Explaining the Galilean Satellites' Density Gradient by Hydrodynamic Escape

The Galilean satellites exhibit a monotonic decrease in density (and increase in ice mass fraction) with distance from Jupiter. Whether this is because of the background conditions when they formed, the process of accretion itself, or later loss due to tidal heating has been in dispute for 40 yr. We find that a hitherto largely neglected process—vapor loss driven by accretional heating—can reproduce the observed density trend for accretion timescales ≳300 kyr, consistent with gas-starved satellite formation models. In this model, both Io and Europa develop an early surface liquid water ocean. Vapor escape from this ocean causes the water inventories of Io and Europa to be completely and mostly lost, respectively. Isotopic fractionation arising from vapor loss means that Europa will develop a higher D/H ratio compared with Ganymede and Callisto. We make predictions that can be tested with in situ measurements of the D/H of potential Europa plumes by the Europa Clipper spacecraft or infrared spectroscopic determinations of D/H at all three bodies.