External photoevaporation of the solar nebula: Jupiter's noble gas enrichments

We present a model explaining the elemental enrichments in Jupiter's atmosphere, particularly the noble gases Ar, Kr, and Xe. While He, Ne, and O are depleted, seven other elements show similar enrichments (∼3 times solar, relative to H). Being volatile, Ar is difficult to fractionate from H₂. We argue that external photoevaporation by far-ultraviolet (FUV) radiation from nearby massive stars removed H₂, He, and Ne from the solar nebula, but Ar and other species were retained because photoevaporation occurred at large heliocentric distances where temperatures were cold enough (≲30 K) to trap them in amorphous water ice. As the solar nebula lost H, it became relatively and uniformly enriched in other species. Our model improves on the similar model of Guillot & Hueso. We recognize that cold temperatures alone do not trap volatiles; continuous water vapor production is also necessary. We demonstrate that FUV fluxes that photoevaporated the disk generated sufficient water vapor in regions ≲30K to trap gas-phase species in amorphous water ice in solar proportions.We find more efficient chemical fractionation in the outer disk: whereas the model of Guillot & Hueso predicts a factor of three enrichment when only <2% of the disk mass remains, we find the same enrichments when 30% of the disk mass remains. Finally, we predict the presence of ∼0.1 M_⊕ of water vapor in the outer solar nebula and protoplanetary disks in H II regions.