[M-IS06] アストロバイオロジー

[MIS06-08] What we have learned from Hayabusa2-returned Ryugu samples – From an astrobiological perspective

★Invited Papers

*橘 省吾¹、Hayabusa2 Initial Analysis Team (1.東京大学大学院理学系研究科宇宙惑星科学機構)

キーワード：はやぶさ２、リュウグウ、サンプルリターン、C型小惑星、有機物、太陽系

The JAXA’s asteroid sample return mission Hayabusa2 aims to explore C-type near-Earth asteroid (162173) Ryugu to unveil the origin of the Solar System and the Earth’s ocean and life through proximity observation and sample analysis [1]. Hayabusa2 explored Ryugu from June 2018 to November 2019, including touchdown operations to collect samples at two surface locations on the asteroid. The spacecraft delivered its reentry capsule to the Earth on December 2020. Returned particles, ~5 g in total, well represent the surface of Ryugu from both spectroscopic and morphological point of views [2-4]. The samples show the featureless low-reflectance nature in the visible wavelength range, consistent with the surface of Ryugu [3]. The near infrared spectroscopy [3] and spectroscopic imaging [4] found that returned particles show absorption features of hydrated minerals, carbonates, and organic matter. A particle with N-H vibration features at 3.06 and 3.24 micrometers was also found [4]. Further detailed sample analysis by the Hayabusa2 initial analysis team has found that Ryugu is most similar to CI carbonaceous chondrites in many aspects. As in CI chondrites, Ryugu samples show an elemental composition well matching the Sun’s elemental abundance [5]. Ryugu samples consist mainly of hydrated silicates such as serpentine and saponite associated with carbonates (dolomite and breunnerite), iron sulfide (pyrrhotite), and iron oxide (magnetite) [6, 7], which is also consistent with mineralogy and petrology of CI chondrites that experienced severe aqueous alteration. The volatile concentrations are less than the solar composition, but the concentrations of C, H, and N are as high as CI chondrites [8]. Various types of organic components are found in the sample, including prebiotic molecules [8, 9], and some of these organics are likely to be the host of planetary-type (Q) noble gases in the sample [10, 11]. The close association between hydrated silicates and diffuse organic matter [9] and heterogeneous distribution of soluble molecules [12] suggests fluid mediated organic evolution in Ryugu’s parent planetesimal. Despite heavy aqueous alteration, some macromolecuar organics in Ryugu showing enrichments of heavy isotopes of H and N are also present [8], which could have formed in low temperature environments prior to formation of Ryugu’s parent planetesimal. The history of organic matter in Ryugu samples is expected to be still recorded in their chemical, compositional, structural, and morphological features from the chemistry of the Sun’s parent molecular cloud to molecular evolution within the planetesimal. If C-type asteroids brought organics into the proto Earth, the organics in Ryugu samples would represent those delivered to the Earth. In this talk we will review the results of Ryugu sample analysis to discuss from an astrobiological perspective.

References: [1] Tachibana S. et al. (2014) Geochem J. 38, 571. [2] Tachibana S. et al. (2022) Science doi.org/10.1126/science.abj8624. [3] Yada T. et al. (2021) Nature Astronomy doi.org/10.1038/s41550-021-01550-6. [4] Pilorget C. et al. (2021) Nature Astronomy doi.org/10.1038/s41550-021-01549-z. [5] Yurimoto H. et al. (2022) 53rd Lunar Planet Sci. [6] Nakamura T. et al. (2022) 53rd Lunar Planet Sci. [7] Noguchi T. et al. (2022) 53rd Lunar Planet Sci. [8] Naraoka H. et al. (2022) 53rd Lunar Planet Sci. [9] Yabuta H. et al. (2022) 53rd Lunar Planet Sci. [10] Okazaki R. et al. (2022) 53rd Lunar Planet Sci. [11] Marty et al. (2022) This meeting. [12] Hashiguchi M. et al. (2022) 53rd Lunar Planet Sci.