²³⁴U and ²³⁸U are naturally occurring radioisotopes, and the ratio of their decay rates is expressed as activity ratio (AR). AR is an index of the degree of deviation from the secular equilibrium and has been used as a tracer in shallow groundwater systems. This study examined the distribution of ²³⁸U and ²³⁴U in deep groundwater in the northern Hokkaido, Japan, to assess the usefulness of AR as a geochemical indicator in the reducing groundwater. Samples consist of saline groundwater derived from seawater at the time of deposition of the strata, and meteoric water infiltrated during the glacial period before about 12,000 years ago. The results showed that AR increased in the sampling depths of 350 < 160 < 250 meters up to ~11 depending on the percentage of meteoric water infiltration, and positively correlated with inverses of uranium concentrations. In addition, uranium concentration in groundwater showed negative and positive correlations with meteoric water infiltration percentages and chloride ion concentrations, respectively. These results indicate that the uranium concentration in the fossil seawater was diluted by the infiltration of the meteoric water, resulting in a decrease in ionic strength. As the consequence, the desorption of sorbed ions and dissolution of precipitates in groundwater may have progressed at higher proportions of meteoric water, leading to increases in AR. Furthermore, the timing of the infiltration was estimated from initial AR values of carbonate precipitation observed in the formation. ²³⁴U/²³⁸U isotope ratio will be a useful indicator to understand the changes in water flow environment for low-flow reducing groundwater.

The Hayabusa2 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.

The latest and ongoing sample return missions from extraterrestrial objects present an opportunity to expand our knowledge of the chemical composition and subsequent evolution of the Solar System, beyond the information that can be obtained from the study of meteorites. Nevertheless, a potential limitation of such missions, at least in the foreseeable future, would be the recovery of a limited amount of sample. In light of these considerations, we have developed a multi-element chemical separation protocol utilizing a sample amount of approximately 25 mg. With this sample amount (and less), we can successfully separate elements including Mg, K, Ca, Ti, Cr, Fe, Ni, Zn, Sr, Zr, Mo, Ba, REEs, Hf, W, Pb, and U. This includes the crucial elements Ti and Cr, whose isotopic compositions are used for the latest classification of meteorites, and in turn the parent asteroidal materials. The new method achieved chemical yields for these elements after separation of greater than 90% for both major and trace elements in the Murchison meteorite (with an exception for Pb with 85%). The corresponding total procedural blanks were negligible, representing less than 0.1% for the majority of the elements. This method was specifically designed for the analysis of small samples (<30 mg) of extraterrestrial materials from sample return missions, such as Hayabusa2 and OSIRIS-REx. It will be of significant application in future missions such as MMX and Artemis where limited quantities of asteroidal, cometary, planetary, and other primitive Solar System solids will be returned.