Circumplanetary disk ices. I. Ice formation vs. viscous evolution and grain drift

TL;DR

This study investigates ice formation in circumplanetary disks, revealing that water ice can form rapidly from atomic conditions and that dust dynamics influence satellite composition, with implications for moon formation.

Contribution

It provides a detailed analysis of ice formation timescales versus dust drift and viscous evolution in CPDs, highlighting the importance of chemical and dynamical processes in moon formation.

Findings

Water ice forms efficiently within 1 year from atomic conditions.

Radial grain drift is slower than ice formation timescales.

CPD midplane must be significantly depleted in dust to match satellite compositions.

Abstract

The large icy moons of Jupiter formed in a circumplanetary disk (CPD). CPDs are fed by infalling circumstellar gas and dust which may be shock-heated upon accretion or sublimated while passing through an optically thin gap. Accreted material is then either incorporated into moons, falls into the planet, or is lost beyond the disk edge on relatively short timescales. If ices are sublimated during accretion onto the CPD we know there must be sufficient time for them to recondense or moons such as Ganymede or Callisto could not form. The chemical timescale to form sufficiently icy solids places a novel constraint on the dynamical behaviour and properties of CPDs. We use the radiation thermochemical code ProDiMo to analyze how the radial ice abundance evolves in CPDs. We consider different initial chemical conditions of the disk to explore the consequences of infalling material being inherited from the circumstellar disk or being reset to atomic conditions by shock-heating. We contrast the timescales of ice formation with those of viscous evolution and radial dust drift. Water ice can form very efficiently in the CPD from initially atomic conditions, as a significant fraction is efficiently re-deposited on dust grains within < 1 yr. Radial grain drift timescales are in general longer than those of ice formation on grains. Icy grains of size $a < 3$ mm retain their icy mantles while crossing an optically thin circumstellar disk gap at 5 au for $L_* < 10 $ L$_{\odot}$. Three-body reactions play an important role in water formation in the dense midplane condition of CPDs. The CPD midplane must be depleted in dust relative to the circumstellar disk by a factor 10-50 to produce solids with the ice to rock ratio of the icy Galilean satellites. The CPD snowline is not erased by radial grain drift, which is consistent with the compositional gradient of the Galilean satellites being primordial.