Ancient dust grains reveal the life story of a Solar System asteroid.

According to analyses of grains collected from the asteroid Ryugu, at least some of the carbon-rich rock began its life much farther from the Sun before reaching the asteroid belt and ultimately roughly the distance of Earth from the Sun.

These findings suggest that asteroids may have complex histories involving multiple migrations across the Solar System before arriving at their current location, and that they contain valuable records of different eras throughout Solar System history.

Ryugu is an interesting, if fairly standard, piece of space rock. It is a C-type asteroid, the most common type, rich in carbon and water, and is scattered across the Main Belt asteroids in the space between Mars and Jupiter.

Ryugu, however, is not in the Main Belt. Its orbit around the Sun is too close to Earth's, suggesting some kind of disruption that sent it outside the asteroid belt long ago.

Launched in 2014 as a sample return mission, the Japan Aerospace Exploration Agency's (JAXA) Hayabusa2 probe studied Ryugu from 2018 to 2019 and finally returned home in 2020. In addition to collecting samples, Hayabusa2 also discovered that the 900-metre (2,950-foot) asteroid was not a single large chunk of rock, but something called a "rubble pile", a loose collection of smaller rocks bound together by gravity.

Ryugu has much in common with asteroids in the asteroid belt, especially the Polana and Eulalia families. However, an increasing number of studies of samples brought to Earth from Ryugu show that at least some of the minerals from the rubble pile are more consistent with material from the outer Solar System.

For example, it contains a lot of organic matter, generally similar to comets from the outer Solar System. Oxygen isotopes are also consistent with the outer Solar System.

An international team led by astrophysicist Rosario Brunetto of the University of Paris-Sarclay in France has now taken a closer look at grains rich in olivine, pyroxene and amorphous silicates - the ones least altered by the water the asteroid is rich in.

Brunetto and his team used infrared spectrometry to study these grains and found that the profile of infrared light they reflect resembles objects in the outer Solar System with origins more distant from the Main Belt. These include the asteroid Hector, a Trojan that shares Jupiter's orbit; Comet Hale-Bopp; and interplanetary dust of possible cometary origin.

The researchers say these results show that Ryugu's primary parent body was a planetoid - a "seed" that could have grown into a planet - that originally formed in the outer Solar System and contained outer Solar System components. However, something interrupted its growth, perhaps tearing it apart and sending it to the Main Belt, where it was exchanged for water and transformed.

The asteroid Ryugu therefore represents an important record of the evolution of the Solar System, the researchers say.

"The samples returned from Ryugu are one of the keys to accessing the source of asteroid spectral diversity, because their more pristine lithology contains anhydrous grains that are spectrally similar to some outer solar system primordial bodies," the researchers write in their paper.

"This suggests that Ryugu potentially contains several reservoirs of anhydrous primordial dust, which may carry valuable information about the formation and evolution of planetary bodies in the protoplanetary disc."

 

Source: https://www.sciencealert.com/