Ryugu samples challenge previous ideas about the formation of carbon-rich asteroids

Asteroid Ryugu may not have traveled as far from its place of origin to its near-Earth orbit as previously thought. New research published in the journal Scientific advances suggests that Ryugu formed near Jupiter.

Previous studies had indicated an origin beyond Saturn’s orbit. Four years ago, the Japanese space probe Hayabusa 2 brought samples from Ryugu back to Earth. Researchers led by the Max Planck Institute for Solar System Research (MPS) in Germany compared the types of nickel found in these samples as well as in typical carbon-rich meteorites.

The results show an alternative to previous ideas about the birthplaces of these bodies: different carbon-rich asteroids could have formed in the same region near Jupiter, although partly by different processes and around two million years away. years apart.

Since December 2020, when the samples from the asteroid Ryugu were brought back to Earth, the few grams of material have undergone many tests. After initial examinations in Japan, some of the tiny jet-black grains were sent to research centers around the world.

There they were measured, weighed, chemically analyzed and exposed to, among other things, infrared, X-ray and synchroton radiation. At MPS, researchers examine the ratios of certain metal isotopes in samples, as in the current study. Scientists consider isotopes to be variations of the same element that differ only in the number of neutrons in the nucleus. Research like this can help understand where Ryugu formed in the solar system.

Ryugu’s journey through the solar system

Ryugu is a near-Earth asteroid. Its orbit around the Sun crosses that of the Earth (without risk of collision). However, researchers assume that, like other near-Earth asteroids, Ryugu did not originate in the inner solar system, but traveled there from the asteroid belt between the orbits of Mars and Jupiter. The true birthplaces of the asteroid belt population are likely even further from the sun, outside Jupiter’s orbit.

Ryugu’s “family connections” may help shed light on his origin and future development. How much does Ryugu resemble representatives of well-known classes of meteorites? These are fragments of asteroids that traveled from space to Earth.

Investigations carried out in recent years have revealed a surprise: Ryugu fits as expected into the large crowd of carbon-rich meteorites, carbonaceous chondrites. However, detailed studies of its composition attribute it to a rare group: the so-called CI chondrites. These are also known as Ivuna-type chondrites, named after the Tanzanian location where their best-known representative was found.

Besides the Ivuna chondrite itself, only eight other such exotic specimens have been discovered to date. Because their chemical composition is similar to that of the sun, they are considered a particularly pristine material that formed at the outermost edge of the solar system.

“Until now, we had assumed that Ryugu’s place of origin was also outside Saturn’s orbit,” says MPS scientist Dr. Timo Hopp, co-author of the present study, who already conducted previous research on the isotopic composition of Ryugu.

The latest analyzes by Göttingen scientists now paint a different picture. For the first time, the team studied nickel isotope ratios in four samples from the asteroid Ryugu and six samples from carbonaceous chondrites. The results confirm the close relationship between Ryugu and CI chondrites. However, the idea of ​​a common birthplace at the edges of the solar system is no longer convincing.

A missing ingredient

What had happened? Until now, researchers considered carbonaceous chondrites as mixtures of three “ingredients” visible even to the naked eye in cross sections. Embedded in the fine-grained rock, millimeter-sized round inclusions as well as smaller, irregularly shaped inclusions are densely grouped. The irregular inclusions were the first materials to condense into solid clumps in the disk of hot gas that once orbited the sun. The round, silicate-rich chondrules formed later.

Until now, researchers attributed differences in isotopic composition between CI chondrites and other groups of carbonaceous chondrites to different mixing ratios of these three ingredients. CI chondrites, for example, are primarily made of fine-grained rocks, while their siblings are significantly richer in inclusions. However, as the team describes in the current publication, the nickel measurement results do not fit this pattern.

The researchers’ calculations now show that their measurements can only be explained by a fourth ingredient: tiny grains of iron-nickel, which must also have accumulated during the formation of asteroids. In the case of Ryugu and the CI chondrites, this process must have been particularly efficient.

“Completely different processes must have been at work in the formation of the Ryugu and CI chondrites on the one hand and the other groups of carbonaceous chondrites on the other,” explains Fridolin Spitzer of the MPS, first author of the new study, summarizing the basic ideas.

According to the researchers, the first carbonaceous chondrites began to form about two million years after the formation of the solar system. Attracted by the gravitational force of the still young sun, dust and the first solid clumps made their way from the outer edge of the disk of gas and dust into the inner solar system, but encountered an obstacle along the way: the Newly formed Jupiter.

Outside its orbit, the heaviest and largest clusters accumulated and thus transformed into carbonaceous chondrites with their numerous inclusions. Towards the end of this evolution, after about two million years, another process took over: under the influence of the sun, the original gas gradually evaporated outside Jupiter’s orbit, resulting in an accumulation mainly of dust and iron-nickel grains. This led to the birth of CI chondrites.

“The results surprised us a lot. We had to completely rethink, not only with regard to Ryugu, but also with regard to the entire CI chondrite group,” explains Dr. Christoph Burkhard from the MPS.

CI chondrites no longer appear as distant and somewhat exotic relatives of other carbonaceous chondrites from the outermost edge of the solar system, but rather as younger siblings that may have formed in the same region, but by a different process and later.

“The current study shows how crucial laboratory research can be in deciphering the history of the formation of our solar system,” says Professor Thorsten Kleine, director of the MPS Department of Planetary Sciences and co-author of the paper. ‘study.