Immigrants from the periphery –

Asteroid Ryugu revealed new secrets: astronomers report that the angular segment that orbits the sun near the Earth may have originally come from the edge of our solar system. This arises from isotope signatures in a material sample provided by the Hayabusa 2 probe. The unusual ratio of different iron isotopes in the Ryugu rocks indicates a close relationship with a rare meteorite class, which is also assigned to the outer solar system. Researchers say they are the intriguing remnants of early objects that formed at the edge of the solar system in ancient times.

The small celestial bodies in our solar system are the bearers of astronomical secrets. Because much is still made up of the primitive materials from which the celestial bodies of the solar system were once formed. To gain insight into their properties, scientists have long relied on studying pieces that have fallen naturally to the Earth’s surface: meteorites. But that has now changed: On December 5, 2020, a capsule sample containing about five grams of rock landed from the asteroid Ryugu in Australia. The scientific mail came from the Japanese spacecraft Hayabusa 2, which previously collected samples from the asteroid Ryugu.

It is a celestial body with a size of about one kilometer, and its shape resembles a round double pyramid. Ryugu orbits the Sun so close to Earth and at times gets so close that a visit to the probe and the return of rock samples are possible. Investigations that can provide more accurate insights into the composition, nature, origin and evolution of the cosmic segment are only possible in terrestrial laboratories. Meanwhile, the precious materials have been subject to various investigations. Among other things, they testify that the carbon-rich material has a granular and loose structure and a history in which minerals have interacted with water over a long period of time. Also, Ryugu has been shown to contain amino acids and other complex organic molecules.

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On the way home

Scientists around the University of Chicago’s Timo Hopp have now looked closely at the question of where Ryugu may have originated. Because it is assumed that some asteroids close to Earth could be migrating from distant regions in the solar system. For clues about the origin of Ryugu, scientists have now examined iron isotopes in the samples. Isotopes are variants of an element that differ only in the number of neutrons in the nucleus and therefore in their weight. Early investigations showed that the isotopes of some elements were unevenly distributed in the early formation of the Solar System. This means that depending on where the body originated, matter with different isotopic ratios formed its structural substance.

“The ratio of some isotopes of iron to each other is an excellent marker for distinguishing groups of celestial bodies according to their places of origin,” explains co-author Thorsten Kleine of the Max Planck Institute for Solar System Research in Göttingen. In their study, the researchers compared the results of the analysis of sample material from Ryugu with those of 13 different meteorites representing different groups. Most of them, like Ryugu, are rich in carbon – they are called carbon chondrites.

As it turned out, the ratios of iron isotopes in the case of the asteroid Ryugu differ significantly from most of the meteorites studied. Only one species was the exception: CI chondrites, which are also known as Ivuna-type meteorites after the Tanzanian site of their most famous representative. “There is a surprising relationship between the asteroid Ryugu and the relatively rare meteorites of the CI chondrite, also known as the Ivuna type,” says Hope. The results are in line with previous findings and analysis results just published in parallel: they show that there are also clear chemical and mineral similarities between the Ivuna-type meteorites and the asteroid Ryugu.

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Beyond Jupiter and Saturn

“Our measurements now show that the Ryugu and Ivuna meteorites formed in the same region of the early Solar System and that this region does not match the origin of other carbonaceous chondrites,” says Hope. Specifically, according to them, a greater distance appears. The underlying assumption is that today’s near-Earth asteroids once emerged from the asteroid belt between the orbits of Mars and Jupiter.

However, in some cases, this was just a pause: physical evidence and simulations suggest that more carbonaceous chondrites formed. It is believed to have originated near the home of Jupiter and Saturn, and then swept away by the gravitational forces of the planets into the solar system. But some may have migrated from further regions – from the sphere of influence of Uranus and Neptune. According to the results of the study, this could be exactly what celestial bodies could be in the eyes of researchers. “In general, there is a lot that suggests that we are dealing with Ryugu and Ivuna-type meteorites with the remains of early bodies that formed at the edge of the solar system,” says co-author Andreas Pack of the Georg-August University in Göttingen.

Source: Max Planck Institute for Solar System Research, Article: Science Advance, doi: 10.1126/sciadv.add8141

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