NASA's New Horizons spacecraft made a close flyby of Arrokoth in 2019. From those data, Stern and his coauthors identified 12 mounds on Arrokoth's larger lobe, Wenu, which are almost the same shape, size, color and reflectivity. They also tentatively identified three more mounds on the object's smaller lobe, Weeyo.
"It's amazing to see this object so well preserved that its shape directly reveals these details of its assembly from a set of building blocks all very similar to one another," said Lowell Observatory's Dr. Will Grundy, co-investigator of the New Horizons mission. "Arrokoth almost looks like a raspberry, made of little sub-units."
Arrokoth's geology supports the streaming instability model of planetesimal formation where collision speeds of just a few miles per hour allowed objects to gently accumulate to build Arrokoth in a local area of the solar nebula undergoing gravitational collapse.
"Similarities including in sizes and other properties of Arrokoth's mound structures suggest new insights into its formation," Stern, the Principal Investigator of the New Horizons mission, said. "If the mounds are indeed representative of the building blocks of ancient planetesimals like Arrokoth, then planetesimal formation models will need to explain the preferred size for these building blocks."
There is a good chance that some of the flyby targets for NASA's Lucy mission to Jupiter's Trojan asteroids and ESA's comet interceptor will be other pristine planetesimals, which could contribute to the understanding of accretion of planetesimals elsewhere in the ancient solar system and whether they differ from processes New Horizons found in the Kuiper Belt.
"It will be important to search for mound-like structures on the planetesimals these missions observe to see how common this phenomenon is, as a further guide to planetesimal formation theories," Stern said.
The Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, designed, built and operates the New Horizons spacecraft, and manages the mission for NASA's Science Mission Directorate. Southwest Research Institute, based in San Antonio, directs the mission via Principal Investigator Stern, who leads the science team, payload operations and encounter science planning. New Horizons is part of the New Frontiers Program managed by NASA's Marshall Space Flight Center in Huntsville, Al
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