In 2019, NASA’s OSIRIS-REx spacecraft sent back images of a geological phenomenon no one had ever seen before: pebbles were flying off the surface of the asteroid Bennu. The asteroid appeared to be shooting off swarms of marble-sized rocks. Scientists had never seen this behavior from an asteroid before, and it’s a mystery exactly why it happens.
But in a new paper, researchers show the first evidence of this process in a meteorite.
”It’s fascinating to see something that was just discovered by a space mission on an asteroid millions of miles away from Earth, and find a record from the same geological process in the museum’s meteorite collection,” said Philipp Heck, the Robert A. Pritzker Curator of Meteoritics at Chicago’s Field Museum, professor at the University of Chicago, and the senior author of the study, published in Nature Astronomy study.
Meteorites are pieces of rock that fall to Earth from outer space; they can be made of pieces of moons and planets, but most often, they're broken-off bits of asteroids. The Aguas Zarcas meteorite is named after the Costa Rican town where it fell in 2019; it came to the Field Museum as a donation from Terry and Gail Boudreaux. Heck and collaborator Xin Yang were preparing the meteorite for another study when they noticed something strange.
“We were trying to isolate very tiny minerals from the meteorite by freezing it with liquid nitrogen and thawing it with warm water, to break it up,” said Yang, a graduate student at the Field Museum and the University of Chicago and the paper’s first author. “That works for most meteorites, but this one was kind of weird—we found some compact fragments that wouldn’t break apart.”
Heck says that finding bits of meteorite that won’t disintegrate isn’t unheard of, but scientists usually just shrug and break out the mortar and pestle. “Xin had a very open mind, he said, ‘I’m not going to crush these pebbles to sand, this is interesting,’” said Heck.
Instead, the researchers devised a plan to figure out what these pebbles were and why they were so resistant to breaking apart.
“We did CT scans to see how the pebbles compared to the other rocks making up the meteorite,” said Heck. “What was striking is that these components were all squished— normally, they’d be spherical—and they all had the same orientation. They were all deformed in the same direction, by one process.”
Something had happened to the pebbles that didn’t happen to the rest of the rock around them.
“This was exciting, we were very curious about what it meant,” said Yang.
The scientists had a clue, though, from the 2019 OSIRIS-REx findings. From there, they put together a hypothesis, which they supported with physical models.