Asteroids may shoot pebbles into shallow temporary orbits, boffins believe

OSIRIS-Rex saw this happening. An odd meteorite suggests why and how

In 2019, scientists clocked something they'd never seen before: an asteroid named Bennu appeared to be popping off swarms of pebbles. Research published Thursday may go some way to explain why.

The pebbles were observed by NASA's Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS-Rex) mission. Scientists investigating the pebble phenomenon considered the Aguas Zarcas meteorite – a lump of space rock that landed in Costa Rica in 2019.

The University of Chicago's Philip Heck and his graduate student Xin Yang noticed Aguas Zarcas didn't break up after being frozen in liquid nitrogen and thawed with warm water.

The pair decided to find out why it held together so well. Using CT scans they discovered that pebbles within the rock were spherical and aligned – as if some process had deformed them all in the same way in its previous life as a part of an asteroid. But whatever that process was, it didn't affect the rest of the rock.

The boffins concluded that a high-speed collision had deformed the rock, which broke apart thanks to surface temperature differences that can swing through hundreds of degrees as an asteroid rotates.

The resulting pebbles were then projected away from the surface – either by another impact or thermal stress – into an orbit around an asteroid.

The energies produced by asteroids are sufficient to allow pebbles to leave the surface, but their orbits degrade and the rocks fall back to coat the surface. As if that weren't enough, another impact transformed the pebbles that comprise Aguas Zarcas into solid cohesive rock.

"It provides a new way of explaining the way that minerals on the surfaces of asteroids get mixed," Yang explained.

"We would expect this in other meteorites," Heck said. "People just haven't looked for it yet."

The pair's research was published in Nature Astronomy. ®