Planetocentric versus heliocentric impacts in the Jovian and Saturnian Satellite System

Abstract

Readily applicable equations are derived for calculating the impact velocities, collision intervals, and cumulative crater frequencies in a satellite system for planetocentric and heliocentric impactors. Observed cumulative crater frequencies in the Saturnian satellite system and a sometimes observed lack of apex‐antapex asymmetry of crater frequencies favor crater‐producing projectiles orbiting initially in elliptic orbits round the parent planet (planetocentric impactors). On the other hand, a sometimes observed asymmetry of crater size distribution and short collision intervals (<104 years) of planetocentric projectiles favor heliocentric impactors. We conclude that cratering on the Jovian and Saturnian satellites is produced by heliocentric projectiles as well as by captured planetocentric impactors. Small satellites could be crushed several times by major impacts and reaccrete from planetocentric orbits.

Orbital Evolution of Impact Ejecta from Ganymede

“…Eventually they impact in time frames ranging from a few months to a few thousand years. The time scales are consistent with those obtained by Horedt and Neukum (1984), who for example give a mean time interval for collisions with Ganymede of 1200 years for a particle with eccentricity e = 0.6, inclination i ∼ 15…”

“…Impact craters on Ganymede are more isotropically distributed than would be expected if they were mostly made by comets falling from heliocentric orbits (Shoemaker and Wolfe 1982, Horedt and Nuekum 1984, Zahnle et al 2001; had this been the case, we would expect to see 15-to 70-fold asymmetry between crater counts on the leading side vs the trailing side (Shoemaker and Wolfe 1982, Horedt and Neukum 1984, Zahnle et al 2001. A similar statement could be made for almost every other satellite in the outer Solar System.…”

“…According to Horedt and Neukum (1984), "when satellites are cratered by planetocentric projectiles of moderate eccentricity, no marked crater frequency asymmetry is expected because the relative velocity between projectile and satellite is approximately symmetric with respect to the satellite's circular orbital velocity." In other words, we expect the distribution of impact sites from these planetocentric impactors to be random on prograde, synchronously rotating satellites such as Ganymede.…”

“…By planetocentric one means that the impacting bodies were themselves satellites of the planet in orbits akin to those of the satellites that remain. Craters made by such a population are isotropically distributed across the surface of a prograde, synchronously locked satellite (Horedt and Neukum 1984). They can help explain what we see in the outer satellites by diluting the expected asymmetry.…”

Effects of the External Environment on Icy Satellites

“…The oldest surface on any of the Saturnian satellites is found on Iapetus and is estimated to be on the order of 4.4 Ga Castillo-Rogez et al 2007). Lunar-like model chronologies for every satellite can then be derived comparing inter-satellite impact and cratering rates within each satellite system (e,g., Horedt and Neukum 1984b). For the Zahnle et al (1998Zahnle et al ( , 2003 models, the uncertainties of each model are also included in the diagrams.…”