Sylvain Douté

Rain, winds and haze during the Huygens probe's descent to Titan's surface

The irreversible conversion of methane into higher hydrocarbons in Titans stratosphere implies a surface or subsurface methane reservoir. Recent measurements from the cameras aboard the Cassini orbiter fail to see a global reservoir, but the methane and smog in Titans atmosphere impedes the search for hydrocarbons on the surface. Here we report spectra and high-resolution images obtained by the Huygens Probe Descent Imager/Spectral Radiometer instrument in Titans atmosphere. Although these images do not show liquid hydrocarbon pools on the surface, they do reveal the traces of once flowing liquid. Surprisingly like Earth, the brighter highland regions show complex systems draining into flat, dark lowlands. Images taken after landing are of a dry riverbed. The infrared reflectance spectrum measured for the surface is unlike any other in the Solar System; there is a red slope in the optical range that is consistent with an organic material such as tholins, and absorption from water ice is seen. However, a blue slope in the near-infrared suggests another, unknown constituent. The number density of haze particles increases by a factor of just a few from an altitude of 150 km to the surface, with no clear space below the tropopause. The methane relative humidity near the surface is 50 per cent.

Evidence for Methane Segregation at the Surface of Pluto

Abstract In May 1995, a set of spectrophotometric curves of the system Pluto–Charon were recorded with the UKIRT telescope equipped with the spectrometer CGS4. The spectra cover the near-infrared range between 1.4 and 2.55 μm with a resolution of approximately 700. The existence of solid methane is confirmed by numerous absorption bands, and carbon monoxide and nitrogen ices are identified by their respective signatures at 2.35 and 2.15 μm. We have modeled the spectrum of May 15 that corresponds to the maximum of Plutos visible lightcurve using a radiative transfer algorithm dealing with compact and stratified media. A geographical mixture of three distinct units is required to explain all the significant structures of the analyzed spectrum. The first unit is a thin, fine-grained layer of pure CH4 covering a compact polycrystalline substratum of N2–CH4–CO, which are in a molecular mixture (concentrations of CH4 and CO of the order of 0.5 and 0.1–0.2% respectively). It covers about 70% of the observed area and corresponds to volatile deposits that are sublimating under solar illumination. The second unit is either (a) a single thick layer of pure large-grained methane or (b) a unit with large-grained CH4 forming a substratum and the N2–CH4–CO mixture a superficial layer of fine grains covering 20% of the surface. Finally, the third unit is bright and spectrally neutral and is first modeled as a layer of very fine grains of nearly pure N2. Tholin, suggested to explain the red slope in the visible, is also found to be spectrally compatible with this unit. It covers the remainder of the surface (about 10–15%). All these results allow a better understanding of the processes of deposition, metamorphism, sublimation, and transport affecting the different ices detected on Pluto during its climatic cycles.

A multilayer bidirectional reflectance model for the analysis of planetary surface hyperspectral images at visible and near‐infrared wavelengths

We present a practical, timely, and effective radiative transfer algorithm, suitable for qualitative and quantitative analyses of high-resolution hyperspectral images of planetary surfaces in the visible and near-infrared domains. The bidirectional reflectance of a plane parallel, absorbing, scattering, and slightly stratified medium is generated. The local mean properties of scattering and absorption of such media are obtained apart, using semiempirical approaches. The functions which express the diffuse reflection and transmission behaviors of each homogeneous layer are then derived. For the multiple scattering term, we numerically resolve the equations appearing in the H, X and Y function method of radiative transfer, reducing the real phase function to a simplified one which can nevertheless be anisotropic. A better approach to the physical realism is obtained for the single and double scattering contributions, using their real analytical expressions. This contrasts with the Hapke model dedicated to homogeneous and semi-infinite media, where only an isotropic reduced phase function is adopted and the single scattering correction is applied. The bidirectional reflectance and the derived quantities (albedos) of an optically semi-infinite homogeneous medium are then easily derived from these quantities. For a stratified medium, a simple adding algorithm based on principles of invariance is presented. Compared to earlier and more complete theoretical developments, this model in most cases reproduces the dependence of the bidirectional reflectance according to the different geometrical and radiative parameters with a maximum of 10% relative error. It leads to important gains of computation time and significantly extends the validity of Hapkes or similar practical approaches.

Winter and spring evolution of northern seasonal deposits on Mars from OMEGA on Mars Express

The OMEGA visible/near-infrared imaging spectrometer on Mars Express has observed the retreat of the northern seasonal deposits during Martian year 27-28 from the period of maximum extension, close to the northern winter solstice, to the end of the retreat at L s 95°. We present the temporal and spatial distributions of both CO 2 and H 2O ices and propose a scenario that describes the winter and spring evolution of the northern seasonal deposits. During winter, the CO 2-rich condensates are initially transparent and could be in slab form. A water ice annulus surrounds the sublimating CO 2 ice, extending over 6° of latitude at L s 320°, decreasing to 2° at L s 350°, and gradually increasing to 4.5° at L s 50°. This annulus first consists of thin frost as observed by the Viking Lander 2 and is then overlaid by H 2O grains trapped in the CO 2-rich ice layer and released during CO 2 sublimation. By L s 50, H 2O ice spectrally dominates most of the deposits. In order to hide the still several tens of centimeters thick CO 2 ice layer in central areas of the cap we propose the buildup of an optically thick top layer of H 2O ice from ice grains previously embedded in the CO 2 ice and by cold trapping of water vapor from the sublimating water ice annulus. The CO 2 ice signature locally reappears between L s 50 and 70. What emerges from our observations is a very active surface-atmosphere water cycle. These data provide additional constraints to the general circulation models simulating the Martian climate. Copyright 2011 by the American Geophysical Union.

No signature of clear CO2 ice from the 'cryptic' regions in Mars' south seasonal polar cap

The seasonal polar ice caps of Mars are composed mainly of CO2 ice. A region of low (< 30%) albedo has been observed within the south seasonal cap during early to mid-spring. The low temperature of this ‘cryptic region’ has been attributed to a clear slab of nearly pure CO2 ice, with the low albedo resulting from absorption by the underlying surface. Here we report near-infrared imaging spectroscopy of the south seasonal cap. The deep and broad CO2 absorption bands that are expected in the near-infrared with a thick transparent slab of CO2 ice are not observed. Models of the observed spectra indicate that the low albedo results from extensive dust contamination close to the surface of a CO2 ice layer, which could be linked to atmospheric circulation patterns. The strength of the CO2 absorption increases after mid-spring, so part of the dust is either carried away or buried more deeply in the ice layer during the CO2 ice sublimation process.

Spectrogonio radiometer for the study of the bidirectional reflectance and polarization functions of planetary surfaces. 1. Design and tests

We have developed a spectrogonio radiometer to measure in the laboratory (-35 degrees C to +30 degrees C) the bidirectional reflectance and polarization distribution functions of various types of planetary material from the UV to the near-IR (310-4800 nm). The major, to our knowledge, novel feature of this instrument is that it is capable of measuring dark to translucent materials with a high degree of radiometric accuracy under most viewing geometries. The sample surface is illuminated with a large monochromatic and polarized parallel beam (incidence: 0 degrees-90 degrees ), and the total intensity and the two polarized components of the reflected light are measured (observation, 0 degrees-80 degrees; azimuth, 0 degrees-180 degrees). The scientific and technical constraints, the design, and the performances and limitations of the system are presented in this first paper.

Empirical Automatic Estimation of the Number of Endmembers in Hyperspectral Images

In this letter, an eigenvalue-based empirical method is proposed in order to estimate the number of endmembers in hyperspectral data. This method is based on the distribution of the differences of the eigenvalues from the correlation and the covariance matrices, respectively. The eigenvalues corresponding to the noise are identical in the covariance and the correlation matrices, while the eigenvalues corresponding to the signal (the endmembers) are larger in the correlation matrix than in the covariance matrix. The proposed method is totally parameter free and very fast. It is validated by experiments carried on both synthetic and real data sets.

Retrieval of Mars surface physical properties from OMEGA hyperspectral images using Regularized Sliced Inverse Regression

Hyperspectral remote sensing, also known as imaging spectroscopy, is a promising space technology regularly selected by agencies with regard to the exploration and observation of planets, to earths geology or to the monitoring of the environment. It allows to collect for each pixel of a scene, the intensity of light energy reflected from planets as it varies across different wavelengths. More than one hundred spectels in the visible and near infra-red are typically recorded, making it possible to observe a continuous spectrum for each image cell. Usually, in space exploration, the analysis of these spectral signatures allows to retrieve the physical, chemical or mineralogical properties of surfaces and of atmospheres that may help to understand the geological and climatological history of planets. We propose in this paper a statistical method to evaluate the physical properties of surface materials on Mars from hyperspectral images collected by the OMEGA instrument aboard the Mars express spacecraft. The approach we develop is based on the estimation of the functional relationship F between some physical parameters and observed spectra. For this purpose, a database of synthetic spectra is generated by a physical radiative transfer model and used to estimate F. The high dimension of spectra is reduced by using Gaussian regularized sliced inverse regression (GRSIR) to overcome the curse of dimensionality and consequently the sensitivity of the inversion to noise (ill-conditioned problems). Compared with a naive spectrum matching approach such as the k-nearest neighbors algorithm, estimates are more accurate and realistic.

A Monte Carlo ray‐tracing model for scattering and polarization by large particles with complex shapes

We present a new model based on Monte Carlo ray tracing which simulates scattering and linear polarization by particles with arbitrary shapes and sizes much larger than the wavelength. The model, called S-Scat, provides a powerful tool for exploring the relationship between actual physical properties of large particles and the single-particle parameters used in multiple-scattering radiative transfer models, and will be particularly valuable for studies of icy outer solar system surfaces. We describe the model algorithm and apply the model to examine absorption and scattering behavior of single, irregular particles as functions of particle size, shape, and optical constants. Single-scattering albedos are investigated first, with results used to test the validity of the widely used equivalent slab model. Single-scattering phase functions are examined next, along with possibilities of parameterization via simple analytic expressions. Finally, the behaviors of linear polarization functions are explored, along with internal path lengths and the spatial distribution of scattered light in close proximity to the particle.

Galileo imaging of SO2 frosts on Io

Ios visible appearance changes dramatically with solar phase angle. The polar regions and some plume deposits near active volcanic centers become comparatively bright with increasing phase angle, while the equatorial band grows relatively dark. We suggest that the areas of Io that appear unusually bright at high phase are covered by thin frosts of SO2 that are transparent under normal illumination. A global disk-resolved photometric analysis indicates that the frosts exhibit more nearly isotropic or forwardscattering behavior and less opposition brightening than average Ionian materials. Comparison with Near-Infrared Mapping Spectrometer (NIMS) results suggests that these frosts have relatively strong 4.1 μm absorptions indicative of fine-grained SO2.