Scientists studying samples from the asteroid Bennu have found that it contains a remarkable mix of materials — some of which formed long before the sun even existed.
Taken together, the findings, described in a trio of recently published papers, show how Bennu has preserved clues about the earliest days of our solar system.
“It’s very fascinating to see that Bennu is a time capsule of the material that was throughout the solar system in the really, really early stages of our solar system,” Pierre Haenecour of the University of Arizona, who analyzed the samples for presolar grains and co-authored the new studies, told Space.com in a recent interview.
The samples, which were scooped up by NASA’s OSIRIS-REx spacecraft during a brief yet dramatic touchdown on Bennu in 2020, contain dust that formed in our solar system, organic matter from interstellar space, and stardust older than the sun itself.
Scientists say these tiny grains may have traveled enormous distances before becoming part of Bennu’s parent asteroid — a much larger body that was shattered in a collision millions of years ago in the asteroid belt between Mars and Jupiter.
“We see that the Bennu sample is this leftover of the material that was basically all around the solar system,” Haenecour told Space.com. Some of these grains survived extreme heat and reactions with water, as well as “several generations of impact events,” including the catastrophic collision that broke the parent asteroid apart, he said.
One of the studies, published in the journal Nature Astronomy, shows that ice inside the parent asteroid melted and reacted with dust, forming the minerals that now make up about 80% of Bennu. Certain grains, such as silicon carbide, carry unique chemical signatures that reveal the types of stars they came from — stars that no longer exist.
“They’re long gone,” Haenecour told Space.com. “We wouldn’t be able to observe the stars that particular grains came from.”
These presolar grains are incredibly tiny, often smaller than a micrometer, and are identified by unusual chemical fingerprints left by the nuclear reactions in their parent stars. Mapping them is like searching for a “needle in a haystack,” but it allows scientists to trace the ancient origins of Bennu’s material, Haenecour said.
Another study, published in the journal Nature Geoscience, highlights how Bennu’s airless surface has been shaped by space weathering, including tiny micrometeorite impacts and the solar wind. The upper layer of Bennu’s surface has been exposed to cosmic rays for 2 million to 7 million years, the study reports. These processes created microscopic craters and splashes of molten rock on the asteroid’s surface, according to the paper.
Comparison with samples from the asteroid Ryugu, which was sampled by Japan’s Hayabusa2 mission, suggest that impacts may play a larger role in reshaping asteroid surfaces than previously thought, scientists say.
“The surface weathering at Bennu is happening a lot faster than conventional wisdom would have it,” Lindsay Keller, a scientist at NASA’s Johnson Space Center in Houston who led the paper on space weathering, said in a statement.
“Space weathering is an important process that affects all asteroids, and with returned samples, we can tease out the properties controlling it and use that data and extrapolate it to explain the surface and evolution of asteroid bodies that we haven’t visited,” Keller added.
Because many asteroids burn up in Earth’s atmosphere, collecting samples directly from space is essential to piece their history together. Meteorites that fall to Earth can provide clues about an asteroid’s orbit, but they rarely reveal its full history, Haenecour said.
OSIRIS-REx studied Bennu up close for over a year before collecting samples, carefully mapping its surface and analyzing its minerals, which provided “very valuable geological context that we cannot get from meteorites,” Haenecour noted.
“We could only get the answers we got because of the samples,” Jessica Barnes of the University of Arizona, who led one of the new papers, added in the statement.
“It’s super exciting that we’re finally able to see these things about an asteroid that we’ve been dreaming of going to for so long.”