Collisions of Small Kuiper Belt Objects With (486958) Arrokoth: Implications for Its Spin Evolution and Bulk Density

The orientation and morphology of the bilobate, cold classical Kuiper belt object (486958) Arrokoth (formerly 2014 MU₆₉) is consistent with a slow, tidal merger of a close binary. However, the discrepancy between Arrokoth's present-day rotation (15.9 hr) and synchronous rotation for nominal cometary densities near ∼500 kg/m³ implies reduction (up to 30%) in post-merger spin angular momentum. We investigate how collisions with dynamically cold and hot classical Kuiper belt objects might have affected Arrokoth's post-merger spin. Using a dynamically equivalent triaxial ellipsoid, 5000 Monte Carlo simulations of 100 impacts each, consistent with Arrokoth's cratering record, were carried out. Starting from the assumption of critical, synchronous rotation for a given density, these simulations rarely reproduce Arrokoth's present spin period, unless its true density is near 250 kg/m³. We explore in greater depth the effects of formation of Arrokoth's largest crater (now officially named Sky, previously informally named Maryland). We adopt point-source scaling and randomly select impact parameters that lead to the crater, using Arrokoth's full bilobate shape. Results from Sky's formation alone are similar to those considering a full range of impactor sizes, unless we adopt low cratering efficiency due to high porosity, which implies substantially larger Sky-forming impactors. Overall, results imply that the probability of substantial angular momentum change due to impacts alone over Solar System history is unlikely, and spindown from a synchronous, tidal rotation rate to a 15.9-hr period unlikely unless Arrokoth itself was and is a very low density object (∼250 kg/m³), though we cannot statistically rule out densities up to 400 kg/m³.