The solar nebula origin of (486958) Arrokoth, a primordial contact binary in the Kuiper Belt

Abstract

The New Horizons spacecraft's encounter with the cold classical Kuiper belt object (486958) Arrokoth (formerly 2014 MU69) revealed a contact-binary planetesimal. We investigate how it formed, finding it is the product of a gentle, low-speed merger in the early Solar System.Its two lenticular lobes suggest low-velocity accumulation of numerous smaller planetesimals within a gravitationally collapsing, solid particle cloud. The geometric alignment of the lobes indicates the lobes were a co-orbiting binary that experienced angular momentum loss and subsequent merger, possibly due to dynamical friction and collisions within the cloud or later gas drag. Arrokoth's contact-binary shape was preserved by the benign dynamical and collisional environment of the cold classical Kuiper belt, and so informs the accretion processes that operated in the early Solar System.Main Text: Following its encounter with Pluto in 2015 (1), the New Horizons spacecraft continued further into the Kuiper belt (2). This included a flyby of (486958) Arrokoth (also informally known as Ultima Thule), discovered in a dedicated Hubble Space Telescope campaign (3). Arrokoth's orbit has a semimajor axis a⨀ = 44.2 astronomical units (au), Submitted Manuscript: Confidential 3 eccentricity e = 0.037, and inclination i = 2.54°, making it a member of the cold classical Kuiper belt (CCKB), a reservoir of mainly small bodies on dynamically cold orbits, i.e., those with lowto-moderate e and low i (typically i < 5°), in the outer Solar System (4). CCKB objects have a steeper size-frequency distribution, higher binary fraction, higher albedos, and redder optical colors than the dynamically hot and Neptune-resonant populations of the Kuiper belt, implying a distinct formation mechanism and/or evolutionary history (4). CCKB objects are thought to have formed in place and remained largely undisturbed by the migration of the Solar System's giant planets (4, 5, 6), making them unperturbed remnants of the original protoplanetary disk.The encounter showed Arrokoth is a bi-lobed object, consisting of two discrete, quasiellipsoidal lobes (equivalent spherical diameters 15.9 and 12.9 km, respectively) joined at a narrow contact area or "neck" (Fig. 1) (7,8). We interpret this geometric, co-joined object as a contact binary, i.e., two formerly separate objects that have gravitated towards each other until they touch. The larger lobe (hereafter LL) is more oblate than the smaller lobe (hereafter SL) (8).Arrokoth rotates with a 15.92-hr period at an obliquity of 99° (the angle between its rotation axis and heliocentric orbital plane). The short axes of both lobes are aligned to within a few degrees of each other and with the spin axis of the body as a whole (8). The average visible and nearinfrared colors of both lobes are indistinguishable (9). Near-infrared spectral absorptions on both lobes indicate the presence of methanol ice-a common, relatively thermally stable component (for an ice) of cometary bodies and extrasolar protoplanetary disks (10). The very red optical c...

The geology and geophysics of Kuiper Belt object (486958) Arrokoth

“…S3 and Table S3)). Comets 19P, 67P, and 103P appear to be highly elongated bilobate objects, suggesting the merger of two distinct bodies, as has been proposed for Arrokoth (1,18), though for comets it is also possible that thermal evolution has generated this shape (e.g., 60). Except for 67P, whose bulk density is 538 ± 1 kg m -3 (16), the densities of the other JFC nuclei are uncertain by a factor of two or more, but all are consistent with ~500 kg m -3 (61), which implies average bulk porosities of ~50-80%.…”

A statistical review of light curves and the prevalence of contact binaries in the Kuiper Belt

“…The ~ 5 km diameter of Arrokoth's neck corresponds to n ≈ 50%. It suggests that the two lobes merged very slowly (McKinnon et al, 2020). Higher-speed mergers in the Kuiper Belt would probably result in larger necks.…”

“…The New Horizons spacecraft completed the first close reconnaissance of a small Kuiper Belt object (KBO) in January 2019. The target body, (486958) Arrokoth (also identified as 2014 MU 69 ) was revealed to be a contact binary, which resulted from the merger of two flattened, spheroidal components (McKinnon et al, 2020;Spencer et al, 2020;Stern et al, 2019; see Fig. 1).…”

Comparing the reflectivity of ungrouped carbonaceous chondrites with those of short-period comets like 2P/Encke

“…All primitive bodies in the Solar System date back to the formation era in which protoplanetary dust evolved into planetesimals. In the past years, evidence was gathered that comets in general (Skorov & Blum 2012;Blum et al 2014), comet 67P/Churyumov-Gerasimenko (Blum et al 2017;Fulle et al 2016) in particular and Kuiper-Belt object (486958) Arrokoth (McKinnon et al 2020) were formed through the gentle gravitational collapse of a cloud of cm-sized "pebbles" (dust agglomerates). If we also apply such a formation mechanism for the progenitor of the members of the TC, we can state that its maximum size is constrained by several processes: (i) Collisional destruction of the pebbles during the gravitational collapse.…”