NASA / JHU APL / SWRI / Roman Tkachenko
After its successful rendezvous with Pluto, the spaceship New Horizons was sent to a smaller object in the Kuiper belt. As the spacecraft shot past, it took pictures of a small world consisting of two very different lobes, the properties of which the scientists found somewhat confusing. But details had to wait because the combination of distance and power budget meant that transferring much of the data from New Horizons to Earth was a slow process.
The wait for this data is now over as the high-resolution images are now available and scientists have tried to use them to better understand the origin and structure of today's Arrokoth (named after the Powhatan word for "sky"). ). The data may not answer all of the questions we have about Arrokoth, but it does give us some very good ideas about how such a strange structure could have been formed.
Can't stay long
New Horizons was the fastest probe to be launched from Earth (others have since gained more speed thanks to gravity support), and Arrokoth is very small, which means the probe had to get pretty close before it could be imaged in detail. That left a narrow window for collecting data while flying by, but the newspapers published today describe how narrow it was. Two days before the next pass, Arrokoth still appeared as a single pixel in New Horizons cameras. It only got bigger than 10 pixels about half a day earlier. So the vast majority of the data comes from a window that is only 12 hours wide.
During this time, the cameras on board New Horizons took pictures that examined the composition of Arrokoth and were able to resolve features with a diameter of only 33 meters on the surface. The following works describe the structure of the body, model its history and make some assumptions about its chemistry.
Arrokoth comes from a region of the solar system, the Kuiper Belt, especially from an area outside the gravitational influence of Neptune, the outermost large planet. There was enough material in this region to form icy bodies, but it was so thinly distributed that the bodies appeared to have remained small without interacting often enough to form larger planets. Neptune's influence further scattered some of the Kuiper belt objects inward, where collisions with other bodies were more likely and the influence of the sun was stronger. But Arrokoth is currently circling beyond the point where this would likely happen.
If so, it means that the object is likely to be made of material that has remained largely unchanged since the solar system was formed. And according to all information, it is true. The evaluation of the crater density on the surface of Arrokoth corresponds to 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, which indicates that its surface does not undergo any significant chemical changes experienced.
The red color appears to come from an intricate mixture of longer chain hydrocarbons, collectively known as tholine. These arise through chemical reactions between shorter molecules that are driven by radiation exposure. In Arrokoth's case, these shorter molecules appear to contain methanol, an alcohol with a carbon, and the only single chemical that was clearly identified in the New Horizons data. Methanol may have arisen from chemical reactions between methane and water, but there is little evidence of the presence of water on Arrokoth and no clear signature of methane. It is possible – even probable considering what we know about the Kuiper Belt and other objects – that they are below the surface, but this was not confirmed by this flyby.
Whatever it is, Arrokoth is not very dense and is likely to resemble comets in this regard. If it is less than half the density of a typical comet, it will spin fast enough to fall apart. Too much denser, and the two lobes would have squeezed more if they came together.
How two became one
One of the things that needs explanation is Arrokoth's unusual shape. It appears to be a so-called "contact binary", which means that two objects are pressed gently against each other. In this case, the objects themselves appeared to be somewhat shredded, so that their flattened, elongated shape also had to be explained.
One of the most important results of the flyby was the creation of two successive images from slightly different perspectives, which allowed a stereoscopic view of Arrokoth. The 3D reconstruction created from this view shows that the two flaps are not flattened as they originally appeared. This degree of flattening could be explained by the rotation of each object, and the rounded shape means that the rotation of each part would only need to be slightly higher than Arrokoth's current rotation to produce the appropriate degree of flatness.
Modeling the types of collisions that could bring two separate bodies together showed that any approach speed above about five meters per second would cause the two bodies to break rather than the clean two-lobed structure that we see. This indicates that the two bodies must have formed close to each other from the same collapsing cloud of material. Anything else is unlikely to achieve this type of gradual approach speed.
But even a slow approach like the two objects experienced would have required something to lose their original momentum. The researchers therefore looked at a variety of points that could have done this. But many of the simple options just don't work. Arrokoth is just too far from the sun – more than 40 times the distance from the sun – for light to have a significant impact on the movement of the body. Collisions were not frequent enough to lose momentum.
The researchers were left with the gas that originally orbited the sun at the beginning of the history of the solar system. While the sun's energy was driving off most of this gas, it would have been there when the two bodies were originally formed, and would have been more slowly orbiting the sun. This would have given the two bodies that formed Arrokoth a friction that would allow them to approach slowly enough to fuse without breaking either of them where they first made contact.
No rings, big crater
The biggest feature on Arrokoth is a crater, nicknamed "Maryland" after the location of the New Horizons Control Center. Maryland lies on the smaller of the two lobes and is about six kilometers wide and at least half a kilometer deep. Its otherwise round outline is interrupted by an outcrop that extends into the crater; How it came about is not clear. There are many smaller depressions that appear to be craters, but none of them are more than a kilometer in diameter.
Craters usually mean that material has been blasted off the surface of a small body like Arrokoth, so the researchers searched for their remains: small moons or rings found on other smaller objects in the solar system. But there were no signs of moons, and if there is a ring, it is incredibly sparse.
There are many other details that have already been examined – three articles mean a lot of text and additional data. However, the publication of the data also means that people who examine other processes in the solar system and objects in the Kuiper belt will rethink their objects of interest in view of what we now know about Arrokoth. And the publications on it will probably be published for a decade or more.
Science, 2020. DOI: 10.1126 / science.aay3705, 10.1126 / science.aay6620, 10.1126 / science.aay3999 (About DOIs).
