Space weathering record and pristine state of Ryugu samples from MicrOmega spectral analysis

The samples returned from asteroid Ryugu were collected both at its surface and at its subsurface by Hayabusa2 and can, thus, provide information on the space weathering of C-type asteroids at different depths without terrestrial contamination. The near-infrared hyperspectral microscope MicrOmega gathered data on the –OH feature at ~2.7 μm for 177 individual grains from the two collection sites. Here, through a spectral analysis of these data, we show that the position of the band peak can be used as an indicator of the degree of space weathering. Most subsurficial grains do not present space weathering features, indicating that Ryugu’s subsurface layers have never been exposed to the interplanetary medium. Moreover, the ~2.7 μm feature for the Ryugu samples is narrower than that observed for CI chondrites, which are the closest meteorite analogues to Ryugu, suggesting that these contain more absorbed molecular water than Ryugu due to terrestrial aqueous contamination. We conclude that Ryugu samples should be considered as a reference for the primordial water abundance within primitive bodies.