Constraints on an Outer Solar System Late Heavy Bombardment from Callisto’s Interior

Introduction: One difficulty in explaining the lunar late heavy bombardment (LHB) lies in identifying a source of impactors that survives at ~3.8 to 4 Gyr, so long after planet formation [1,2]. A leading theory is that the LHB was triggered by events in the outer solar system [3,4]. Any model that invokes an outer solar system source for the lunar LHB will also produce an intense bombardment on the Galilean satellites because their effective " target areas " are larger than that of Earth's moon due to strong gravitational focusing by Jupiter [3]. During an outer solar system LHB, Jupiter's outer-most regular satellite Callisto would receive an impacting mass some ~ 40 times larger than Earth's moon [3]. The mass would be delivered in the form of a few thousand icy impactors with radii r p ~10 to a few hundreds of km. The objects would come from heliocentric orbit and have characteristic impact velocities v i ~ 15 km/s [5]. If outer solar system impactors produced the entire lunar LHB, the bombardment of Callisto would deliver about twice the amount of energy required to melt all of Callisto's ice. Plausible interior structures for Callisto based on Galileo radio tracking data include some component of mixed ice/rock in its interior, assuming hydrostatic equilibrium [6]. This implies that Callisto has never experienced widespread melting in its interior, including during its formation, long-term thermal evolution, and a possible outer solar system LHB. Thus, limits on the size of a rocky core in Callisto based on its moment of inertia constant can be used to constrain the contribution of outer solar system impactors to the lunar LHB. Impact-Induced Core Formation: Impacts with v i > 3 km/s onto a surface composed of a mixture of ice and rock will create pools of H 2 O melt. At locations where the peak shock pressure is high enough to melt ice, rock suspended in the ice will sink to the base of the melt pool and consolidate into a large rock body that can sink rapidly to the satellite's center [cf. 7]. We have constructed a simple model of core formation due to repeated impacts of icy objects with a homogeneous ice/rock callisto. Callisto begins as a homogeneous ice/rock sphere with mean density ρ=1.834 g/cm 3 [8] composed of rock with density ρ r =3.0 g/cm 3 and ice with a representative density ρ i …