Strange behavior of particles on near-Earth asteroid Bennu

Bennu is a near-Earth (its orbit around the Sun is within Jupiter’s orbit) asteroid approximately twice as far from Earth as the distance from Earth to the Sun. The size of the asteroid is around 500 meters in diameter and it has a porous structure with a composition of carbon-rich materials. Furthermore, scientists believe that analyzing these materials might shed some light on the origins of the Solar system.

Data about the asteroid has been collected by NASA’s spacecraft OSIRIS-Rex from December 2018 to March 2021 and the spacecraft is currently on its way back to Earth. Bennu was chosen as a target for the OSIRIS-REX’s mission because it’s believed to be an active asteroid, meaning that there is evidence that it loses mass. During this observation, there was spotted a strange behaviour of particles (small rocks), each of size less than 10 cm, on its surface particles were suddenly ejected from Bennu’s surface. The mechanism behind these particle ejections is still controversial among scientists. And in this article I am going to tell you about the possible reasons behind this particle phenomenon on Bennu:

During the observation, there were multiple large ejections of particles, the largest happening on 6^th January 2019, when 200 observed particles launched off Bennu’s surface, with a maximum velocity of around 10 km/h. These large particle ejections happened roughly in 2-week intervals, usually on the side facing the Sun. There were three common scenarios that happened to the particles after the ejection – some fell straight back to the surface, due to lack of speed; others began to orbit Bennu, the longest orbit was 16 revolutions and lasted a few days; lastly, some particles completely left Bennu.

The first possible reason behind these mysterious ejections might be the collisions of other space rubble with Bennu. This would mean that the objects colliding with Bennu would transfer their kinetic energies to particles on the surface causing them to launch off the surface. To clarify this mechanism, you may imagine throwing a ball into a swimming pool, once the ball hits the surface of the water it creates a splash – the ball transferring its kinetic energy to the water causes some water to launch off the water’s surface.

Another possible cause of this particle phenomenon might be water vapour, as the face of Bennu facing the Sun might reach almost 130 ˚C, this might cause some water-bearing minerals to release the water as a vapour that might build pressure in the cracks of Bennu’s porous structure. If the gas is then released it may cause the particles to eject as the gas escapes through the cracks. You can compare this to inflating a balloon, if the pressure in the balloon is too high, the material breaks and the air escapes.

And lastly, the ejection of particles might be caused by thermal stress fracturing. Bennu makes one revolution around its axis in around 4 hours, leading to frequent temperature changes on its surface as the temperatures may change every 4 hours from -70 ˚C to 130 ˚C. These sudden changes might cause rocks on Bennu’s surface to crack which may result in breakage and energy release which may eject the particles.

To sum up, scientists are still unsure about the actual cause of these ejections, but it’s probable that it might be a combination of some reasons mentioned earlier. Nevertheless, the samples from Bennu might help scientists understand the behaviour of asteroids and also the origin of our Solar system.

Jakub Greguš

Paper:

Lauretta, D. S., Hergenrother, C. W., Chesley, S. R., Leonard, J. M., Pelgrift, J. Y., Adam, C. D., … Becker, K. J. (2019). Episodes of particle ejection from the surface of the active asteroid (101955) Bennu. Science, 366(6470). doi:10.1126/science.aay3544