Context: The Galilean moons are expected to have formed in a circumplanetary disk (CPD) and exhibit a characteristic compositional gradient: an increasing ice mass fraction with increasing distance from Jupiter. Ice released from hydrated silicates formed inside planetesimals might be the cause of this compositional gradient. One of the mechanisms known to cause significant heat in the early solar system is radiogenic heating by 26𝐴𝑙, so a different formation time for each Galilean moon could potentially explain a fully dehydrated Io, a low ice mass faction on Europa and ∼ 50% ice mass fractions on Ganymede and Callisto.
Aims: We aim to determine the mass fraction of hydrated silicates of the planetesimals in Jupiter’s CPD formed by radiogenic heating of 26𝐴𝑙 to constrain the formation times of the Galilean moons.
Methods: This is done using a numerical thermal evolution model capable of evolving planetesimals over time and altering the composition based on the internal temperature. Planetesimals are initiated with an ice mass fraction, temperature and ammonia ice concentration similar to the dust in the CPD. Their interior compositions alter by aqueous alteration, differentiation and dehydration.
Results: Hydration of rocks inside planetesimals does not occur for formation times later than 4 Myr after Ca-Al inclusion (CAI) formation, and a mass fraction of hydrous rocks of 73% is formed for 3 Myr after CAI formation. Larger planetesimals able that produce hydrated silicates dominate the compositional evolution of the population. A reduction in ammonia concentration in the ice by 50% increases the time needed to create hydrated silicates at 3 Myr after CAI formation from 1.5 Myr to > 2 Myr.
Conclusions: If Io formed before 4 Myr, it should have lost its ice no later than 1 Myr to prevent aqueous alteration. The low ice mass fraction on Europa could be explained by planetesimals formed at 3 Myr after CAI formation over a period of 1.5 Myr with a hydrous rock mass fraction of 66%, able to release 6.3% ice mass by dehydration. If the interior of Ganymede differentiated by radiogenic heating, the planetesimals should be formed before 4 Myr after CAI formation. Callisto’s partially differentiated interior requires formation times of planetesimals after 4 Myr after CAI formation.