How Did Arrokoth Get Its Mounds?

Large mounds abound on the surface of the Kuiper Belt object Arrokoth. Using images from the New Horizons flyby, researchers have pieced together a story of how these features came to be.

A Close Look at a Distant Object

New Horizons image of Arrokoth

New Horizons image of Arrokoth, with its two distinct lobes and several other prominent features labeled. [Stern et al. 2023]

After New Horizons made its historic flyby of Pluto in 2015, the spacecraft set its sights on another first: a close flyby of an object in the Kuiper Belt — the ring of icy objects orbiting beyond Neptune. On New Year’s Day in 2019, New Horizons flew within 3,500 kilometers (about the distance between Washington, DC, and Los Angeles) of an object named Arrokoth, giving us our first close look at a Kuiper Belt object.

Arrokoth consists of two separate bodies, or lobes, that fused together at some point in the past. Though the two lobes, named Wenu and Weeyo, appear spherical from the flyby images, observations taken from farther away suggest that they’re actually rather flat, more like walnuts or pancakes. (You can explore Arrokoth’s shape using an interactive three-dimensional model here.) In addition to being curiously flattened, the larger lobe is covered with a series of interlocking mounds, raising even more questions about how this oddly shaped object was assembled in the cold, dark outskirts of our solar system.

Mapping Mounds

Alan Stern (Southwest Research Institute) and collaborators analyzed two New Horizons images of Arrokoth to assess the origins of the mounds. In total, the team identified 12 mounds on the larger lobe, Wenu. The mounds are roughly the same size and color and have similar ratios of length to width, suggesting that they share a common origin. Using computer simulations, Stern’s team explored two scenarios that could account for Wenu’s lumpy appearance: 1) multiple objects about 3 kilometers wide colliding with a larger object, and 2) a rotating cloud of many 5-kilometer-wide objects gently collapsing to form a single object.

Comparison of model output and Wenu's structure

Comparison of the final model output for the second scenario (left) and the structure of Wenu (right). Click to enlarge. [Stern et al. 2023]

The first scenario generated an object that is too uniform, the mounds having been splattered and flattened in the collision. The second scenario, though, resulted in a distinctly Wenu-like shape; because the objects came together gently, the mounds remained raised rather than flattened. This scenario also predicts other characteristics of the Wenu lobe, such as mounds of similar area that are arranged in an orderly way. How exactly a gravitationally bound, rotating group of 5-kilometer-wide objects might arise in the first place remains unknown, but future high-resolution simulations should provide clues as to whether it’s plausible.

As Wenu, So Weeyo?

New Horizons images of Arrokoth and maps made in this study

New Horizons images of Arrokoth (left column) and resultant maps created in this study (right). Mound regions have labels beginning with “t”. Click to enlarge. [Stern et al. 2023]

Wenu appears to have formed from multiple smaller objects coming together — could Weeyo be made the same way? At first glance, the geology of the two lobes is very different, possibly because of Weeyo’s single large impact crater, the creation of which blanketed the nearby surface with ejected material. Stern’s team picked out three possible mounds along the visible edge of the lobe, farthest from the influence of the crater, but this designation is only tentative.

With no missions to the Kuiper Belt currently planned, our best hope of learning more about Arrokoth is by studying similar objects targeted in upcoming missions: the trojan asteroids in Jupiter’s orbit, which will be visited by NASA’s Lucy mission, and a comet approaching Earth’s orbit, visited by the European Space Agency’s Comet Interceptor.


“The Properties and Origin of Kuiper Belt Object Arrokoth’s Large Mounds,” S. A. Stern et al 2023 Planet. Sci. J. 4 176. doi:10.3847/PSJ/acf317