矢野 創

This paper reports the results of the estimation of the probabilities of impact on Mars, Earth, and the Moon for the asteroid probe Hayabusa2 in its extended mission. The probabilities of impact were estimated using an analytical method instead of a computationally expensive Monte Carlo simulation. The results show that the probability of impacting Mars is, which is sufficiently small to meet the planetary protection requirements of the Committee on Space Research. The probabilities of impacting Earth and the Moon are also estimated as and, respectively, which are found to be sufficiently small.

The presence of microorganisms within meteorites has been used as evidence for extraterrestrial life, however, the potential for terrestrial contamination makes their interpretation highly controversial. Here, we report the discovery of rods and filaments of organic matter, which are interpreted as filamentous microorganisms, on a space‐returned sample from 162173 Ryugu recovered by the Hayabusa 2 mission. The observed carbonaceous filaments have sizes and morphologies consistent with microorganisms and are spatially associated with indigenous organic matter. The abundance of filaments changed with time and suggests the growth and decline of a prokaryote population with a generation time of 5.2 days. The population statistics indicate an extant microbial community originating through terrestrial contamination. The discovery emphasizes that terrestrial biota can rapidly colonize extraterrestrial specimens even given contamination control precautions. The colonization of a space‐returned sample emphasizes that extraterrestrial organic matter can provide a suitable source of metabolic energy for heterotrophic organisms on Earth and other planets.

Hydrated asteroids are likely to be main source of water for the terrestrial planets. The controls on the extent of asteroid hydration, however, are poorly understood. Here we report the discovery of multiple fracture and vein sets in a sample from the C-type asteroid 162173 Ryugu that acted as pathways for the migration of distal fluids during its aqueous alteration. Early veins in Ryugu are decorated with framboidal magnetite, while later veins caused metasomatism of wall rocks. Both veins and fractures have cuspate geometries and complex intersecting geometries consistent with freeze–thaw fractures formed during experiments. We show that freeze–thaw is effective in fracturing C-type asteroids to up to 300 km in diameter and is thus crucial in the outwards migration of fluids in ice-bearing asteroids. Freeze–thaw is likely, therefore, to determine the distribution of mineral-hosted water in asteroids throughout the Cosmos.

Raman spectroscopy is a non-destructive analytical technique for characterizing organic and inorganic materials with spatial resolution in the micrometer range. This makes it a method of choice for space-mission sample characterization, whether on return or in situ. To enhance its sensitivity, we use signal amplification via interaction with plasmonic silver-based colloids, which corresponds to surface-enhanced Raman scattering (SERS). In this study, we focus on the analysis of biomolecules of prebiotic interest on extraterrestrial dust trapped in silica aerogel, jointly with the Japanese Tanpopo mission. The aim is twofold: to prepare samples as close as possible to the real ones, and to optimize analysis by SERS for this specific context. Serpentinite was chosen as the inorganic matrix and adenine as the target biomolecule. The dust was projected at high velocity into the trapping aerogel and then mechanically extracted. A quantitative study shows effective detection even for adenine doping from a 5·10−9mol/L solution. After the dust has been expelled from the aerogel using a solvent, SERS mapping enables unambiguous adenine detection over the entire dust surface. This study shows the potential of SERS as a key technique not only for return samples, but also for upcoming new explorations.