Asteroid 101955 Bennu Has a Young and Impact-Armored Surface

Image of 101955 Bennu taken by the OSIRIS-REx spacecraft. Credit: NASA/Goddard/University of Arizona.

Impact cratering involves the displacement of material when a surface is impacted by an external object at high velocity. Impact cratering occurs frequently on airless solar system bodies, and counting craters can help determine the age of a planetary surface. For example, older planetary surfaces like the Moon’s tend to have more impact craters preserved, compared to the surface of Venus, which has been resurfaced more recently by volcanism and tectonics.

Armoring of a surface occurs when the impactor and a particle of a surface body are of similar size, preventing craters at the small end of the size-frequency distribution from forming. Armoring occurs on what are known as rubble-pile asteroids, which consist of fragments of rocky material held together only by gravity. Studying the relationship between crater size and impactor properties provides crucial input into the methods for determining the ages of planetary surfaces from crater counting.

Edward Bierhaus (Lockheed Martin) and colleagues analyzed images taken of 101955 Bennu, a rubble-pile asteroid visited by the Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS-REx) mission. Although the surface of 101955 Bennu contains many craters, the number was found to be lower than expected based on the frequency and intensity of crater-forming impacts elsewhere in the solar system. The team measured the diameters of 1,560 craters on the surface of 101955 Bennu and found that the impact-armoring effect is responsible for shielding the surface from the significant development of craters. Based on crater counts determined in this study, 101955 Bennu is likely undergoing continuous resurfacing due to the disruption of boulders by impacts and has one of the youngest crater-derived surface ages in our solar system. Using this information in age models to understand the dynamics of impact cratering and armoring, these findings have implications for planetary protection, such as preventing asteroids from impacting Earth. READ MORE