Early impact chronology of the icy regular satellites of the outer solar system

The dating of the surfaces of the giant planets' regular satellites is hampered by two issues. The first is the lack of known samples from which radiometric ages can be measured. The second is a potentially different cratering chronology compared to those of the Moon and Mars, due to a difference in the impactor population and planetary system architecture, making a direct comparison and absolute ages derivation difficult. That leaves modelling as the only way to obtain something close to absolute surface ages. Here we applied and reported an improved chronological model of the outer solar system during an episode of dynamical instability caused by giant planet migration. We have combined a series of dynamical and Monte Carlo simulations with various crater scaling laws, impactor size-frequency distributions and observational crater densities to compute the expected crater densities on the surfaces of the regular satellites of Jupiter, Saturn and Uranus. From this, we derived their oldest surface ages. Our impactors are sourced from heliocentric planetesimals that originated from the disk of small bodies exterior to the orbit of the giant planets, and the irregular satellites, which were likely captured by the mutual encounters between the giant planets. With both crater scaling laws and the assumption that the giant planets began their migration at 4.5 Ga, most satellites' surfaces were dated to be older than 4.3 Ga, apart from the inner and small satellites, i.e. Mimas, Enceladus, Hyperion and Miranda. Their estimated surface ages are ca. 4 Ga, although these bodies' masses are expected to have been heavily eroded and if they existed prior to giant planet migration they were likely to be destroyed. Our prediction of old surface ages for the mid-sized Saturnian satellites has implications for their formation, their tidal evolution, the interior of Saturn and the ages of the rings.