Evidence for “pebble accretion” in Jupiter’s moon Callisto—Partial differentiation provides clues to the origin of planets and moons

Specially Appointed Assistant Professor Yuhito Shibaike at the Amanogawa Galaxy Astronomy Research Center investigated how the internal temperature of Jupiter’s giant moon Callisto evolved under various formation conditions. He found that its internal structure can only be explained by a formation mechanism known as “pebble accretion.”

Two main theories have been proposed for satellite formation: accretion of satellitesimals (objects 1–100 km in size) and accretion of pebbles (1 cm to 1 m in size). However, it remains unclear which mechanism is dominant.

Shibaike clearly showed that Callisto’s partially differentiated interior cannot be explained by satellitesimal accretion, which would inevitably lead to a fully differentiated structure. In contrast, pebble accretion can maintain partial differentiation. Pebbles release their impact energy at the surface, allowing efficient radiative cooling, and their impact speeds are reduced by aerodynamic drag from the gas disk surrounding proto-Jupiter.

If JUICE (JUpiter ICy moons Explorer) confirms that Callisto is only partially differentiated, it could provide the first observational evidence for the pebble accretion mechanism—not only in satellite formation but also in the broader context of planet formation.

　More details about the study can be found here