In 2019, the OSIRIS-REX space probe of the NASA, sent images of a geological phenomenon that no one had seen before: stones flying from the surface of the asteroid Bennu. The asteroid did indeed appear to be ‘shooting’ swarms of small, marble-sized pebbles. Scientists have never seen similar behavior in an asteroid, and they still don’t quite know why it happens. But now, in a paper just published in Nature Astronomy, two researchers show, for the first time, evidence of the same phenomenon in a meteorite that fell to Earth three years ago.
“It’s fascinating,” he says. Philipp Heckcurator of meteorites at Chicago’s Field Museum and lead author of the study – seeing something just discovered by a space mission on an asteroid millions of miles from Earth, and finding a record of the same geological process in the meteorite collection at the museum”.
space rocks
Meteorites are rocks that fall to Earth from outer space; they may be made from fragments of ancient moons and planets, but most often they are broken off pieces of larger asteroids. One of them is the Aguas Zarcas meteorite, named after the Costa Rican town where it fell in 2019. It later came to the Field Museum as a donation, and as they prepared the meteorite for study, Heck and his student, Xin Yang, noticed in him something very strange.
“We were trying to isolate very small minerals from the meteorite, first freezing them with liquid nitrogen and then thawing them with warm water to break them down,” recalls Yang, the first author of the article. That works for most meteorites, but something weird happened with this one: we found some compact fragments that didn’t break up.”
According to Heck, it’s not too unusual to find meteorite fragments that don’t disintegrate, but when they do, scientists usually shrug their shoulders and turn to the mortar and pestle to crush them by hand. “But Xin is very open-minded,” says Heck, and she didn’t want to turn those pebbles into sand because she found them interesting. So the researchers devised a plan to find out what those pebbles were and why they were so resistant to breaking.
“We did CT scans to compare the pebbles to the other rocks that make up the meteorite,” Heck continues. What is surprising is that these components were all flattened (normally, they would be spherical) and all had the same orientation. That is, they were all deformed in the same direction by some kind of process.” Something, then, had happened to those pebbles that had not happened to the rest of the rock around them. “It was exciting,” says Yang for his part, “we were very curious about what this meant.”
The track of the asteroid Bennu
The two researchers found a clue where they least expected it: in the data from the 2019 OSIRIS-REX mission. And from there they put together a hypothesis, which they supported with physical models. According to Heck and Yang, the asteroid from which the Aguas Zarcas meteorite emerged suffered a high-speed collision and the impact area was deformed. That warped rock eventually broke apart due to the huge temperature differences the asteroid experiences as it spins, with the side facing the Sun being about 150 degrees hotter than the opposite. “This constant thermal cycling,” Heck says, “causes the rock to become brittle and break down into gravel.”
Afterward, the pebbles were ejected from the asteroid’s surface. “We still don’t know what the process that expels the stones is,” admits the scientist. They may be dislodged by smaller impacts, or they could simply be released by thermal stress on the asteroid. But once the pebbles start moving, it doesn’t take much for them to be ejected, as the escape velocity is so low.” In fact, a recent study of Bennu revealed that its surface is made up of fragments that are barely attached to each other, somewhat like popcorn in a bowl.
The pebbles later entered a very slow orbit around the asteroid, eventually falling back to its surface further out, where there was no longer any deformation. Finally, Heck and Yang explain in their study, the asteroid suffered another collision, the loose pebbles that were mixed on the surface transformed into a solid rock. “Basically,” says Heck, “it was all packed together, and this loose gravel became a cohesive rock.” The same impact may have dislodged the new rock, sending it hurtling into space. In the end, that fragment fell to Earth as the Aguas Zarcas meteorite, carrying with it the evidence of that mixture of pebbles.
The first physical evidence
In this way, the Aguas Zarcas meteorite has become the first physical evidence of the geological process observed by OSIRIS-REx in Bennu. And in Yang’s words, “it provides a new way to explain how minerals are mixed on asteroid surfaces.”
Scientists have long assumed that the main way minerals on asteroid surfaces rearrange themselves is through large impacts, which, however, doesn’t happen very often. “But thanks to OSIRIS-REx,” Heck says, “we know that these pebble ejection events are much more frequent than high-velocity impacts, so they probably play a bigger role in the composition of asteroids and meteorites.”
Aguas Zarcas, then, is the first meteorite to show signs of this behavior, although it is probably not the only one. “We would expect to see the same thing in other meteorites,” says Heck. But people just haven’t looked for this yet.”