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

Abstract

revealed a contact binary planetesimal, likely the most primitive object yet visited by spacecraft (Stern et al., 2019). From Arrokoth's detailed shape, the principal axes of each individual lobe were found to be aligned to within a few degrees (Spencer et al., 2020), a configuration that suggests a co-orbiting Arrokoth before a tidally aligned, gentle (≲few m s −1

Snow Crash: Compaction Craters on (486958) Arrokoth and Other Small KBOs, With Implications

“…The differences between Figure S6a in Supporting Information and the results of the model simulations in Mao et al. (2021) are not significant in terms of final spin distributions. However, because ejecta are effectively suppressed in the compaction regime, it simplifies modeling of cratering using Arrokoth's true, complicated bilobate shape.…”

“…If we instead simply assume compaction cratering (for a highly porous target) in the limit in which all ejecta is retained (and thus angular momentum changes are a matter of simple vector addition), our results are modified as shown in Figure S6 in Supporting Information S1. The differences between Figure S6a in Supporting Information S1 and the results of the model simulations in Mao et al (2021) are not significant in terms of final spin distributions. However, because ejecta are effectively suppressed in the compaction regime, it simplifies modeling of cratering using Arrokoth's true, complicated bilobate shape.…”

“…The retention of ejecta makes spin and other dynamical evolution modeling easier, as collisions can be treated as completely inelastic, without recoil. For example, previously Mao et al (2021) studied the spin evolution of Arrokoth under its bombardment history in the cold classical region of the Kuiper belt. Ejecta escape was treated explicitly for each impact in a Monte Carlo model using standard ejecta scaling for a modestly porous granular target (i.e., a sand-like porosity closer to ∼30%-40%; Housen & Holsapple, 2011).…”

“…The slow spin of the bilobate, cold classical Kuiper belt object (CCKBO) Arrokoth (formerly 2014 MU 69 )—revealed by NASA's New Horizons in 2019—as well as its gravitational surface slope distribution and structural integrity, suggest that Arrokoth is a remarkably low‐density body, ∼250–500 kg m −3 (see Hirabayashi et al., 2020; Keane et al., 2022; Mao et al., 2021; McKinnon et al., 2020; Spencer et al., 2020; Stern et al., 2019; and below). Such a density is at the lower end of estimates for cometary nuclei (with which Arrokoth likely shares a similar formation history; Nesvorný, 2018; Morbidelli & Nesvorný, 2020), and is even lower than that of 67P/Churyumov‐Gerasimenko (67P), 532 ± 7 kg m −3 (Groussin et al., 2019).…”

Detection of Close Kuiper Belt Binaries with HST WFC3

“…The present-day Kuiper Belt consists almost exclusively of objects that formed outside of the giant planets and were either pushed outward by giant planet migration (the Neptune-resonant and scattered populations) or are still in the original orbits where they formed around the Sun (the classical Kuiper Belt objects, KBOs; McKinnon et al 2020). KBOs, and especially the cold classical KBOs (CCKBOs), which have the least perturbed heliocentric orbits, have suffered very little bombardment since their formation relative to asteroids interior of Jupiter (Mao et al 2021) and no real thermal processing like comets (Grundy et al 2020). The best constraints on the surfaces of KBOs come from the New Horizons mission, which flew past both the Pluto system and the CCKBO Arrokoth.…”