Scientists reviewing data of Bennu from OSIRIS-REx have found six very bright boulders that have a make-up similar to that found on Vesta, which suggests they initially came from that asteroid.
The unusual boulders on Bennu first caught the team’s eye in images from the OSIRIS-REx Camera Suite. They appeared extremely bright, with some almost ten times brighter than their surroundings. They analyzed the light from the boulders using the OSIRIS-REx Visible and Infrared Spectrometer instrument to get clues to their composition. … The signature from the boulders was characteristic of the mineral pyroxene, similar to what is seen on Vesta and the vestoids, smaller asteroids that are fragments blasted from Vesta when it sustained significant asteroid impacts.
Of course it’s possible that the boulders actually formed on Bennu’s parent asteroid, but the team thinks this is unlikely based on how pyroxene typically forms. The mineral typically forms when rocky material melts at high-temperature. However, most of Bennu is composed of rocks containing water-bearing minerals, so it (and its parent) couldn’t have experienced very high temperatures. Next, the team considered localized heating, perhaps from an impact. An impact needed to melt enough material to create large pyroxene boulders would be so significant that it would have destroyed Bennu’s parent-body. So, the team ruled out these scenarios, and instead considered other pyroxene-rich asteroids that might have implanted this material to Bennu or its parent.
The make-up of Vesta matches. While these rocks might have been flung from Vesta during an impact there, eventually to settle on the surface of Bennu, Vesta is not the only possibility. We do not have a good census of the asteroids in the solar system. Others whose make-up is not yet determined could be a source, as well as an asteroid that no longer exists, destroyed by a collision long ago.
Regardless, these rocks confirm that in the process of formation in the early days of the solar system, asteroids of all types exchanged material.
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