C-type asteroids¹ are considered to be primitive small Solar System bodies enriched in water and organics, providing clues to the origin and evolution of the Solar System and the building blocks of life. C-type asteroid 162173 Ryugu has been characterized by remote sensing^2,3,4,5,6,7 and on-asteroid measurements^8,9 with Hayabusa2 (ref. ¹⁰). However, the ground truth provided by laboratory analysis of returned samples is invaluable to determine the fine properties of asteroids and other planetary bodies. We report preliminary results of analyses on returned samples from Ryugu of the particle size distribution, density and porosity, spectral properties and textural properties, and the results of a search for Ca–Al-rich inclusions (CAIs) and chondrules. The bulk sample mainly consists of rugged and smooth particles of millimetre to submillimetre size, confirming that the physical and chemical properties were not altered during the return from the asteroid. The power index of its size distribution is shallower than that of the surface boulder observed on Ryugu¹¹, indicating differences in the returned Ryugu samples. The average of the estimated bulk densities of Ryugu sample particles is 1,282 ± 231 kg m⁻³, which is lower than that of meteorites¹², suggesting a high microporosity down to the millimetre scale, extending centimetre-scale estimates from thermal measurements^5,9. The extremely dark optical to near-infrared reflectance and spectral profile with weak absorptions at 2.7 and 3.4 μm imply a carbonaceous composition with indigenous aqueous alteration, matching the global average of Ryugu^3,4 and confirming that the sample is representative of the asteroid. Together with the absence of submillimetre CAIs and chondrules, these features indicate that Ryugu is most similar to CI chondrites but has lower albedo, higher porosity and more fragile characteristics.

C型小行星¹被认为是富含水和有机物的原始太阳系小型天体，为太阳系的起源和演化以及生命的组成部分提供线索。C 型小行星 162173 Ryugu 的特点是遥感^2,3,4,5,6,7和小行星上的测量^8,9与 Hayabusa2（参考文献 ¹⁰）。然而，实验室对返回样本的分析所提供的基本事实对于确定小行星和其他行星体的优良特性是无价的。我们报告了对来自 Ryugu 的返回样品的粒度分布、密度和孔隙率、光谱特性和结构特性的初步分析结果，以及对富含 Ca-Al 夹杂物 (CAI) 和球粒的搜索结果。大块样品主要由毫米到亚毫米大小的粗糙和光滑的颗粒组成，证实在从小行星返回期间物理和化学性质没有改变。其尺寸分布的功率指数比在龙宫^{11上观测到的地表巨石要浅}，表示返回的 Ryugu 样本的差异。Ryugu 样品颗粒的估计体积密度平均值为 1,282 ± 231 kg m ^-3，低于陨石¹²，这表明低至毫米级的高微孔率，扩展了热测量的厘米级估计值^5,9 . 在 2.7 和 3.4 μm 处具有微弱吸收的极暗光学至近红外反射率和光谱分布表明碳质成分具有本地水蚀变，与 Ryugu ^{3,4的全球平均值相匹配}并确认样本是小行星的代表。再加上没有亚毫米级 CAI 和球粒陨石，这些特征表明 Ryugu 与 CI 球粒陨石最相似，但具有较低的反照率、较高的孔隙率和更易碎的特征。