As NASA’s New Horizons spacecraft presses on into the dark and distant reaches of the Kuiper Belt, scientists have been exploring the trove of data collected during its recent encounter with Arrokoth, a mysterious and ancient space rock that, at 4.1 billion miles from Earth, is the farthest object ever visited by a spacecraft.
This week, three papers published in the journal Science report new findings on the ancient object that give us a unique window into deep cosmic time, including insights into how it formed and details on the exotic organic molecules that give it its red color.
With roughly ten times more data than was available for an initial report last year, the studies explore the distant rock’s geology, composition, and history in far greater detail. Among the highlights are a clearer picture of how Arrokoth formed in the early solar nebula, a cloud of gas and dust that surrounded the sun 4.5 billion years ago, and details of its chemistry, which seems to include both simple organic molecules and “tholins,” a class of carbon-based polymers that are believed to be prevalent throughout the Kuiper Belt.
Overall, the new findings support the idea that Arrokoth is a primordial time capsule that can help lift the fog on the earliest chapters of our cosmic history.
Most of the new clues to Arrokoth’s origin story come from its outward appearance. As New Horizons approached Arrokoth in the lead up to the flyby, it became clear the object was bi-lobed; a “contact binary” that formed when two rocks touched. Using numerical models, a team led by astronomer Will McKinnon of Washington University in St. Louis has now fleshed out the picture further. Their results suggest the merger was less of a collision and more a very gentle joining of two objects that co-evolved as a cloud of nebula material collapsed under its own gravity.
According to McKinnon, the idea that objects like Arrokoth can form from collapsing dust clouds has been around for a while. But without seeing one up close, it was hard to rule out other theories, like a series of more violent collisions. With the alignment and shape of Arrokoth’s two lobes matching this calmer formation model, astronomers can be more confident that other Kuiper Belt Objects formed similarly.
A separate paper focused on Arrokoth’s geology argues that its cratering history is consistent with an object that formed over 4 billion years ago in the solar nebula. Finally, Arrokoth’s relatively uniform chemical composition and color support the idea that it evolved from a single blob of particles rather than a more jumbled assortment of rocks.
“All the characteristics of Arrokoth fit this theory,” McKinnon said. “And they don’t fit the other theory of something building up from small to large in a sequence of impacts.”
“I think Arrokoth is really neat because we’ve never seen an object that looks the way it would have right after forming by this mechanism,” said Lowell Observatory astronomer Will Grundy, who led up the analysis of Arrokoth’s chemistry.
But while Grundy’s results help solve the mystery of how Arrokoth formed, they also raise new questions about where the space rock’s exotic organic compounds came from.
As early as 2016 scientists had hints that Arrokoth was red. A study published last year confirmed this to be the case. At the time, the researchers’ best explanation was that Arrokoth was rich in tholins, complex organic polymers that are reddish in color and form in chilly outer solar system environments—including, scientists think, in Pluto’s atmosphere—when simpler carbon-bearing molecules interact with UV light or cosmic rays.
The new analysis, which examines far more of the data collected by the color camera and infrared spectrometer on New Horizons’ Ralph Instrument, supports the idea that Arrokoth is chock full of tholins. The study also found that Arrokoth is rich in methanol ice, a simpler organic molecule, and surprisingly little, if any, water ice.
Unpacking the history of Arrokoth’s exotic organics offers a different sort of window into deep time. Tholins, Grundy says, could have formed on Arrokoth’s surface or in the solar nebula. Or they could hail from even further back in time, to the giant molecular cloud that collapsed to form our Sun.
Ashley Walker, an aspiring astrochemist who did her senior thesis studying tholins on Saturn’s moon Titan with Johns Hopkins planetary scientist Sarah Hörst, said that even though we expect to these molecules to form in the outer solar system, “it’s really intriguing to see them in further out, colder places.” Sleuthing out their origins in the ancient proto-solar system, she said, “can give us clues to our very own Earth.”
While these latest papers “wrap up the [Arrokoth] encounter with a nice bow,” as McKinnon put it, the story isn’t over for New Horizons. The spacecraft continues to sail deeper into the Kuiper Belt on its way to interstellar space. And while there are no further encounters on the agenda for now, this summer, astronomers will use the Subaru telescope in Hawaii to scan the sky for any additional objects in the spacecraft’s vicinity. If they find one, they might be able to arrange a third rendezvous.
“It’s a low probability, but if we don’t look, the chances of us finding an object are zero,” McKinnon said. “So, we’re going to look, and we’ll tell you in a few months if we’ve found a target. It’s not over till it’s over.”