Proximity observations of (162173) Ryugu by the telescopic Optical Navigation
Camera onboard Hayabusa2 and (101955) Bennu by MapCam onboard Origins, Spectral
Interpretation, Resource Identification, and Security-Regolith Explorer found
opposite spectral trends of space weathering on these carbonaceous asteroids.
Whether the space weathering trends on these asteroids evolved from the same
starting spectra would place an important constraint for understanding their
relation. However, systematic error between data obtained by the two imagers
needed to be reduced for accurate comparison. To resolve this problem, we cross
calibrated albedo and color data using the Moon as the common standard. We show
that the cross-calibrated reflectance can be obtained by upscaling the
pre-cross-calibrated reflectance of Bennu by 12 +/- 2% at v-band, reducing the
systematic errors down to 2%. The cross-calibrated data show that Bennu is
brighter by 16 +/- 2% at v-band and bluer in spectral slope by 0.19 +/- 0.05
(/um) than Ryugu. The spectra of fresh craters on Ryugu and Bennu before cross
calibration appeared to follow two parallel trend lines with offset, but they
converged to a single trend after cross calibration. Such a
post-cross-calibration perspective raise the possibility that Ryugu and Bennu
evolved from materials with similar visible spectra but evolved in diverging
directions by space weathering. The divergent evolution can be caused by the
difference in space weathering dose/process and/or composition of the starting
material. Thus, comparing the composition of samples returned from Ryugu and
Bennu may change the way we interpret the spectral variation of C-complex
asteroids