Researchers at AIST, in collaboration with Tohoku University, the Japan Atomic Energy Agency, the University of Tokyo, Kyushu University, and others, conducted a direct comparison of remote-sensed data of the Cb-type asteroid Ryugu’s surface observed by the Asteroid Explorer Hayabusa2 and lab-measured data using Ryugu samples brought back to the Earth by Hayabusa2 spacecraft without exposure to the Earth's atmosphere. The remote-sensed and lab-measured reflectance spectra of Ryugu are quite similar, however, a clear difference is the OH absorption band depth: the OH band of remote-sensed spectra is more than half weaker than that of lab-measured spectra. To clarify what causes that difference, we performed lab experiments and data comparisons using primitive carbonaceous chondrites similar to Ryugu. Then we revealed that the most likely cause is that the surface of Ryugu, about 1/100 mm in depth, has been affected by space weathering, alteration caused by exposure to cosmic rays and cosmic dusts, resulting in partial dehydration and the OH band weakening. Our result has been enabled for the first time by the combination of remote sensing of asteroid Ryugu and laboratory measurements using collected samples by Hayabusa2, and suggests the importance of sample return missions playing an important role in planetary science.

Asteroid Ryugu is one of the primitive small bodies that are expected to preserve information about the formation and evolution process of our Solar System. To understand what kind of materials such bodies are composed of, sample return, collecting materials of a target object and bringing them back to the Earth by a spacecraft, is a powerful tool in addition to telescopic observations and meteorite analyses. Once a direct linkage between the collected samples and their parent body is established, it becomes possible to estimate accurate information of materials on primitive asteroids. Ryugu contains water and organic matter, therefore, it has been expected that detailed investigation using Ryugu samples collected by Hayabusa2 will help us to understand the origin of the Earth and life.