Arrokoth, an object far beyond Pluto, a haunting image reminiscent of a Stone Age carving, has remained undisturbed since it first formed billions of years ago. Data from NASA’s New Horizons mission are providing new insight into how planets and planetesimals – the building blocks of the planets – were formed in a local collapse of a cloud in the solar nebula.
“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 Alan Stern, of the Southwest Research Institute in Boulder, Colorado.
An intricate dance
“Just as fossils tell us how species evolved on Earth, planetesimals tell us how planets formed in space,” said William McKinnon, a New Horizons co-investigator from Washington University in St. Louis, and lead author of an Arrokoth formation paper in Science this week. “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.”
The New Horizons spacecraft flew past the ancient Kuiper Belt object Arrokoth (2014 MU69) on Jan. 1, 2019, providing humankind’s first close-up look at one of the icy remnants of solar system formation in the vast region beyond the orbit of Neptune. Using detailed data on the object’s shape, geology, color and composition – gathered during a record-setting flyby that occurred more than four billion miles from Earth – researchers have apparently answered a longstanding question about planetesimal origins.
The image above shows the Kuiper Belt object resembling a prehistoric stone carving formed from a small, uniform, cloud of material in the solar nebula, rather than a mishmash of matter from more separated parts of the nebula, bolstering the theory that Arrokoth formed in a local collapse of a cloud in the solar nebula.(NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Roman Tkachenko)
The team reports those findings in a set of three papers in the journal Science, and at a media briefing Feb. 13 at the annual American Association for the Advancement of Science meeting in Seattle.
Revealed in first post-flyby images
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 Science last May.
“This is truly an exciting find for what is already a very successful and history-making mission” said 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.”
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 22-mile long object New Horizons observed.
Gravity-driven collapse of a cloud
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.
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 Will Grundy, New Horizons composition theme team lead from Lowell Observatory in Flagstaff, Arizona, and the lead author of a second Science paper. “An object like Arrokoth wouldn’t have formed, or look the way it does, in a more chaotic accretion environment.”
The latest Arrokoth reports significantly expand on the May 2019 Science paper, led by Stern. The three 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,” Stern said.
The New Horizons spacecraft is now 4.4 billion miles (7.1 billion kilometers) from Earth, operating normally and speeding deeper into the Kuiper Belt at nearly 31,300 miles (50,400 kilometers) per hour.
The Daily Galaxy, Sam Cabot, via New Horizons Mission