Analysis of 70-M Crater on Bennu Indicates Rubble Pile Surfaces May Be Much Weaker than Previously Estimated

Credit: NASA/Goddard/University of Arizona.

The surface characteristics of asteroids are invaluable to understanding their geologic history and material properties. The Origins Spectral Interpretation Resource Identification Security-Regolith Explorer (OSIRIS-REx) mission to asteroid 101955 Bennu revealed it to be covered with boulders and impact craters. Bennu is a rubble pile asteroid, meaning it is composed of loosely gravitationally bound sediment and fractured rocks known as regolith.

Mark Perry (Johns Hopkins University) and colleagues studied the distribution of impact ejecta around Bennu’s Bralgah Crater to better understand the asteroid’s regolith properties. A particle’s ejection after an impact depends on both the cohesion (the force of attraction between adjacent particles) and the strength (the force needed to break apart a rock) of the material. Because higher forces are needed to fracture stronger, more cohesive rocks, increasing either cohesion or surface strength will increase the speed and distance traveled by an ejected particle. However, Bennu’s escape velocity is only 20 cm s-1, indicating that both cohesion and surface strength must be relatively low or the majority of impact ejecta would be lost to space.

Analysis of Bralgah Crater’s ejecta indicates that the Bennu’s surface strength is 0.1 – 2 pascals (Pa), several orders of magnitude lower than that of coherent rocks. Such low surface strength suggests that resurfacing processes like landslides occur more frequently on rubble-pile asteroids than on those made of intact rock. This is critical because if resurfacing mechanisms are more common, craters will be erased from the surface, possibly skewing age determinations based on crater counting. Finally, the low surface strength of rubble-pile asteroids leads to the formation of larger craters compared to similarly sized impact events on intact asteroids. This effect, in conjunction with higher resurfacing rates, necessitates revising crater-scaling relationships used for asteroids. These new revelations indicate that Bennu and other rubble-pile asteroids have likely been resurfaced multiple times throughout their histories. READ MORE