Comparison of optical spectra between asteroids Ryugu and Bennu: II. High-precision analysis for space weathering trends

Various natural effects gradually alter the surfaces of asteroids exposed to the space environment. These processes are collectively known as space weathering. The influence of space weathering on the observed spectra of C-complex asteroids remains uncertain. This has long hindered our understanding of their composition and evolution through ground-based telescope observations. Proximity observations of (162173) Ryugu by the telescopic Optical Navigation Camera (ONC-T) onboard Hayabusa2 and that of (101955) Bennu by MapCam onboard Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) found opposite spectral trends of space weathering; Ryugu darkened and reddened while Bennu brightened and blued. How the spectra of Ryugu and Bennu evolved relative to each other would place an important constraint for understanding their mutual relationship and differences in their origins and evolutions. In this study, we compared the space weathering trends on Ryugu and Bennu by applying the results of cross calibration between ONC-T and MapCam obtained in our companion paper. We show that the average Bennu surface is brighter by 18.0 +/- 1.5% at v band (550 nm) and bluer by 0.18 +/- 0.03 mu m(-1) (in the 480-850 nm spectral slope) than Ryugu. The spectral slopes of surface materials are more uniform on Bennu than on Ryugu at spatial scales larger than similar to 1 m, but Bennu is more heterogeneous at scales below similar to 1 m. This suggests that lateral mixing of surface materials due to resurfacing processes may have been more efficient on Bennu. The reflectance-spectral slope distributions of craters on Ryugu and Bennu appeared to follow two parallel trend lines with an offset before cross calibration, but they converged to a single straight trend without a bend after cross calibration. We showthat the spectra of the freshest craters on Ryugu and Bennu are indistinguishable within the uncertainty of cross calibration. These results suggest that Ryugu and Bennu initially had similar spectra before space weathering and that they evolved in completely opposite directions along the same trend line, subsequently evolving into asteroids with different disk-averaged spectra. These findings further suggest that space weathering likely expanded the spectral slope variation of C-complex asteroids, implying that they may have formed from materials with more uniform spectral slopes.