Origin of molecular oxygen in Comet 67P/Churyumov-Gerasimenko

Molecular oxygen has been detected in the coma of comet 67P/Churyumov-Gerasimenko with abundances in the 1-10% range by the ROSINA-DFMS 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, and that its incorporation as crystalline ice is highly implausible because this would imply much larger abundances of Ar and N2 than those observed in the coma. Assuming that radiolysis has been the only O2 production mechanism at work, we conclude that the formation of comet 67P/Churyumov-Gerasimenko is possible in a dense and early protosolar nebula 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 O2 and H2O observed in 67P/C-G's coma while the latter scenario requires that clathrates formed from ISM icy grains that crystallized when entering the protosolar nebula.Comment: The Astrophysical Journal Letters, in pres

Limits on the Contribution of Endogenic Radiolysis to the Presence of Molecular Oxygen in Comet 67P/Churyumov-Gerasimenko

Radiolytic production has been proposed as a potential source for the molecular oxygen observed in comet 67P/Churyumov-Gerasimenko. Radiolysis can be exogenic or endogenic, the latter due to radionuclides present in the dust constitutive of the comet nucleus. We investigated the possibility of forming a significant amount of molecular oxygen through endogenic radiolysis. We applied a model of radiolytic production, developed for an Earth rock-water mixture, and improved it to account for the effect of the size of a radionuclide-bearing grain on the net radiation deposited in its ice mantle. We calculated the possible production of molecular oxygen considering the available experimental values of radiolytic yields. We found that endogenic radiolysis cannot account for the totality of the 3.8% (relative to water) O2abundance derived from the ROSINA observations, with an end member case of our model producing at most a 1% abundance. By contrast, we predict H2O2production leads to an abundance up to two orders of magnitude above observed values

The magma ocean was a huge helium reservoir in the early Earth

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Gas-phase sequestration of noble gases in the protosolar nebula: possible consequences on the outer solar system composition

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Protonated ions as systemic trapping agents for noble gases: From electronic structure to radiative association

International audienceThe deficiencies of argon, krypton, and xenon observed in the atmosphere of Titan as well as anticipated in some comets might be related to a scenario of sequestration by H+3H3+ in the gas phase at the early evolution of the solar nebula. The chemical process implied is a radiative association, evaluated as rather efficient in the case of H+3H3+, especially for krypton and xenon. This mechanism of chemical trapping might not be limited to H+3H3+ only, considering that the protonated ions produced in the destruction of H+3H3+ by its main competitors present in the primitive nebula, i.e., H2O, CO, and N2, might also give stable complexes with the noble gases. However the effective efficiency of such processes is still to be proven. Here, the reactivity of the noble gases Ar, Kr, and Xe, with all protonated ions issued from H2O, CO, and N2, expected to be present in the nebula with reasonably high abundances, has been studied with quantum simulation method dynamics included. All of them give stable complexes and the rate coefficients of their radiative associations range from 10−16 to 10−19 cm3 s−1, which is reasonable for such reactions and has to be compared to the rates of 10−16 to 10−18 cm3 s−1, obtained with H+3H3+. We can consider this process as universal for all protonated ions which, if present in the primitive nebula as astrophysical models predict, should act as sequestration agents for all three noble gases with increasing efficiency from Ar to Xe

Stability of Sulphur Dimers (S 2 ) in Cometary Ices

International audienceS 2 has been observed for decades in comets, including comet 67P/Churyumov–Gerasimenko. Despite the fact that this molecule appears ubiquitous in these bodies, the nature of its source remains unknown. In this study, we assume that S 2 is formed by irradiation (photolysis and/or radiolysis) of S-bearing molecules embedded in the icy grain precursors of cometsand that the cosmic ray flux simultaneously creates voids in ices within which the produced molecules can accumulate. We investigate the stability of S 2 molecules in such cavities, assuming that the surrounding ice is made of H 2 S or H 2 O. We show that the stabilization energy of S 2 molecules in such voids is close to that of the H 2 O ice binding energy, implying that they can only leave the icy matrix when this latter sublimates. Because S 2 has a short lifetime in the vapor phase, we derive that its formation in grains via irradiation must occur only in low-density environments such as the ISM or the upper layers of the protosolar nebula, where the local temperature is extremely low. In the first case, comets would have agglomerated from icy grains that remained pristine when entering the nebula. In the second case, comets would have agglomerated from icy grains condensed in the protosolar nebula and that would have been efficiently irradiated during their turbulent transport toward the upper layers of the disk. Both scenarios are found consistent with the presence of molecular oxygen in comets