RESEARCH ARTICLE SUMMARY
◥OUTER SOLAR SYSTEM
The solar nebula origin of (486958) Arrokoth,
a primordial contact binary in the Kuiper Belt
W. B. McKinnon*,D.C.Richardson,J.C.Marohnic,J.T.Keane,W.M.Grundy,D.P.Hamilton,D.Nesvorný,
O. M. Umurhan, T. R. Lauer, K. N. Singer, S. A. Stern, H. A. Weaver, J. R. Spencer, M. W. Buie, J. M. Moore,
J. J. Kavelaars, C. M. Lisse, X. Mao, A. H. Parker, S. B. Porter, M. R. Showalter, C. B. Olkin, D. P. Cruikshank,
H. A. Elliott, G. R. Gladstone, J. Wm. Parker, A. J. Verbiscer, L. A. Young, the New Horizons Science Team
INTRODUCTION:ThecloseflybyoftheKuiper
Belt object (486958) Arrokoth (formerly 2014
MU 69 )byNASA’sNewHorizonsspacecraft
revealed details of the body’sstructure,geol-
ogy, 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 dynami-
cally or collisionally disturbed, unlike the as-
teroids 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 accre-
tion, largely unobscured by later geological or
dynamical processes.
RATIONALE:Planetesimal formation is an un-
solved problem in planetary science. Many
mechanisms have been proposed in which
small solid particles (dust and pebbles) agglom-
erate 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 (deter-
mined 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 ofArrokoth do not display any obvious signs of
catastrophic or subcatastrophic collision, and
the join or neck between the two lobes is nar-
row. 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 struc-
tural integrity of Arrokoth for densities (sev-
eral 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 typi-
cal cometary nuclei. We performed numerical
simulations of collisions between two bodies
on the scale of the two lobes of Arrokoth, as-
suming 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 evolu-
tion 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 con-
centrations 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 charac-
teristics that are a good match to binaries
observed in the Kuiper Belt today. We exam-
ined a range of mechanisms to extract or trans-
fer 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 ejectionofthirdbodies,asym-
metric radiation forces, and gas drag. We found
that for bodies the size of Arrokoth, gas drag
maybemosteffectiveinthismergerprocess
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 ac-
cretional processes and binary planetesimal
formation likely occurred throughout the early
Solar System.
▪RESEARCH
McKinnonet al.,Science 367 , 1000 (2020) 28 February 2020 1of1
The list of author affiliations is available in the full article online.
*Corresponding author. Email: [email protected]
Cite this article as W. B. McKinnonet al.,Science 367 ,
eaay6620 (2019). DOI: 10.1126/science.aay6620Acceleration (cm s - 2)0.035 44 880 5
km 04 530. 488Simulated 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.
ON OUR WEBSITE◥Read the full article
at http://dx.doi.
org/10.1126/
science.aay6620
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