Physical Properties of (2) Pallas

We acquired and analyzed adaptive-optics imaging observations of asteroid (2) Pallas from Keck II and the Very Large Telescope taken during four Pallas oppositions between 2003 and 2007, with spatial resolution spanning 32-88 km (image scales 13-20 km/pix). We improve our determination of the size, shape, and pole by a novel method that combines our AO data with 51 visual light-curves spanning 34 years of observations as well as occultation data. The shape model of Pallas derived here reproduces well both the projected shape of Pallas on the sky and light-curve behavior at all the epochs considered. We resolved the pole ambiguity and found the spin-vector coordinates to be within 5 deg. of [long, lat] = [30 deg., -16 deg.] in the ECJ2000.0 reference frame, indicating a high obliquity of ~84 deg., leading to high seasonal contrast. The best triaxial-ellipsoid fit returns radii of a=275 km, b= 258 km, and c= 238 km. From the mass of Pallas determined by gravitational perturbation on other minor bodies [(1.2 +/- 0.3) x 10-10 Solar Masses], we derive a density of 3.4 +/- 0.9 g.cm-3 significantly different from the density of C-type (1) Ceres of 2.2 +/- 0.1 g.cm-3. Considering the spectral similarities of Pallas and Ceres at visible and near-infrared wavelengths, this may point to fundamental differences in the interior composition or structure of these two bodies. We define a planetocentric longitude system for Pallas, following IAU guidelines. We also present the first albedo maps of Pallas covering ~80% of the surface in K-band. These maps reveal features with diameters in the 70-180 km range and an albedo contrast of about 6% wrt the mean surface albedo.Comment: 16 pages, 8 figures, 6 table

Identification of Ammonium Salts on Comet 67P/C-G Surface from Infrared VIRTIS/Rosetta Data Based on Laboratory Experiments. Implications and Perspectives

The nucleus of comet 67P/Churyumov-Gerasimenko exhibits a broad spectral reflectance feature around 3.2 $\mu$m, which is omnipresent in all spectra of the surface, and whose attribution has remained elusive since its discovery. Based on laboratory experiments, we have shown that most of this absorption feature is due to ammonium (NH4+) salts mixed with the dark surface material. The depth of the band is compatible with semi-volatile ammonium salts being a major reservoir of nitrogen in the comet, which could dominate over refractory organic matter and volatile species. These salts may thus represent the long-sought reservoir of nitrogen in comets, possibly bringing their nitrogen-to-carbon ratio in agreement with the solar value. Moreover, the reflectance spectra of several asteroids are compatible with the presence of NH4+ salts at their surfaces. The presence of such salts, and other NH4+-bearing compounds on asteroids, comets, and possibly in proto-stellar environments, suggests that NH4+ may be a tracer of the incorporation and transformation of nitrogen in ices, minerals and organics, at different phases of the formation of the Solar System

Analysing spectral signatures with multiscale methods

International audienceA new spectral analysis method based on wavelet decomposition and on a multiscale vision model is presented here. This method was developed to process reflectance spectra from planetary surfaces and to extract the relevant information from highly correlated data, where it only represents a small fraction of the overall variance. The outcomes of the analysis are a description of the bands detected, and a quantitative and reliable confidence parameter. The bands can be described either by the most appropriate wavelet scale only (for rapid analyses) or after reconstruction from all scales involved (for more precise measurements). An interesting side effect is the ability to separate even narrow features from random noise, as well as to identify low - frequency variations i.e., wide and shallow bands

Temperature and reflectance retrieval from NIR spectra

International audienceThe spectral range of NIR detectors has extended towards longer wavelengths in the recent years, and now currently encompasses the 3-5 mum region and the onset of the thermal emission of most Solar System bodies. This range contains absorptions from minerals, ices, and organic materials providing information on surface composition that is not available at shorter wavelengths. The spectral contrast is however greatly reduced because spectral features appear as absorptions in reflected light, and as peaks in emission. Study of the composition in this range therefore involves separating reflected from emitted light. Naive modeling using a constant spectral emissivity would not retrieve the original spectral contrast and precludes quantitative analyses. The procedure used here is to fit both the spectral reflectance and the temperature in one pass. We use a basic radiance model in which reflectance and emissivity are related through a photometric function, and a single temperature is used for each pixel. However, inversions of such models are known to be numerically instable. A regularization scheme is proposed here

Physical relevance of Independent Component Analysis of planetary radiance

International audienceResults of Virtis / Venux Express observations analysed through ICA are compared with an explicit physical modeling of the spectra performed to map surface temperature and its variation

ICA applied to imaging spectroscopy remote sensing

International audienceA comparison of Principal Components Analysis and Independent Components Analysis is made in the context of imaging spectroscopy of the Solar System bodies. Specific behaviors are outlined and explained, using examples from recent space-borne experiments and telescopic observations. ICA is in general a much more efficient tool to analyze spectral data cubes

International Data Archiving Collaborations

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Highlights of VIRTIS/Rosetta observations 
- 67P/Churyumov-Gerasimenko seen from orbit

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Physical relevance of Independent Component Analysis of planetary radiance

International audienceResults of Virtis / Venux Express observations analysed through ICA are compared with an explicit physical modeling of the spectra performed to map surface temperature and its variation