Meteorite provides record of asteroid 'spitting out' pebbles

Meteorite provides record of asteroid ‘spitting out’ pebbles

The main mass of the Aguas Zakas meteorite in the Field Museum.Credit: John Weinstein, Field Museum

In 2019, NASA’s OSIRIS-REx spacecraft sent back images of a never-before-seen geological phenomenon: pebbles flying off the surface of asteroid Bennu. The asteroid appears to be shooting out of a marble-sized rock mass. Scientists have never seen this behavior on an asteroid before, and exactly why it happens remains a mystery.but in a new paper natural astronomythe researchers showed the first evidence of this process in a meteorite.

“It’s fascinating to see what a space mission has just discovered on an asteroid millions of miles from Earth, and to find records from the same geological process in the museum’s meteorite collection,” Curator Robert A. Pritzker Philip Heck speaks of Meteoritics at Chicago’s Field Museum and natural astronomy study.

Meteorites are rock fragments that fall to Earth from outer space; they can consist of fragments of moons and planets, but most often, they’re fragments of asteroids. The Aguas Zarcas meteorite is named after the town in Costa Rica where it fell in 2019; it came to the Field Museum as a gift from Terry and Gail Boudreau. Heck and his student Xin Yang were preparing the meteorite for another study when they noticed something strange.

“We were trying to isolate very tiny minerals from meteorites by freezing them with liquid nitrogen and freezing them with warm water to break them down,” said Yang, a graduate student at the Field Museum and the University of Chicago and the author. “This works for most meteorites, but this one is a little weird – we found some dense fragments that don’t break apart.”

Meteorite provides record of asteroid 'spitting out' pebbles

Artistic depiction of the mixing process of pebbles from the parent body of Aguas Zarcas. Pebble-sized fragments were ejected and redeposited on the asteroid’s surface. Image credit: April I. Neander. Asteroid image: NASA/Goddard/University of Arizona.

It’s not unheard of to find pieces of meteorites that don’t break down, Heck said, but scientists usually just shrug and break up mortars and pestles. “Shin had a very open mind and he said, ‘I’m not going to grind these pebbles into sand, it’s fun,'” Heck said. Instead, the researchers devised a plan to figure out what these pebbles are and why they are so difficult to break down.

“We did CT scans to see how the pebbles compared to the other rocks that make up the meteorite,” Heck said. “It’s shocking that these components are all squashed — usually, they’re spherical — and they all have the same orientation. They’re all deformed in the same direction, through a process.” Something Happened to Cobblestone things, while the other rocks around them didn’t.

Meteorite provides record of asteroid 'spitting out' pebbles

NASA’s OSIRIS-REx spacecraft regularly observes pebbles ejected from the surface of asteroid Bennu. This observation inspired the study. Image credit: NASA/Goddard/University of Arizona/Lockheed Martin.

“It’s exciting, and we’re curious what that means,” Young said.

However, scientists took a clue from the 2019 OSIRIS-REx discovery. From there, they came up with a hypothesis, which they backed up with physical models. The asteroid underwent a high-speed collision that deformed the impact area. Due to the huge temperature difference the asteroid experiences as it rotates, the deformed rock eventually breaks apart because the side facing the sun is 300°F higher than the side facing away from the sun. “This constant thermal cycle brittles the rock and breaks it down into gravel,” Heck said.

These pebbles were then ejected from the asteroid’s surface. “We don’t yet know what the process of ejecting the pebbles is,” Heck said—they could be sloughed off by smaller impacts from other space collisions, or they could just be released by the thermal stress the asteroid is experiencing. But once the pebbles are disturbed, Heck says, “you don’t need a lot of stuff to jet things—the escape velocity is very low.” A recent study of Bennu showed that its surfaces are loosely bound and behave like barrels popcorn in.

Meteorite provides record of asteroid 'spitting out' pebbles

Sampling the Aguas Zarcas meteorite at the Field Museum of Natural History.Credit: Drew Carhart, Field Museum

The pebbles then went into a very slow orbit around the asteroid, and eventually, they fell back to the surface of the asteroid farther away without deformation.Then, Heck and Young said, the asteroid experienced other On impact, the loose-faced mixed pebbles turned into solid rock. “It basically packs everything together, and this loose gravel becomes a sticky rock,” Heck said. The same impact may have dislodged the new rock, sending it into space. Eventually, the meteorite fell to Earth as the Aguas Zarcas meteorite, carrying evidence of pebbles mixing.

This could explain the presence of pebbles in Aguas Zarcas, making the meteorite the first physical evidence of the geological process OSIRIS-REx observed on Bennu. “It provides a new way to explain how minerals mix on asteroid surfaces,” Yang said.

That’s important, Heck said, because scientists have long believed that the main way minerals on the surface of asteroids rearrange is through large collisions, which don’t happen very often. “We know from OSIRIS-REx that these particle ejection events are much more frequent than these high-velocity impacts,” Heck said, “so they may 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 may not be the only one. “We expect this to be the case for other meteorites,” Heck said. “People just haven’t looked for it yet.”

‘Fireball’ meteorite contains primitive extraterrestrial organic compounds

More information:
Yang Xin, Post-accretion asteroid surface mixing records preserved in the Aguas Zarcas meteorite, natural astronomy (2022). DOI: 10.1038/s41550-022-01746-4.

Citation: Meteorite provides record of asteroid ‘spitting out’ pebbles retrieved from on August 12, 2022 (August 11, 2022 )

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