Early stages of Galilean moon formation in a water-depleted environment

TL;DR

This paper explores how Galilean moons could form with low water content in a water-depleted environment, proposing a dehydration and vapor diffusion process that leads to ice condensation in the circumplanetary disk.

Contribution

It introduces a novel model where dehydration of ice-poor minerals and water vapor diffusion enable ice formation in a water-depleted circumplanetary disk, explaining the moons' compositions.

Findings

Ganymede and Callisto could form with significant ice despite initial water depletion.

Dehydration of minerals releases water vapor that condenses into ice in the disk.

The model suggests moons can be ice-rich even in water-depleted environments.

Abstract

A key feature of the Galilean satellite system is its monotonic decrease in bulk density with distance from Jupiter, indicating an ice mass fraction that is zero in the innermost moon Io, and about half in the outer moons Ganymede and Callisto. Jupiter formation models, and perhaps the Juno spacecraft water measurements, are consistent with the possibility that the Jovian system may have formed, at least partly, from ice-poor material. And yet, models of the formation of the Galilean satellites usually assume abundant water ice in the system. Here, we investigate the possibility that the Jovian circumplanetary disk was populated with ice-depleted chondritic minerals, including phyllosilicates. We show that the dehydration of such particles and the outward diffusion of the released water vapor allow condensation of significant amounts of ice in the formation region of Ganymede and Callisto in the Jovian circumplanetary disk. Our model predicts that Europa, Ganymede and Callisto should have accreted little if any volatiles other than water ice, in contrast to the comet-like composition of Saturn's moon Enceladus. This mechanism allows for the presence of ice-rich moons in water-depleted formation environments around exoplanets as well.