Meteorite asteroids provide a record of “spitting” cobbles

Meteorite asteroids provide a record of "spitting" cobbles

NEWSWISE – In 2019, NASA’s OSIRIS-REx spacecraft sent back images of an underground phenomenon no one had seen before: boulders flying off the surface of the asteroid Bennu. The asteroid appeared to be shooting marble-sized rocks. Scientists have never seen this behavior from an asteroid before, and why it happens is a mystery. But in a new paper Natural AstronomyResearchers show the first evidence of this process in a meteorite.

“It’s fascinating to see something discovered by a space mission on an asteroid millions of miles from Earth and find a record from the same geological process in the museum’s meteorite collection,” says Robert A. Pritzker Curator Philip Heck. and Senior Scribe of Meteorites at the Field Museum, Chicago Natural Astronomy Research.

Meteorites are pieces of rock that fall to Earth from space; They can be made of pieces of moons and planets, but most often they are fragments of asteroids. The Aguas Circas meteorite is named after the Costa Rican town where it hit in 2019. It was donated to the Field Museum by Terry and Gail Boudreaux. Hek and his student Xin Yang were preparing the meteorite for another study when they noticed something strange.

“We isolated very small minerals from the meteorite, froze them with liquid nitrogen and tried to melt them in warm water,” says Yang, a graduate student at the Field Museum and the University of Chicago and first on the paper. the author. “It works for most meteorites, but this one was kind of weird — we found some compact fragments that didn’t split.”

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It’s not unheard of to find fragments of a meteorite that haven’t broken down, but Heck says scientists usually just shrug off the mortar and pestle. “Zinn was very open-minded and said, ‘I’m not going to crush these pebbles into sand, it’s fun,'” says Heck. Instead, the researchers devised a plan to figure out what these pebbles were and why they were so resistant to splitting.

“We did a CT scan to see how the pebbles looked compared to the rest of the meteorite’s rocks,” Heck says. “What’s amazing is that all these elements were squeezed — normally, they’d be spherical — and they all had the same orientation. They all deformed in the same direction through a process. Something happened to the pebbles that didn’t happen to the rest of the surrounding rock.

“It was exciting, and we were very curious about what it meant,” Yang says.

Scientists had a clue from the 2019 OSIRIS-REx findings. From there, they formulated a theory, which they supported with physical models. The asteroid underwent a high-speed collision, deforming the impact region. Because the side facing the Sun is over 300° F hotter than the side facing the Sun, the large temperature difference the asteroid experiences as it rotates eventually splits the deformed rock. “This constant thermal cycling cracks the rock, causing it to break down into gravel,” says Heck.

These pebbles are then ejected from the surface of the asteroid. “We don’t yet know what ejects the rocks,” Heck says–they could be scattered by smaller impacts from other space collisions, or they could be released by the thermal stress the asteroid is exposed to. But if the stones are disturbed, Heck says, “you don’t need much to eject anything– the escape velocity is very low.” A recent study revealed that Bennu’s surface is loosely bound and behaves like popcorn in a bucket.

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The stones then entered a very slow orbit around the asteroid, and eventually, they fell back to its surface, further away, without deformation. Then, Hek and Yang say that the asteroid has passed Another one Upon collision, the loose cobbles on the surface were transformed into solid rock. “It basically packed everything together and this loose gravel became a cohesive rock,” Heck says. The same impact sent the new rock into space. Eventually, the piece hit Earth as the Aguas Circas meteorite, carrying evidence of the Pebble mix.

This would explain the cobbles found at Aguas Circas, making the meteorite the first physical evidence of a geologic process observed by OSIRIS-REx in Bennu. “This provides a new way to explain the way minerals on the surface of asteroids coalesce,” says Yang.

That’s a big deal, Heck says, because scientists have long assumed that the main way asteroids’ surface minerals are rearranged is through large crashes, which doesn’t happen very often. “We know from OSIRIS-REx that these particle ejection events are much more frequent than these high-velocity impacts, so they play a more important role in determining the composition of asteroids and meteorites,” says Heck.

Aguas Circas is the first meteorite to show signs of this behavior, but perhaps not the only one. “We would expect this in other meteorites,” Heck says. “People haven’t looked for it yet.”

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