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

Abstract

The Cold Classical Kuiper Belt, a class of small bodies in undisturbed orbits beyond Neptune, is composed of primitive objects preserving information about Solar System formation. In January 2019, the New Horizons spacecraft flew past one of these objects, the 36-kilometer-long contact binary (486958) Arrokoth (provisional designation 2014 MU69). Images from the flyby show that Arrokoth has no detectable rings, and no satellites (larger than 180 meters in diameter) within a radius of 8000 kilometers. Arrokoth has a lightly cratered, smooth surface with complex geological features, unlike those on previously visited Solar System bodies. The density of impact craters indicates the surface dates from the formation of the Solar System. The two lobes of the contact binary have closely aligned poles and equators, constraining their accretion mechanism.

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

“…The very red optical colors of both lobes are similar to other CCKB objects (9), and consistent with space weathering of simple ices to produce organic compounds, although other sources of reddening are also possible [e.g., iron and sulfur compounds (9)]. LL and SL both appear to be intact, or at least little disturbed, with no obvious morphological signs of a violent or energetic merger (7,8).We examine the implications of these findings for the planetesimal formation process within the Kuiper belt, which might be broadly applicable throughout the primordial Solar System. We focus on binary formation in the outer Solar System, which appears to have been common in the Kuiper belt, based on the abundance of binaries detected there in telescopic surveys (11,12).…”

“…The generally ellipsoidal to lenticular shapes of Arrokoth's two lobes, and their general smoothness at scales resolved by the available images (7,8), nevertheless resemble equilibrium figures, perhaps obtained in the past. It is possible that the flattened shapes of both lobes were acquired as they rapidly accreted in a pebble cloud undergoing gravitational collapse, We infer that before their final merger, the LL and SL lobes were already aligned.…”

“…References (101-118) estimates scale inversely with the assumed contact area (we adopted 23 km 2 [8]). The minimum mean slope occurs for a bulk density of ~240 kg m -3 (cf.…”

“…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.…”

“…Figure 2 shows the gravitationally induced stress at the neck, either compressional or tensile, as a function of the assumed bulk density. The contact area between the two lobes is approximately 23 km 2 (8). We calculated the gravitational attraction between the lobes from the external gravitational potential of a homogeneous triaxial ellipsoid (32), integrated over the mass distribution of the other ellispsoid.…”