Origin of craters on Phoebe: comparison with Cassini's data

TL;DR

This study analyzes Phoebe's craters using Cassini data, comparing observed features with simulated impacts from trans-Neptunian objects, suggesting most craters formed early in Solar System history.

Contribution

It provides a new method to estimate crater numbers and rates on Phoebe from simulations of Centaur impacts, linking crater features to early Solar System events.

Findings

Main craters likely formed early, not in current Solar System configuration.

Current impact rates are too low to account for observed craters.

Craters probably formed when Phoebe was captured early in Solar System evolution.

Abstract

Phoebe is one of the irregular satellites of Saturn; the images taken by Cassini-Huygens spacecraft allowed us to analyze its surface and the craters on it. We study the craters on Phoebe produced by Centaur objects from the Scattered Disk (SD) and plutinos escaped from the 3:2 mean motion resonance with Neptune and compare our results with the observations by Cassini. We use previous simulations on trans-Neptunian Objects and a method that allows us to obtain the number of craters and the cratering rate on Phoebe. We obtain the number of craters and the greatest crater on Phoebe produced by Centaurs in the present configuration of the Solar System. Moreover, we obtain a present normalized rate of encounters of Centaurs with Saturn of $\dot F = 7.1 \times 10^{-11}$ per year, from which we can infer the current cratering rate on Phoebe for each crater diameter. Our study and the comparison with the observations suggest that the main crater features on Phoebe are unlikely to have been produced in the present configuration of the Solar System and that they must have been acquired when the SD were depleted in the early Solar System. If this is what happened and the craters were produced when Phoebe was a satellite of Saturn, then it had to be captured, very early in the evolution of the Solar System.