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.

Automated summary

(English Summary:) Using observations from Hayabusa2, the study shows that some bright boulders on the asteroid Ryugu have compositional signatures consistent with ordinary chondrites, likely resulting from collisional mixing with anhydrous silicate-rich asteroids. Another type of bright boulders on Ryugu is more similar to thermal metamorphosed carbonaceous meteorites. These findings provide new insights into the collisional evolution and accumulation of subkilometre rubble-pile asteroids.