The asteroid Ryugu has revealed new secrets: astronomers report that the angular chunk that orbits the sun in the vicinity of Earth probably originally came from the edge of our solar system. This emerges from isotope signatures in sample material provided by the Hayabusa 2 probe. The unusual ratio of different iron isotopes in the rocks of Ryugu indicates a close relationship with a rare meteorite class, which is also assigned to the outer solar system. The researchers say they are exciting remnants of the early bodies that formed at the edge of the solar system in ancient times.
The small celestial bodies of our solar system are regarded as bearers of astronomical secrets. Because many still consist of the primeval material from which the celestial bodies of the solar system once formed. To gain insight into their characteristics, scientists have long depended on studying the chunks that naturally fell to Earth's surface: meteorites. But that has now changed: On December 5, 2020, a sample capsule containing around five grams of rock from the asteroid Ryugu landed in Australia. The scientific mail came from Japan's Hayabusa 2 spacecraft, which had previously collected sample material from the asteroid Ryugu.
It is a celestial body about one kilometer in size, the shape of which is reminiscent of a rounded double pyramid. Ryugu orbits the sun as closely as the earth and at times comes so close to us that the probe visit and the return of rock samples became possible. Investigations that can provide more precise insights into the composition, nature, origin and development of the cosmic chunk are only possible in earthly laboratories. The precious material has meanwhile been subjected to various investigations. Among other things, they certify that the carbon-rich substance has a granular and loose structure and a history in which minerals reacted with water over a long period of time. Also, it has been proven that Ryugu contains amino acids and other complex organic molecules.
On the trail of home
The scientists around Timo Hopp from the University of Chicago have now looked more closely at the question of where Ryugu could have originated. Because it is assumed that some near-Earth asteroids could be immigrants from more distant areas of the solar system. To get clues about the origin of Ryugu, the 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 thus in their weight. Early investigations showed that the isotopes of certain elements were unevenly distributed in the early formation phase of the solar system. This means that depending on where a body originated, material with different isotope ratios formed its structural substance.
"The ratio of certain iron isotopes to each other is an excellent marker for distinguishing groups of celestial bodies according to their places of origin," explains co-author Thorsten Kleine from the Max Planck Institute for Solar System Research in Göttingen. For their study, the researchers compared the analysis results of the sample material from Ryugu with results from 13 different meteorites representing different groups. Most of them, like Ryugu, are high in carbon - they are so-called carbonaceous chondrites.
As it turned out, the ratios of the iron isotopes in the case of the asteroid Ryugu differ significantly from those of most of the meteorites studied. Only one type was the exception: the CI chondrites, which are also known as Ivuna-type meteorites after the Tanzanian location of their best-known representative. "There is a striking relationship between the asteroid Ryugu and the comparatively rare meteorites of the CI chondrites, which are also known as the Ivuna type," says Hopp. The results are in line with earlier findings and analysis results that have just been published in parallel: They show that there are also clear chemical and mineralogical similarities between the asteroid Ryugu and Ivuna-type meteorites.
Beyond Jupiter and Saturn
"Our measurements now show that Ryugu and Ivuna-type meteorites formed in the same area of the early solar system and that this area does not coincide with the origin of other carbonaceous chondrites," says Hopp. Specifically, according to them, a greater distance is emerging. The basic assumption is that the asteroids that are close to Earth today once came out of the asteroid belt between the orbits of Mars and Jupiter.
In some cases, however, this was only a stopover: physical evidence and simulations suggest that carbonaceous chondrites formed further out. They are believed to have originated near the birthplaces of Jupiter and Saturn, and were then swept into the interior of the solar system by the planets' gravitational forces. But some might have immigrated from further afield - from the sphere of influence of Uranus and Neptune. According to the study results, this could be exactly what the celestial bodies in the researchers' sights could be. "All in all, there is a lot to suggest that we are dealing with Ryugu and the Ivuna-type meteorites with remnants of the early bodies that formed at the very edge of the solar system," says co-author Andreas Pack of the Georg- August University of Goettingen.
Source: Max Planck Institute for Solar System Research, Article: Science Advances, doi: 10.1126/sciadv.add8141