On January 1, 2019, NASA’s New Horizons spacecraft flew past the Kuiper Belt object (KBO) Arrokoth — provisional designation 2014 MU69, previously nicknamed ‘Ultima Thule’ — at a distance of 3,538 km (2,200 miles). In three new studies published in the journal Science, researchers analyzed new data and images from the historic flyby and found that: Arrokoth is uniformly red, cold, and covered with methanol ice and unidentified complex organic molecules; it has no detectable rings, and no satellites within a radius of 8,000 km (5,000 miles); it has a lightly-cratered smooth surface with complex geological features, unlike those on previously visited solar system bodies; Arrokoth’s binary lobes are less flat than initially inferred and have larger volumes than previous estimates suggested; the lobes were previously independent bodies formed close together that assembled into the present-day object very gently.
This composite image of Arrokoth was compiled from data obtained by NASA’s New Horizons spacecraft as it flew by the object on January 1, 2019. The image combines enhanced color data (close to what the human eye would see) with detailed high-resolution panchromatic pictures. Image credit: NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute / Roman Tkachenko.
“Arrokoth is the most distant, most primitive and most pristine object ever explored by spacecraft, so we knew it would have a unique story to tell,” said New Horizons principal investigator Dr. Alan Stern, a researcher in the Southwest Research Institute.
“It’s teaching us how planetesimals formed, and we believe the result marks a significant advance in understanding overall planetesimal and planet formation.”
The first post-flyby images transmitted from New Horizons last year showed that Arrokoth had two connected lobes, a smooth surface and a uniform composition, indicating it was likely pristine and would provide decisive information on how bodies like it formed. These first results were published in the journal Science in May 2019.
Over the following months, working with more and higher-resolution data as well as sophisticated computer simulations, the mission team assembled a picture of how Arrokoth must have formed.
Their analysis indicates that the lobes of this contact binary object were once separate bodies that formed close together and at low velocity, orbited each other, and then gently merged to create the 36-km (22 miles) long object New Horizons observed.
This indicates Arrokoth formed during the gravity-driven collapse of a cloud of solid particles in the primordial Solar Nebula, rather than by the competing theory of planetesimal formation called hierarchical accretion.
Unlike the high-speed collisions between planetesimals in hierarchical accretion, in particle-cloud collapse, particles merge gently, slowly growing larger.
“Just as fossils tell us how species evolved on Earth, planetesimals tell us how planets formed in space,” said New Horizons co-investigator Dr. William McKinnon, a scientist at Washington University in St. Louis.
“Arrokoth looks the way it does not because it formed through violent collisions, but in more of an intricate dance, in which its component objects slowly orbited each other before coming together.”
Two other important pieces of evidence support this conclusion.
The uniform color and composition of Arrokoth’s surface shows the KBO formed from nearby material, as local cloud collapse models predict, rather than a mishmash of matter from more separated parts of the nebula, as hierarchical models might predict.
The flattened shapes of each of Arrokoth’s lobes, as well as the remarkably close alignment of their poles and equators, also point to a more orderly merger from a collapse cloud.
Further still, Arrokoth’s smooth, lightly cratered surface indicates its face has remained well preserved since the end of the planet formation era.
“Arrokoth has the physical features of a body that came together slowly, with ‘local’ materials in the Solar Nebula,” said New Horizons project member Dr. Will Grundy, a researcher at Lowell Observatory.
“An object like Arrokoth wouldn’t have formed, or look the way it does, in a more chaotic accretion environment.”
The new papers are based on 10 times as much data as the first report, and together provide a far more complete picture of Arrokoth’s origin.
“All of the evidence we’ve found points to particle-cloud collapse models, and all but rule out hierarchical accretion for the formation mode of Arrokoth, and by inference, other planetesimals,” Dr. Stern said.
“This is truly an exciting find for what is already a very successful and history-making mission,” said Dr. Lori Glaze, director of NASA’s Planetary Science Division.
“The continued discoveries of NASA’s New Horizons spacecraft astound as it reshapes our knowledge and understanding of how planetary bodies form in solar systems across the Universe.”
_____
W.M. Grundy et al. Color, composition, and thermal environment of Kuiper Belt object (486958) Arrokoth. Science, published online February 13, 2020; doi: 10.1126/science.aay3705
J.R. Spencer et al. The geology and geophysics of Kuiper Belt object (486958) Arrokoth. Science, published online February 13, 2020; doi: 10.1126/science.aay3999
W.B. McKinnon et al. The solar nebula origin of (486958) Arrokoth, a primordial contact binary in the Kuiper Belt. Science, published online February 13, 2020; doi: 10.1126/science.aay6620
