Asteroid Bennu, subject of NASA’s OSIRIS-REx asteroid sampling mission, is a big pile of rubble. The latest analysis from OSIRIS-REx suggests that the rubble might have not a single origin. It found evidence from asteroid Vesta, the second-largest body in the Asteroid Belt.
The researchers believe Bennu likely formed as a result of a collision between asteroids, one of which was a fragment of Vesta. When the scattered debris of the collision ended up forming Bennu, some of the Vesta rocks ended up on the surface of this small body. The discovery is reported in Nature Astronomy.
“We found six boulders ranging in size from 5 to 14 feet (about 1.5 to 4.3 meters) scattered across Bennu’s southern hemisphere and near the equator,” lead author Daniella DellaGiustina of the Lunar & Planetary Laboratory, University of Arizona said in a statement. “These boulders are much brighter than the rest of Bennu and match material from Vesta.”
It is possible that they formed from the parent body of Bennu but it is more likely they were pieces of Vesta. The boulders are made of pyroxene, which forms at high temperatures from the melting of rocky material, and can be 10 times brighter than the surrounding surface. Bennu's rocks are made from water-bearing minerals, so it's unlikely it, or its parent body, experienced high temperatures.
It's not unusual to see remnants of an asteroid splashed across the surface of another. NASA’s Dawn spacecraft saw craters on Vesta where different asteroids had collided, breaking away pieces and leaving dark material behind. Similarly, a large black boulder was seen by the Japanese probe Hayabusa on asteroid Itokawa. Just yesterday, a study revealed fellow pile-of-rubble asteroid Ryugu, visited by Hayabusa2, is also the product of a cosmic collision.
“Our leading hypothesis is that Bennu inherited this material from its parent asteroid after a vestoid (a fragment from Vesta) struck the parent,” said Hannah Kaplan of NASA’s Goddard Space Flight Center. “Then, when the parent asteroid was catastrophically disrupted, a portion of its debris accumulated under its own gravity into Bennu, including some of the pyroxene from Vesta.”
Asteroids moving through the Solar System interact with one another, and can over time end up in a collision. The gravitational interaction can also take them from the Asteroid Belt (between Mars and Jupiter) to near the Earth’s orbit, which is the case for both Bennu and Ryugu.
“Future studies of asteroid families, as well as the origin of Bennu, must reconcile the presence of Vesta-like material as well as the apparent lack of other asteroid types. We look forward to the returned sample, which hopefully contains pieces of these intriguing rock types,” added Dante Lauretta, OSIRIS-REx principal investigator. “This constraint is even more compelling given the finding of S-type material on asteroid Ryugu. This difference shows the value in studying multiple asteroids across the Solar System.”
OSIRIS-REx is scheduled to fly down to the surface of Bennu and collect a sample of soil next month, so stay tuned.