The Morphometry of Impact Craters on Bennu

Abstract

Bennu is an~500-m-diameter rubble-pile asteroid that is the target of detailed study by the Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) mission. Here we use data from the OSIRIS-REx Laser Altimeter to assess depth-to-diameter ratios (d/D) of 108 impact craters larger than 10 m in diameter. The d/D of craters on Bennu ranges from 0.02 to 0.19. The mean is 0.10 ± 0.03. The smallest craters show the broadest range in d/D, consistent with d/D measurements on other asteroids. A few craters have central mounds, which is interpreted as evidence that a more competent substrate lies a few meters beneath them. The range of d/D narrows as crater size increases, with craters larger than 80 m tending toward smaller d/D. At large scales, increases in target strength with depth, combined with target curvature, may affect crater morphometry.

Impact-crater ejecta on Bennu indicate a surface with very low strength

“…The well-defined topographic expression and morphology suggest that Bralgah Crater has undergone little degradation 14 . It has a depth-diameter ratio of 0.07 ± 0.01 with respect to elevation and a volume of 9x10 3 m 3 ±50% 14 . The crater resides on a ~23° regional slope (Fig.…”

“…The crater itself is fully encircled by a well-defined, raised rim and has a slightly asymmetric bowl-shaped interior 14 . From among similarly sized craters on Bennu 14 , Bralgah Crater's morphology is most reminiscent of classical simple craters on larger bodies such as Earth's Moon. The well-defined topographic expression and morphology suggest that Bralgah Crater has undergone little degradation 14 .…”

“…From among similarly sized craters on Bennu 14 , Bralgah Crater's morphology is most reminiscent of classical simple craters on larger bodies such as Earth's Moon. The well-defined topographic expression and morphology suggest that Bralgah Crater has undergone little degradation 14 . It has a depth-diameter ratio of 0.07 ± 0.01 with respect to elevation and a volume of 9x10 3 m 3 ±50% 14 .…”

“…Buried structures near the surface can complicate crater formation 8,14 , but there is little evidence of this at Bralgah Crater. Its circular rim and relatively smooth floor indicate that the near-surface material was initially uniform and did not contain large boulders or regions of higher strength to interfere with crater formation.…”

“…A distinguishing characteristic of the event that created Bralgah Crater is that the impact occurred into a deep reservoir of finer material. Most large craters on Bennu have rocky floors 14 , so gravity-scaled crater formation would have transitioned to the strength regime when encountering a coherent subsurface. The low ejection speeds that enabled retention in the case of the crater we studied would not have occurred.…”