Impact of self-shadowing on the Jovian Circumplanetary disk volatile ices distribution

Modelling the origin of Jupiter’s Galilean moons remains a significant challenge. While it is widely accepted that the moons formed within a circumplanetary disk (CPD) that surrounded Jupiter during the final stages of its formation, the physical properties and composition of this disk remain poorly constrained in theoretical models.

An approach to deduce the CPD’s properties and composition is by using the bulk composition of the Galilean moons as a reference to infer compositional trends within the disk. A notable example is the gradient in water content with distance from Jupiter: from the completely dry Io to a 1:1 water-to-rock ratio on Ganymede and Callisto. This gradient strongly suggests that the CPD exhibited a corresponding water abundance gradient during its formation. With the JUICE and Europa Clipper missions currently cruising to the Jovian system, the next decade will provide an unprecedented opportunity to study Europa, Ganymede, and Callisto, providing new constraints for CPD models based on improved understanding of the moons' bulk compositions.

In this context, we have previously developed a 2-dimensional model of the circumplanetary disk, which highlighted the existence and of self-shadowing within the CPD. We demonstrated that regions within the shadow are up to 100 K cooler than their surroundings, allowing for the formation of potential cold traps in which volatile ices could accumulate. This process would bring ice closer to Jupiter on timescales of up to 30-50 kyr. However, this phenomenon had not been tested with a volatile transport model, as previous studies had focused only on simple iceline location analysis.

To address this, we employed a volatile transport model to simulate the evolution of volatiles within the midplane of the CPD. The results show that the enrichment in H2O, CO2, and NH3 remains close to unity, without the peaks at iceline locations commonly observed in protoplanetary disk models. Additionally, the model reveals that self-shadowing creates cold traps, with ice surrounded by vapor around 150 kyr into CPD evolution. These cold traps lead to volatile enrichments between 2 and 3 times the initial values in regions near 10 R_J. However, since the disk is already heavily depleted at this stage, the surface density of volatiles has dropped by a factor of a thousand compared to the initial condition.

Thus, we demonstrate that cold traps influence the radial distribution of volatiles in the Jovian circumplanetary disk, which could affect the composition of the building blocks of the Galilean moons. However, due to the transient nature of these cold traps and their effectiveness only when the disk is heavily depleted, their impact on the composition of a moon that forms within the cold trap region centred around 10 R_J may be limited

 

 

Figure 1: Abundance enrichment of H2O, CO2 and NH3 in the circumplanetary disk midplane (from top to bottom) compared to, their initial abundances, at 50, 150 and 200 kyr (from left to right) after the jovian circumplanetary disk formation.