Experimental studies of gas trapping in amorphous ice and thermal modelling of comets: Implications for Rosetta

The trapping of mixtures of CO, CH4, N2 and Ar in amorphous water ice was studied experimentally. It is shown that the ice particles could not have been formed at a higher temperature and, subsequently, cool down. Experiments where ice was deposited at elevated temperatures, then cooled down and gas was flowed into the ice, showed that the amount of trapped gas depends only on the highest temperature at which the ice was formed, or resided, prior to cooling and gas flow into it. Consequently, the cometary ice had to be formed at approx. 48 K and the ice is therefore amorphous. The thermal profile of a comet in Halley's orbit was calculated, including the build-up of an insulating dust layer. It was found that an insulating dust layer a few cm thick is enough to choke most of the water emission from the surface. A similar thermal model was calculated for comet P/Temple-1, a candidate for both CRAF and Rosetta (CNSR) missions. The temperature at a depth of 10 m is approx. 160 K for all models considered and, hence, the ice at this depth is crystalline. A crystalline ice layer 40 to 240 m thick was found to overly the gas-laden amorphous ice. Consequently, it should be difficult for the probes of the two comet missions to sample pristine amorphous ice, unless they are aimed at the bottom of an active crater

Modifications of comet materials by the sublimation process: Results from simulation experiments

An active comet like comet Halley loses by sublimation a surface layer of the order of 1 m thickness per perihelion passage. In situ measurements show that water ice is the main constituent which contributes to the gas emission although even more volatile species (CO, NH3, CH4, CO2 etc.) have been identified. Dust particles which were embedded in the ices are carried by the sublimating gases. Measurements of the chemical composition of cometary grains indicate that they are composed of silicates of approximate chondritic composition and refractory carbonaceous material. Comet simulation experiments show that significant modifications of cometary materials occur due to sublimation process in near surface layers which have to be taken into account in order to derive the original state of the material

Simulating the one‐dimensional structure of Titan's upper atmosphere: 1. Formulation of the Titan Global Ionosphere‐Thermosphere Model and benchmark simulations

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94638/1/jgre2819.pd

Simulating the one‐dimensional structure of Titan's upper atmosphere: 2. Alternative scenarios for methane escape

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94984/1/jgre2821.pd

Simulating the one‐dimensional structure of Titan's upper atmosphere: 3. Mechanisms determining methane escape

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94596/1/jgre2822.pd

Detection of argon in the coma of comet 67P/Churyumov-Gerasimenko

Comets have been considered to be representative of icy planetesimals that may have contributed a significant fraction of the volatile inventory of the terrestrial planets. For example, comets must have brought some water to Earth. However, the magnitude of their contribution is still debated. We report the detection of argon and its relation to the water abundance in the Jupiter family comet 67P/Churyumov-Gerasimenko by in situ measurement of the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) mass spectrometer aboard the Rosetta spacecraft. Despite the very low intensity of the signal, argon is clearly identified by the exact determination of the mass of the isotope 36Ar and by the 36Ar/38Ar ratio. Because of time variability and spatial heterogeneity of the coma, only a range of the relative abundance of argon to water can be given. Nevertheless, this range confirms that comets of the type 67P/Churyumov-Gerasimenko cannot be the major source of Earth’s major volatiles

Three-micron extinction of the Titan haze in the 250–700 km altitude range: Possible evidence of a particle-aging process

Context. The chemical nature of the Titan haze is poorly understood. The investigation carried out by the Cassini-Huygens suite of instruments is bringing new insights into this question. Aims. This work aims at deriving the vertical variation of the spectral structure of the 3.3–3.4 μm absorption feature of the Titan haze from Cassini VIMS solar occultation data recorded between 250 and 700 km altitude. Methods. We computed the transmittance of Titan’s atmosphere using a spherical shell model and a radiative transfer code including the influence of CH4, CH3D, and C2H6, as well as the effects of absorption and scattering by the haze particles. We derived the haze extinction from a comparison of the synthetic spectra with the VIMS solar occultation spectra. Results. We find a marked change in the relative amplitudes of the 3.33 and 3.38 μm features, which are characteristic of aromatic (double C=C chains or rings) or aliphatic (single C–C chains) structural groups, respectively. The pseudo-ratio of aromatics to aliphatics (uncorrected for the absolute band strengths) varies from 3.3 ± 1.9 at 580−700 km to 0.9 ± 0.1 at 350−450 km, and is 0.5 ± 0.1 around 250 km. The structural change from the aromatic to the aliphatic type between 580 and 480 km appears to correspond to a spontaneous aging of the particles – a transition between unannealed and hardened particles – while the further decrease of the pseudo-ratio of aromatics to aliphatics below 480 km may be related to the coating of the core particles by condensates such as heavy alkanes

Cometary Nitrogen-Noble gases and the Origin of the Oceans: Waiting for ROSINA-Rosetta Data (invited)

International audienceThe origin of the terrestrial oceans and atmosphere is still a matter of intensive debate. For a while high D/H ratios in comets compared to both meteorites and Earth’s oceans (which share approximately the same range of values) has been taken as evidence for an asteroidal origin of the oceans. This possibility is in line with the N isotope composition of meteorites encompassing the terrestrial atmosphere value. Recently, Earth-like D/H ratios have been reported for two comets, thus re-vitalizing the possibility of cometary contribution to terrestrial volatiles. Nitrogen in cometary CN, HCN, and NH2 (the N-bearing molecules that can be analyzed remotely) is enriched by a factor of approx. 80% in 15N compared to terrestrial N. The question is whether or not these N-bearing molecules are representative of bulk N in comets. If nitrogen is trapped as N2 in cometary ices, its isotope composition could reflect that of the protosolar nebula, which was depleted by 40% relative to Earth. A cometary cocktail consisting of 1/3 reduced, 15N-rich N and 2/3 of nebula-like N2 would make the terrestrial N isotope composition. Another strong constraint will arise from the analysis of cometary noble gases which, if trapped at sufficiently low temperature in ice, could account for the elevated noble gas/H,C ratio of the terrestrial surface reservoirs. Hopefully, the ROSINA instrument on board of the Rosetta spacecraft which will analyze volatile elements degassed by comet 67/P Churyumov-Gerasimenko, which should permit to shed light on this fundamental issue