Yada, and 90 co-authors, reported on December 20, 2021 that C-type asteroids1 are considered to be primitive small Solar System bodies enriched in water and organics, providing clues to the origin and evolution of the Solar System and the building blocks of life. C-type asteroid 162173 Ryugu has been characterized by remote sensing2,3,4,5,6,7 and on-asteroid measurements8,9 with Hayabusa2 (ref. 10). However, the ground truth provided by laboratory analysis of returned samples is invaluable to determine the fine properties of asteroids and other planetary bodies.
We report preliminary results of analyses on returned samples from Ryugu of the particle size distribution, density and porosity, spectral properties and textural properties, and the results of a search for Ca–Al-rich inclusions (CAIs) and chondrules. The bulk sample mainly consists of rugged and smooth particles of millimeter to submillimeter size, confirming that the physical and chemical properties were not altered during the return from the asteroid. The power index of its size distribution is shallower than that of the surface boulder observed on Ryugu11, indicating differences in the returned Ryugu samples.
The average of the estimated bulk densities of Ryugu sample particles is 1,282 ± 231 kg m−3, which is lower than that of meteorites12, suggesting a high microporosity down to the millimeter scale, extending centimeter-scale estimates from thermal measurements5,9. The extremely dark optical to near-infrared reflectance and spectral profile with weak absorptions at 2.7 and 3.4 μm imply a carbonaceous composition with indigenous aqueous alteration, matching the global average of Ryugu3,4 and confirming that the sample is representative of the asteroid. Together with the absence of submillimeter CAIs and chondrules, these features indicate that Ryugu is most similar to CI chondrites but has lower albedo, higher porosity and more fragile characteristics.
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From another publication, Strickland indicated that Yada said “As the mission team member, I felt so happy to handle and analyze the first C-type asteroid samples with my hands,” Yada said.
“This was (and still is!) an incredible experience,” said Pilorget via email. “The team work and international collaboration is really exemplary and I am really happy and proud to be part of this great mission.”
Some of the samples have been shared with other research teams and more findings about Ryugu are expected soon, Yada said. Samples from Bennu, an asteroid that may share similarities to Ryugu, are currently en route to Earth after NASA’s OSIRIS-REx mission collected them in October 2020. The samples will return to Earth in September 2023.
“We would like to compare Ryugu samples with Bennu samples to see what is similar and what is different between them,” Yada said.
Patrick Michel, director of research at the French National Centre for Scientific Research in Paris, is an investigator for both missions. While he did not participate in this research, he regards these first results as historic “because they give the very first results of the preliminary analysis of samples from a carbonaceous asteroid.”
Samples from asteroids like these provide a different look at space rocks.
“Our collection is biased towards the strongest component of asteroids that can survive the entry in the atmosphere,” Michel said. “We need to get to asteroids and return samples to access the weakest, and probably most primitive, component of these bodies.” Previous research has suggested that both Ryugu and Bennu are the fragments from a once much-larger celestial body that was hit by something.
“The possibility that both bodies have a common origin, or that their parent bodies come from the same region still holds,” Michel said. “We can check whether they have a common history or determine what differs and why.”
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