Development of preparation methods of polished sections of returned samples from asteroid Ryugu by the Hayabusa2 spacecraft

Preparation procedures of polished sections of the Ryugu samples returned by the Hayabusa2 spacecraft were established through tests using CI and CM chondrites as analog materials of the Ryugu samples and processing of the Ryugu samples. The procedures consisted of four steps: epoxy-coating, embedding in epoxy cylinders, cutting with a wire saw, and dry polish by hand. There are three key points for successful preparation of the polished sections: (1) ethanol-mixed epoxy with low viscosity for reinforcing the fragile samples, (2) handling under dry conditions to avoid breakup of the samples on contact with liquids due to their highly porous nature, and (3) X-ray computed tomography data for exposing maximum surface areas of target mineral phases and clasts. These key points may also be important for processing of samples returned from asteroid Bennu and the Martian Moon Phobos, as those samples are likely to be hydrous carbonaceous chondrite-like materials. The established procedures induce two side effects: zoning of the polished surface of the Ryugu samples in scanning electron microscope images reflecting differences in carbon contents due to permeation of low viscosity epoxy resin into the sample surface and fractures in anhydrous minerals possibly due to shear stress during dry polishing.