Reflectance spectra measurement plan of captured samples from Ryugu: current status

Abstract:

We report the current status of our spectral analysis plans by the Hayabusa2 MINeralogy and PETrology of coarse grains (MIN-PET CG) team led by T. Nakamura, and the latest rehearsal analysis results using carbonaceous chondrites. Our goal is to clarify Ryugu’s mineralogical and physicochemical properties and their variation, effective for linking to the spectral characteristics obtained by an Optical Navigation Camera (ONC) and Near-InfraRed Spectrometer (NIRS3) onboard Hayabusa2 spacecraft. In order to demonstrate the ability to obtain detailed spectral properties of Ryugu grains with diverse spatial resolutions ranging from μm to mm scale or larger, we analyzed a few mm size polished CM samples with several microscopic imaging spectrometers; 5–200 μm spatial resolution measurements in the wavelength range of 0.4–1.0 μm, 1.25–20 μm, and 20–50 μm at synchrotron SOLEIL and at Institut d’Astrophysique Spatiale, ~150 μm spatial resolution measurements in the wavelength range of ~1.6–16 μm at Brown University, and 40 μm and larger spatial resolution in the 2.2–15 μm wavelength range at Tohoku University. In addition, measurements were taken using mm-size CM and CI grains with a diffuse reflectance spectrometer covering ~0.38–15 μm in wavelength with several mm resolution at Tohoku University. To prevent exposing returned samples to Earth’s atmosphere, thus avoiding any terrestrial contamination, we have prepared an air shutoff cell kept in a N₂ purged condition to handle samples and standard materials (Amano et al., 2020). Samples examined include Murchison, showing typically ~5% reflectance with a strong OH absorption band at ~2.8 μm, and laboratory heated CI chondrites showing ~3% reflectance and a weak OH absorption band at ~2.7 μm. We will perform (1) laboratory standard, fixed angle measurements setting incident, emission, and phase angles at 30°, 0°, 30°, or 0°, 30°, 30°, respectively, and (2) photometric correction measurements under various incident, emission, and phase angle conditions. Future lab analyses of Ryugu samples will provide direct and detailed information on the mineralogical and physicochemical properties of Ryugu, and a method to correlate the sample findings with the spectral propeties obtained by the remote sensing instruments.