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Did Io Accrete Wet or Dry? Exploring Bounds on Water Loss from Io During Accretion

Presentation #103.03 in the session Fire and Ice: Io and Beyond (Oral Presentation)

Published onOct 23, 2023
Did Io Accrete Wet or Dry? Exploring Bounds on Water Loss from Io During Accretion

The Galilean satellites are a remarkably well-ordered system, and their similar masses and radial compositional gradient are key constraints that any formation model or scenario must satisfy. Classically, Io & Europa formed in hotter/warmer regions of the protojovian circumplanetary disk (CPD), and thus formed ice free or ice-depleted. The Laplace resonance and the recognition of planetary migration allow for other scenarios in which the satellites accrete in more distant (nominally colder) regions of the CPD and migrate inward. If so, they must lose all or most of their water (ice). Tidally induced volcanism (e.g., Io) is woefully inefficient in the presence of an overlying ocean, though could dry out a hot silicate mantle. Recent work has focused on hydrodynamic loss of water vapor from a surface ocean, driven by accretional and/or CPD background heating. Success in these models depends critically on accretion timescale (the shorter the better) as well as on background temperature (the latter must be well above the 200 K ice condensation temperature to begin with in order to transform a Ganymede into an Io or Europa). In published models, formation timescales for Io & Europa range from as long as 106 year (gas-starved accretion) or several×105 year (heliocentric planetesimal sourced pebble accretion) to as little as a few×103 year (oligarchic growth in a cold decretion disk). The pressure at the sonic radius needs to exceed the CPD ambient pressure as well in order for the escaping wind to blow (e.g., the ocean surface Ts must be ≳ 450 K for isothermal escape from proto-Io against a 1 Pa ambient background, appropriate to some gas-starved models, with greater Ts necessary for more realistic, condensing outflows; similar escape from proto-Europa requires Ts ≳ 350 K). The headwind pressure from the sub-Keplerian CPD can suppress hydrodynamic escape as well, if it exceeds the escape ram pressure at the sonic radius, but this can be neglected for gas-starved CPDs. Volatile loss from an accreting icy satellite around a giant planet, or lack thereof, passes through up to four distinct phases, which we will describe. In general, loss of water vapor can be efficient as a satellite accretes, but because of protojovian nebular backpressure, becomes increasingly difficult as a body approaches an Io-like like size and mass. Substantial water vapor loss from Io or Europa should not, however, be assumed, but be evaluated in the specific context of a given satellite formation model or scenario.

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