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Outer Space

Scientists find out that meteorites spit this into space

The bizarre behavior was identified in the Aguas Zarcas meteor that fell to Earth three years ago.

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Pebbles ejected off the surface of asteroid Bennu were observed frequently by NASA.
(NASA/Goddard/University of Arizona/Lockheed Martin via SWNS).

By Mark Waghorn via SWNS

Meteorites 'spit' pebble-sized rocks into space, according to new research.

They flick out fragments as they hurtle through the cosmos - at up to 160,000 miles an hour.

The bizarre behavior was identified in Aguas Zarcas - which fell to Earth three years ago.

It is more than 4.5 billion years old - and formed out of dust from the early Milky Way.

Philipp Heck, the Robert A. Pritzker Curator of Meteoritics at Chicago’s Field Museum and the senior author of the Nature Astronomy study said: "We would expect this in other meteorites. People just haven't looked for it yet."

In 2019, NASA's OSIRIS-REx spacecraft sent back images of swarms of marble-like rocks flying off the asteroid Bennu as it traveled through the solar system.

Now the phenomenon has been detected in meteorites that break off from asteroids and occasionally moons - or even planets.

Heck said: "It's fascinating to see something that was just discovered by a space mission on an asteroid millions of miles away from Earth, and find a record from the same geological process in the museum's meteorite collection."

The cause is a mystery. One theory is ice just under the surface turns to gas - and acts as a propellant.

Aguas Zarcas is named after the Costa Rican town where it dropped. The meteorite was donated to the museum by private collectors Terry and Gail Boudreaux.

Heck and his student Xin Yang were preparing it for another study when they noticed something strange.

First author Yang said: "We were trying to isolate very tiny minerals from the meteorite by freezing it with liquid nitrogen and thawing it with warm water, to break it up.

"That works for most meteorites, but this one was kind of weird - we found some compact fragments that wouldn't break apart."

The main mass of the Aguas Zarcas meteorite at the Field Museum. (John Weinstein, The Field Museum via SWNS)

Finding bits that won't disintegrate sometimes happens. Scientists usually just shrug and break out the mortar and pestle.

Heck said: "Xin had a very open mind. He said, 'I'm not going to crush these pebbles to sand, this is interesting.'"

Instead, the researchers devised a plan to figure out what they were - and why they were so strong.

Explained Heck: "We did CT (computed tomography) scans to see how the pebbles compared to the other rocks making up the meteorite.

"What was striking is these components were all squished - normally, they'd be spherical - and they all had the same orientation. They were all deformed in the same direction, by one process."

Something had happened to the pebbles that didn't happen to the rest of the rock around them.

Yang said: "This was exciting, we were very curious about what it meant."

By analyzing the OSIRIS-REx findings they put together a hypothesis - supported by physical models.

They showed the asteroid underwent a high-speed collision, and the area of impact got deformed.

That rock eventually broke apart due to the huge temperature differences the asteroid experiences when it rotates. The side facing the sun is more than 300° F warmer.

Heck said: "This constant thermal cycling makes the rock brittle, and it breaks apart into gravel."

Artistic depiction of pebble mixing process from the Aguas Zarcas parent body. (April I. Neander/NASA/Godd via SWNS)

These pebbles are then ejected from the asteroid's surface. Heck said: "We don't yet know what the process is that ejects the pebbles."

They might be dislodged by smaller impacts other space collisions, or just get released by the thermal stress the asteroid undergoes.

But once the pebbles are disturbed, "you don't need much to eject something - the escape velocity is very low," said Heck.

A recent study of Bennu revealed its surface is loosely bound and behaves like 'popcorn in a bucket.'

The pebbles then entered a very slow orbit around the asteroid and, eventually, fell back down to its surface further away where there was no deformation.

Then the asteroid underwent another collision, and the loose mixed pebbles on the surface got transformed into solid rock.

Heck said: "It basically packed everything together, and this loose gravel became a cohesive rock."

The same impact may have dislodged the new rock, sending it careening into space. Eventually, that chunk fell to Earth as the Aguas Zarcas meteorite, carrying evidence of the pebble mixing.

This could explain the pebbles present in Aguas Zarcas, making the meteorite the first physical evidence of the geological process observed by OSIRIS-REx on Bennu.

Yang said: "It provides a new way of explaining the way that minerals on the surfaces of asteroids get mixed."

Heck said that's the big deal: For a long time, it was assumed the main way the minerals on the surfaces of asteroids get rearranged is through big crashes, which don't happen very often.

Added Heck: "From OSIRIS-REx we know these particle ejection events are much more frequent than these high-velocity impacts, so they probably play a more important role in determining the makeup of asteroids and meteorites."

The study was published in the journal Nature Astronomy.

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