One of the more surprising discoveries of the OSIRIS-REx spacecraft was that the asteroid Bennu was shooting swarms of pebbles the size of marbles at it. It might have felt like a sentient being forcefully asking to be left alone, but rationality tells us there must be another explanation, even if we haven’t yet worked out what it is. 

Such “pebble spitting” had never been seen before 2019, but now astronomers can confirm Bennu isn’t alone. In Nature Astronomy they provide evidence of the same thing once happening to the parent body of the Aguas Zarcas (AZ) meteorite.

The AZ meteorite fell near the Costa Rican town of the same name, coincidentally also in 2019, and was donated to the Chicago Field Museum. As a CM2 carbonaceous chondrite, its composition is similar to Bennu’s. While preparing it for study Field curator Professor Philipp Heck and student Xin Yang noticed something unusual about the space rock.

“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,” explained Yang in a statement. “That works for most meteorites, but this one was kind of weird – we found some compact fragments that wouldn’t break apart.”

Such robust pebbles have been seen before in meteorites, but have been broken apart by researchers to understand their composition. Spotting oddities that might prove important is a crucial scientific skill, however. “Xin had a very open mind, he said, ‘I’m not going to crush these pebbles to sand, this is interesting,’” Heck said.

The pair conducted CT scans comparing the pebbles with the rest of the meteorite and noticed their deformation from the spherical, and that all had the same orientation. Somehow, the same process had affected all of them but left the rest of the rock untouched.

Without the Bennu observations the pair might have been stumped, but with them, they proposed an explanation.

First, AZ’s parental asteroid must have experienced a high-speed collision, deforming one portion of it. Temperature differences between the Sun and night-facing sides of the asteroid made the asteroid brittle and eventually caused it to break apart.

The pebbles were thrown off the night surface and began to orbit the asteroid. Despite its low gravity, with nothing else in the vicinity, the pebbles eventually fell back onto the asteroid’s surface in an undeformed area. A subsequent collision, “Basically packed everything together, and this loose gravel became a cohesive rock,” Heck said. 

Perhaps in the same collision, perhaps in some later event, that section of the meteorite became detached, eventually falling on Central America.

”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,” Heck said.

The process described is elaborate, but in lifespans of billions of years, asteroids will experience multiple small collisions of suitable sizes. This is something the authors argue astronomers have tended to ignore when explaining asteroid evolution, instead focusing on much rarer larger impacts. There’s an easy way to test if the authors are right.  “We would expect this in other meteorites,” Heck said. “People just haven't looked for it yet.” 

Nevertheless, there is one aspect the paper does not explain – why AZ's parent body, or Bennu for that matter, spat the pebbles out. With such low mass bodies, little force is needed, but little isn’t zero. We might need more missions like OSIRIS-REx to sort that out.