The Chemical Composition of Ryugu: Prospects as a Reference Material for Solar System Composition

The Hayabusa 2 spacecraft sampled approximately 5.4 g of asteroid material from the Cb-type asteroid Ryugu. Initial analysis of the Ryugu materials revealed a mineralogical, chemical, and isotopic kinship to the CI chondrites. The pristine nature of Ryugu makes the returned samples ideal for constraining the composition of the Solar System. However, some elements (e.g., P, Ca, Mn, and rare earth elements) show large relative dispersions compared to the other elements in the returned materials studied so far, most likely due to the presence of aqueously formed secondary minerals (e.g., carbonates, phosphates) in Ryugu. Therefore, the estimation of the Solar System composition using currently available Ryugu data is challenging due to the so-called nugget effect of carbonates, phosphates, and possibly other accessory minerals. The nugget effect can be mitigated by analyzing a homogenized, relatively large amount of sample. We estimate that for approximately 0.1 g of Ryugu sample, the dispersion (2SD) of the bulk Mn/Cr and Rb/Sr ratios are +/-13% and +/-15%, respectively, while they will be improved to be better than +/-5% for approximately 1 g of homogenized Ryugu sample. To further constrain the Solar System composition and to evaluate if previous estimates based on CI chondrites stored in museums for decades to centuries are reliable, it is strongly recommended to determine the chemical and isotopic compositions of Ryugu using a homogenized sample prepared from relatively large (approx. 1 g) returned material. Determining Ryugu reference compositions will be used by multidisciplinary communities, including Earth and planetary sciences, astronomy, physics, and chemistry.

隼鸟2号航天器从Cb型小行星龙宫采集了约5.4克的小行星物质。对龙宫物质的初步分析显示，其在矿物学、化学和同位素方面与CI球粒陨石有亲缘关系。龙宫的原始性质使得带回的样本非常适合用于确定太阳系的组成。然而，与目前所研究的带回物质中的其他元素相比，一些元素（例如，磷、钙、锰和稀土元素）显示出较大的相对离散度，这很可能是由于龙宫存在水成次生矿物（例如，碳酸盐、磷酸盐）。因此，由于碳酸盐、磷酸盐以及可能的其他副矿物的所谓“块金效应”，利用目前可用的龙宫数据来估算太阳系组成具有挑战性。通过分析均匀化的、相对大量的样本可以减轻块金效应。我们估计，对于约0.1克的龙宫样本，整体锰/铬和铷/锶比值的离散度（2倍标准差）分别为±13%和±15%，而对于约1克均匀化的龙宫样本，它们将提高到优于±5%。为了进一步确定太阳系的组成，并评估基于在博物馆中保存了几十年到几百年的CI球粒陨石的先前估算是否可靠，强烈建议使用由相对大量（约1克）带回物质制备的均匀化样本确定龙宫的化学和同位素组成。确定龙宫的参考组成将被包括地球与行星科学、天文学、物理学和化学在内的多学科领域所使用。