Rock and Dust Plucked from Asteroid Ryugu: Primitive, Hydrated, and Porous

Samples returned from the asteroid Ryugu (a) are composed primarily of highly porous, sub-mm and mm-scale fragments. Reflectance measurements (b) of returned samples (red and blue lines) possess absorption features at wavelengths 2.7 and 3.4 µm, consistent with the presence of OH-bonded clay minerals and organic molecules. These features suggest that asteroid Ryugu is composed of primitive, hydrated materials that formed near the beginning of solar system history. Credit: Yada et al., 2021.

On February 22, 2019, the Hayabusa2 mission of the Japan Aerospace Exploration Agency (JAXA) collected samples of rock and dust from the surface of near-Earth asteroid Ryugu, but it has taken nearly three years for these samples to be returned to Earth and prepared for researchers to safely analyze these materials. These pristine samples offer a rare opportunity to investigate ancient planetary building blocks unmarred by terrestrial contamination, in contrast to their meteorite counterparts.

The first analyses at JAXA’s Extraterrestrial Sample Curation Center in Sagamihara, Japan, found that the returned samples contain clays, which are hydrous (OH-bearing) minerals and therefore show that water was once present within the parent body of asteroid Ryugu. Researchers also found evidence for carbonate minerals and/or organic molecules among returned samples, which would have been destroyed if Ryugu’s parent body had experienced high-temperature geologic processes such as metamorphism or melting. These characteristics are consistent with those of the most primitive carbonaceous chondrite meteorites called CI chondrites. However, the returned samples differ from CI chondrites in having much lower density due to their high porosity (i.e., void space) and are therefore more fragile than any known meteorites. Such materials would normally be destroyed during the atmospheric entry of meteors, leaving only the densest constituents to be recovered as meteorites. As such, the samples returned by Hayabusa2 provide the most representative view we have ever had of the compositional and physical characteristics of primitive solar system materials.

The first sets of analyses of the Hayabusa2 samples were exclusively non-destructive. Using these preliminary findings, researchers can efficiently plan and propose more detailed investigations that may consume some of these valuable materials. Future work on these samples, and others arriving soon with the return of NASA’s OSIRIS-REx mission, will seek to understand the origins of water and habitability at the birth of the solar system. READ MORE