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鈥檚 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鈥檚 surface known as super-volatiles from solid to gas 鈥� a process known as sublimation 鈥� when it first formed 4 billion years ago.
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鈥淲e think the sublimation process of the super-volatiles on the body produced this flattening,鈥� says Zhao, noting it would have occurred in Arrokoth鈥檚 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掳C. While it is unclear which ices would have been present, Arrokoth鈥檚 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 鈥� what are now the 鈥渇ront鈥� and 鈥渂ack鈥� 鈥� are exposed to the sun more than its other sides, resulting in prolonged heating on them that produced its flattened peanut shape.
Whether Arrokoth鈥檚 shape is unique in the Kuiper belt would depend on how rare super-volatiles and its unusual rotation are. 鈥淚t should not be unique,鈥� says Ladislav Rezac at the Max Planck Institute in Germany, a co-author on the paper. 鈥淏ut that depends on these two probabilities, which we do not know.鈥�
Nature Astronomy
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