External Photoevaporation of the Solar Nebula: Jupiter's Noble Gas Enrichments

TL;DR

This paper presents a model explaining Jupiter's noble gas enrichments through external photoevaporation of the solar nebula, showing how UV radiation removed lighter gases while trapping noble gases in ice at cold temperatures.

Contribution

The model improves previous explanations by incorporating continuous water vapor production and predicts specific water vapor amounts in the outer solar nebula and similar disks.

Findings

Photoevaporation removed H, He, Ne from the nebula.

Noble gases like Ar, Kr, Xe were trapped in ice at <30 K.

Enrichment factors depend on disk mass remaining.

Abstract

We present a model explaining 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 ($\sim$3 times solar, relative to H). Being volatile, Ar is difficult to fractionate from ${\rm H}_{2}$. We argue that external photoevaporation by far ultraviolet (FUV) radiation from nearby massive stars removed ${\rm H}_{2}$, 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 ($\lt 30$ 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 (2006). We recognize that cold temperatures alone do not trap volatiles; continuous water vapor production also is necessary. We demonstrate that FUV fluxes that photoevaporated the disk generated sufficient water vapor, in regions $\lt 30$ K, 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 (2006) predicts a factor of 3 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 $\sim 0.1 \, M_{\oplus}$ of water vapor in the outer solar nebula and in protoplanetary disks in H II regions.