally, we validate the technique over a subset of CRISM images over the Jezero crater and NE Syrtis regions of Mars.

¹Jemma Davidson,²Meenakshi Wadhwa,²Richard L.Hervig,^1,2AliceStephant
Earth and Planetary Science Letters 552, 116597 Link to Article [https://doi.org/10.1016/j.epsl.2020.116597]
¹Center for Meteorite Studies, Arizona State University, 781 East Terrace Road, Tempe, AZ 85287-6004, USA
²School of Earth and Space Exploration, Arizona State University, 781 East Terrace Road, Tempe, AZ 85287-6004, USA
Copyright Elsevier

Determining the source of planetary water from the hydrogen isotope compositions of crustal samples is complicated by the overprinting of isotopically diverse source material by geologic and atmospheric processes. As Mars has no plate tectonics, crustal material, which may have isotopically exchanged with the martian atmosphere, is not recycled into the mantle keeping the water reservoirs in the mantle and atmosphere mostly isolated, buffered by the crust. As the only known martian samples that are regolith breccias with a composition representative of the average crust of Mars, Northwest Africa (NWA) 7034 and its paired stones provide an important opportunity to investigate the water content and hydrogen isotope composition of the martian crust. In particular, apatites in distinct clasts as well as the brecciated matrix of NWA 7034 record a complex history including magmatic and impact processes, and exchange with crustal fluids.