Asteroid loses weight and may gain some back

On 6 January 2019, physicists noticed some 200 pebbles escaping from the surface of the near-Earth asteroid Bennu. NASA’s OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer) spacecraft had arrived at the asteroid in December 2018 and was taking pictures and collecting samples.

Over the next two months, mission scientists observed several more surprising particle-ejection events. Of the million or so known asteroids in the solar system’s main, crowded belt, astronomers have only found a few dozen that show such activity, and the images taken were much more detailed than what can be seen through a telescope. Now Dante Lauretta and Carl Hergenrother of the University of Arizona and their colleagues have analyzed the events and narrowed down the possible mechanisms to explain Bennu’s change in mass.

The composite image here shows the particles ejected from Bennu’s surface during the 6 January event. That one and the other two largest events included dozens of centimeter-sized particles moving at maximum velocities of 1–3 m/s.

The research team used the images to test multiple ejection hypotheses. Each of the three events lasted a few hours, which is consistent with the time it takes for water-bearing minerals on Bennu’s surface to dehydrate, shrink, and crack. That process could supply the ejection particles. The events could also arise from meteoroid impacts, and the pebbles that don’t escape from Bennu could hit the surface and generate still more particles. Independent analyses of Bennu’s surface by other researchers show that it has a thermal gradient that cycles about every four hours, and that frequency would allow for rocks to fracture from thermal stress.

The authors were able to rule out other hypotheses. They determined that the observed particle sizes and velocities couldn’t have resulted from Bennu’s rotational instability or the electrostatic force acting on the particles. Ice sublimation can produce ejection forces, but the authors found no evidence for ice on Bennu’s surface. Lauretta and his colleagues conclude that mineral dehydration, meteoroid impacts, and thermal stress could all contribute toward particle-ejection events. But they may be able to narrow down the possibilities in 2023: That’s when rock samples collected from Bennu’s surface are brought to Earth for further analysis. (D. S. Lauretta et al., Science 366, eaay3544, 2019 .)