C. de Bergh

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.

The Color Distribution in the Edgeworth-Kuiper Belt*

In 1997 we began the Meudon Multicolor Survey of Outer Solar System Objects with the aim of collecting a large and homogeneous set of color data for trans-Neptunian objects (TNOs) and Centaurs. Here we present our latest B-V, V-R, and R-I color measurements obtained with the CFH12K mosaic camera of the 3.6 m Canada-France-Hawaii Telescope. With the colors of 30 objects reported in this work, we have a combined sample of 52 B-R color measurements for eight Centaurs, 22 classical TNOs, 13 Plutinos, eight scattered objects, and one object of unidentified dynamical class. This is the largest single and homogeneous data set published to date, and it is large enough to search for compositional structures, interrelations between dynamical classes of objects, and correlations with physical and orbital parameters. The color-color diagrams show that all the classes of objects share the same wide color diversity. No significant correlations are seen for the whole population of TNOs and Centaurs, or for individual subpopulations, except for the classical objects. Indeed, we found a significant and strong correlation of the colors of classical TNOs with inclination, eccentricity, and perihelion, but nothing with semimajor axis and absolute magnitude. Most of these results are common to previous works and do not seem to be due to sampling bias. Moreover, a strong correlation with mean excitation velocity [VK(e2 + i2)1/2] points toward a space weathering or impact origin for the color diversity. However, thorough modeling of the collisional/dynamical environment in the Edgeworth-Kuiper belt needs to be done in order to confirm this scenario. We found also that the classical TNOs are made up of a superposition of two distinct populations: the dynamically cold classical TNOs (red colors, low i, small sizes) and the dynamically hot classical TNOs (diverse colors, moderate and high i, larger sizes). Furthermore, the latter population displays a strong correlation between color and mean excitation velocity. The dynamically cold classical TNOs may be primordial, while the dynamically hot classical TNOs, whose surface colors may be the result of space weathering and/or impact processes, have possibly been injected from the inner regions of the disk. Our specific observation strategy to repeat color measurements with no rotation artifacts have permitted us to highlight a few objects suspected to have true compositional and/or texture variation on their surfaces. These TNOs are 1998 HK151, 1999 DF9, 1999 OY3, 2000 GP183, 2000 OK67, and 2001 KA77, and they should be prime targets for further observations in order to study and confirm the color variation with the rotation. Finally, our survey has also highlighted some peculiar objects, such as 1998 SN165, whose colors and dynamical properties put this object in a new dynamical class distinct from the classical TNOs, its previously assigned dynamical class.

WATER IN THE DEEP ATMOSPHERE OF VENUS FROM HIGH-RESOLUTION SPECTRA OF THE NIGHT SIDE

High-resolution, near-infrared (1.09 to 2.5 μm) spectra of the night side of Venus have been obtained in 1990 and 1991 using the Fourier Transform Spectrometer at the 3.6-m Canada-France-Hawaii telescope. Absorptions due to H2O were detected in spectral windows near 2.3, 1.74, and 1.18 μm. Our analysis of these absorptions constrains the abundance of water vapor in three different altitude ranges located between the clouds and the surface: 30–40 km, 15–25 km and 0–15 km. A constant water vapor mixing ratio of 30±15 ppm below the clouds can fit the observations. These values are consistent with recent near-infrared studies of the night side of Venus at lower spectral resolution. The atmosphere of Venus appears to be dryer than originally suggested by the in-situ measurements made by the Pioneer Venus and Venera mass-spectrometers and gas-chromatographs.

Decoding the Domino: The Dark Side of Iapetus

Abstract We present new spectra of the leading and trailing hemispheres of Iapetus from 2.4 to 3.8 μm. We have combined the leading hemisphere spectra with previous observations by others to construct a composite spectrum of the dark side (leading) hemisphere from 0.3 to 3.8 μm. We review attempts to deduce the composition of the dark material from previously available spectrophotometry. None of them (numbering more than 20 million!) leads to a synthetic spectrum that matches the new data. An intimate mixture of water ice, amorphous carbon, and a nitrogen-rich organic compound (modeled here as Triton tholin) can fit the entire composite dark side spectrum. Observations in this spectral region have not revealed this mix of material on any other object observed thus far. We propose that this dark material may have originated on Titan, where atmospheric photochemistry has been producing nitrogen-rich organic compounds for 4.5 GY.

Is sedna another triton

Sedna is, so far, the largest and most distant trans-neptunian object. It was observed at visible and near-infrared wavelengths using simultaneously two 8.2 m telescopes at the Very Large Telescope of the European Southern Observatory. The spectrum of Sedna suggests the presence on its surface of different ices (total abundance >50%). Its surface composition is different from that determined for other trans-neptunian objects, and apparently resembles that of Triton, particularly in terms of the possible presence of nitrogen and methane ices.

ESO Large Program on physical studies of Trans-Neptunian objects and Centaurs: Final results of the visible spectrophotometric observations

The Large Program on physical studies of TNOs and Centaurs, started at ESO Cerro Paranal on April 2001, has recently been concluded. This project was devoted to the investigation of the surface properties of these icy bodies through photometric and spectroscopic observations. In this paper we present the latest results on these pristine bodies obtained from the spectrophotometric investigation in the visible range. The newly obtained spectrophotometric data on 3 Centaurs and 5 TNOs, coming from 2 observing runs at the Very Large Telescope (VLT), show a large variety of spectral characteristics, comprising both gray and red objects in the two different populations. A very broad and weak absorption feature, centered around 7000 A , has been revealed in the spectrum of the gray TNO 2003 AZ84. This absorption is very similar to a feature observed on low albedo main belt asteroids and attributed to the action of the aqueous alteration process on minerals. This process was previously also claimed as the most plausible explanation for some peculiar visible absorption bands observed on 2000 EB173 and 2000 GN171 in the framework of the Large Program (Lazzarin et al. 2003; de Bergh et al. 2004). This detection seems to reinforce the hypothesis that aqueous alteration might have taken place also at such large heliocentric distances. We also report the results of a spectroscopic investigation performed outside the Large Program on the very interest- ing TNO 2000 GN171 during part of its rotational period. This object, previously observed twice in the framework of the Large Program, had shown during the early observations a very peculiar absorption band tentatively attributed to aqueous alteration processes. As this feature was not confirmed in a successive spectrum, we recently repeated the investigations of 2000 GN171, finding out that it has an heterogeneous composition. Finally an analysis of the visible spectral slopes is reported for all the data coming from the Large Program and those available in literature.

Monodeuterated methane in the outer solar system. II. Its detection on Uranus at 1. 6 microns

Deuterium in the atmosphere of Uranus has been studied only via measurements of the exceedingly weak dipole lines of hydrogen-deuteride (HD) seen in the visible region of the spectrum. The other sensitive indicator of deuterium in the outer solar system is monodeuterated methane (CH3D) but the two bands normally used ot study this molecule, NU sub 2 near 2200 1/cm and NU sub 6 near 1161 1/cm, have not been detected in Uranus.

ESO large program on physical studies of Transneptunian Objects and Centaurs: Visible photometry – First results

We present the first results of BVRI photometry of Transneptunian Objects (TNOs) and Centaurs obtained through the ESO Large Program on physical studies of these icy bodies in the outer solar system. In total 28 objects were observed of which 18 are new measurements. Combining our new BVRI photometry with the data summary published by Hainaut & Delsanti (2002) results in a database of 94 objects: 45 Cubewanos, 22 Plutinos, 13 scattered disk objects, 14 Centaurs. The reddening range seems to be similar among the four dynamical classes (− 5t o 55%/l00 nm) and only one outlier (1994 ES2) exists. The spectral gradient distribution of the Cubewanos peaks between 25 to 35%/l00 nm, while for the three other types the maximum seems to fall below 20%/l00 nm. A clustering of red Cubewanos with perihelia beyond ∼41 AU in low eccentricity and low inclination orbit suggests that these objects are less affected by the physical processes that potentially produce neutral colors, i.e. resurfacing by collision and by intrinsic activity. For Cubewanos and scattered disk objects, the range of reddening increases with decreasing perihelion distance and with increasing orbital excitation. A correlation of the spectral slope with inclination is present for Cubewanos and scattered disk objects, and is non-existent for the other dynamical types. It is unclear whether these trends (or their absence) are discriminative for the correctness of the resurfacing scenarios. If intrinsic activity is responsible for resurfacing, the start of the effect inside ∼41 AU from the Sun may be indicative for the driving agent, while in the collision scenario the survival of the red Cubewano cluster in the central region of the Kuiper-Belt argues for the existence of a population of bodies the surface of which is heavily radiation processed without impact resurfacing.

Colors and taxonomy of Centaurs and trans-Neptunian objects

Context. The study of the surface properties of Centaurs and trans-Neptunian objects (TNOs) provides essential information about the early conditions and evolution of the outer Solar System. Due to the faintness of most of these distant and icy bodies, photometry currently constitutes the best technique to survey a statistically significant number of them. Aims. Our aim is to investigate color properties of a large sample of minor bodies of the outer Solar System, and set their taxonomic classification. Methods. We carried out visible and near-infrared photometry of Centaurs and TNOs, making use, respectively, of the FORS2 and ISAAC instruments at the Very Large Telescope (European Southern Observatory). Using G-mode analysis, we derived taxonomic classifications according to the Barucci et al. (2005a, AJ, 130, 1291) system. Results. We report photometric observations of 31 objects, 10 of them have their colors reported for the first time ever. 28 Centaurs and TNOs have been assigned to a taxon. Conclusions. We combined the entire sample of 38 objects taxonomically classified in the framework of our programme (28 objects from this work; 10 objects from DeMeo et al. 2009a, A&A, 493, 283) with previously classified TNOs and Centaurs, looking for correlations between taxonomy and dynamics. We compared our photometric results to literature data, finding hints of heterogeneity for the surfaces of 4 objects.