矢野 創

The Tanpopo experiment is Japan's first astrobiology mission aboard the Japanese Experiment Module Exposed Facility on the International Space Station. The Tanpopo-1 mission exposed silica aerogel panels to low Earth orbit from 2015 to 2016 to capture micrometeoroids. We identified an impact track measuring approximately 8 mm long, which contained terminal grains in the silica aerogel panel oriented toward space. The impact track exhibited a bulbous cavity with two thin, straight tracks branching from it, each preserving a terminal grain at their ends. The terminal grains were extracted from the silica aerogel and analyzed using scanning transmission electron microscopy and scanning transmission X-ray microscopy to investigate their X-ray absorption near-edge structure (STXM-XANES). Both grains are Fe-bearing and relatively homogeneous orthopyroxene crystals (En88.4±0.4 and En88.2±1.8). The recovery of Fe-bearing low-Ca pyroxene aligns with previous studies of micrometeoroids captured in LEO. Micrometeoroids containing Fe-bearing olivine and low-Ca pyroxene are likely abundant in LEO.

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.