Evidence of Global Space Weathering by Solar Wind on Asteroid 162173 Ryugu

Reflectance spectra (0.3–4 μm) of powder samples of the third largest stone C0002 returned from the asteroid Ryugu by JAXA Hayabusa2 mission were measured and compared with proximity observation data taken by the Optical Navigation Camera - Telescopic (ONC-T) and the Near-Infrared Spectrometer (NIRS3) onboard the Hayabusa2 spacecraft. Although the characteristics of the 2.7 μm absorption band due to structural hydroxyl of the Ryugu powder samples are consistent with CI1 chondrite meteorites, NIRS3 spectra of the global surface of Ryugu exhibit a wavelength shift of +6 nm in the 2.7 μm absorption band center in comparison with those of both the powder samples and an artificial crater on the asteroid formed by the Small Carry-on Impactor (SCI). This is consistent with effects of solar wind-induced space weathering being present across the global surface of asteroid Ryugu. The darkness of the Ryugu surface in comparison with the returned samples and an upturn in reflectance observed in the average ONC-T spectrum at ultraviolet wavelengths may also be due to space weathering.