A protosolar nebula origin for the ices agglomerated by Comet 67P/Churyumov-Gerasimenko

TL;DR

This study suggests that the ices in comet 67P/Churyumov-Gerasimenko originated from the protosolar nebula, specifically from clathrates and crystalline ices, based on gas ratio measurements, impacting our understanding of solar system formation.

Contribution

It provides evidence supporting a nebular origin for cometary ices, challenging interstellar origin scenarios, and discusses implications for volatile delivery to Jupiter.

Findings

N2/CO and Ar/CO ratios match clathrate predictions

Comet 67P's ices formed in the nebula, not from interstellar medium

Additional sources needed for Jupiter's volatile enrichments

Abstract

The nature of the icy material accreted by comets during their formation in the outer regions of the protosolar nebula is a major open question in planetary science. Some scenarios of comet formation predict that these bodies agglomerated from crystalline ices condensed in the protosolar nebula. Concurrently, alternative scenarios suggest that comets accreted amorphous ice originating from the interstellar cloud or from the very distant regions of the protosolar nebula. On the basis of existing laboratory and modeling data, we find that the N$_2$/CO and Ar/CO ratios measured in the coma of the Jupiter family comet 67P/Churyumov-Gerasimenko by the ROSINA instrument aboard the European Space Agency's Rosetta spacecraft match those predicted for gases trapped in clathrates. If these measurements are representative of the bulk N$_2$/CO and Ar/CO ratios in 67P/Churyumov-Gerasimenko, it implies that the ices accreted by the comet formed in the nebula and do not originate from the interstellar medium, supporting the idea that the building blocks of outer solar system bodies have been formed from clathrates and possibly from pure crystalline ices. Moreover, because 67P/Churyumov-Gerasimenko is impoverished in Ar and N$_2$, the volatile enrichments observed in Jupiter's atmosphere cannot be explained solely via the accretion of building blocks with similar compositions and require an additional delivery source. A potential source may be the accretion of gas from the nebula that has been progressively enriched in heavy elements due to photoevaporation.