Ryugu, a near-Earth asteroid, has an orbit around the Sun that crosses Earth's orbit but poses no collision risk. However, like many near-Earth asteroids, Ryugu is not believed to have formed in the inner Solar System. Scientists hypothesize that it originated from the asteroid belt between Mars and Jupiter and perhaps from even further, beyond Jupiter's orbit.

One area of investigation into Ryugu's origins involves its comparison to meteorites that have fallen to Earth. Recent research has confirmed that Ryugu fits into the category of carbon-rich meteorites, specifically the rare CI chondrites. These meteorites, also called Ivuna-type chondrites, have a chemical composition similar to the Sun, which makes them particularly valuable for studying the early Solar System. Until now, it was believed that Ryugu's birthplace lay beyond Saturn's orbit. However, new research challenges this assumption.

A New Picture of Ryugu's Origin
The latest study by scientists in Gottingen marks a major step forward. For the first time, researchers examined the ratios of nickel isotopes in four Ryugu samples and six carbonaceous chondrite samples. While the results confirmed a strong relationship between Ryugu and CI chondrites, the previous assumption that both formed at the Solar System's outer edge may no longer hold.

Past models have viewed CI chondrites as a mix of three visible ingredients: fine-grained rock, round inclusions, and smaller, irregular inclusions. These inclusions represent early solid material in the Solar System's protoplanetary disk. Differences in isotopic composition among different chondrite types were thought to result from varying amounts of these three components. CI chondrites, for instance, contain more fine-grained material. However, new nickel isotope data doesn't align with this three-ingredient model.

Instead, the research suggests a fourth, previously unrecognized component: tiny iron-nickel grains. In the case of Ryugu and CI chondrites, these grains must have been particularly abundant during formation.

"Completely different processes must have been at work in the formation of Ryugu and the CI chondrites compared to other carbonaceous chondrites," explained Fridolin Spitzer from MPS, lead author of the new study.

The formation of carbonaceous chondrites likely began about two million years after the Solar System itself began to take shape. As dust and solid clumps moved inward, they encountered Jupiter's growing gravitational influence. This caused larger, heavier materials to accumulate outside its orbit, leading to the formation of the chondrites. By the end of this period, dust and iron-nickel grains dominated the region, leading to the formation of CI chondrites.

"The results surprised us very much. We had to completely rethink not only Ryugu's origin but also that of all CI chondrites," said Dr. Christoph Burkhard from MPS. CI chondrites, once thought to be distant relatives from the Solar System's outermost edges, may actually have formed closer to Jupiter but through a distinct process.

"This study highlights how essential laboratory investigations are for understanding the history of our Solar System," added Prof. Dr. Thorsten Kleine, Director of the Department of Planetary Sciences at MPS and co-author of the study.

Research Report:The Ni isotopic composition of Ryugu reveals a common accretion region for carbonaceous condrites

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