Collisional history of Ryugu’s parent body from bright surface boulders

The asteroid (162173) Ryugu and other rubble-pile asteroids are likely re-accumulated fragments of much larger parent bodies that were disrupted by impacts. However, the collisional and orbital pathways from the original parent bodies to subkilometre rubble-pile asteroids are not yet well understood 1 – 3 . Here we use Hayabusa2 observations to show that some of the bright boulders on the dark, carbonaceous (C-type) asteroid Ryugu 4 are remnants of an impactor with a different composition as well as an anomalous portion of its parent body. The bright boulders on Ryugu can be classified into two spectral groups: most are featureless and similar to Ryugu’s average spectrum 4 , 5 , while others show distinct compositional signatures consistent with ordinary chondrites—a class of meteorites that originate from anhydrous silicate-rich asteroids 6 . The observed anhydrous silicate-like material is likely the result of collisional mixing between Ryugu’s parent body and one or multiple anhydrous silicate-rich asteroid(s) before and during Ryugu’s formation. In addition, the bright boulders with featureless spectra and less ultraviolet upturn are consistent with thermal metamorphism of carbonaceous meteorites 7 , 8 . They might sample different thermal-metamorphosed regions, which the returned sample will allow us to verify. Hence, the bright boulders on Ryugu provide new insights into the collisional evolution and accumulation of subkilometre rubble-pile asteroids. The Hayabusa2 team has discovered two types of bright boulder on the dark, carbonaceous asteroid Ryugu. One type has a spectrum consistent with material from an anhydrous silicate-rich asteroid, likely introduced by one or more collisions in Ryugu’s past.