Improved method of hydrous mineral detection by latitudinal distribution of 0.7-μm surface reflectance absorption on the asteroid Ryugu
Shingo Kameda, Yasuhiro Yokota, Toru Kouyama, Eri Tatsumi, Marika Ishida, Tomokatsu Morota, Rie Honda, Naoya Sakatani, Manabu Yamada, Moe Matsuoka, Hidehiko Suzuki, Yuichiro Choe, Masahiko Hayakawa, Chikatoshi Honda, Hirotaka Sawada, Kazuo Yoshioka, Kazunori Ogawa, Seiji Sugita
In Press, Journal Pre-proof, Available online 29 January 2021
• The 0.7-μm absorption maps of Ryugu were generated by the new flat correction.
• The 0.7-μm absorption strength at the equator is stronger than the average.
• It is suggested that the materials at the equator are fresher than the average.
• The returned samples should contain hydrous minerals with 0.7-μm absorption.”
“Global multiband images of the C-type asteroid (162173) Ryugu were obtained by the optical navigation camera telescope (ONC-T) onboard Hayabusa2. The 0.7-μm absorption depth of the surface reflectance spectrum, which indicates the presence of hydrous minerals, was not clearly seen on Ryugu using flat field correction data obtained in the preflight measurement. The flat field correction data were obtained in the preflight calibration test only at room temperatures (24–28 °C), whereas most observations around Ryugu were performed at a charge-coupled device (CCD) temperature of approximately −30 °C. To obtain higher accuracy measurements, we used a new flat field correction method using the Ryugu surface reflection data. We confirmed that the flat-field patterns are different in high and low temperature conditions. The 0.7-μm absorption map generated by the new method shows that the 0.7-μm absorption near the equator (5°N–5°S) is stronger than that from 30°N to 30°S. We found that the excess of the absorption depth at low latitudes was 0.072%, corresponding to 2.7σ. The spectral analysis also shows that the Ryugu surface at low latitudes is bluer than that at high latitudes and bluer materials tend to show stronger 0.7-μm absorption than redder materials, suggesting that this region has been subjected to less space weathering and less solar heating.”