It seems that asteroid Ryugu may not have traveled as far from its origin to its current location near Earth as was once thought. Recent research published in the journal Science Advances indicates that Ryugu was likely formed near Jupiter, contradicting earlier beliefs that it originated beyond Saturn’s orbit. Four years ago, the Japanese space probe Hayabusa 2 returned with samples from Ryugu. A team of researchers from the Max Planck Institute for Solar System Research (MPS) in Germany has analyzed the types of nickel present in these samples, comparing them with typical carbon-rich meteorites. The findings provide a new perspective on the locations where these celestial bodies formed: various carbon-rich asteroids may have originated in the same area close to Jupiter, albeit through different processes and approximately two million years apart.
In December 2020, the Hayabusa 2 probe successfully delivered samples from asteroid Ryugu to Earth. Since then, these few grams of material have undergone extensive analysis. After preliminary examinations in Japan, the small, dark grains were sent to research institutions worldwide, where they were measured, weighed, chemically analyzed, and subjected to infrared, X-ray, and synchrotron radiation testing, among other studies. At the MPS, researchers are measuring the ratios of specific metal isotopes within the samples, as is done in the current study. Isotopes are different versions of the same element that only vary in the number of neutrons in their nuclei. Such investigations help scientists understand where in the Solar System Ryugu was formed.
Ryugu’s journey through the Solar System
Ryugu is classified as a near-Earth asteroid, meaning its orbit around the Sun crosses that of Earth (though there is no risk of collision). Researchers believe that like other near-Earth asteroids, Ryugu is not originally from the inner Solar System but migrated there from the asteroid belt positioned between the orbits of Mars and Jupiter. The true origins of the asteroids in the belt are likely even further from the Sun, beyond Jupiter’s orbit. Initially, it was believed that Ryugu had traveled a long distance to get to its current near-Earth orbit. However, current research suggests that Ryugu formed near Jupiter instead of beyond Saturn’s orbit, as previous studies proposed.
To better understand Ryugu’s origin and subsequent evolution, scientists compare it with well-known meteorite classes. These classes consist of fragments from asteroids that have fallen to Earth. Recent studies have surprisingly found that, while Ryugu is indeed part of the broader category of carbon-rich meteorites known as carbonaceous chondrites, it belongs to a less common subgroup known as CI chondrites. The CI chondrites, also referred to as Ivuna-type chondrites, are named after a region in Tanzania where one of the best-known examples was found. So far, only eight of these unique specimens have been identified, and their chemical makeup closely resembles that of the Sun, indicating they are some of the most pristine materials formed at the farthest edge of the Solar System. “Up until now, we believed that Ryugu’s origin was also located past Saturn’s orbit,” says Dr. Timo Hopp, an MPS scientist and co-author of this study, who has previously led research on Ryugu’s isotopic makeup.
However, the latest analyses from the Göttingen team tell a different story. This investigation was the first to explore the ratios of nickel isotopes in four samples of asteroid Ryugu and six samples of carbonaceous chondrites. The findings confirm a close connection between Ryugu and CI chondrites but make a common origin at the edge of the Solar System less credible.
A missing ingredient
What has changed? Previously, scientists understood carbonaceous chondrites as combinations of three “ingredients,” observable even in cross-sections. These components include fine-grained rock, round inclusions about a millimeter in size, and smaller irregularly shaped inclusions. The irregular inclusions are thought to be the first materials that condensed into solid clumps within the hot gas disk that once surrounded the Sun. The round chondrules, rich in silicates, formed afterward. Research previously attributed isotopic differences between CI chondrites and other carbonaceous chondrites to varying mixing ratios of these three components. For example, CI chondrites primarily consist of fine-grained rock, while other varieties have more inclusions. However, the current study describes how the nickel measurements do not fit this framework.
The researchers have concluded that a fourth component must exist: tiny iron-nickel grains formed during the asteroids’ creation. In the cases of Ryugu and the CI chondrites, this formation process needed to be especially effective. “Clearly distinct processes were at play in forming Ryugu and CI chondrites, as opposed to other types of carbonaceous chondrites,” summarizes Fridolin Spitzer from the MPS, the lead author of the new study.
The study indicates that the first carbonaceous chondrites began forming approximately two million years after the Solar System’s formation. Dust and initial solid clumps were drawn into the inner Solar System by the gravitational pull of the young Sun, but they faced a challenge with the forming Jupiter in their path. Heavier materials primarily accumulated beyond Jupiter’s orbit, leading to the growth of carbonaceous chondrites rich in inclusions. Toward the end of this formation, around two million years later, the Sun’s influence caused the original gas to evaporate outside Jupiter’s orbit, resulting in the concentration of mainly dust and iron-nickel grains and giving rise to the CI chondrites.
“Our findings took us by surprise. We had to completely reassess our views—not only concerning Ryugu but regarding the entire group of CI chondrites,” remarks Dr. Christoph Burkhard from the MPS. The CI chondrites now seem less like distant relatives of other carbonaceous chondrites from the outer Solar System and more like younger siblings formed in the same area, but through different processes and later. “This study illustrates the vital role that laboratory investigations play in piecing together the formation history of our Solar System,” notes Prof. Dr. Thorsten Kleine, Director of the Department of Planetary Sciences at the MPS and a co-author of the research.
