The longevity of water ice on Ganymedes and Europas around migrated giant planets

TL;DR

This study models the longevity of water ice on icy moons orbiting migrating giant exoplanets, showing that larger moons can retain surface water indefinitely beyond a certain orbital distance, while smaller moons lose water over Gyr timescales.

Contribution

It provides a quantitative analysis of how stellar flux and moon size influence the survival of surface ice on moons around migrating giant planets.

Findings

Ganymede-sized moons can retain water ice indefinitely outside the runaway greenhouse limit.

Europa-sized moons lose their surface water within 1 Gyr at certain orbital distances.

Small moons may create detectable hydrogen tori due to water loss.

Abstract

The gas giant planets in the Solar System have a retinue of icy moons, and we expect giant exoplanets to have similar satellite systems. If a Jupiter-like planet were to migrate toward its parent star the icy moons orbiting it would evaporate, creating atmospheres and possible habitable surface oceans. Here, we examine how long the surface ice and possible oceans would last before being hydrodynamically lost to space. The hydrodynamic loss rate from the moons is determined, in large part, by the stellar flux available for absorption, which increases as the giant planet and icy moons migrate closer to the star. At some planet-star distance the stellar flux incident on the icy moons becomes so great that they enter a runaway greenhouse state. This runaway greenhouse state rapidly transfers all available surface water to the atmosphere as vapor, where it is easily lost from the small moons. However, for icy moons of Ganymede's size around a Sun-like star we found that surface water (either ice or liquid) can persist indefinitely outside the runaway greenhouse orbital distance. In contrast, the surface water on smaller moons of Europa's size will only persist on timescales greater than 1 Gyr at distances ranging 1.49 to 0.74 AU around a Sun-like star for Bond albedos of 0.2 and 0.8, where the lower albedo becomes relevant if ice melts. Consequently, small moons can lose their icy shells, which would create a torus of H atoms around their host planet that might be detectable in future observations.