Surface Modifications of a Space-weathered Ryugu Sample and the Reflectance Spectral ResponseOPEN ACCESS 

Hotaka Onuma, Megumi Matsumoto, Tomoki Nakamura, Kana Amano, Taiga Takase, Kazuhiko Ninomiya and Hideko Nomura

The Planetary Science Journal, Volume 7, Number 1, Published: 16 January 2026

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“Reflectance spectra from the surface of asteroid Ryugu, measured by the Hayabusa2 spacecraft, exhibit a metal-OH absorption at ∼2.7 μm that is approximately half as deep as the returned samples measured in the laboratory and shifted toward longer wavelengths. These spectral differences likely result from the effects of space weathering; however, this interpretation has not been confirmed. In this study, we conducted a detailed observation of the surface of the Ryugu sample A0283, which shows reflectance spectra indicative of space weathering. Electron microscope observations on the A0283 surface identified space-weathering features (e.g., smooth and frothy layers and melt splashes) as previously reported. At these modified surfaces, the 2.7 μm band was approximately 50% shallower than that from unweathered surfaces and the position shifted up to 8 nm longer wavelengths. Additionally, the Reststrahlen band of phyllosilicates at ∼10 μm shifted by up to 0.8 μm from the position of unweathered samples, and its peak broadened. In particular, the regions covered by the frothy layer exhibited significant spectral changes. The frothy layer consists of vesicular amorphous silicate, with a thickness of tens to thousands of nanometers, and exhibits a lower Mg/Fe atomic ratio than the interior. These results indicate that space weathering causes amorphization, dehydration, Mg sputtering of the asteroid surface, and consequently, modification of reflectance spectra of asteroid Ryugu. In contrast, the collected samples are dominated by newly exposed surfaces, resulting in the observed spectral differences between the asteroid surface and its returned samples.”