Asteroids are Constantly Spitting out Pebbles

Asteroids are commonly thought of as solid balls of rock and metal – a place where Bruce Willis can land and stick a nuke into the middle of it. But as we learn more about them, it is becoming more evident that many asteroids are just piles of rubble held loosely together by gravity. So it might not be too big of a surprise when some of those pieces of rubble fly off of the asteroid itself. But it surprised scientists who first observed the phenomena on the asteroid Bennu when OSIRIS-REx visited it in 2019. Now a team led by researchers at the field museum found a meteorite that shows signs it underwent the same process.

The meteorite, known as Aguas Zarcas for the town in Costa Rica it was found in, was originally going to be studied for a separate research project. But when Phillip Heck and Xin Yang his graduate student, they had some trouble. They used a standard technique of freezing the system with liquid nitrogen and warming it up quickly with warm water, which usually breaks the meteorite apart into small enough pieces to study.

But parts of Aguas Zarcas wouldn’t break apart. That does occasionally happen, and typically the scientists would go about grinding up the tougher nodules with a mortar and pestle. But something about these nodules seemed strange to Heck and Yang, so they decided to do what any self-respecting scientist would do: give it a CT scan.

UT Video discussing the OSIRIS-REx mission

Yes, CT scans are helpful for things other than just diagnosing medical conditions. In this case, the scientists found that these nodules weren’t only weird because they wouldn’t break apart – they weren’t entirely spherical and all orientated in a specific direction. It was like something had forced them into that orientation.

With that in mind, the researcher had heard about OSIRIS-REx’s experience with Bennu and how it had observed pebbles flying off the asteroid. So far, no one had been able to explain why exactly that happened, but the pair seemed to be holding an ancient example of it right in front of them.

So they started modeling. They found that, as with so many other aspects of an asteroid’s life, the pebble ejections were most likely caused by an impact – in both Bennu’s and Aguas Zarcas’ cases, most likely a small one. That impact deformed the part of the asteroid near where it hit. That embrittled rock was put under even more stress by the asteroid’s rotation.

OSIRIS-REx's backup sampling site Osprey on asteroid Bennu.
OSIRIS-REx’s backup sampling site Osprey on asteroid Bennu.

As asteroids rotate, parts of their surface alternately face away from the Sun or toward it. This creates substantial temperature differentials, which can break the rocks affected by the impact into gravel. That gravel is most likely what OSIRIS-REx saw being blown off of Bennu. The process by which it leaves the asteroid’s surface isn’t yet understood, but it doesn’t exactly take much force to push off an asteroid. Expanding gas pockets and other features found elsewhere in the solar system could do the trick.

Many of those pebbles would be pulled back down to the asteroid’s surface, as the minimal gravity it exerts on bodies near it is still more than the gravity of every other body in the solar system. The pebbles seen in the Aguas Zarcas meteorite seem to have been collected back on the surface and then hit by another impactor that caused the pebbles to fuse into the object that became the meteorite. There’s a good chance that the same impact formed the meteorite.

All this has plenty of implications for the geology and formation processes of asteroids. And while physical models and one example are a start, these theories are far from solidified. But, as Dr. Heck points out, there are probably plenty more meteorites with these kinds of pebbles embedded in them. People just haven’t started looking for them yet.

Learn More:
Field Museum – Meteorite provides record of asteroids “spitting out” pebbles
Yang et al – A record of post-accretion asteroid surface mixing preserved in the Aguas Zarcas meteorite
UT – OSIRIS-REx Would Have Sunk Deep into Asteroid Bennu if it Tried to Land
UT – Bennu is Constantly Getting Sandblasted by Tiny Meteoroids

Lead Image:
Artist depiction of pebbles being blasted off the surface of an asteroid.
Credit – April I Neader / NASA / Goddard / University of Arizona