Geophysics of (486958) Arrokoth revealed by New Horizons

On 1 January 2019, NASA’s New Horizons spacecraft performed the first flyby of a small Kuiper Belt Object: (486958) Arrokoth (formerly 2014 MU₆₉). This ~35-km long, bilobed contact binary is a member of the cold classical Kuiper belt population—a dynamically stable reservoir of small bodies that has remained relatively untouched since the earliest epochs of solar system formation. In short, Arrokoth is a relic planetesimal—a leftover building block of planet formation—and by studying its geology and geophysics we can explore the processes and environment that built our solar system. In this work, we present the current state of knowledge of Arrokoth’s shape and geophysical environment, and quantitative comparisons to other small bodies across the solar system. Arrokoth’s flattened, bilobate nature yields an unintuitive physical environment, making geophysical analyses critical to interpreting the observed geology. Several surface features correlate with the modeled geophysical environment. Most notably, bright surface features appear preferentially in geopotential lows—like Arrokoth’s neck and the bright annulus on the encounter hemisphere of the large lobe—perhaps indicative of mass-wasting or other geologic processes. While New Horizons was not able to directly measure Arrokoth’s mass, several lines of evidence suggest that it has a very low bulk density. The statistics of surface slopes, required neck strength, and overall configuration of the contact binary suggest a bulk density between 200-500 kg/m³. While this density is low compared to characterized (i.e., larger) Kuiper Belt objects and many comets (e.g., 67P/Churyumov-Gerasimenko density = 532 kg/m³), it is suspiciously similar to some small ring moons of Saturn (e.g., Atlas, Pan, Methone have densities between 300-400 kg/m³). While not conclusive, this supports the hypothesis that Arrokoth formed in a gentle environment (e.g., Stern et al. 2019, Spencer et al. 2020, McKinnon et al. 2020).