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Current models suggest the five regular moons of Uranus formed rapidly from a planetary disk after a giant impactor hit Uranus and caused its large spin axial tilt of ~98 (Ida et al., 2020; Woo et al., 2022). A power-law relationship between size and density of moons evidences varying rock/ice ratios in the moons. This relationship is not well described by differential diffusion of rock and ice in the disk (Woo et al., 2022). We find that this relationship is well explained by a mild enrichment of rock with respect to ice in the solids that aggregate to form the moons, following Rayleigh law for distillation (Rayleigh, 1896). In this model, moon composition and density reflect their order of formation in a closed-system circumplanetary disk. For Uranus, the largest and densest moons Titania and Oberon (R ~ 788 and 761 km, respectively) first formed, then the mid-sized Umbriel and Ariel (585 and 579 km), satellites in each pair forming simultaneously with similar composition, and finally the small rock-depleted Miranda (236 km). Fractionation of rock and ice during aggregation of planets and moons may occur in other planetary disks. Rayleigh distillation in the Saturnian disk may for instance account for early formation of rock-poor Rhea, Iapetus and Tethys. This mechanism adds to those affecting the composition of accreting planets and moons in disks such as temporal/spatial variation of disk composition due to differential diffusion, advection, and large impacts.Ida, S., Ueta, S., Sasaki, T., Ishizawa, Y., 2020. Uranian satellite formation by evolution of a water vapour disk generated by a giant impact. Nature Astronomy 4, 880-885.Rayleigh, L., 1896. L. Theoretical considerations respecting the separation of gases by diffusion and similar processes. The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science 42, 493-498.Woo, J.M.Y., Reinhardt, C., Cilibrasi, M., Chau, A., Helled, R., Stadel, J., 2022. Did Uranus' regular moons form via a rocky giant impactor? Icarus 375, 114842.
Reynard et al. (Wed,) studied this question.