The Bombardment History of the Giant Planet Satellites

Abstract

The origins of the giant planet satellites are debated, with scenarios including formation from a protoplanetary disk, sequential assembly from massive rings, and recent accretion after major satellite–satellite collisions. Here, we test their predictions by simulating outer solar system bombardment and calculating the oldest surface ages on each moon. Our crater production model assumes the projectiles originated from a massive primordial Kuiper Belt (PKB) that experienced substantial changes from collisional evolution, which transformed its size frequency distribution into a wavy shape, and Neptune’s outward migration, which ejected most PKB objects onto destabilized orbits. The latter event also triggered an instability among the giant planets some tens of Myr after the solar nebula dispersed. We find all giant planet satellites are missing their earliest crater histories, with the likely source being impact resetting events. Iapetus, Hyperion, Phoebe, and Oberon have surface ages that are a few Myr to a few tens of Myr younger than when Neptune entered the PKB (i.e., they are 4.52–4.53 Gyr old). The remaining midsized satellites of Saturn and Uranus, as well as the small satellites located between Saturn’s rings and Dione, have surfaces that are younger still by many tens to many hundreds of Myr (4.1–4.5 Gyr old). A much wider range of surface ages are found for the large moons Callisto, Ganymede, Titan, and Europa (4.1, 3.4, 1.8, and 0.18 Gyr old, respectively). At present, we favor the midsized and larger moons forming within protoplanetary disks, with the other scenarios having several challenges to overcome.

Geologic Constraints on the Formation and Evolution of Saturn’s Mid-Sized Moons

“…Overall, the impactor size frequency distribution (SFD) represented by Case A matches expectations for a heliocentric population coming from the primordial Kuiper belt that has experienced substantial collisional evolution (e.g., Bottke et al. 2023 , 2024 ). In addition, the New Horizons mission acquired crater data at Pluto and Charon that is a much closer match to Case A than Case B (Singer et al.…”

“…The bombardment predictions were revised downward by Bottke et al. ( 2024 ), who found the expected impact flux was a factor of 5 smaller than previously suggested, enough that volatiles on the icy worlds could survive early bombardment.…”

“… 2019 , 2021 , 2023 ; Bottke et al. 2024 ). The models estimate the surface ages of the moons to be >4 Gyr old.…”

“…However, the lack of craters at the largest sizes, as compared with Iapetus and model predictions for early heliocentric bombardment, suggest that the surfaces are not primordial either (Kirchoff and Schenk 2010 ; Bottke et al. 2024 ).…”

“… 2015 ; Bottke et al. 2023 , 2024 ), particularly if all of the moons formed contemporaneously in the CPD. Moons that emerged from the rings (e.g., Charnoz et al.…”