Galilean moon formation in a water-depleted environment

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Montage des quatre lunes galiléennes de Jupiter. De gauche à droite : Io, Europe
Montage des quatre lunes galiléennes de Jupiter. De gauche à droite : Io, Europe, Ganymède et Callisto

While models describing the formation of Galilean moons assume the omnipresence of ice in their building blocks, an international team of researchers led by French scientists from the Origins Institute of Aix-Marseille Université and the Laboratoire d’astrophysique de Marseille (CNRS/Aix-Marseille Université) provide an original scenario where these bodies would have agglomerated from solids initially depleted in water. These results were published on February 17, 2023 in The Astrophysical Journal Letters.

One of the fundamental characteristics of the Galilean moons (Io, Europa, Ganymede and Callisto) is that the two most distant satellites of Jupiter are also the least dense. This difference in density is caused by different proportions of rock and ice, the latter being lighter. Thus, the fraction of ice would be negligible in Io, the innermost moon, but would constitute up to half the mass of the outer satellites Ganymede and Callisto. Models of Jupiter’s formation, as well as measurements of water abundance made by the NASA/Juno mission, are consistent with the possibility that the Jovian system formed, at least in part, from ice-depleted material. However, traditional models explaining the genesis of the Galilean satellites require that they would have been formed from initially ice-rich building blocks from the protosolar nebula.

Against this classical view, this study provides an original scenario where the Jovian circumplanetary disk would have been populated only by chondritic minerals devoid of ice, but rich in phyllosilicates. These minerals are clayey materials that can contain up to 10% of water trapped in their interspace. The migration of such particles in the inner region of the disc would have led to their dehydration and the release of water vapour, which would diffuse towards the outside. Io would have been the product of the accretion of these particles, which would have dried by migrating towards the interior of the disc. Europa, on the other hand, would have been formed from the same particles, but before they would have dehydrated because of a too pronounced migration in the internal and hot regions of the disc.

The diffusion of the water vapour released by the phyllosilicates would have allowed the formation of important quantities of ice in the region of formation of Ganymede and Callisto within the circumplanetary disc. These two moons would have been the product of the accretion of minerals directly from the protosolar nebula and of ice condensed from the water vapour coming from the internal regions of the circumplanetary disc. Some of these moons, Europa in particular, contain a liquid water ocean under the surface, the way they were formed has important consequences for the chemical composition of this ocean, and thus for their potential to harbour life. The scenario can be tested by the ESA/JUICE and Europa/Clipper missions that will be launched to the Jupiter system in April 2023 and spring 2024, respectively.

Reference: Early stages of Galilean moon formation in a water-depleted environment. Olivier Mousis, Antoine Schneeberger, Jonathan I. Lunine, Christopher R. Glein, Alexis Bouquet, and Steven D. Vance. The Astrophysical Journal Letters, February 17, 2023.