Samples taken from asteroid Ryugu have once again surprised scientists and challenged previous ideas about how carbon-rich asteroids form. A new study has been published Science Advances suggests that Ryugu may have formed near Jupiter rather than orbiting Saturn as previous studies suggested. Four years ago, Japan’s Hayabusa 2 mission returned samples of Ryugu to Earth.

Researchers at the Max Planck Institute for Solar System Research (MPS) in Germany compared the nickel species found in these samples to typical carbon-rich meteorites. The findings point to a new possibility: Different carbon-rich asteroids may have formed in the same region near Jupiter, albeit by different processes and about two million years apart.

Global journey of Ryugu samples

After the Hayabusa 2 probe delivered samples of Ryugu to Earth in December 2020, the material was extensively analyzed. The tiny pitch-black grains were first studied in Japan and later sent to research institutions around the world. Here, scientists measured, weighed and chemically analyzed samples by exposing them to infrared, X-ray and synchrotron radiation. At MPS, researchers focused on isotope ratios of metals, including nickel. These isotope studies, which involve examining elements with different numbers of neutrons, help scientists determine where in the solar system Ryugu formed.

Ryugu’s journey through the solar system

Ryugu is a near-Earth asteroid: its orbit around the Sun crosses Earth’s orbit (without risk of collision). However, researchers suggest that Ryugu, like other near-Earth asteroids, is not native to the inner Solar System, but came there from the asteroid belt located between the orbits of Mars and Jupiter. The actual birthplaces of the asteroid belt population are probably much farther from the Sun, beyond the orbit of Jupiter.

Ryugu’s “family relationship” may help shed light on his origins and subsequent evolution. How similar is Ryugu to representatives of known classes of meteorites? These are asteroid fragments that fell to Earth from space.

Research in recent years has revealed a surprise: Ryugu fits, as expected, into a large group of carbon-rich meteorites, the carbonaceous chondrites. However, detailed studies of its composition link it to a rare group: the so-called XI-chondrites. They are also known as Iwuna-type chondrites, named after the region in Tanzania where their most famous representatives are found. Besides the Ivuna chondrite, only eight other exotic specimens have been discovered to date. Because their chemical composition is similar to that of the Sun, they are considered particularly pristine material formed at the outermost edge of the Solar System.

Co-author of the current study, MPS scientist Dr. “Until now, we had assumed that Ryugu’s origin was also outside the orbit of Saturn,” explains Timo Hopp.

The latest analysis by scientists from Göttingen paints a different picture. For the first time, the team examined the ratio of nickel isotopes in four samples of the Ryugu asteroid and six samples of carbonaceous chondrites. The results confirm a close relationship between Ryugu and CI chondrites. But the idea of ​​a common birthplace at the edge of the solar system is no longer convincing.

A revised understanding of cosmic ingredients

What happened? Until now, researchers viewed carbonaceous chondrites as a mixture of three “components” that were visible in cross-sections even with the naked eye. Millimeter-sized rounded inclusions are embedded in fine-grained rock, as well as smaller, irregularly shaped inclusions packed tightly together. The irregular inclusions are the first material to condense into solid clumps in the disk of hot gas that once orbited the Sun. Later, round silicate chondrules were formed.

So far, researchers have attributed the isotopic composition differences between CI chondrites and other carbonaceous chondrite groups to different mixing ratios of these three components. For example, CI chondrites consist predominantly of fine-grained rocks, while their counterparts are much richer in inclusions. But as the team explains in the current publication, the results of the nickel measurements do not fit this pattern.

The researchers’ calculations now show that their measurements can only be explained by a fourth component: tiny iron-nickel grains that must also have accumulated during asteroid formation. In the case of the Ryugu and CI chondrites this process must have been particularly effective. “Completely different processes must have occurred in the formation of Ryugu and CI chondrites on the one hand, and other groups of carbonaceous chondrites on the other,” says Fridolin Spitzer from MPS, first author of the new study.

Surprising discoveries in asteroid research

According to the researchers, the first carbonaceous chondrites began to form approximately two million years after the formation of the solar system. Dust and the first solid clumps, pulled by the young Sun’s gravity, moved from the outer edge of the gas-dust disk into the inner Solar System, but they encountered an obstacle on the way: the newly formed Jupiter.

Outside its orbit, heavier and larger clusters accumulated, turning into carbonaceous chondrites containing many debris. Towards the end of this development, approximately two million years later, a different process occurred: Under the influence of the Sun, the source gas slowly evaporated outside Jupiter’s orbit, leading to the accumulation of primarily dust and iron-nickel particles. This led to the birth of CI chondrites.

“The results really surprised us. We had to completely rethink not only Ryugu, but also the entire CI chondrite group,” says MPS’s Dr. Christoph Burkhard. CI chondrites no longer resemble distant, somewhat exotic relatives of other carbonaceous chondrites in the outer Solar System; Rather, they appear to be younger siblings that may have formed in the same region, but by a different process and at a later time.

Director of the MPS Department of Planetary Sciences and co-author of the study, Professor Dr. “The current study shows how critical laboratory research can be for unraveling the formation history of our Solar System,” says Thorsten Kleine.