Constraining the final merger of contact binary (486958) Arrokoth with soft-sphere discrete element simulations

In 2019, the New Horizons spacecraft flew by the Kuiper belt object (486958) Arrokoth. Images returned from the New Horizons flyby reveal that Arrokoth is a contact binary composed of two distinct lobes joined by a narrow contact region (the “neck”). The long axis of the whole body measures about 35 km (Stern et al., Icarus 2019). Cold classical Kuiper belt objects like Arrokoth are thought to be among the most primitive bodies in our solar system, forming in their current positions from the solar nebula over 4 Gya and remaining nearly undisturbed since. The mechanism by which Arrokoth formed is thus of great interest and may lend unique insight into the formation of terrestrial bodies in the outer solar system. Under the assumption that Arrokoth is the union of two distinct progenitor bodies, we use a numerical code to model the final stage of the merger that created this unique object. We investigate a range of possible merger parameters and use the results to determine the most plausible scenarios. The work presented here is described in greater detail in Marohnic et al. 2020, in press for the Icarus special issue on Kuiper belt science. We use the parallel N-body code “pkdgrav” to model the Arrokoth merger. pkdgrav uses a soft-sphere discrete element method to compute collisions between spherical particles. Restoring forces are modeled as damped springs and static, rolling, and twisting friction are accounted for as well. The progenitor bodies are modeled as “rubble piles” composed of up to 200,000 spherical particles altogether. Our simulations test the effects of varying impact angle and speed, material cohesion strength, body shape, bulk density, and spin. The results of our suite of simulations suggest that any merger that could produce an object like Arrokoth would have to have been quite slow (roughly equal to the mutual escape speed of the progenitor objects or slower) and at a near-grazing angle. We find that the most plausible scenario is a gradual inspiral of two bodies in a close synchronous orbit leading to final, gentle merger.

The simulations described here were performed on the Deepthought 2 cluster at the University of Maryland, College Park. This work was supported by NASA grant NNX15AH90G awarded by the Solar System Workings program, by a University of Maryland Graduate School Research and Scholarship Award, and by NASA’s New Horizons project via contracts NASW-02008 and NAS5-97271/TaskOrder30.