Anomalously porous boulders on (162173) Ryugu as primordial materials from its parent body

Planetesimals—the initial stage of the planetary formation process—are considered to be initially very porous aggregates of dusts 1 , 2 , and subsequent thermal and compaction processes reduce their porosity 3 . The Hayabusa2 spacecraft found that boulders on the surface of asteroid (162173) Ryugu have an average porosity of 30–50% (refs. 4 – 6 ), higher than meteorites but lower than cometary nuclei 7 , which are considered to be remnants of the original planetesimals 8 . Here, using high-resolution thermal and optical imaging of Ryugu’s surface, we discovered, on the floor of fresh small craters (<20 m in diameter), boulders with reflectance (~0.015) lower than the Ryugu average 6 and porosity >70%, which is as high as in cometary bodies. The artificial crater formed by Hayabusa2’s impact experiment 9 is similar to these craters in size but does not have such high-porosity boulders. Thus, we argue that the observed high porosity is intrinsic and not created by subsequent impact comminution and/or cracking. We propose that these boulders are the least processed material on Ryugu and represent remnants of porous planetesimals that did not undergo a high degree of heating and compaction 3 . Our multi-instrumental analysis suggests that fragments of the highly porous boulders are mixed within the surface regolith globally, implying that they might be captured within collected samples by touch-down operations 10 , 11 .