Jupiter's "Cold" Formation in the Protosolar Disk Shadow: An Explanation for the Planet's Uniformly Enriched Atmosphere

TL;DR

This paper proposes that a shadow cast by dust pileup at the H2O snow line in the protosolar disk cooled the region around Jupiter, allowing volatile gases like N2 and noble gases to freeze and enrich Jupiter's atmosphere.

Contribution

It introduces a novel mechanism involving disk shadowing and dust pileup to explain Jupiter's atmospheric composition, supported by radiative transfer and condensation calculations.

Findings

The disk shadow could cool the Jupiter-forming region to ~30 K.

Shadow-induced condensation of N2 and Ar explains atmospheric abundances.

Shadow effects may influence interpretation of exoplanet atmospheres.

Abstract

Atmospheric compositions offer valuable clues to planetary formation and evolution. Jupiter has been the most well-studied giant planet in terms of its atmosphere; however, the origin of the Jovian atmospheric composition remains a puzzle as the abundances of nitrogen and noble gases as high as those of other elements could only originate from extremely cold environments. We propose a novel idea for explaining the Jovian atmospheric composition: Dust pileup at the H$_2$O snow line casts a shadow and cools the Jupiter orbit so that N$_2$ and noble gases can freeze. Planetesimals or a core formed in the shadowed region can enrich nitrogen and noble gases as much as other elements through their dissolution in the envelope. We compute the temperature structure of a shadowed protosolar disk with radiative transfer calculations. Then, we investigate the radial volatile distributions and predict the atmospheric composition of Jupiter with condensation calculations. We find that the vicinity of the current Jupiter orbit, approximately $3$--$7~{\rm AU}$, could be as cold as $30~{\rm K}$ if the small-dust surface density varies by a factor of $\gtrsim30$ across the H$_2$O snow line. According to previous grain growth simulations, this condition could be achieved by weak disk turbulence if silicate grains are more fragile than icy grains. The shadow can cause the condensation of most volatile substances, namely N$_2$ and Ar. We demonstrate that the dissolution of shadowed solids can explain the elemental abundance patterns of the Jovian atmosphere even if proto-Jupiter was formed near Jupiter's current orbit. The disk shadow may play a vital role in controlling atmospheric compositions. The effect of the shadow also impacts the interpretation of upcoming observations of exoplanetary atmospheres by JWST.