We now have not one but two explanations for how the Kuiper belt object Arrokoth, visited by NASA’s New Horizons spacecraft in early 2019, was able to keep its curious two-lobed shape despite being hit by a large rock at some point in its history.
A large portion of Arrokoth’s smaller 15-kilometre-wide lobe is taken up by the Maryland crater, which measures about seven kilometres across (see picture). The object that made it must have been several hundred metres wide, and hit it at a relative speed of about 6400 kilometres per hour.
But the force of such an impact should have broken the two lobes apart. Instead, they remain joined by a “neck”, with the impact believed to have happened at some unknown point after the binary object formed 4 billion years ago.
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Strong neck?
Masatoshi Hirabayashi at Auburn University, Alabama, and his colleagues propose two scenarios. It is possible that the impact broke the original neck of the object, causing Arrokoth to reform in a different orientation. Alternatively, the neck survived the impact, suggesting contact binary objects like this are stronger than we thought.
“The first scenario is the neck breaks structurally, and it has a reconfiguration of the shape,” says Hirabayashi. “Another scenario is the neck doesn’t break. In this case, we can assume the strength of the neck is strong enough that [it] can resist the disturbance of the impact.”
While both scenarios are plausible, Hirabayashi notes that bright material seen around the neck may be the result of landslides resulting from vibrations as the neck broke. However, Arrokoth’s comparable density to the similarly two-lobed comet 67P/Churyumov–Gerasimenko suggests that if the neck didn’t break, these objects are stronger than we thought.
“In order to keep the structure of the neck intact, our prediction is a cohesive strength [100 times higher] than proposed for other asteroids and comets,” says Hirabayashi.
The Astrophysical Journal Letters
