Mound Mysteries in the Kuiper Belt: Shared Origin of Arrokoth’s Giant Structures?

The large mound structures that dominate one of the lobes of the Kuiper belt object Arrokoth are similar enough to suggest a common origin, according to a new study led by Southwest Research Institute (SwRI) Planetary Scientist and Associate Vice President Dr. Alan Stern. Credit: Southwest Research Institute

These “Building blocks” of this Kuiper belt object may point to key details of streaming instability model of planetesimal formation.

A new study led by Southwest Research Institute (SwRI) Planetary Scientist and Associate Vice President Dr. Alan Stern posits that the large, approximately 5-kilometer-long mounds that dominate the appearance of the larger lobe of the pristine Kuiper Belt object Arrokoth are similar enough to suggest a common origin. The SwRI study suggests that these “building blocks” could guide further work on planetesimal formational models. Stern presented these findings this week at the American Astronomical Society’s 55th Annual Division for Planetary Sciences (DPS) meeting in San Antonio. These results were published on September 26 in the peer-reviewed Planetary Science Journal.

NASA’s Observations and Analysis

In 2019, NASA’s New Horizons spacecraft made a close flyby of Arrokoth. 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.”

Observations of the Kuiper Belt object Arrokoth suggest it formed from an assembly of similarly sized objects, brought together at low speeds in a local area undergoing gravitational collapse. These findings support the streaming instability model of planetesimal formation and are described in a new study led by Southwest Research Institute (SwRI) Planetary Scientist and Associate Vice President Dr. Alan Stern. Credit: New Horizons/NASA/JHUAPL/SwRI/James Tuttle Keane

The Streaming Instability Model

Arrokoth’s geology supports the streaming instability model of planetesimal formation where collision speeds of just a few miles per hour allow 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.

Reference: “The Properties and Origin of Kuiper Belt Object Arrokoth’s Large Mounds” by S. A. Stern, O. L. White, W. M. Grundy, B. A. Keeney, J. D. Hofgartner, D. Nesvorný, W. B. McKinnon, D. C. Richardson, J. C. Marohnic, A. J. Verbiscer, S. D. Benecchi, P. M. Schenk and J. M. Moore, 26 September 2023, The Planetary Science Journal.
DOI: 10.3847/PSJ/acf317

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, Alabama.