A newly revised estimation of bulk densities and examination of the shape of individual Ryugu grains

Abstract The bulk density of grains from a celestial body is a fundamental property related to its composition and structure, providing a clue to understand the evolutionary history of the body. Here, we provide the bulk density of 637 grains returned from the C-type near-Earth asteroid 162173 Ryugu. This is the largest number of grains to date as a part of the curation activity, and corresponds to 38 wt.% of total returned samples (⁓5 g). Although several densities of the Ryugu grains were reported, some showed uncertainties in sample volume estimation. In this study, we applied a new volume estimation model calibrated by X-ray micro-computed tomography (XCT) to Ryugu grains to estimate their bulk density more accurately. The averaged bulk density of the Ryugu grains was 1.79±0.31 g/cm3 (error is 1 standard deviation) for a wide range of weight of 0.5‒100 mg (sub-mm ‒10 mm in size) and for a variety of their 3D shapes characterized by the three axial length ratios. We also found that the average bulk densities of the two sampling sites on Ryugu, stored separately in the Chambers A and C of the sample container, were almost the same within error. The log plots in the weight (≥1.0 mg) and size (≥1.5 mm) distributions were approximately linear, and the slopes were approximately -1.1 and -3.4, respectively. The average bulk density of 637 Ryugu grains with >0.5 mg was almost the same as that of 16 Ryugu grains with XCT, and also consistent with the densities of CI chondrites (1.57 ‒2.12 g/cm3). The axial ratios of the 724 Ryugu grains and the particles ejected during the sampling operations (flying particles) were similar, which suggests relatively small Ryugu materials (mm‒cm in size) are similar in shape. The minor difference between Rygu grains and flying particles could be attributed to subsequent events such as scraping during sampling operations and transportation.