Cratering Rates in the Outer Solar System

Abstract

Synopsis: We use several independent constraints on the number of ecliptic comets (aka JFCs) to determine impact cratering rates from Jupiter to Pluto. The impact rate on Jupiter by 1.5-km diameter ecliptic comets is currently ˙. N d > () 1 5 km = 0 005 0 003 0 006. .. − + per an-num. Long period comets and asteroids are currently unimportant on most worlds at most sizes. The size-number distribution of comets smaller than 20 km is inferred from size-number distributions of impact craters on Europa, Ganymede, and Triton; while the size-number distribution of comets bigger than 50 km is equated to the size-number distribution of Kuiper Belt Objects. The gap is bridged by interpolation. It is notable that small craters on Jupiter's moons indicate a pronounced paucity of small impactors, while small craters on Triton imply a collisional population rich in small bodies. However it is unclear whether the craters on Triton are of heliocentric or planetocentric origin. We therefore consider two cases for Saturn and beyond: a Case A in which the size-number distribution is like that inferred at Jupiter, and a Case B in which small objects obey a more nearly collisional distribution. Known craters on Saturnian and Uranian satellites are consistent with either Case, although surface ages are much younger in Case B, especially at Saturn and Uranus. At Neptune and especially at Saturn our cratering rates are much higher than rates estimated by Shoemaker and colleagues [1], presumably because Shoemaker's estimates mostly predate discovery of the Kuiper Belt. We also estimate collisional disruption rates of moons and compare these to estimates in the literature [1,2]. Discussion: By placing a heavy weight on the historical record of close encounters with Jupiter we favor generally high impact rates, especially for comets larger than a few km diameter. In particular we conclude that the satellite systems of Saturn, Uranus, and Neptune are unstable against collisionally-induced evolution over the age of the solar system. At the smaller scale we reach the opposite conclusion: comets smaller than km-size are relatively rare and small primary craters are produced less frequently than one might expect. This latter conclusion is based on data at Jupiter, where the result is not really in doubt, but we have attempted to show that the same paucity of small comets is allowed by crater counts on the moons of the more distant planets (yet neither …