After NASA’s OSIRIS-REx spacecraft began orbiting the asteroid Bennu in 2019, its navigation camera witnessed multiple episodes where small rocks were being ejected from the surface at speeds up to 10 feet per second (3 metres per second). More than 300 particle ejection events have been seen to date.
Surprised by the observations, the science team came up with three possible explanations: meteoroid impacts, thermal stress and the release of water vapour. In research papers published 9 September in the Journal of Geophysical Research: Planets, the team has concluded the most likely explanation is a combination of thermal stress and meteoroid impacts.
“We thought that Bennu’s boulder-covered surface was the wild-card discovery at the asteroid, but these particle events definitely surprised us,” said Dante Lauretta, OSIRIS-REx principal investigator. “We’ve spent the last year investigating Bennu’s active surface, and it’s provided us with a remarkable opportunity to expand our knowledge of how active asteroids behave.”
Bennu rotates every 4.3 hours. As the temperature rises and falls, rocks can crack and break down, possibly ejecting smaller fragments from the surface in the extreme low-gravity environment. The observed particle showers occurred more often in the late afternoon on Bennu, when surface rocks heat up.
The timing of the events also matches up well with head-on meteoroid impacts in the inner solar system thanks to an unseen population of small fragments released from comets as they near the Sun and heat up or even break apart.
“One of the most significant things we’ve noticed is that the asteroid is frequently ejecting materials into space,” said Southwest Research Institute scientist William Bottke, lead author of a paper in JGR: Planets. “Tiny rocks are just flying off its surface, yet there is no evidence that they are propelled by sublimating ice, as one might expect from a comet.”
Using a computer model developed by NASA’s Meteoroid Environment Office at the Marshall Space Flight Center, Bottke found that sand-size meteoroids, hitting with the force of a shotgun blast, could explain many of the ejection events seen on Bennu. The model works best for asteroids with weak, porous surfaces, which pebble-strewn Bennu resembles.