Sky crater impact induced structural behavior of (486958) Arrokoth

NASA’s New Horizons spacecraft flew by (486958) Arrokoth, which belongs to the cold classic Kuiper belt object and may be the most primordial object in the solar system. The Arrokoth image taken by New Horizons revealed that this object is a bilobated shape – two lobes connected with a narrow contact region (called ‘neck’) – that might be formed via a slow and tidal merger of a binary planetesimal. One geological feature identified in New Horizons images is that Arrokoth has the largest crater-like feature (named ‘sky’) on the small lobe and is probably an impact crater based on its bowl shape. The sky crater takes up ~7% of the size of the small lobe, which is 7 km wide and 1 km deep. Given that the sky crater has a significant size, it raises an interesting question of how the structurally weak neck region of the bilobated Arrokoth responded to the sky impact-induced structural disturbance if the impact occurred after the bilobate shape had formed. If it was subject to the catastrophic effects of the sky impact, such as a neck breakup, Arrokoth might not sustain the primordial dynamical conditions, which contradicts the current state of knowledge.

To explore the question of how Arrokoth responds to this impact event, we numerically investigated the evolution of stress fields of the entire structure after the sky impact. Using a pi-scaling law, we first estimated the impulse velocity felt by the small lobe given the final crater size of 7 km. The bulk density is set as 2.0–5.0 g/cc, and the projectile is assumed to have the same material as the target body. We then measured the stress field across the Arrokoth structure when applying the impulse velocity into the small lobe. The stress field is computed from our Finite Element Model (FEM), which is developed to calculate the stress distribution when the body rotates uniformly as its initial condition. Our preliminary results show that Arrokoth’s neck region could likely have a few kilopascal stress variations when the small lobe has the velocity impulse of 1 m/s (Figure 1). This stress variation may reach its failure limit of a weak structure like Arrokoth, which is a highly porous body possibly having lower strengths, although the impact mechanism of the porous icy body is still not well explored yet.