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A consensus has progressively developed that the path towards complex organic molecules could be favoured by the icy mantles of interstellar dust particles. However, the question of whether activation energy is required to promote those reactions is open. This work deals with the simplest process thought to increase complexity i.e. successive additions of a single atom (H). Two situations can be considered, the direct additions of H in the gas phase and the atomic hydrogenation processes on water ice surfaces available in the interstellar medium (ISM). The synthesis of methanol, largely discussed in the literature is revisited as a case study. Computational investigations of the reactions leading from CO to CH3OH show that, with [H...(H2O)n] complexes as hydrogenation vectors, all steps of the process are barrier-less, contrary to the gas phase process and this, whatever the level of theory considered. The key parameter is the position of the CO + [H...(H2O)n] initial system on the global energy scale. At the present level of theory, when H is pre-adsorbed on the ice, the system happens to be high enough above the reaction path, namely, above any intermediates and possible transition states. This is true whatever the dimension of the ice support, even for the simplest model of one H2O molecule. Application of such a simplified synthetic approach, here validated for the synthesis of methanol, could be generalized, providing a simple way to get a fair insight into the important class of atomic hydrogenations on ices in the ISM.

International audience; Context. Almost 20% of the ~200 different species detected in the interstellar and circumstellar media present a carbon atom linked to nitrogen by a triple bond. Of these 37 molecules, 30 are nitrile R-CN compounds, the remaining 7 belonging to the isonitrile R-NC family. How these species behave in their interactions with the grain surfaces is still an open question. Aims. In a previous work, we have investigated whether the difference between nitrile and isonitrile functional groups may induce differences in the adsorption energies of the related isomers at the surfaces of interstellar grains of various nature and morphologies. This study is a follow up of this work, where we focus on the adsorption on carbonaceous aromatic surfaces. Methods. The question is addressed by means of a concerted experimental and theoretical approach of the adsorption energies of CH3CN and CH3NC on the surface of graphite (with and without surface defects). The experimental determination of the molecule and surface interaction energies is carried out using temperature-programmed desorption in an ultra-high vacuum between 70 and 160 K. Theoretically, the question is addressed using first-principle periodic density functional theory to represent the organised solid support.Results. The adsorption energy of each compound is found to be very sensitive to the structural defects of the aromatic carbonaceous surface: these defects, expected to be present in a large numbers and great diversity on a realistic surface, significantly increase the average adsorption energies to more than 50% as compared to adsorption on perfect graphene planes. The most stable isomer (CH3CN) interacts more efficiently with the carbonaceous solid support than the higher energy isomer (CH3NC), however.

International audience; The heavy noble gas deficiencies observed in Titan's atmosphere and in comets have been proposed to be related to a sequestration process by H-3(+) in the gas phase at the early protosolar nebula. Chemical thermodynamics and astrophysics modeling are favorable to this hypothesis, as presented in preceding papers. However, there is a point still to be raised, i.e., that no dynamical study of the efficiency of the collisional processes had been performed so far. Here, we show that, apart from the expected exception of Ne, the rate constants obtained, i.e., 0.7 x 10(-18), 0.5 x 10(-16), and 10(-16) (cm(3) s(-1)) for Ar, Kr, and Xe, respectively, are reasonably high for such processes, particularly in the case of Kr and Xe. The temperature dependence is also considered, showing a similar behavior for all noble gases with a peak efficiency in the range 50-60 K. Globally, we can conclude that the scenario of sequestration by H-3(+) is definitively comforted by the results of our quantum dynamical treatment. This process may also be responsible of the Ar impoverishment just measured in comet 67P/Churyumov-Gerasimenko by the ROSINA mass spectrometer on board the Rosetta spacecraft.

International audience; The origin of the neutral sodium comet tail discovered in comet Hale-Bopp in 1997 is still a matter of discussion. Here we propose a scenario which is based on chemical grounds. The starting point is the chemical trapping of the Na+ ion in the refractory material during the condensation phase of the protosolar nebula, followed by its incorporation in the building blocks of the comets parent bodies. In the next step, the Na+ ions are washed out of the refractory material by the water formed by the melting of the ice due to the heat released in the radioactive decay of short period elements. When the water freezes again, the Na+ ion looses its positive charge to evolve progressively toward a neutral atom when approaching the surface of the ice. As shown by high-level numerical simulations based on first principle periodic density functional theory (DFT) to describe the solid structure of the ice, it is a neutral Na that is ejected with the sublimation of the ice top layer

We address the problem of the sequestration of Ar, Kr, and Xe by H in the gas-phase conditions encountered during the cooling of protoplanetary disks when H is competing with other species present in the same environment. Using high-level ab initio simulations, we try to quantify other sequestration possibilities involving He, H, H2O, and H3O+ present in the protosolar nebula. Apart from the fact that H complexes formed with heavy noble gases are found to be by far much more stable than those formed with He or H2O, we show that H2D+ and H3O+, both products of the reactions of H with HD and H2O, can also be efficient trapping agents for Ar, Kr, and Xe. Meanwhile, the abundance profile of H in the outer part of the nebula is revisited with the use of an evolutionary accretion disk model that allows us to investigate the possibility that heavy noble gases can be sequestered by H at earlier epochs than those corresponding to their trapping in planetesimals. We find that H might be abundant enough in the outer protosolar nebula to trap Xe and Kr prior their condensation epochs, implying that their abundances should be solar in Saturn's current atmosphere and below the observational limit in Titan. The same scenario predicts that comets formed at high heliocentric distances should also be depleted in Kr and Xe. In situ measurements, such as those planed with the Rosetta mission on 67P/Churyumov-Gerasimenko, will be critical to check the validity of our hypotheses.

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The origin of Titan and its atmosphere dominated by nitrogen and methane is still uncertain. A puzzling feature of the satellite's atmosphere is the non-detection of the primordial noble gases krypton and xenon by the Huygens probe, present in the atmosphere of Jupiter and presumably Saturn. The carbon monoxide to methane ratio is ∼ 10-3 in Titan's atmosphere, which also places a strong constraint on the satellite's origin, since carbon monoxide should have been more abundant than methane in the primitive nebula. Here we present a model for the origin of Titan consistent with these observations: Titan was formed from planetesimals initially produced in the primitive nebula that were partially devolatilized during their migration within Saturn's satellite-forming subnebula. In contrast with krypton and xenon which remained trapped, most of the carbon monoxide and argon was lost from icy planetesimals during their migration within the subnebula. The observed deficiency of Titan's atmosphere in krypton and xenon could result from various sequestration processes that may have occurred prior or after the satellite's completion. On the basis of our model, we predict that all regular satellites interiors to the orbit of Titan are depleted in primordial carbon monoxide and nitrogen.

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We study the efficiency of the noble gases sequestration by the ion H3+ in the form of XH3+ complexes (with X = argon, krypton or xenon) in gas phase conditions similar to those encountered during the cooling of protoplanetary disks, at the epoch of icy planetesimals formation. We show that XH3+ complexes form very stable structures in the gas phase and that their binding energies are much higher than those involved in the structures of X-H2O hydrates or pure X-X condensates. This implies that, in presence of H3+ ions, argon, krypton or xenon are likely to remain sequestrated in the form of XH3+ complexes embedded in the gas phase rather than forming ices during the cooling of protoplanetary disks. The amount of the deficiency depends on how much H3+ is available and efficient in capturing noble gases. In the dense gas of the mid-plane of solar nebula, H3+ is formed by the ionization of H2 from energetic particles, as those in cosmic rays or those ejected by the young Sun. Even using the largest estimate of the cosmic rays ionization rate, we compute that the H3+ abundance is two and three orders of magnitude lower than the xenon and krypton abundance, respectively. Estimating the ionization induced by the young Sun, on the other hand, is very uncertain but leaves the possibility to have enough H3+ to make krypton and xenon trapping efficent. Finally, additional source of H3+ formation may be provided by the presence of a nearby supernova, as discussed in the literature. Recent solar system observations show a deficiency of Ar, and, even more, of Kr and Xe in Titan and in comets. In this article, we consider the possibility that this deficiency is caused by the afore-mentioned process, namely trapping of those noble gases by H3+ ions in the solar nebula., Comment: Accepted for publication in Astrophysical Journal

Molecular oxygen has been detected in the coma of comet 67P/Churyumov-Gerasimenko with abundances in the 1%-10% range by the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis-Double Focusing Mass Spectrometer instrument on board the Rosetta spacecraft. Here we find that the radiolysis of icy grains in low-density environments such as the presolar cloud may induce the production of large amounts of molecular oxygen. We also show that molecular oxygen can be efficiently trapped in clathrates formed in the protosolar nebula (PSN), and that its incorporation as crystalline ice is highly implausible, because this would imply much larger abundances of Ar and N-2 than those observed in the coma. Assuming that radiolysis has been the only O-2 production mechanism at work, we conclude that the formation of comet 67P/Churyumov-Gerasimenko is possible in a dense and early PSN in the framework of two extreme scenarios: (1) agglomeration from pristine amorphous icy grains/particles formed in ISM and (2) agglomeration from clathrates that formed during the disk's cooling. The former scenario is found consistent with the strong correlation between O-2 and H2O observed in comet 67P/Churyumov-Gerasimenko's coma while the latter scenario requires that clathrates formed from ISM icy grains that crystallized when entering the PSN.

The origin of cometary volatiles remains a major open question in planetary science. Comets may have either agglomerated from crystalline ices condensed in the protosolar nebula (PSN) or from amorphous ice originating from the molecular cloud and interstellar medium. Here, based on the recent argon, krypton, and xenon measurements performed by the ROSINA mass spectrometer on board the European Space Agency’s Rosetta spacecraft in the coma of 67P/Churyumov–Gerasimenko, we show that these noble gas relative abundances can be explained if the comet’s building blocks formed from a mixture of gas and H2O grains resulting from the annealing of pristine amorphous ice (i.e., originating from the presolar cloud) in the PSN. In this scenario, the different volatiles released during the amorphous-to-crystalline ice phase transition would have been subsequently trapped at lower temperatures in stoichiometric hydrate or clathrate hydrate forms by the crystalline water ice generated by the transition. Once crystalline water was completely consumed by clathration in the ∼25–80 K temperature range, the volatile species remaining in the gas phase would have formed pure condensates at lower temperatures. The formation of clathrates hydrates and pure condensates to explain the noble gas relative abundances is consistent with a proposed interstellar origin of molecular oxygen detected in 67P/Churyumov–Gerasimenko, and with the measured molecular nitrogen depletion in comets.

International audience; The origin of cometary volatiles remains a major open question in planetary science.

Non-empirical calculations have been performed to analyze the effects of fluorine substitution on the geometry and electronic properties of two series of π and σ radicals. Both UHF and RHF + perturbation methods have been used and the results are compared as a function of the basis set quality. As concerns geometry and spin-free electronic properties the results are independent of the UHF or RHF formalism, but highly sensitive to the basis set. The STO-3G basis is unable to reproduce even general trends. Polarization functions always play a relevant role and correlation effects seem not negligible at least for fluorine-containing radicals. The molecular shape of π radicals changes from a planar to a pyramidal geometry when increasing the electronegativity of the substituents. On the contrary, σ radicals always remain planar. Unprojected UHF spin densities are closer to the RHF + perturbation results for small spin contamination (X 2 NO). On the contrary, it is the projected UHF spin density which is in better agreement with the RHF + perturbation value for large spin contamination (X 2 CN). No simple correlation can be found between spin densities and gross atomic spin populations. For H 2 NO the spin density at nitrogen is smaller than at the oxygen nucleus, but substitution may enhance or reverse this trend.