Research Article

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

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Science  28 Feb 2020:
Vol. 367, Issue 6481, eaay6620
DOI: 10.1126/science.aay6620

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Examining Arrokoth

The New Horizons spacecraft flew past the Kuiper Belt object (486958) Arrokoth (also known as 2014 MU69) in January 2019. Because of the great distance to the outer Solar System and limited bandwidth, it will take until late 2020 to downlink all the spacecraft's observations back to Earth. Three papers in this issue analyze recently downlinked data, including the highest-resolution images taken during the encounter (see the Perspective by Jewitt). Spencer et al. examined Arrokoth's geology and geophysics using stereo imaging, dated the surface using impact craters, and produced a geomorphological map. Grundy et al. investigated the composition of the surface using color imaging and spectroscopic data and assessed Arrokoth's thermal emission using microwave radiometry. McKinnon et al. used simulations to determine how Arrokoth formed: Two gravitationally bound objects gently spiraled together during the formation of the Solar System. Together, these papers determine the age, composition, and formation process of the most pristine object yet visited by a spacecraft.

Science, this issue p. eaay3999, p. eaay3705, p. eaay6620; see also p. 980

Structured Abstract

INTRODUCTION

The close flyby of the Kuiper Belt object (486958) Arrokoth (formerly 2014 MU69) by NASA’s New Horizons spacecraft revealed details of the body’s structure, geology, and composition. Arrokoth is a member of the cold classical component of the Kuiper Belt, a population of dwarf planets and smaller bodies thought to be only modestly dynamically or collisionally disturbed, unlike the asteroids of the inner Solar System, comets, or other groups of Kuiper Belt objects. Data from this flyby provides the opportunity to observe the results of primordial planetesimal accretion, largely unobscured by later geological or dynamical processes.

RATIONALE

Planetesimal formation is an unsolved problem in planetary science. Many mechanisms have been proposed in which small solid particles (dust and pebbles) agglomerate into planetesimals and ultimately into planets. The flyby of Arrokoth provides data that constrain planetesimal formation theories and allow us to construct models of Arrokoth’s specific physical characteristics. The accretion processes that operated in the cold classical region of the Kuiper Belt during the formation of the Solar System are expected to have also occurred elsewhere in the protosolar nebula.

Arrokoth is a contact binary about 35 km long composed of two unequally sized lobes. Each lobe is flattened or lenticular in shape, and the planes of flattening of both (determined from their principal axes) are closely aligned, to within 5°. The smaller lobe is slightly oblong, with its long axis pointing down the long axis of the binary as a whole (to within 5°). The surface and overall structure of Arrokoth do not display any obvious signs of catastrophic or subcatastrophic collision, and the join or neck between the two lobes is narrow. Each lobe is compositionally similar to within the precision of spectral measurements.

RESULTS

We show that stresses in the neck region today are compatible with the structural integrity of Arrokoth for densities (several 100 kg m−3) and material strengths (a few kilopascals) similar to those observed in comets, but at mass scales ~1000 times the mass of typical cometary nuclei. We performed numerical simulations of collisions between two bodies on the scale of the two lobes of Arrokoth, assuming those density and strength parameters. We found that impacts at or greater than their mutual escape speed (a few meters per second) would have been highly damaging. The close geometric alignment of the lobes is highly unlikely to the be the result of a chance collision alone but can be readily understood as the result of tidal evolution of a tight, co-orbiting binary. This requires a mechanism to extract angular momentum from the binary orbit, causing the orbit to shrink, and the two components to gently merge.

Numerical models show that overdense concentrations of particles in the protosolar gas nebula can become gravitationally unstable and collapse to form planetesimals. The angular momentum in the simulated pebble clouds is high enough that formation of co-orbiting binaries is efficient and with binary characteristics that are a good match to binaries observed in the Kuiper Belt today. We examined a range of mechanisms to extract or transfer angular momentum from a co-orbiting binary and drive an ultimate merger, including mutual tides, tidal effects of the Sun (Kozai-Lidov oscillations), collisions with smaller Kuiper Belt objects, the ejection of third bodies, asymmetric radiation forces, and gas drag. We found that for bodies the size of Arrokoth, gas drag may be most effective in this merger process over the lifetime of the protosolar nebula.

CONCLUSION

We show that models of Arrokoth’s formation and evolution support accretion of the binary through the gravitational collapse of an overdense pebble cloud in the presence of protosolar nebular gas, either as a contact binary initially or as a co-orbiting binary that later inspiraled and gently merged. Similar accretional processes and binary planetesimal formation likely occurred throughout the early Solar System.

Simulated maximum accelerations experienced by particles during low-velocity impact of two spheres, approximating the scale of the two lobes of Arrokoth.

Spheres are modeled as granular aggregates with bulk densities of 500 kg m−3 and an impact speed of 2.9 m s−1 at a tangent angle of 80°; such gentle collisional conditions are necessary to preserve Arrokoth’s overall undamaged shape. Extreme reds and blues correspond to the greatest and least maximum accelerations experienced, respectively. The maximum disturbance is concentrated in the narrow contact area, or neck, between the two bodies.

Abstract

The New Horizons spacecraft’s encounter with the cold classical Kuiper Belt object (486958) Arrokoth (provisional designation 2014 MU69) revealed a contact-binary planetesimal. We investigated how Arrokoth formed and found that 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 cloud of solid particles. The geometric alignment of the lobes indicates that they were a co-orbiting binary that experienced angular momentum loss and subsequent merger, possibly because of 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 therefore informs the accretion processes that operated in the early Solar System.

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