Magnetic minerals were also present in the sample, which behave like a natural hard disk, storing the past magnetic field.
Ryugu’s formation history has been revealed through the analysis of samples returned by the Hayabusa2 spacecraft of the Japan Aerospace Exploration Agency. According to a study led by Tohoku University’s Tomoki Nakamura and dubbed “Formation and evolution of carbonaceous asteroid Ryugu: Direct evidence from returned samples,” the parent body of the carbonaceous asteroid Ryugu was formed in the outer Solar System based on the laboratory analysis of 17 individual grains collected by Hayabusa2.
Two samplings were performed by Hayabusa2: one on Feb. 21, 2019, and another on July 11, 2019. As part of the first sample, the undisturbed surface was sampled. In contrast, part of the second sample was collected from the regolith that was excavated by the artificial impact that occurred earlier in the mission. This study examined grains from both sample sites, providing samples that can provide insight into Ryugu’s evolution,” said Planetary Science Institute Senior Scientists Deborah Domingue. In addition, using reflectance spectroscopy, finer-grained powder samples less than 1 millimeter in size were also examined.
Ryugu’s formation history will be understood and characterized with the help of these early studies. “We discovered that there were high levels of phyllosilicates in the regolith grains based on the orbital data. Still, the sample analysis provided us with details about their mineral composition and physical properties,” Domingue said. Based on these results, numerical simulations indicate Ryugu’s parent body formed about 2 million years after the birth of our Solar System, and it came into existence outside of our early star system.
Mineralogy and petrology indicate that the parent body formed more than three times the distance between the Sun and Earth, possibly even beyond Jupiter’s orbit, in the early Solar System when water and carbon dioxide existed as solids. Based on orbital dynamical calculations of Ryugu’s origin, this was followed by scattering inward to the main asteroid belt, somewhere about to the current positions of the Polana and Eulalia asteroid families, about 2.5 times as far from Earth as the Sun. As a result of orbital dynamical calculations of Ryugu’s origin, the Polana and Eulalia asteroid families are likely to be Ryugu’s parent families.
In addition to creating a great complement to other meteorite studies, Ryugu samples will help scientists better understand this specific asteroidal object. “For most meteorites, the precise parent body is unknown,” co-author Amanda Hendrix said. “That allows us to connect the dots then and understand better the formation of the rubble pile, near-Earth asteroid Ryugu.”
CI chondrites are a carbon-rich meteorites collected here on Earth, whose mineralogy has a great deal in common with the Ryugu samples. “Understanding the formation history of Ryugu has real implications for understanding the origin of these meteorites and where their parent bodies formed in our Solar System,” Domingue said.
By comparing the spectral reflectances of the samples with meteorite measurements, the researchers were able to establish the link between the Ryugu and CI meteorites. Furthermore, as a result of these measurements, the Hayabusa2 spacecraft’s camera and spectrometer were able to connect the mineralogy of the sample with remote sensing observations from the spacecraft.
“The camera system displays a brighter first touchdown site than the second sample site, and this difference in reflectance can also be observed between samples collected from each site,” Vilas said. In addition, a comparison of Hayabusa2’s Near Infrared Spectrometer (NIRS3) observations and sample measurements reveals differences in overall reflectance (NIRS3 data appear darker) and clay absorption (NIRS3 spectra appear shallower). “This difference is attributed, in part, to the particle size range and porosity differences between the surface and the samples, providing important information on the role of dust in the spectral properties of asteroid regoliths,” Vilas explained.
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