The asteroid underwent a high-speed collision, and the area of impact got 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 300 ° F warmer than the side facing away. “This constant thermal cycling makes the rock brittle, and it breaks apart into gravel,” said Heck.
These pebbles are then ejected from the asteroid’s surface.
“We don’t yet know what the process is that ejects the pebbles,” said Heck — they might be dislodged by smaller impacts other space collisions, or they might just get released by the thermal stress the asteroid undergoes. But once the pebbles are disturbed, Heck says, “you don’t need much to eject something — the escape velocity is very low.” A recent study of Bennu revealed that its surface is loosely bound and behaves like popcorn in a bucket.
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 say, the asteroid underwent another collision, the loose mixed pebbles on the surface got transformed into a solid rock. “It basically packed everything together, and this loose gravel became a cohesive rock,” said Heck.
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,” said Yang.
That’s a big deal, Heck says, because for a long time, scientists assumed that the main way that the minerals on the surfaces of asteroids get rearranged is through big crashes, which don’t happen very often.
“From OSIRIS-REx we know that these particle ejection events are much more frequent than these high-velocity impacts,” said Heck, “so they probably play a more important role in determining the makeup of asteroids and meteorites.”
Aguas Zarcas is the first meteorite to show signs of this behavior, but it’s probably not the only one. “We would expect this in other meteorites,” said Heck. “People just haven’t looked for it yet.”
—Adapted from an article by Kate Golembiewski first published by the Field Museum.
Citation: “A record of post-accretion asteroid surface mixing preserved in the Aguas Zarcas meteorite. ” Yang et al, Nature Astronomy, Aug. 11, 2022.