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 porosity³. The Hayabusa2 spacecraft found that boulders on the surface of asteroid (162173) Ryugu have an average porosity of 30–50% (refs. ^4,5,6), higher than meteorites but lower than cometary nuclei⁷, which are considered to be remnants of the original planetesimals⁸. 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 average⁶ and porosity >70%, which is as high as in cometary bodies. The artificial crater formed by Hayabusa2’s impact experiment⁹ 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 compaction³. 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.

小行星——行星形成过程的初始阶段——最初被认为是非常多孔的尘埃聚集体^1,2，随后的热和压实过程会降低它们的孔隙率³。隼鸟 2 号宇宙飞船发现小行星 (162173) Ryugu 表面的巨石平均孔隙率为 30-50%（参考文献^4,5,6），高于陨石但低于彗核⁷，后者被认为是残余物原始小行星⁸ . 在这里，我们使用 Ryugu 表面的高分辨率热和光学成像，在新鲜的小陨石坑（直径<20 m）的底部发现了反射率（~0.015）低于 Ryugu 平均值^6的巨石孔隙率>70%，与彗星体一样高。隼鸟 2 号撞击实验⁹形成的人造陨石坑与这些陨石坑的大小相似，但没有如此高孔隙率的巨石。因此，我们认为观察到的高孔隙率是固有的，而不是由随后的冲击粉碎和/或开裂产生的。我们认为这些巨石是 Ryugu 上加工最少的材料，代表了没有经过高度加热和压实³的多孔小行星的残余物。我们的多仪器分析表明，高度多孔的巨石碎片在全球地表风化层中混合，这意味着它们可能通过着陆操作^10,11在收集的样本中被捕获。