The Astrophysical Birth Environment of the Solar System Inferred From Cometary Noble Gases

The stellar sources that contributed to the chemical and isotopic composition of the solar system, their mixing in the Galactic environment, and the evolution and longevity of the molecular cloud from which our solar system formed can be probed with isotopic measurements of presolar grains, meteorites, and comets. The isotopic compositions of cometary xenon (Xe) and krypton (Kr) measured in the coma of 67P/Churyumov-Gerasimenko (67P/C-G) are distinct from those of the Sun. As noble gases in cometary ices are thought to be derived from the same molecular cloud from which the Sun formed, such nonsolar isotopic compositions suggest that a compositionally distinct reservoir of noble gases was present in the presolar molecular cloud that is currently absent from 67P/C-G. This likely requires that a noble gas component was cotrapped in volatile ice which was later destroyed by processing within the protoplanetary disk. Here, we show that this hypothetical component was enriched in r-process and potentially p-process isotopes compared to the solar composition, and that these isotopes formed over a hundred million years before coalescence and cosmochemical isolation of the solar system. We discuss a scenario where this component was accreted by the presolar molecular cloud sometime after the formation of water ice containing cotrapped Xe and Kr with the composition of 67P/C-G. Such a component could have been derived from a diffuse, heterogeneous molecular portion of the interstellar medium accreted by the presolar molecular cloud, or incorporated from another molecular cloud via a cloud-cloud collision. These observations support two distinct r-processes contributing to the overall solar r-process abundance pattern.