NASA’s OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer) spacecraft returned images of a geological phenomenon observed for the first time in 2019: pebbles flying off the surface of the asteroid Bennu.
It seemed as if the asteroid was "shooting off" swarms of marble-sized rocks – and scientists were stumped as to how and why this was happening. But in a new peer-reviewed study in Nature Astronomy, researchers presented 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 and senior author of the study.
Meteorites are pieces of rock that fall to Earth from outer space. While they're often broken-off bits of asteroids, they can also be made of pieces of moons and planets.
The Aguas Zarcas meteorite, which is named after the Costa Rican town where it fell in 2019, came to the Field Museum as a donation from Terry and Gail Boudreaux. Heck and his student, 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 Xin, a graduate student at the 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 said 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.
Meteorites, asteroids and pebbles: What's the new approach?
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.”
“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.”
Philipp Heck
Something had happened to the pebbles that didn’t happen to the rest of the rock around them.
Utilizing the findings of 2019 OSIRIS-REx, the researchers put together a hypothesis, which they supported with the use of physical models.
The asteroid underwent a high-speed collision, and the area of impact became deformed. That deformed rock eventually broke apart due to the huge temperature differences the asteroid experiences when it rotates, since the side facing the sun is more than 150°C (300°F) warmer than the side facing away.
“This constant thermal cycling makes the rock brittle, and it breaks apart into gravel,” Heck said. These pebbles are then ejected from the asteroid’s surface. “We don’t yet know what the process is that ejects the pebbles.”
The pebbles might be dislodged by smaller impacts or other space collisions, or they might just get released by the thermal stress the asteroid undergoes. But once they are disturbed, “you don’t need much to eject something – the escape velocity is very low.”
How the asteroid pebbles behave
A recent study of Bennu revealed that its surface is loosely bound.
The pebbles then entered a very slow orbit around the asteroid, and eventually, they fell back down to its surface further away where there was no deformation. Then, Heck and Yang said, the asteroid underwent another collision, the loose mixed pebbles on the surface got transformed into solid rock.
“It basically packed everything together, and this loose gravel became a cohesive rock,” Heck said. The same impact may have dislodged the new rock, sending it careening into space. Eventually, that chunk fell to Earth as the Aguas Zarcas meteorite, carrying evidence of the pebble mixing.
This could explain the pebbles present in Aguas Zarcas, making the meteorite the first physical evidence of the geological process observed by OSIRIS-REx on Bennu.
“It provides a new way of explaining the way that minerals on the surfaces of asteroids get mixed,” Yang said.