Are Rocks from Asteroid Vesta Hitch-hiking Through Space on Asteroid Bennu?

NASA’s OSIRIS-REx spacecraft captured these images of asteroid Bennu’s surface during Spring 2019. They show strangely bright rocks that are now know to be fragments of asteroid Vesta. Credits: NASA/Goddard/University of Arizona

Long gone are the days when the asteroids were just faint points of light in the sky. Long gone too are the days when asteroids were just lumps of rock left over from the formation of the Solar System. Now, planetary scientists are beginning to reconstruct their very histories and in the case of asteroid Bennu, it’s a fascinating tale of catastrophe and rebirth.

Such analyses are a step up in the sophistication of asteroid science and are being made possible by the missions that fly to asteroids in order to characterize their physical and chemical compositions. The latest fascinating result was announced by NASA on 21 September, and on the face of it, it’s a real mystery.

Six rocks on the surface of Bennu appear to come from a completely different asteroid altogether called Vesta. The six imposters instantly stood out because they are much brighter than the other rocks in their surroundings.

They were spotted on images taken by the NASA spacecraft OSIRIS-REx, and range in size from about 1.5 metres to 4.5 metres across. An analysis of the light bouncing off the rocks, made with OSIRIS-REx’s Visible and Infrared Spectrometer (OVIRS), showed that the rocks contained pyroxene. This was a puzzle because pyroxene is not common on Bennu but is common in the rocks found on asteroid Vesta.

The analysis was performed by Asteroid Day LIVE 2020 alumnus Daniella DellaGiustina, Lunar & Planetary Laboratory, University of Arizona, Tucson and collaborators and the results were published in Nature Astronomy on 21 September.

To explain the puzzling boulders from one asteroid appearing on another, Daniella and colleagues propose an extraordinary scenario. Astronomers know that Bennu is a ‘rubble pile’ asteroid. This means it is not a solid, ‘monolithic’ asteroid. Instead it is a collection of fragments of a larger, shattered body that is now held together by gravity.

Additionally, they know that the asteroid Vesta possesses two impact basins, Veneneia and Rheasilvia, that were created roughly two and one billion years ago respectively. During those impacts, fragments of Vesta were blasted into space and now form a family of small asteroids called vestoids.

Daniella and colleagues propose that one of these vestoids struck the surface of Bennu’s parent asteroid. Later, when Bennu parent asteroid was shattered by a giant impact, pieces of it and the vestoids on its surface coalesced into the rubble pile asteroid that we call Bennu.

If confirmed, it’s a fascinating scenario that hints at the rich and varied histories that many of the asteroids are hiding behind their rocky facades. And it is these hidden histories that spacecraft like OSIRIS-REx and Japan’s Hayabusa2 are gradually bringing to life by returning samples of asteroids to Earth for laboratory analysis.

OSIRIS-REx will collect samples from the surface of Bennu on 20 October. Hayabusa2 will return its samples from asteroid Ryugu to Earth on 6 December.

Read the original NASA story here.