Asteroid Bennu surprised astronomers in 2019 by “spitting out” small pebbles, and now a meteorite may provide clues about this strange behavior.

One of the most surprising findings of the OSIRIS-REx spacecraft was that asteroid Bennu was throwing swarms of marble-sized pebbles at it.

Such phenomena had never been observed before 2019, but now astronomers can confirm that Bennu is not alone. In the Nature Astronomythey provide evidence that the same thing once happened to the parent of the Aguas Zarcas (AZ) meteorite.

The AZ meteorite fell near the Costa Rican city of the same name, coincidentally also in 2019, and was donated to the Field Museum in Chicago. In addition, its composition is similar to that of Bennu. While preparing it for study, curator Field, professor Philipp Heck and student Xin Yang noticed something unusual about the space rock.

Asteroid Bennu surprised astronomers in 2019

“We were trying to isolate very small minerals from the meteorite by freezing it with liquid nitrogen and thawing it with warm water to break it up,” Yang explained in a statement. “This works for most meteorites, but this one was quite strange – we found some compact fragments that were not decaying.”

Such robust pebbles have been seen in meteorites before, but were broken apart by researchers to understand their composition. Still, spotting oddities that might prove important is a crucial scientific skill.

The researchers performed CT scans, comparing the pebbles to the rest of the meteorite, and noted their distortion from the spherical ones and the fact that they all had the same orientation. Somehow the same process had affected them all but left the rest of the rock untouched.

Without the observations on Bennu, they might have remained puzzled, but with their help, they proposed an explanation.

The AZ meteorite and its story

First, AZ’s parent asteroid must have suffered a high-speed collision, warping a portion of it. Temperature differences between the sun-facing and night-facing side caused the asteroid to become more brittle and eventually break into pieces.

Pebbles were thrown from the night surface and began orbiting the asteroid. Despite its low gravity, with nothing else nearby, the pebbles eventually fell back to 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 a later event, that section of the meteorite broke off, eventually falling in Central America.

New missions like OSIRIS-REx?

“It’s fascinating to see something that was just discovered by a space mission on an asteroid millions of kilometers away from Earth and find a record of the same geological process in the museum’s meteorite collection,” Heck said.

The process described is elaborate, but over lifetimes of billions of years, asteroids will undergo multiple smaller collisions. This is something the authors say astronomers have tended to ignore when explaining the evolution of asteroids, focusing instead on larger, much rarer impacts, they write IFL Science.

However, there is one aspect the paper does not explain – why AZ’s parent body, or Bennu for that matter, spat out the pebbles. Thus, we may need more missions like OSIRIS-REx to clarify this point.

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