The Elemental Abundances of Ryugu: Assessment of Chemical Heterogeneities and the Nugget Effect

TL;DR

This study assesses the elemental composition of Ryugu asteroid samples, highlighting variability due to mineral nuggets, and proposes a consortium approach to obtain representative, homogenized elemental abundances for better understanding of Solar System materials.

Contribution

It provides a comprehensive evaluation of Ryugu's elemental variability, identifies the nugget effect as a source of uncertainty, and proposes a standardized sampling method to improve compositional accuracy.

Findings

Large dispersions in P, Ca, Mn, and REE due to nugget effect.

Estimated sample sizes (750 mg and 400 mg) needed for +/-5% heterogeneity.

Ryugu's elemental abundances are similar to CI chondrites, aiding Solar System studies.

Abstract

The Hayabusa 2 spacecraft sampled ~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. In this study, we have summarized the elemental abundances of Ryugu samples published to date, and evaluated their compositional variability associated with the CI chondrite data. The abundances of some elements (e.g., P, Ca, Mn, and rare earth elements) in individual Ryugu particles were found to show large relative dispersions compared to the other elements, presumably due to the nugget effect of aqueously formed minor secondary minerals (e.g., dolomite, apatite, magnetite, and pyrrhotite). Consequently, the mean abundances of Ryugu for these elements, calculated using currently available Ryugu data, are accompanied by a certain degree of uncertainties. We suggest establishing a consortium to determine the representative elemental abundances of Ryugu by measuring aliquots from a large homogenized powder sample that can mitigate the nugget effect. Our statistical calculation shows that at least 750 and 400 mg of homogenized samples from Chambers A and C, respectively, are needed to achieve within +/-5% compositional heterogeneity. The data obtained throughout the consortium activity complement the scientific objectives of the Hayabusa2 mission. Moreover, we anticipate that the obtained Ryugu data, coupled with the elemental abundances of CI chondrites, provide new insights into the chemical composition of the Solar System, which will be used by multidisciplinary communities, including Earth and planetary sciences, astronomy, physics, and chemistry.