Spectral investigation of bright and dark areas on Ryugu surface

The JAXA Hayabusa2 spacecraft approached the C-type Near-Earth asteroid 162173 Ryugu on 27th June 2018 [1] and acquired images and spectral data of surface until November 2019, when the spacecraft departed from Ryugu to return to Earth with collected samples. The payload of spacecraft included a Thermal Infrared Imager TIR [2], the NIRS3 spectrometer [3] and the Optical Navigation Camera-Telescopic, with a wideband and seven narrow band filters (ONC-T) [4]. Ryugu is a top-shaped Cb type asteroid and ONC images revealed a surface covered by a large number of boulders, characterized by different roughness and albedo [5,6]. Reflectance spectra acquired by NIRS3 ranges from 1.8 to 3.1 µm, and detected a narrow absorption feature centered at 2.72 µm across the entire observed surface, indicating the ubiquitous occurrence of hydroxyl (OH)-bearing minerals on the surface of Ryugu [7]. NIRS3 data also detected Ryugu as a very dark object with a globally-averaged reflectance value at 2.0 µm of about 0.017 [7]. The aim of this work was to detect bright and dark areas by using spectral data acquired by NIRS3. We used calibrated, thermally and photometrically corrected data acquired on 10 and 11 July 2018 and on 19 July 2018, when NIRS3, operating in scanning mode, obtained a near-global coverage of Ryugu surface. The data acquired on 10 and 11 July are characterized by a spatial resolution of 40 m, since the spacecraft was at an altitude of 20 km (Home Position), whereas the data of 19 July have a spatial resolution of 20 m, since the spacecraft's altitude was of 13 km. We used a method yet validated for Ceres and Vesta [8,9] to detect bright and dark areas on Ryugu surface. For each pixel, we obtained the reflectance factor at 1.9 µm and we estimated the mean value of reflectance at wavelength of 1.9 µm, i.e. 0.017. Bright areas have been defined as regions with a reflectance factor at 1.9 µm larger than the 5.5% than the mean value; dark areas are the regions with a reflectance factor lower than the 8% of the mean value and larger than 0.01 (to avoid false positive due to low S/N). A total of 36 Bright areas and 28 Dark areas have been detected by the application of method. A longitudinal dichotomy is observed in the distribution of bright areas, mainly localized in the 240-360°E region, whereas a latitudinal variation characterizes the distribution of dark areas, mostly focused in the northern hemisphere. By comparing dark and bright areas with the NIR spectral slope estimated between 1.9 and 2.5 µm, a more positive slope can be observed in darker areas. Both dark and bright area spectra show a weak absorption band at 2.72 µm, suggesting a widespread occurrence of hydroxylated minerals on the surface of Ryugu and a secondary absorption at 2.8 µm. The two bands are moderately correlated and appear to be stronger in brighter areas. NIRS3 and ONC data were compared in order to characterize the bright and dark areas in NIR and VIS range. The spectral darkening and brightening observed in the NIR range (by using the reflectance factor at 1.9 µm) reflects in the VIS range (by observing the 0.55 µm reflectance). Furthermore, the increasing strength of both the 2.7-µm and 2.8-µm band is related to a VIS spectral reddening (estimated in the 0.48-0.86 µm range). As a result, material composing both dark and bright areas are spectrally similar to heated Ivuna at 400-500°C, revealing the thermal metamorphism experienced by Ryugu as a consequence of the fragmentation of its parent body. References: [1] Watanabe S. et al. (2017) Space Sci. Rev., 208, 3-16. [2] Okada T. et al., (2017) Space Sci. Rev., 208, 255-286. [3] Iwata T. et al. (2017) Space Sci. Rev., 208, 317-337. [4] Kameda S. et al. (2017) Space Sci. Rev., 208, 17-31. [5] Sugita S. et al., (2019) Science, 364, eaaw0422. [6] Watanabe S. et al. (2019) Science, 364, 268-272. [7] Kitazato K. et al. (2019) Science, 364, 272-275. [8] Palomba E. et al. (2014) Icarus, 240, 58-72. [9] Palomba E. et al. (2019) Icarus, 320, 202-212.