Space weathering acts strongly on the uppermost surface of Ryugu

Returned samples from Cb-type asteroid (162173) Ryugu exhibit very dark spectra in visible and near-infrared ranges, generally consistent with the Hayabusa2 observations. A critical difference is that a structural water absorption of hydrous silicates is around twice as deep in the returned samples compared with those of Ryugu's surface, suggesting Ryugu surface is more dehydrated. Here we use laboratory experiments data to indicate the spectral differences between returned samples and asteroid surface are best explained if Ryugu surface has (1) higher porosity, (2) larger particle size, and (3) more space-weathered condition, with the last being the most effective. On Ryugu, space weathering by micrometeoroid bombardments promoting dehydration seem to be more effective than that by solar-wind implantation. Extremely homogeneous spectra of the Ryugu's global surface is in contrast with the heterogeneous S-type asteroid (25143) Itokawa's spectra, which suggests space weathering has proceeded more rapidly on Cb-type asteroids than S-type asteroids. Samples returned from Ryugu contain fresher material than the space weathered outer asteroid surface observed by remote sensing, according to a comparison of spectral data from returned samples, orbital observations and Murchison meteorite samples.

Automated summary

The returned samples from asteroid Ryugu show that its surface is more dehydrated and undergoes more rapid space weathering compared to S-type asteroids. The spectral differences between the returned samples and asteroid surface can be explained by higher porosity, larger particle size, and more space weathered condition of Ryugu's surface.