Following its successful rendezvous with Pluto, the New Horizons spacecraft was sent on towards a smaller object out in the Kuiper Belt. As it shot past, the spacecraft captured images of a small world consisting of two very distinct lobes, with properties that scientists found a bit confusing. But details would have to wait, as the combination of distance and power budget meant that transmitting much of New Horizons’ data back to Earth was a slow process.
The wait for that data is now over, as the high-resolution imagery is now available, and scientists have used it to try to better understand the formation and structure of what is now known as Arrokoth (named for the Powhatan word for “sky”). While the data doesn’t answer every question we might have about Arrokoth, it does give us some very good ideas about how such a strange structure could have formed.
Can’t stay long
New Horizons was the fastest probe as launched from Earth (others have since picked up more speed thanks to gravity assists), and Arrokoth is very small, meaning the probe had to get rather close before it could be imaged in any detail. That left a narrow window for gathering data during the flyby, but the papers released today details just how narrow it was. As little as two days before the closest pass, Arrokoth was still showing up as a single pixel in New Horizons’ cameras. It didn’t grow larger than 10 pixels until about a half day before. So the vast majority of the data comes from a window that’s just 12 hours wide.
Still, during that time, the cameras on board New Horizons captured images that probed the composition of Arrokoth, and they were able to resolve features as small as 33 meters across on its surface. The papers that ensued describe the body’s structure, model its history, and take some guesses at its chemistry.
Arrokoth comes from a region of the Solar System called the Kuiper Belt, specifically from an area that’s outside the gravitational influence of Neptune, the outermost large planet. In this region, there was enough material to form icy bodies, but it was spread so thinly that the bodies seem to have remained small, without interacting frequently enough to form larger planets. Neptune’s influence scattered some of the Kuiper Belt objects further inward, where collisions with other bodies were more likely and the influence of the Sun was stronger. But Arrokoth currently orbits beyond the point where that was likely to happen.
If true, that means the object is likely to be comprised of material that is largely unchanged since the formation of the Solar System. And, by all indications, it is true. Evaluations of the crater density on the surface of Arrokoth is consistent with an age of four billion years, that of the Solar System itself. And the surface has the red color typical of other objects from this region of the Kuiper belt, suggesting that its surface hasn’t seen significant chemical modification.
The red color seems to come from a complicated mix of longer-chain hydrocarbons collectively called tholins. These are built by chemical reactions among shorter molecules driven by radiation exposure. In Arrokoth’s case, those shorter molecules appear to include methanol, a single-carbon alcohol, and the only individual chemical clearly identified in the New Horizons data. Methanol could have formed by chemical reactions between methane and water, but there’s only weak indications of the presence of water on Arrokoth, and no clear signature of methane. It’s possible—even likely, given what we know about the Kuiper Belt and other objects from it—that they’re present below the surface, but that hasn’t been confirmed by this flyby.
Whatever it’s made of, Arrokoth isn’t very dense, and is likely similar to comets in this regard. If it’s less than half as dense as a typical comet, it’s spinning fast enough that it would come apart. Too much more dense, and the two lobes would have crushed each other more when they came together.
How two became one
One of the things that demands an explanation is Arrokoth’s unusual shape. It appears to be what’s called a “contact binary,” meaning it formed by two objects gently being smushed against each other. But in this case, the objects themselves had appeared to be a bit smushed, requiring their flattened, elongated shape to also be explained.
One of the key results of the flyby was the generation of two successive images from slightly different perspectives, allowing a stereoscopic view of Arrokoth. The 3D reconstruction built from that view indicates that the two lobes aren’t as flattened as they had originally appeared. This level of flattening could be explained by the spin of each object, and the more rounded shape means that the spin of each part would only have to be slightly higher than the current spin of Arrokoth in order to create the appropriate degree of flatness.
Modeling of the sorts of collisions that might bring two separate bodies together indicated that any approach speed over about five meters a second would lead to some fracturing of the two bodies, rather than the neat two-lobed structure we see. This suggests the two bodies must have formed in proximity to each other, from the same collapsing cloud of material. Anything other than that is unlikely to provide this sort of gradual approach speed.
But even a slow approach like the two objects experienced would have required something to bleed away their original momentum. So, the researchers considered a variety of items that could have done so. But a lot of the easy options just won’t work. Arrokoth is simply too far from the Sun—over 40 times the Earth’s distance from the Sun—for light to have had a significant affect on the bodies’ motion. Collisions haven’t been frequent enough to bleed off enough of the momentum.
What they researchers were left with is the gas that originally orbited the Sun early in the Solar System’s history. While the Sun’s energy drove off most of this gas, it would have been present at the time the two bodies originally formed and, more critically, would have orbited the Sun more slowly. This would have provided a friction to the two bodies that formed Arrokoth, allowing them to approach slowly enough to fuse without shattering either of them at the point where they first contacted.
No rings, big crater
The single largest feature on Arrokoth is a crater that’s picked up the nickname “Maryland,” after the site of the New Horizons control center. Maryland is on the smaller of the two lobes and is about six kilometers across and at least a half-kilometer deep. Its otherwise round outline is interrupted by an outcrop that extends into the crater; how it formed is not clear. There are plenty of smaller depressions that appear to be craters, but none of them are more than a kilometer across.
Craters typically mean that material was blasted off the surface of a small body like Arrokoth, so the researchers scanned for their remains: small moons or rings, which have been found on other minor Solar System objects. But there were no signs of any moons, and if a ring is present, it’s incredibly sparse.
There are lots of further details that have already been explored—three papers mean a lot of text and supplemental data. But the publication of the data also means that people who study other Solar System processes and Kuiper belt objects will start rethinking their objects of interest in light of what we now know about Arrokoth. And the publications on those will probably keep coming for a decade or more.