M. Combes

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.

Release of volatiles from a possible cryovolcano from near-infrared imaging of Titan

Titan is the only satellite in our Solar System with a dense atmosphere. The surface pressure is 1.5 bar (ref. 1) and, similar to the Earth, N2 is the main component of the atmosphere. Methane is the second most important component, but it is photodissociated on a timescale of 107 years (ref. 3). This short timescale has led to the suggestion that Titan may possess a surface or subsurface reservoir of hydrocarbons to replenish the atmosphere. Here we report near-infrared images of Titan obtained on 26 October 2004 by the Cassini spacecraft. The images show that a widespread methane ocean does not exist; subtle albedo variations instead suggest topographical variations, as would be expected for a more solid (perhaps icy) surface. We also find a circular structure ∼30 km in diameter that does not resemble any features seen on other icy satellites. We propose that the structure is a dome formed by upwelling icy plumes that release methane into Titans atmosphere.

An estimate of the PH3, CH3D, and GeH4 Abundances on Jupiter from the Voyager IRIS data at 4.5 μm

Abstract The abundances of PH 3 , CH 3 D, and GeH 4 are derived from the 2100- to 2250-cm −1 region of the Voyager 1 IRIS spectra. No evidence is seen for large-scale variations of the phosphine abundance over Jovian latitudes between −30 and +30°. In the atmospheric regions corresponding to 170–200°K, the derived PH 3 /H 2 value is (4.5 ± 1.5) × 10 −7 or 0.75 ± 0.25 times the solar value. This result, compared with other PH 3 determinations at 10 μm, suggests than the PH 3 /H 2 ratio on Jupiter decreases with atmospheric pressure. In the 200–250°K region, we derive, within a factor of 2, CH 3 D/H 2 and GeH 4 /H 2 ratios of 2.0 × 10 −7 and 1.0 × 10 −9 , respectively. Assuming a C/H value of 1.0 × 10 −3 , as derived from Voyager, our CH 3 D/H 2 ratio implies a D/H ratio of 1.8 × 10 −5 , in reasonable agreement with the interstellar medium value.

The Jovian stratosphere in the ultraviolet.

The center-of-disk reflectivity of Jupiter in the wavelength range from 1450 to 3150 angstroms has been computed from 30 low-dispersion IUE spectra taken during solar maximum in 1978-1980. A vertically inhomogeneous radiative transfer program is used to compute model reflectivities of various stratospheric compositions for comparison. Ammonia and acetylene are well determined because they show narrow absorption bands in the ultraviolet. Above 1800 angstroms, these two gases provide a good fit to the data, but not below. At shorter wavelengths the fit would be much improved by a small amount (0.5-1.5 ppb) of propadiene/allene (C3H4). Voyager IRIS spectra show that the IR bands of allene are not strong enough to be detected in such a small amount. Additional absorption around 1600 angstroms can be reproduced best with the presence of cyclopropane (C3H6, <15 ppb), although other absorbers (e.g., hydrocarbon molecules with more than three carbon atoms, oxygen- or nitrogen-containing molecules, or a high-altitude haze) could also explain the spectrum in this region. The data are too noisy to detect possible CO Cameron band absorption near 2000 angstroms.

Monitoring atmospheric phenomena on Titan

For the past 8 years (1998-2005), we have used adaptive optics imaging (with VLT/NACO and CFHT/PUEO) to explore Titans atmosphere, which is currently scrutinized in situ by the Cassini-Huygens mission. In the course of our work, we have found variations, such as as seasonal and diurnal effects, as well as temporary features in the southern polar region. The north-south asymmetry is shown to have changed since 2000 in the near-IR and to be currently organized in a brighter northern than southern pole. We study this evolution here. With our data, we also have new significant statistical evidence of diurnal effects in Titans stratosphere, with a brighter (as much as 19%) morning limb appearing in our images in many cases, when the phase effect is expected on the evening side. The southern bright feature is probably a time-limited seasonal and/or meteorological phenomenon, revolving around the south pole (confined in its motion within the 80

VLT/NACO adaptive optics imaging of Titan ?

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A determination of the composition of the Saturnian stratosphere using the IUE

S parallel) and located somewhere in the upper troposphere (18-40 km of altitude). Its behavior and possible nature are discussed here.

Temperature and size of the nucleus of comet P/Halley deduced from IKS infrared Vega 1 measurements

The advent of the NAOS/CONICA adaptive optics system at the ESO Very Large Telescope recently gave us the opportunity to map the surface of Titan and to search for atmospheric variations at high spatial resolution and contrast. We report here the first results from a series of observations of Titan performed with this instrument in a number of near-infrared narrow-band filters, covering various altitude regions and three dierent longitudes (out of the 16 days of Titans orbit). We have achieved unequaled contrast on images showing complex topography on Titans trailing hemisphere and have found robust evidence for the north-south asymmetry inversion. The presence of other interesting atmospheric features at Titans South Pole is described.

Detection of parent molecules in comet P/Halley from the IKS-Vega experiment

Abstract We reduced ultraviolet spectra of Saturn from the IUE satellite to produce a geometric albedo of the planet from 1500 to 3000 A. By matching computer models to the albedo we determined a chemical composition consistent with the data. This model includes C 2 H 2 and C 2 H 6 with mixing ratios and distributions of (9 ± 3) × 10 −8 in the top 20 mbar of the atmosphere with none below for C 2 H 2 and (6 ± 1) × 10 −6 also in the top 20 mbar with none below for C 2 H 6 . The C 2 H 2 and C 2 H 6 distributions and the C 2 H 6 mixing ratio are taken directly from the Voyager IRIS model [R. Courtin et al., Bull. Amer. Astron. Soc. 13 , 722 (1981), and private communication]. The Voyager IRIS model also includes PH 3 , which is not consistent with the uv albedo from 1800 to 2400 A. Our model requires a previously unidentified absorber to explain the albedo near 1600 A. After considering several candidates, we find that the best fit to the data is obtained with H 2 O, having a column density of (6 ± 1) × 10 −3 cm-am.

Observations of water vapour anomaly above Tharsis volcanoes on Mars in the ISM (Phobos-2) experiment

The infrared thermal emission of the nucleus of Halley’s comet was measured by the imaging channel of the IKS spectrometer in two wavelengths bands (7–10 µm and 9–14 µm) and in two perpendicular directions of analysis. The effective dimensions of the infrared emissive region in these directions are estimated from a simulation of the flyby configuration, and compared to observations made in the visible range. A colour temperature larger than 360 K was deduced using on-ground preflight calibrations. A temperature of the same order is obtained independently, comparing the absolute value of the measured fluxes with model simulations. Such temperatures, significantly higher than the sublimating temperature of ices, and near the blackbody temperatures at 0.8 au, suggest the existence of an essentially inactive dark crust covering a significant part of the surface of the nucleus.