The surface of Arrokoth, an oddly flat object on the edge of the solar system, may have been boiled away by heat from the sun.
This body, more than 6 billion kilometres from the sun in the Kuiper belt beyond Neptune, was visited by NASA’s New Horizons spacecraft in 2019. The probe flew past during an extended mission after visiting Pluto in 2015, taking images that revealed Arrokoth as a strange world with two flattened lobes joined by a neck.
Now Yuhui Zhao from the Chinese Academy of Sciences and her colleagues think they know how this strange shape came to be: the sun turned ices on Arrokoth’s surface known as super-volatiles from solid to gas – a process known as sublimation – when it first formed 4 billion years ago.
Advertisement
“We think the sublimation process of the super-volatiles on the body produced this flattening,” says Zhao, noting it would have occurred in Arrokoth’s first 100 million years, after its two near-spherical lobes had joined together.
Super-volatiles are ices that have a particularly low sublimation point. Carbon monoxide, for example, sublimates at about -240°C. While it is unclear which ices would have been present, Arrokoth’s distance from the sun and resulting low temperature means that only super-volatiles would probably sublimate.
It is likely that this sublimation resulted in a flattened appearance because of the way Arrokoth spins. It rotates around the path of its orbit around the sun in a way that means two sides – what are now the “front” and “back” – are exposed to the sun more than its other sides, resulting in prolonged heating on them that produced its flattened peanut shape.
Whether Arrokoth’s shape is unique in the Kuiper belt would depend on how rare super-volatiles and its unusual rotation are. “It should not be unique,” says Ladislav Rezac at the Max Planck Institute in Germany, a co-author on the paper. “But that depends on these two probabilities, which we do not know.”
Nature Astronomy
Sign up to our free Launchpad newsletter for a voyage across the galaxy and beyond, every Friday
