The bright boulders (circled in the images) on the surface of near-Earth asteroid Bennu are pyroxene-rich material from Vesta; some bright material appear to be individual rocks (left) while others appear to be clasts within larger boulders (right). Image credit: NASA’s Goddard Space Flight Center / University of Arizona.
“We found six boulders ranging in size from 1.5 to 4.3 m (4.9-14.1 feet) scattered across Bennu’s southern hemisphere and near the equator,” said team member Dr. Daniella DellaGiustina, a researcher in the Lunar & Planetary Laboratory at the University of Arizona.
“These boulders are much brighter than the rest of Bennu and match material from Vesta.”
“Our leading hypothesis is that Bennu inherited this material from its parent asteroid after a vestoid (a fragment from Vesta) struck the parent,” said team member Dr. Hannah Kaplan, a scientist at 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.”
The team analyzed the spectral data from the OVIRS instrument and found that the signature from the boulders was characteristic of the mineral pyroxene, similar to what is seen on Vesta and the vestoids.
“It’s possible that the boulders actually formed on Bennu’s parent asteroid, but we think this is unlikely based on how pyroxene typically forms,” the authors said.
“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 researchers 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, they ruled out these scenarios, and instead considered other pyroxene-rich asteroids that might have implanted this material to Bennu or its parent.
Several studies indicate Bennu was delivered from the inner region of the main asteroid belt via a well-known gravitational pathway that can take objects from the inner main belt to near-Earth orbits.
There are two inner main belt asteroid families — Polana and Eulalia — that look like Bennu: dark and rich in carbon, making them likely candidates for Bennu’s parent.
Likewise, the formation of the vestoids is tied to the formation of the Veneneia and Rheasilvia impact basins on Vesta, at roughly about two billion years ago and approximately one billion years ago, respectively.
“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,” said OSIRIS-REx principal investigator Dr. Dante Lauretta, a researcher in the Lunar & Planetary Laboratory at the University of Arizona.
“We look forward to the returned sample, which hopefully contains pieces of these intriguing rock types.”
“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.”
The findings appear in the journal Nature Astronomy.
D.N. DellaGiustina et al. Exogenic basalt on asteroid (101955) Bennu. Nat Astron, published online September 21, 2020; doi: 10.1038/s41550-020-1195-z
This article is based on a press-release provided by the National Aeronautics and Space Administration.
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