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...

Critical measurements for understanding accretion and the dust/gas ratio in the solar nebula, where planets were forming 4.5 billion years ago, are being obtained by the GIADA (Grain Impact Analyser and Dust Accumulator) experiment on the European Space Agency's Rosetta spacecraft orbiting comet 67P/Churyumov-Gerasimenko. Between 3.6 and 3.4 astronomical units inbound, GIADA and OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System) detected 35 outflowing grains of mass 10(-10) to 10(-7) kilograms, and 48 grains of mass 10(-5) to 10(-2) kilograms, respectively. Combined with gas data from the MIRO (Microwave Instrument for the Rosetta Orbiter) and ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) instruments, we find a dust/gas mass ratio of 4 ± 2 averaged over the sunlit nucleus surface. A cloud of larger grains also encircles the nucleus in bound orbits from the previous perihelion. The largest orbiting clumps are meter-sized, confirming the dust/gas ratio of 3 inferred at perihelion from models of dust comae and trails.

Abstract. The Optical, Spectroscopic, and Infrared Remote Imaging System OSIRIS is the scientific camera system onboard the Rosetta spacecraft (Figure 1). The advanced high performance imaging system will be pivotal for the success of the Rosetta mission. OSIRIS will detect 67P/Churyumov-Gerasimenko from a distance of more than 10 6 km, characterise the comet shape and volume, its rotational state and find a suitable landing spot for Philae, the Rosetta lander. OSIRIS will observe the nucleus, its activity and surroundings down to a scale of ~2cmpx~1. The observations will begin well before the onset of cometary activity and will extend over months until the comet reaches perihelion. During the rendezvous episode of the Rosetta mission, OSIRIS will provide key information about the nature of cometary nuclei and reveal the physics of cometary activity that leads to the gas and dust coma.OSIRIS comprises a high resolution Narrow Angle Camera (NAC) unit and a Wide Angle Camera (WAC) unit accompanied by three electronics boxes. The NAC is designed to obtain high resolution images of the surface of comet 67P/Churyumov-Gerasimenko through 12 discrete filters over the wavelength range 250-1000 nm at an angular resolution of 18.6 /xradpx -1 . The WAC is optimised to provide images of the near-nucleus environment in 14 discrete filters at an angular resolution of 101 ¡xrad px~1. The two units use identical shutter, filter wheel, front door, and detector systems. They are operated by a common Data Processing Unit. The OSIRIS instrument has a total mass of 35 kg and is provided by institutes from six European countries.