Ryugu samples cast doubt on previous ideas about the formation of carbon-rich asteroids

Asteroid Ryugu may not have migrated as far from its origin location to its current near-Earth orbit as previously assumed. A new study published in the journal Science Advances suggests that Ryugu formed near Jupiter.

Previous research had pointed to an origin outside of Saturn’s orbit. Four years ago, Japan’s space probe Hayabusa2 brought back samples of Ryugu to Earth. Now, researchers led by Germany’s Max Planck Institute for Solar System Research (MPS) compared these samples to what types of nickel are found in typical carbon-rich meteorites.

The results suggest an alternative to previous ideas about the birthplaces of these objects. This means that different carbon-rich asteroids could have formed in the same region close to Jupiter, albeit through partially different processes and about 2 million years apart.

Since samples from the asteroid Ryugu were brought back to Earth in December 2020, significant amounts of several grams of material have been collected. After initial testing in Japan, some of the tiny jet-black particles were shipped to research facilities around the world.

There measurements, gravimetric measurements, chemical analyzes were carried out, and they were exposed to infrared, X-ray and synchroton radiation, among others. In MPS, as in this study, researchers look at the proportions of specific metal isotopes in a sample. Scientists refer to isotopes as variants of the same element that differ only in the number of neutrons in the atomic nucleus. This type of research helps us understand where in the solar system Ryugu formed.

Ryugu’s solar system journey

Ryugu is a near-Earth asteroid. Its orbit around the Sun intersects the Earth’s orbit (there is no risk of collision). However, researchers believe that Ryugu, like other near-Earth asteroids, is not native to the solar system, but may have migrated into the solar system from the asteroid belt between the orbits of Mars and Jupiter. There is. The actual birthplace of the asteroid belt population is probably even further from the Sun, outside of Jupiter’s orbit.

Ryugu’s “family relationships” help reveal its origins and further evolution. How similar is Ryugu to typical meteorites of a well-known class?These are fragments of asteroids that came to Earth from space.

Recent research has revealed something surprising. As expected, Ryugu is nestled within a large cluster of carbon-rich meteorites, carbonaceous chondrites. However, a detailed study of its composition places it in an unusual group – the so-called CI chondrites. These are also known as Ivna-type chondrites, named after the location in Tanzania where their best-known representatives were discovered.

In addition to the Ivna chondrites themselves, only eight of these exotic specimens have been discovered to date. Because its chemical composition is similar to that of the Sun, it is considered a particularly primitive material that formed at the outermost edge of the solar system.

“Until now, it was thought that the origin of Ryugu was also outside of Saturn’s orbit,” said MPS, co-author of the study and who has already led initial investigations into Ryugu’s isotopic composition. explains scientist Dr. Timo Hopp.

The latest analysis by Göttingen scientists now paints a different picture. For the first time, the research team examined the nickel isotope ratios in four samples from the asteroid Ryugu and six samples from carbonaceous chondrites. This result confirmed the 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.

missing ingredients

What happened? Previously, researchers understood carbonaceous chondrites to be a mixture of three “components” that can be seen in cross section with the naked eye. In fine-grained rocks, millimeter-sized round inclusions and smaller irregularly shaped inclusions are densely packed. Irregular inclusions are the first material to condense into solid clumps in the hot gas disk that once orbited the Sun. Silicate-rich round chondrules were later formed.

Previously, researchers had assumed that the differences in isotopic composition between CI chondrites and other carbonaceous chondrite groups were due to different mixing ratios of these three components. For example, CI chondrites are primarily composed of fine-grained rocks, but their siblings are significantly enriched in inclusions. However, as the research team explains in their latest publication, the nickel measurements do not fit into this scheme.

The researchers’ calculations show that the measurements can only be explained by a fourth component: small iron-nickel particles that must have accumulated during the asteroid’s formation. In the case of Ryugu and CI chondrites, this process appears to have been particularly efficient.

“Very different processes must have operated for the formation of Ryugu and CI chondrites, on the one hand, and other groups of carbonaceous chondrites, on the other hand,” said MPS’s Fridolin Spitzer, lead author of the new study. He said: Basic idea.

Researchers say the first carbonaceous chondrites began to form about 2 million years after the formation of the solar system. Drawn by the gravity of the still young Sun, the dust and first solid clumps made their way from the outer edge of the disk of gas and dust toward the inner Solar System, but along the way they encountered an obstacle: the newly formed Jupiter. I encountered it.

Outside the orbit, particularly heavy and large masses accumulated and grew into carbonaceous chondrites containing many inclusions. About 2 million years later, towards the end of this development, another process became predominant. Under the influence of the Sun, the original gas gradually evaporated outside Jupiter’s orbit, accumulating mainly dust and iron and nickel particles. This led to the birth of CI chondrites.

“This result really surprised us. We had to completely rethink not only Ryugu, but also the whole group of CI chondrites,” says Dr. Christoph Burkhardt of MPS.

CI chondrites are not seen as distant, somewhat exotic relatives of other carbonaceous chondrites in the outermost reaches of the solar system, but rather as younger, possibly formed later, in the same region, but through different processes. They appear as brothers.

“This study shows how important laboratory investigations are in deciphering the formation history of the solar system,” said Professor Torsten Kleine, MPS Director of Planetary Sciences and co-author of the study. speak