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Ancient asteroids are covered with popcorning pebbles, new study finds

An artist's impression of the popcorning pebbles on the parent body of the Aguas Zarcas meteorite. Small-scale rearrangements may be more important for asteroids than major space collisions.

An artist’s impression of the popcorning pebbles on the parent body of the Aguas Zarcas meteorite. Small-scale rearrangements could be more very important to asteroids than major space collisions.(Image credit: Illustration credit: April I. Neander. Asteroid image: NASA/Goddard/University of Arizona.)

In 2019, when NASA’s OSIRIS-REx spacecraft approached the asteroid Bennu, scientists saw something stunning in the images beamed back again to Earth. The top of space rock wasn’t calm instead, swarms of marble-sized rocks were popcorning off the asteroid.

Now, a fresh study of a meteorite that landed on the planet reveals how this asteroid activity occurs. Small collisions can dislodge the pebbles, which shoot off the asteroid but fall back, used by the area rock’s gravitational pull. Another collision may then smush the loose pebbles back together, developing a sort of cement of minerals from all over the asteroid’s surface.

“It offers a new method of explaining just how that minerals on the surfaces of asteroids get mixed,” Xin Yang, a graduate student at the Chicago Field Museum and the University of Chicago and the initial author of the brand new study, said in a statement.

Meteorite mystery

Previously, astronomers thought that asteroids had to endure dramatic, high-speed, high-pressure collisions to reshape their surfaces, Philipp Heck, the curator of meteoritics at the Field Museum and the studys senior author, said in the statement.

However, the brand new study, published Aug. 11 in the journal Nature Astronomy (opens in new tab), indicates that it actually doesn’t take much to morph an asteroid. The researchers discovered this if they examined a little bit of the Aguas Zarcas meteorite, which fell in Costa Rica in 2019. Fragments of the area rock, which acquired a smooth glassy sheen because of the heating it experienced in the atmosphere, hit the roof of a residence and a nearby doghouse, in accordance with Arizona State University’s Buseck Center for Meteorite Studies.

Related: This weird meteorite crashed by way of a doghouse in Costa Rica. (The dog’s fine.)

“We were attempting to isolate very small minerals from the meteorite by freezing it with liquid nitrogen and thawing it with tepid to warm water, to break it up,” Yang said. “That works for some meteorites, but that one was sort of weird we found some compact fragments that wouldn’t break apart.”

The main mass of the Aguas Zarcas meteorite, which broke up over Costa Rica in 2019.

The primary mass of the Aguas Zarcas meteorite, which split up over Costa Rica in 2019. (Image credit: John Weinstein, Field Museum)

Rather than forcing the fragments apart, the researchers looked deeper to discover why these were so resilient. Using computed tomography (CT), the scientists could actually peer at the grains, or chondrules, within the tough fragments. Generally in most space rocks, these chondrules are spherical, however in the Aguas Zarcas fragments, these were squished, and all in exactly the same direction. This is an obvious sign that the fragments that wouldn’t break apart have been impacted.

Mineral mixing

The 2019 images of Bennu’s popcorning surface helped tell all of those other meteorites story. Bennu and Aguas Zarcas are both carbon-rich rocks that formed early in the solar system’s history. Therefore,the Aguas Zarcas fragment that hit Earth could have broken off an asteroid nearly the same as Bennu.

Putting the space- and lab-based observations together, the researchers figured the parent asteroid of Aguas Zarcas first underwent a high-speed collision, deforming part of the rock. This weakened rock gradually broke apart, likely due to the dramatic temperature changes an asteroid undergoes since it rotates evoking the rock to expand, compress, and ultimately fracture. (The medial side of an asteroid facing sunlight could be 300 degrees Fahrenheit (149 degrees Celsius) hotter compared to the side facing away.)

Related: Whats the difference between asteroids, comets and meteors?

Something then ejects this broken-up gravel from the asteroid surface, Heck said. It isn’t clear whether another collision is essential, or if the same thermal stress from uneven heating can perform the trick. In any event, the pebbles slowly orbit the asteroid. The gravitational pull of the primary asteroid body then gradually causes the pebbles to rain back off onto elements of the top that never underwent a direct effect. Finally, the asteroid experienced another collision that cemented the impacted fragments and the unimpacted fragments into one rock.

“It basically packed everything together,” Heck said. This might have already been the impact that broke off the fragment that eventually reached Earth.

While major space rock crashes are rare, scientists now know from observations of Bennu that asteroids often spit off pebbles. These low-level events are most likely more very important to an asteroid’s composition than big collisions, Heck said.

“We’d expect this in other meteorites,” he said. “People just haven’t looked for this yet.”

Originally published on Live Science.

Stephanie Pappas

Stephanie Pappas is really a contributing writer for Live Science, covering topics which range from geoscience to archaeology to the mind and behavior. She once was a senior writer for Live Science but is currently a freelancer located in Denver, Colorado, and regularly plays a part in Scientific American and The Monitor, the monthlymagazine of the American Psychological Association. Stephanie received a bachelor’s degree in psychology from the University of SC and a graduate certificate in science communication from the University of California, Santa Cruz.

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