Chemical Constraints on the Oxygen Abundances in Jupiter and Saturn

TL;DR

This study compares chemical kinetics models of CO and PH3 in Jupiter and Saturn to constrain their water abundances, finding that updated kinetics limit high water enrichments and support a water-depleted formation environment.

Contribution

It introduces a new chemical scheme for CO kinetics that refines constraints on planetary water abundances, challenging previous models.

Findings

Visscher's scheme limits water enrichment to 2.6 times solar in Jupiter.

High water enrichments are precluded by the updated kinetics.

Results support a water-depleted formation environment for Jupiter.

Abstract

We perform a comparative analysis of the chemical kinetics of CO and $\rm PH_3$ in Jupiter and Saturn to assess the full set of constraints available on the troposphere water abundance in the two giant planets. For carbon monoxide we employ both a widely used CO kinetic scheme from Yung et al, and a newly identified CO chemical scheme from Visscher and Moses. For $\rm PH_3$ chemical scheme, we use the same chemical scheme as in Visscher and Fegley. Yung's chemical scheme for CO yields a water enrichment of 0.95 - 23.0 times solar abundance on Jupiter, and an upper limit of 14.0 for Saturn. Visscher's chemical scheme in contrast produces a water enrichment of 0.24 - 2.6 times solar abundance in Jupiter, and for Saturn an upper limit for water enrichment of 8.0. From this scheme, which takes advantage of the most up-to-date kinetics, we preclude high water enrichments on Jupiter and Saturn, and show that the kinetics approach yields Jovian bulk abundance in which values of C/O elevated relative to solar are admissible. Our result is consistent with recent reinterpretation of Galileo Probe data in which Jupiter formed in a water-depleted portion of the protoplanetary disk (Mousis et al).