Partial Differentiation of Callisto as Possible Evidence for Pebble Accretion

Abstract

“Planetesimal or pebble” is one of the most fundamental open questions in planet formation theory. Similarly, “satellitesimal or pebble” remains unsettled regarding the formation of the Galilean satellites. I focus on a unique characteristic of Callisto—its interior is estimated to be only partially differentiated based on gravitational field measurements. I robustly demonstrate that such a state is not achievable through satellitesimal accretion, which inevitably leads to significant differentiation, but can be maintained with pebble accretion. Pebbles can release their impact energy at the surface of the satellite, allowing efficient radiative cooling, and their impact velocities can be reduced by aerodynamic drag from the circumplanetary gas disk. If future missions such as JUpiter ICy moons Explorer confirm that Callisto is indeed only partially differentiated, it could provide the first observed evidence for the pebble accretion mechanism—not only in the context of satellite formation but also in the broader framework of planet formation.