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Strange bright rocks reveal glimpse of asteroid Ryugu's violent past

The asteroid Ryugu as seen by Japan's Hayabusa2 probe.
The asteroid Ryugu as seen by Japan's Hayabusa2 probe.
(Image: © JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu, AIST)

Shiny boulders on one of the darkest asteroids in the solar system may shed light on the cataclysmic impacts behind that space rock's origins, a new study finds.

The near-Earth asteroids Itokawa, Bennu and Ryugu are each loose piles of rubble held together by their collective gravity. They likely formed after collisions shattered their parent bodies into many fragments. Much remains unknown about these catastrophic disruptions. 

To uncover secrets about rubble-pile asteroids, the Japan Aerospace Exploration Agency (JAXA) dispatched the spacecraft Hayabusa2 to Ryugu, a 2,790-foot-wide (850 meters) asteroid that is one of the darkest celestial bodies in the solar system. Ryugu's name, which means "dragon palace," refers to a magical underwater castle in a Japanese folktale. 

Related: The strangest asteroids in the solar system

In 2018, Hayabusa2 arrived at Ryugu to map it from orbit and deploy rovers on the boulder-covered asteroid. Ryugu is a carbonaceous or C-type asteroid, which means it is primarily composed of rock that contains a lot of carbon and water. C-types are the most common kind of asteroid found in the outer main asteroid belt

Secrets of a dark asteroid

Although Ryugu's surface is uniformly dark, the scientists behind the new research found numerous boulders scattered across the asteroid that were 1.5 or more times brighter than their surroundings — that is, they reflected at least 50% more light than most of the rest of Ryugu. This contrast made the researchers suspect these boulders may have come from outside the asteroid.

By analyzing the spectrum of light reflected off 21 of these boulders, the scientists deduced they were made of minerals known as anhydrous silicates. Prior studies have suggested that such water-poor, silicon-rich rocks make up silicaceous or S-type asteroids, the most common kind of asteroid found in the inner main asteroid belt. The brightness of these boulders also matches the brightness of S-type asteroids.

"One boulder could be just a coincidence, but we found several of them, which cannot be a lucky event," study lead author Eri Tatsumi, a planetary scientist at the University of La Laguna in Spain, told Space.com. "There should be a story behind them."

Japan's Hayabusa2 spacecraft captured these images of unusually bright S-type rocks that stand out from the darker C-type material that makes up the bulk of asteroid Ryugu. (Image credit: 2020 Tatsumi et al.)

One potential explanation for the presence of these bright boulders is that they struck Ryugu after the asteroid formed. To investigate this possibility, Tatsumi and her colleagues analyzed the size and number of these boulders, as well as prior estimates of how often impacts occur in the main asteroid belt.

The scientists estimated that in order for these bright boulders to come to Ryugu via collisions, 10,000 to 15,000 other impacts likely would have struck the asteroid as well, including two to eight projectiles 75 to 92 feet (23 to 28 meters) wide. These larger projectiles would have destroyed Ryugu, so the researchers suggested this origin scenario for the bright boulders was statistically unlikely.

Instead, the researchers suggested these bright boulders are likely the result of Ryugu's parent body colliding with one or more S-type asteroids before or during Ryugu's formation. These S-type asteroids were likely smaller than Ryugu's parent body, since there is only a very low level of S-type contamination on Ryugu, Tatsumi said.

Because S-type asteroids are now seen mostly in the inner asteroid belt, these new findings suggest that Ryugu may have originated there. "Now we have one more piece to consider in its history," Tatsumi said.

Samples incoming

Hayabusa2 is designed to return samples from Ryugu to shed light on the early days of the solar system. As such, these findings are key to proper analysis of these returned samples, "because the story can change very much depending on where it comes from — for example, the inner or outer solar system," Tatsumi said.

Previous research suggested that Ryugu preserves troves of primordial material from the nebula that gave birth to the sun and its planets. As such, samples from it could yield key insights on planetary formation. 

"My ultimate goal is to reveal the conditions of the early solar system leading to the formation of our Earth," Tatsumi said.

The scientists detailed their findings online Sept. 21 in the journal Nature Astronomy.

Follow Charles Q. Choi on Twitter @cqchoi. Follow us on Twitter @Spacedotcom and on Facebook.

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