Jupiter’s “cold” formation in the protosolar disk shadow. An explanation for the planet’s uniformly enriched atmosphere

Vol. 651
1. Letter to the Editor

Jupiter’s “cold” formation in the protosolar disk shadow. An explanation for the planet’s uniformly enriched atmosphere

Spacecraft observations have revealed that Jupiter's atmosphere is uniformly enriched in heavy elements, with elemental abundances of O, C, N, S, P, Ar, Kr, and Xe all a factor of two to four larger than protosolar abundances. While a general enrichment of heavy elements can be expected due to planetesimal and pebble dissolution or to core erosion posterior to its formation, it is intriguing that even highly volatile elements, such as N and Ar, are as enriched as other elements. Previously proposed explanations have invoked a formation of Jupiter in very cold environments (< 30 K) beyond the Ar and N2 snow lines (> 30 au), or the entrapment of hyper-volatiles in water ice ("clathration"), but have faced a number of difficulties. Ohno and Ueda propose a novel idea, namely that the dust pileup that occurs at the H2O snow line casts a shadow on the disk, cooling a 2-10 au wide zone exterior to the H2O snow line and enabling the condensation of N2 and noble gases there. They compute the temperature structure of a shadowed protosolar disk and attendant volatile radial distribution and are able to reproduce a uniform enrichment pattern at Jupiter if the small-dust surface density drops by a factor of at least 30 across the H2O snow line. Unlike in many previous models, a formation of the proto-Jupiter near the current orbit is therefore a viable hypothesis. Interestingly, because the disk shadow barely extends to Saturn's orbit, the new scenario does not predict a uniform elemental enrichment at Saturn and can therefore be tested by in situ measurements from a Saturn entry probe.