Formation of the Ganymede/Callisto Dichotomy by Impacts During the Late Heavy Bombardment

Details Formation of the Ganymede/Callisto Dichotomy by Impacts During the Late Heavy Bombardment Formation of the Ganymede/Callisto Dichotomy by Impacts During the Late Heavy Bombardment

Sep 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009dps....41.5505b&link_type=abstract

American Astronomical Society, DPS meeting #41, #55.05

Mathematics

Logic

Scientific paper

Despite their similar sizes and compositions, Ganymede and Callisto have followed different evolutionary pathways. Ganymede has experienced extensive geological activity and has a large rock core. Callisto's surface is ancient. Core formation in Callisto has apparently been incomplete; its interior has layers of mixed ice and rock (Anderson et al., 2001). We show that the Ganymede/Callisto dichotomy would have likely arisen during an outer solar system late heavy bombardment (LHB). It has been proposed that the lunar LHB was triggered by dynamical events in the outer solar system (Levison et al. 2001; Gomes et al., 2005; Tsiganis et al., 2005). Resulting cometary LHB impacts onto Ganymede and Callisto melt their ice/rock surfaces, allowing denser rock to sink to the satellites’ centers. Once core formation in Ganymede or Callisto is 50% complete, it becomes an energetically self-sustaining process ("runaway differentiation"). We have constructed a 3 dimensional model of impact-induced core formation and find that during an outer solar system LHB, Ganymede undergoes runaway differentiation, but Callisto does not, consistent with their present interior states. The dichotomy is created if the disk supplying LHB cometary impactors contains between about 5 to 21 Earth-masses, consistent with simulations of a disk-scattering event that creates the present day solar system architecture and the lunar LHB. We also derive a limit on the total mass of large objects that have impacted the satellites over their entire history, based on the requirement that Callisto avoids runaway differentiation. Our results imply that the Ganymede/Callisto dichotomy would arise naturally as a consequence of proposed dynamical sculpting processes in the outer solar system.

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