Formation of Water-rich Giant Planet Satellites at Decretion Disk Ice Lines

Abstract

The volatile budgets of giant planet satellites are critical to unraveling the origin of their building blocks within the circumplanetary disks that hosted them. The Galilean moons Ganymede and Callisto, as well as the Saturnian moon Titan, are known to be anomalously water rich on the basis of their mean densities and interior models informed by gravity data from Galileo and Cassini, characterized by ice-to-rock ratios around unity. Here, we show that the water-ice sublimation line in a decreting circumplanetary disk lends itself to the formation of a water-rich solid reservoir, serving as a natural site for the birthplace of icy satellites. Fundamentally, this reflects how interior to the ice line, water vapor is advected outward, while beyond it, water ice drifts inward as pebbles. Using a semi-analytic model for dust and vapor evolution, we simulate vapor and ice accumulation at the ice line, showing that solids just beyond it achieve steady-state ice-to-rock ratios a factor of a few higher than elsewhere in the disk. For typical disk parameters, this ice buildup occurs within a timescale of a few thousand years. We propose this as a first-order process that explains, at least to some extent, the compositions of three aforementioned satellites. We explore the impact of uncertain turbulence parameters on our results, namely the turbulent Schmidt number and Shakura-Sunyaev alpha, before discussing them in the context of icy satellite D/H ratios. We conclude by evaluating alternative scenarios for explaining water-rich satellites, based on the conversion of CO to CH4, with water as a by-product.