Harold Clenet

Dikes of distinct composition intruded into Noachian‐aged crust exposed in the walls of Valles Marineris

[1]xa0Valles Marineris represents the deepest natural incision in the Martian upper crust. Previous studies showed that the upper parts of the walls were made of finely layered probable basalts in most of the chasmata, while the base of the stratigraphy reveals primary Noachian crustal blocks. Exposures of pristine Noachian bedrock are rare on Mars, and mostly observed outside of their geological context. The occurrence of well-preserved and extended outcrops of pristine material in this giant rift could bring valuable information on the early processes that took place at the surface of Mars. Analyses of high resolution data over the best exposures of lower walls in Coprates Chasma, central Valles Marineris, revealed the presence of multiple magmatic intrusions interpreted as dikes. These dikes intrude an old, massive, fractured bedrock interpreted as being preserved ancient Noachian crust. Their composition, determined using CRISM data, and distribution, limited to this ancient crust at the bottom walls, indicate that they might have formed early in the rift formation, and therefore represent exceptionally well-preserved outcrops of the early history of Mars.

A deep crust-mantle boundary in the asteroid 4 Vesta

The asteroid 4xa0Vesta was recently found to have two large impact craters near its south pole, exposing subsurface material. Modelling suggested that surface material in the northern hemisphere of Vesta came from a depth of about 20xa0kilometres, whereas the exposed southern material comes from a depth of 60 to 100xa0kilometres. Large amounts of olivine from the mantle were not seen, suggesting that the outer 100xa0kilometres or so is mainly igneous crust. Here we analyse the data on Vesta and conclude that the crust–mantle boundary (or Moho) is deeper than 80xa0kilometres.

Geological mapping strategy using visible near-infrared–shortwave infrared hyperspectral remote sensing: Application to the Oman ophiolite (Sumail Massif)

[1]xa0An airborne hyperspectral survey of the Oman ophiolite (Sumail Massif) has been conducted using the HyMap airborne imaging spectrometer with associated field measurements (GER 3700). An ASD FieldSpec3 spectrometer was also used in order to constrain the spectral signatures of the principal lithologies cropping out in the surveyed area. Our objective was to identify and map the various igneous lithologies by a direct comparison at high spectral resolution between field and airborne spectra despite strong variations in outcropping conditions such as (1) lighting, (2) surface roughness geometry, (3) blocks coated with red/brown patina and exfoliation products, or (4) deep hydrothermal weathering. On the basis of spectral signatures, we are able to distinguish three end-members of olivine-orthopyroxene bearing assemblages in the mantle sequence: (1) harzburgites, (2) dunites, and (3) a harzburgite with interstitial carbonate. Because plagioclase is spectrally featureless in the wavelength range studied it cannot be detected. In the crustal sequence, we therefore identified four end-members with variable abundance of clinopyroxene: (1) massive gabbros, (2) amphibolized (upper) gabbros associated with intrusive dykes, (3) wehrlite with high serpentine content, and (4) gabbronorite (a lithology not previously recognized in the studied area). With the exception of wehrlite, spectra of olivine-rich end-members display characteristic Mg-OH narrow absorption features caused by their high serpentine content. We take advantage of this observation to split the data into two subsets, corresponding to the mantle and crustal sequences, respectively. Pixels of an image often correspond to heterogeneous areas in the field and a direct comparison between airborne and in situ spectra is not straightforward. However, comparing spectra of pixels associated with the most homogeneous areas in the field with the spectra acquired in situ at the same location, we found a systematic change both in mean intensity and overall spectral shape. Dividing each spectrum by its low-pass trend removes the effects caused by surface light scattering associated with each scale of analysis and results in an exceptional match between field and airborne spectra. However, the albedo information is lost and as a consequence, rock types only characterized by albedo change cannot be discriminated. A spectrum of a mixture of powdered minerals is usually seen as a linear combination of mineral spectra proportional to their abundance. However, this is no longer the case when minerals occur in complex arrangements in rock types. We thus develop a synthetic spectral library of all possible combinations of rock types covering the surface area of a pixel and use a simple distance calculation to identify the best match between each pixel and modeled spectra. This procedure allows the determination of the fractional cover of each rock type in a given pixel and to establish maps for each spectral end-member. The final product is a geological map, derived from the combination of end-member fractional cover maps, and is broadly consistent with the existing geological maps. Beyond this general agreement which demonstrates the potential of this new approach for geological mapping, imaging spectrometry allows (1) to map in detail the outline of the Moho north of Maqsad and (2) to identify a new crustal sequence enriched in silica south of Muqzah, revealing the presence of orthopyroxene, the nature and distribution of which are of relevance to the petrological and tectonic understanding of the Oman ophiolite evolution.

Joint Anomaly Detection and Spectral Unmixing for Planetary Hyperspectral Images

Hyperspectral (HS) images are commonly used in the context of planetary exploration, particularly for the analysis of the composition of planets. As several instruments have been sent throughout the Solar System, a huge quantity of data is getting available for the research community. Among classical problems in the analysis of HS images, a crucial one is unsupervised nonlinear spectral unmixing, which aims at estimating the spectral signatures of elementary materials and determining their relative contribution at a subpixel level. While the unmixing problem is well studied for Earth observation, some of the traditional problems encountered with Earth images are somehow magnified in planetary exploration. Among them, large image sizes, strong nonlinearities in the mixing (often different from those found in the Earth images), and the presence of anomalies are usually impairing the unmixing algorithms. This paper presents a new method that scales favorably with the problem posed by this analysis. It performs an unsupervised unmixing jointly with anomaly-detection capacities and has a global linear complexity. Nonlinearities are handled by decomposing the HS data on an overcomplete set of spectra, combined with a specific sparse projection, which guarantees the interpretability of the analysis. A theoretical study is proposed on synthetic data sets, and results are presented over the challenging 4-Vesta asteroid data set.

Identification of mafic minerals on Mars by nonlinear hyperspectral unmixing

Typically, quantitative interpretation of Mars mineralogy from spectra can be retrieved by analyzing the overlaps of absorption features. It is possible to achieve a thorough description of the abundances of each mineral the considered scene is composed of by applying proper deconvolution techniques such as those based on modified Gaussian model (MGM). However, MGM-based methods are sensitive on initial parameters for statistical distribution definition, or they are very time consuming when fully automatized. In this paper, a new method for identification of minerals on Mars surface by means of higher order nonlinear hyperspectral unmixing framework is introduced. Abundance distribution of magmatic minerals (olivine and pyroxenes) compounds is retrieved according to polytope decomposition algorithm. Experimental results show how the proposed method is able to provide actual abundance maps which are highly correlated to those obtained by an automatized MGM-based technique.

The adapted Modified Gaussian Model: A tool to characterize the composition of magmatic rocks on terrestrial ‘planets’

Here we present an adapted version of the Modified Gaussian Model (MGM), a tool developed to detect and characterize both simple and complex mineralogies in magmatic rocks, and we review the results we obtained over the last years on four different planetary bodies. We first describe the working principle of the adapted MGM technique, with the differences compared to the original version, and the validation process using various laboratory and natural spectra. We then describe the results obtained on the Oman ophiolite (Earth), taken as a planetary spectral analog for magmatic terrains. We finally present spectroscopy-based mineralogical maps and some petrologic implications for key regions of Mars, the Moon and asteroid (4) Vesta.

Data reduction of CRISM data to highlight alteration minerals

CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) hyperspectral data have a spatial resolution ranging from 12 to 36m/pixel allowing the high resolution mapping of minerals at the surface of Mars. However, the signal-to-noise ratio (SNR) makes challenging the discrimination of minerals spectrally close such as certain phyllosilicates and carbonates. Here, we discuss different processing of data reduction used to improve the signal-to-noise ratio and to highlight the alteration minerals at the surface of Mars and their limit. We show that our tool allows to understand trends in global mineralogy present in hyperspectral data cube.