In space, large boulders such as asteroids or comets sometimes lose fragments the size of small pebbles. These pebbles later often sink back to the surface of the chunks or, if they cake together themselves, sometimes fall to earth as meteorites. This process is exciting because researchers previously believed that only collisions with other rocks caused clouds of small pebbles to form. However, a new study now shows that asteroids and meteorites throw small pebbles around much more often than previously thought. This process could therefore have a major impact on the structure and composition of objects such as the asteroid Bennu.
Striking crushed structure of the pebbles on the meteorite
As is so often the case, the discovery by researchers Philipp Heck and Xin Yang from the Field Museum in Chicago was more of a chance find. They studied the Aguas Zarcas meteorite, a boulder that fell from the sky near the Costa Rican town of the same name in 2019 and was later donated to the museum. When the researchers tried to isolate small minerals from the meteorite by alternating freezing and thawing, they noticed small, compact, pebble-sized fragments that proved to be very stable.
The researchers used computed tomography to analyze these pebbles and compare them to the rest of the rock in the meteorite. “What was striking was that these components were all crushed instead of spherical and that they were aligned in the same way. They had all been deformed in the same direction, by a single process,” says Philipp Heck. Something had happened to the pebbles that apparently hadn’t happened to the rest of the rock around them. “It was exciting and we were very curious about what it meant,” says Yang.
Pebbles are thrown up and fall back to the surface in other places
This phenomenon had already been noticed by the Osiris-Rex space probe, which had examined the asteroid Bennu. Fragments the size of pebbles had broken away from Bennu and later sunk back to its surface. Heck and Yang have now developed a physical model that explains the course of this process.
Accordingly, earlier collisions with other bodies lead to the fact that deformed zones arise on asteroids. Then, as these asteroids rotate, with different parts turning toward and away from the Sun, the surface is constantly being heated and cooled. Eventually, these temperature differences cause the deformed zones to crumble and then pebbles to be ejected from the surface, likely by minor impacts from other rocks or temperature stress.
However, many of these pebbles tend to stay in a slow, low orbit around their parent body, eventually falling back to the surface and dispersing minerals on the boulder.