Asteroid Ryugu was born out in the cold, in the solar system's earliest days

rocky grey surface
The surface of the asteroid Ryugu, which dates back to the first 4 or 5 million years of the solar system's history. (Image credit: ISAS/JAXA/Hayabusa2 Team)

The near-Earth asteroid Ryugu formed far from the sun, in the cold depths of the outer solar system, according to new analysis of samples returned from Ryugu by the Japanese Hayabusa2 mission.

Hayabusa2 collected two samples of material from Ryugu's surface in 2019, then returned those samples to Earth in 2020. Early analysis indicated that the samples were the most pristine material ever seen in the solar system, incorporating dust older than the sun. Essentially, Ryugu has remained unchanged since it formed during the first 4 or 5 million years of solar-system history. And the latest research on the samples shows that Ryugu hails from near the orbit of Neptune and was kicked inward by the migrating ice giant planets.

The sample analysis, from a team led by Timo Hopp, a planetary scientist at the Max Planck Institute for Solar System Research in Germany, detected surprising abundances of particular isotopes, which are atoms of an element with different numbers of neutrons. 

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Ryugu is broadly classified as a carbonaceous (C-type) asteroid, meaning that it is rich in carbon. Carbonaceous asteroids are the most common type of asteroid, but Ryugu's composition is notably different to the majority of them. For example, Ryugu's abundance of the iron-54 isotope is lower than in most other C-types except for a kind known as CI chondrites. 

One well-known example of a CI chondrite is a meteorite called Ivuna that was discovered on Earth in 1938. Ivuna was notable because the proportions of its volatile elements heavier than hydrogen and helium were nearly identical to the abundances of said elements that are detectable on the sun's visible surface, the photosphere, a sign that the meteorite formed particularly early.

That's because over time, various chemical and thermal processes have changed the compositions of planets, asteroids and comets close to the sun, so these bodies no longer appear primordial. But in the outer extremities of the solar system, where it is much colder, few chemical reactions take place. That means the composition of these objects still reflects the composition of the sun, which in turn reflects the composition of the solar nebula, the gassy cloud that formed the sun and planets.

Scientists believe that most C-type asteroids formed in the region where Jupiter and Saturn now orbit the sun, but the iron isotope signatures of Ryugu and other CI chondrites indicate that these bodies must have formed farther from the sun. The abundance of deuterium (the form of hydrogen that includes a neutron at the atom's core) and nitrogen-15 isotopes in the Ryugu samples, which are what one would expect from an origin in the cold outer solar system. 

However, a team led by Ryuji Okazaki, a planetary scientist at Kyushu University in Japan, has recognized some differences in the composition of Ryugu compared to CI chondrites. In particular, these researchers found that Ryugu contains higher abundances of some noble gases (these are inert, unreactive gases) including helium, neon, argon, krypton and xenon, but a lower abundance of the isotope nitrogen-15 than CI chondrites. The discrepancies indicate that while these objects may have formed in the same region of the solar system, Ryugu and the CI chondrites did not necessarily come from the same parent object.

An artist's impression of planets in a protoplanetary disk. The solar system formed out of such a disk, and planetesimals such as Ryugu would have formed within the disk quickly from a process called pebble accretion.  (Image credit: NASA/FUSE/Lynette Cook)

"In our model, Ryugu might have formed in the region in which it has also been suggested that Oort Cloud comets formed, before they were scattered into the Oort Cloud," Hopp told The Oort Cloud is a realm of trillions of small, icy objects that extends up to at least a light-year from the sun. "Therefore, one could speculate that some Oort Cloud comets might have a similar isotopic composition to Ryugu."

The objects now inhabiting the Oort Cloud were scattered outward by Uranus and Neptune; some bodies would have been ejected from the solar system altogether to become interstellar objects like 'Oumuamua. Ryugu, in contrast, got kicked inward, orbiting in the main asteroid belt between Mars and Jupiter until gravitational interactions with Jupiter pushed it even closer to the sun and it became a near-Earth asteroid.

Asteroids with similar compositions to Ryugu, based on remote spectroscopic observations, make up 10% 20% of all C-type asteroids in the main asteroid belt, suggesting a sizeable proportion of primordial objects were scattered inward.

Ryugu's location near Earth is strong supporting evidence that the planets in the solar system grew quickly and swiftly began to migrate, Hopp and his colleagues argue. 

"For Ryugu to be scattered within the first 4 to 5 million years after solar system formation, the rocky cores of the gas and ice giant planets must have grown quickly," Hopp said. "The fast growth of the giant planets can best be achieved by so-called pebble accretion."

Pebble accretion is a model of planet formation in which the first step toward building a planet is accomplished by small pebbles and boulders that have gradually built up in the cooling protoplanetary disk rapidly stick together to form increasingly larger objects. However, a sticking point — pardon the pun — for theorists has been getting pebbles in a warm protoplanetary disk to stick together, since their velocities often result in them either smashing each other apart or bouncing off one another. Despite these issues, the existence of Ryugu suggests that pebble accretion of some form did indeed happen to form the planets of the solar system.

The research papers from Hopp's team and Okazaki's team were published Oct. 21 in the journal Science Advances.

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Keith Cooper
Contributing writer

Keith Cooper is a freelance science journalist and editor in the United Kingdom, and has a degree in physics and astrophysics from the University of Manchester. He's the author of "The Contact Paradox: Challenging Our Assumptions in the Search for Extraterrestrial Intelligence" (Bloomsbury Sigma, 2020) and has written articles on astronomy, space, physics and astrobiology for a multitude of magazines and websites.