Present status of initial descriptions and distributions of Ryugu samples returned by Hayabusa2

Introduction: Hayabusa2 spacecraft returned samples from near-Earth C-type asteroid 162173 Ryugu to the Earth on 6 Dec. 2020 [1]. Since the samples had been returned, we continue describing individual Ryugu particles for their images, weights, visible and infrared spectra with an optical microscope, a balance, a monochronic digital microscope with six-bands filters for incident light, an FT-IR and the infrared microscope MicrOmega [1, 2]. So far, 404 individual Ryugu particles have been handpicked and described for all or some of the methods [3-11].

Overview of initial descriptions of individual Ryugu particles:  As mentioned in [3, 4], individual Ryugu particles have been grouped into angular or rounded in their shapes and smooth and rough in their mor-prologues. In total, those having rounded-shape (59%) and rough-surface (80%) are dominant in them. The relationship with their optical morphologies and surface mineralogizes were confirmed for the four individual Ryugu particles with an electron microscope [12], indicating those showing smooth surfaces showing space-weathering features on their surfaces. Although it is still poor in statistics, surface morphologies might reflect their mineralogical fea-ture such as space weathering. More samples should be investigated for better statistics to understand relationship between morphologies and space weathering. Among 404 individual Ryugu particles, 244 of them have been analyzed for their infrared reflectance spectra with the FT-IR. 2.7µm absorption, corresponding to presence of -OH, have been detected from all of them, indicating ubiquitous presence of hydrous minerals in Ryugu samples. 3.4 µm absorption features, corresponding to presence of carbonate and/or -CH, have been detected from 48% of them. As pointed before [3], this should be partially due to their heterogeneous distribution of carbonate and/or organic components in them, and that the spectral feature in 3.4 µm is much weaker than 2.7 µm so that it cannot be detected by the FT-IR.  The MicrOmega will help to detect minor spectral features from such small particles, even though there is limitation of number of analyses per day (2 individual particles per day). Visible spectra of individual Ryugu samples have been also analyzed with the digital microscope of muti-band filters [7]. The average spectrum of 69 individual Ryugu particles are comparable to that of bulk Ryugu samples, indicating there is no spectrally different particle among them.  Due to the schedule of analyses, not all of the individual Ryugu particles have been analyzed for the visible spectra, but we should proceed the analysis to understand overall spectral features of Ryugu samples.

Ryugu sample database and announcement of opportunity (AO) for Ryugu samples: All the obtained initial description data shown above is open in public by the Ryugu Sample Database System (https://darts.isas.jaxa.jp/curation/hayabusa2/) [13]. The first AO is ongoing based on this database, and the samples will be distributed to principal investigators of selected research proposals from this June (see detail in https://jaxa-ryugu-sample-ao.net/). Additionally, the second AO is planned to be released in this summer.

References: [1] Yada T. et al. (2021) Nature Astron. 6: 214. [2] Pilorget C. et al. (2021) Nature Astron. 6: 221. [3] Yada T. et al. (2022) LPS LIII, Abstract #1831. [4] Miyazaki A. et al.  (2022) LPS LIII, Abstract #1816. [5] Hatakeda K. et al. (2022) LPS LIII, Abstract #1828. [6] Yogata K. et al. (2022) LPS LIII, Abstract #1767. [7] Yumoto K. et al. (2022) LPS LIII, Abstract #1326. [8] Yabe Y. et al. (2022) LPS LIII, Abstract #2371. [9] Pilorget C. et al. (2022) LPS LIII, Abstract #2088. [10] Carter J. et al. (2022) LPS LIII, Abstract #2017. [11] Loizeau D. et al. (2022) LPS LIII, Abstract #1495. [12] Nakato A. et al. (2022) LPS LIII, Abstract #1810. [13] Nishimura M. et al. (2022) LPS LIII, Abstract #1731.