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The gas giant planets in the Solar System have a retinue of icy moons, and weexpect giant exoplanets to have similar satellite systems. If a Jupiter-likeplanet were to migrate toward its parent star the icy moons orbiting it wouldevaporate, creating atmospheres and possible habitable surface oceans. Here, weexamine how long the surface ice and possible oceans would last before beinghydrodynamically lost to space. The hydrodynamic loss rate from the moons isdetermined, in large part, by the stellar flux available for absorption, whichincreases as the giant planet and icy moons migrate closer to the star. At someplanet-star distance the stellar flux incident on the icy moons becomes sogreat that they enter a runaway greenhouse state. This runaway greenhouse staterapidly transfers all available surface water to the atmosphere as vapor, whereit is easily lost from the small moons. However, for icy moons of Ganymede'ssize around a Sun-like star we found that surface water (either ice or liquid)can persist indefinitely outside the runaway greenhouse orbital distance. Incontrast, the surface water on smaller moons of Europa's size will only persiston timescales greater than 1 Gyr at distances ranging 1.49 to 0.74 AU around aSun-like star for Bond albedos of 0.2 and 0.8, where the lower albedo becomesrelevant 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 bedetectable in future observations.

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