On the basis of previous ground-based and fly-by information, we knew that Titan's atmosphere was mainly nitrogen, with some methane, but its temperature and pressure profiles were poorly constrained because of uncertainties in the detailed composition. The extent of atmospheric electricity ('lightning') was also hitherto unknown. Here we report the temperature and density profiles, as determined by the Huygens Atmospheric Structure Instrument (HASI), from an altitude of 1,400 km down to the surface. In the upper part of the atmosphere, the temperature and density were both higher than expected. There is a lower ionospheric layer between 140 km and 40 km, with electrical conductivity peaking near 60 km. We may also have seen the signature of lightning. At the surface, the temperature was 93.65 +/- 0.25 K, and the pressure was 1,467 +/- 1 hPa.

The vertical distribution of Titan's neutral atmosphere compounds is calculated from a new photochemical model extending from 40 to 1432 km. This model makes use of many updated reaction rates, and of the new scheme for methane photolysis proposed by Mordaunt et al. [1993]. The model also includes a realistic treatment of the dissociation of N2, of the deposition of water in the atmosphere from meteoritic ablation, and of condensation processes. The sensitivity of the results to the eddy diffusion coefficient profile is investigated. Fitting the methane thermospheric profile and the stratospheric abundance of the major hydrocarbons requires a methane stratospheric mixing ratio of 1.5-2% rather than 3%. Fitting the HCN stratospheric profile requires an eddy diffusion coefficient at 100-300 km that is 5-20 times larger than that necessary for the hydrocarbons. Most species are reasonably well reproduced, with the exception of CH3C2 H and HC3N. The formation of CH3CN may involve the reaction of CN with either CH 4 or (preferably) C2H 6. The observed CO2 profile can be modeled by assuming an external source of water of---6 x 106 cm -2 s -•. For a nominal CO mixing ratio of 5 x 10 -s, the chemical loss of CO exceeds its production by ---15%, and equilibrium is achieved for CO = 1 x 10 -s. 23,261 23,262 LARA ET AL.: PHOTOCHEMICAL MODELING OF TITAN'S ATMOSPHERE ual species were derived and, consequently, no comparison with observational data was possible. The first detailed photochemical model since Voyager was developed by Yung et al. [1984] (and updated by Yung [1987]). This work made use of a very complete set of chemical reactions, based on the compilation of the earlier studies by Strobel [1974, 1982] andAllen et al. [1980], and adding the photochemistry of oxygen compounds in a mildly reducing atmosphere (investigated by Pinto et al. [1980]), as well as new chemical reactions, mainly those forming long-chain hydrocarbons or polyynes. Vertical profiles for all the constituents observed in Titan's atmosphere were derived, and average mixing ratios were compared to early analyses of Voyager infrared observations [Hanel et al., 1981; Maguire et al., 1981; Kunde et al., 1981; Samuelson et al., 1983]. Implications of the model for the composition of the troposphere, the origin and evolution of the atmosphere, and the geochemistry were also assessed. Despite the qualitative and quantitative importance of this work, there are at least two reasons to reconsider Titan's photochemical models today. First, several new observational constraints have become available. The Voyager infrared imaging spectrometer (IRIS) spectra at the equator have been more fully exploited, resulting in improved determinations of the mixing ratios, in well-understood altitude ranges [Coustenis et al., 1989]. Vertical information is also available for some minor constituents observed in the north polar region [Coustenis et al., 1991]. A reanalysis of the Voyager ultraviolet spectrometer (UVS) data has also been performed, resulting in a new vertical...

Context. The complex shape of comet 67P and its oblique rotation axis cause pronounced seasonal effects. Irradiation and hence activity vary strongly. Aims. We investigate the insolation of the cometary surface in order to predict the sublimation of water ice. The strongly varying erosion levels are correlated with the topography and morphology of the present cometary surface and its evolution. Methods. The insolation as a function of heliocentric distance and diurnal (spin dependent) variation is calculated using >10 5 facets of a detailed digital terrain model. Shading, but also illumination and thermal radiation by facets in the field of view of a specific facet are iteratively taken into account. We use a two-layer model of a thin porous dust cover above an icy surface to calculate the water sublimation, presuming steady state and a uniform surface. Our second model, which includes the history of warming and cooling due to thermal inertia, is restricted to a much simpler shape model but allows us to test various distributions of active areas. Results. Sublimation from a dirty ice surface yields maximum erosion. A thin dust cover of 50 µm yields similar rates at perihelion. Only about 6% of the surface needs to be active to match the observed water production rates at perihelion. A dust layer of 1 mm thickness suppresses the activity by a factor of 4 to 5. Erosion on the south side can reach more than 10 m per orbit at active spots. The energy input to the concave neck area (Hapi) during northern summer is enhanced by about 50% owing to self-illumination. Here surface temperatures reach maximum values along the foot of the Hathor wall. Integrated over the whole orbit this area receives the least energy input. Based on the detailed shape model, the simulations identify "hot spots" in depressions and larger pits in good correlation with observed dust activity. Three-quarters of the total sublimation is produced while the sub-solar latitude is south, resulting in a distinct dichotomy in activity and morphology. Conclusions. The northern areas display a much rougher morphology than what is seen on Imhotep, an area at the equator that will be fully illuminated when 67P is closer to the Sun. Self-illumination in concave regions enhance the energy input and hence erosion. This explains the early activity observed at Hapi. Cliffs are more prone to erosion than horizontal, often dust covered, areas, which leads to surface planation. Local activity can only persist if the forming cliff walls are eroding. Comet 67P has two lobes and also two distinct sides. Transport of material from the south to the north is probable. The morphology of the Imhotep plain should be typical for the terrains of the yet unseen southern hemisphere.

Images from the OSIRIS scientific imaging system onboard Rosetta show that the nucleus of 67P/Churyumov-Gerasimenko consists of two lobes connected by a short neck. The nucleus has a bulk density less than half that of water. Activity at a distance from the Sun of >3 astronomical units is predominantly from the neck, where jets have been seen consistently. The nucleus rotates about the principal axis of momentum. The surface morphology suggests that the removal of larger volumes of material, possibly via explosive release of subsurface pressure or via creation of overhangs by sublimation, may be a major mass loss process. The shape raises the question of whether the two lobes represent a contact binary formed 4.5 billion years ago, or a single body where a gap has evolved via mass loss.