New insights on Saturn's formation from its nitrogen isotopic composition

TL;DR

This study revises Saturn's formation models by analyzing its nitrogen isotopic composition and heavy element abundances, suggesting formation at around 45 K with specific oxygen enrichment in the protosolar nebula.

Contribution

It introduces a thermodynamic model of clathrate formation to explain Saturn's chemical and isotopic compositions, linking them to formation conditions in the protosolar nebula.

Findings

Saturn's oxygen abundance is at least 34.9 times protosolar.

The mass of heavy elements in Saturn is about 43.1 Earth masses.

Formation at ~45 K with specific oxygen enrichment explains observed compositions.

Abstract

The recent derivation of a lower limit for the $^{14}$N/$^{15}$N ratio in Saturn's ammonia, which is found to be consistent with the Jovian value, prompted us to revise models of Saturn's formation using as constraints the supersolar abundances of heavy elements measured in its atmosphere. Here we find that it is possible to account for both Saturn's chemical and isotopic compositions if one assumes the formation of its building blocks at $\sim$45 K in the protosolar nebula, provided that the O abundance was $\sim$2.6 times protosolar in its feeding zone. To do so, we used a statistical thermodynamic model to investigate the composition of the clathrate phase that formed during the cooling of the protosolar nebula and from which the building blocks of Saturn were agglomerated. We find that Saturn's O/H is at least $\sim$34.9 times protosolar and that the corresponding mass of heavy elements ($\sim$43.1 \Mearth) is within the range predicted by semi-convective interior models.