Abstract
Abstract 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.
Highlights
The origin of cometary volatiles remains a major open question in planetary science
Results similar to those shown in the present work can be obtained as long as CO2 remains the dominating species in the multiple guest (MG) clathrate hydrate, i.e., for any CO2/CO ratio 2 in the protosolar nebula (PSN) gas phase
Our calculations are based on a statistical thermodynamic approach widely used in the fields of geoscience and industry, and allow derivation of the volatiles relative propensities for trapping in MG clathrate hydrates at lower temperatures than those obtained from experiments
Summary
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; Mousis et al 2016b; Luspay-Kuti et al 2016) or from amorphous ice originating from the molecular cloud and interstellar medium (ISM; Klinger 1980; Bar-Nun & Laufer 2003; Bar-Nun et al 2007; Mumma & Charnley 2011; Rubin et al 2015; Marty et al 2017). 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 (67P/C-G; Rubin et al 2018), 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. 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/C-G (Bieler et al 2015; Mousis et al 2016b, 2018), and with the measured molecular nitrogen depletion in comets (Mumma & Charnley 2011; Rubin et al 2015)
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