Author: Robert Zimmerman

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Using data collected by OSIRIS-REx at the asteriod Bennu, scientists have determined that the ejecta from impacts on a rubble-pile asteroid behaves in a very different manner than on planets with higher gravity.

Instead of flying away at about the same speed as the impactor and escaping into space, as expected in the weak gravity, the material is lifted up at a very slow speed, falls back down, and then rolls downhill like a landslide. The graphic to the right from the press release, reduced and enhanced to post here, illustrates what the scientists think happened when one of Bennu’s larger craters was created.

[M]ost of that material, called ejecta, returned to the surface and slid down the face of the asteroid, starting a wide avalanche that slowly rolled toward Bennu’s equator. Perry said the only way this could happen on a small object like Bennu, which is less than 500 meters (1,640 feet) in diameter and has low gravity, is if the dust had low or next to no cohesion.

“Because Bennu is so small, its escape velocity is less than a few tenths of a mile per hour, so any particle ejected faster than that would leave the surface,” he said. “These slow speeds are possible only if Bennu’s surface is weaker than we thought, even weaker than very loose, dry sand. This extremely low surface strength also means material on a slope is easily disturbed, and that’s what led to the landslide.”

In other words, the low cohesion prevents the impact’s energy from being transferred efficiently to the asteroid’s particles. They move, but only slowly, and thus end up sliding away more or less along the asteroid’s surface.

This discovery helps explain how these rubble-pile asteroids accumulate material, despite their low gravity.