  The influence of space weathering on the observed spectra of C-complex
asteroids remains uncertain. This has long hindered our understanding of their
composition through telescope observations. Multi-band imaging of Ryugu by
ONC-T on Hayabusa2 and that of Bennu by MapCam on OSIRIS-REx found opposite
spectral trends of space weathering; Ryugu darkened/reddened while Bennu
brightened/blued. How the spectra of Ryugu and Bennu evolved relative to each
other would place a constraint for understanding 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. We show
that the average Bennu surface is brighter by 18.0 $\pm$ 1.5% at 550 nm and
bluer by 0.18 $\pm$ 0.03 $\mu$m$^{-1}$ (480-850 nm slope) than Ryugu. The
spectral slopes of surface materials are more uniform on Bennu than on Ryugu at
spatial scales $\gtrsim$1 m, but Bennu is more heterogeneous at $\lesssim$1 m.
This suggests that lateral mixing due to resurfacing may have been more
efficient on Bennu. The reflectance-spectral slope distributions of craters on
Ryugu and Bennu appeared to follow two trend lines with an offset before cross
calibration, but they converged to a single straight trend without a bend after
cross calibration. We show that 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.
