Aiko Nakafuji (Research Scientist, Japan Aerospace Exploration Agency)

Shiori Inada (Department of Earth and Planetary Environmental Science, Faculty of Science, The University of Tokyo, 4th year)

Kanako Sakamoto (Senior Staff, Japan Aerospace Exploration Agency)

Takashi Okazaki (Associate Professor, Graduate School of Science, Kyushu University)

Hirotaka Sawada (Senior Staff, Japan Aerospace Exploration Agency)

Shogo Tachibana (Professor, UTokyo Organization for Planetary and Space Science / Project Professor, Japan Aerospace Exploration Agency)

Key points of the presentation

• Two black particles of about 1 mm in diameter were found outside the sample storage container of Hayabusa2, which brought back a sample of about 5 grams from the asteroid Ryuguu, in the gap between the lid and the container itself.
• These unknown particles, discovered before the container was opened, were subsequently identified as asteroid Ryuguu particles through mineralogical and petrological studies. It was also confirmed that they were not similar meteorites on the ground.
• It is thought that the particles ejected from the sample containment chamber were trapped between the lid and the container body and returned to Earth, a new finding that can be utilized in the design of sample collection mechanisms for future missions.

Announcement Summary

The asteroid probe Hayabusa2 ^{(Note 1)} brought back a sample from the asteroid Ryuguu ^{(Note 2),} which weighed about 5 grams, far exceeding the mission success criterion of 0.1 grams. A research group led by Professor Tachibana of The University of Tokyo's Graduate School of Science, the Hayabusa2 sampler ^{(*3) team}, and the Extraterrestrial Material Analysis Group of the Institute of Space and Astronautical Science found a gap between the container lid and the container itself just before opening the container (sample storage container ^(*4)) in December 2020, which had contained the samples. The research group, led by the extraterrestrial material analysis group of the Institute for Earth Science, conducted histological observations and elemental analysis of the constituent minerals of the two black particles found in the gap between the container lid and the container itself just before the container was opened in December 2020, and clarified that these particles originated from the asteroid Ryuguu. It was also confirmed that they are not meteorites similar to Ryuguu, which has already been discovered on the ground. These particles are thought to have been ejected from Hayabusa2 in space before the sample container was sealed, and returned to the ground after being trapped between the container lid and the container itself. The presence of these particles may affect the container's sealing performance, which is a new finding that can be utilized in the design of sample storage mechanisms for future sample return missions.

Announcement

The asteroid explorer "Hayabusa2" explored Ryuguu, one of the C-type asteroids, which contains minerals and organic matter including water and is expected to remember the beginning of the solar system and the origin of the Earth's oceans and life materials, and collected samples at two locations on its surface. The sample containers in the re-entry capsule, which returned to Earth (Woomera, South Australia) on December 6, 2020, were brought to the Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency (JAXA) on December 8, and the presence of Ryuguu particles was confirmed in the sample containment chamber on December 15. During the preliminary preparation period (December 8 to December 11) for opening the container containing the sample catcher ^{(Note 5} ), two dark-colored particles were found in the space between the sample storage container body and the lid (outside the sealing surface for container sealing). These particles were about 1 mm in size. Different in appearance from the soil near the capsule landing site in the Woomera restricted entry area and from the heat-resistant material (ablator) used to protect the capsule from heat during atmospheric flight, these particles appeared to be Ryuguu samples, but were suspicious (Questionable) because they were found outside the container Therefore, they were named "Q particles" and promptly stored in a nitrogen displacement desiccator (Figure 1).

Figure 1: Image of the discovered Q particle (left) and its contamination outside the container (right)

Subsequently, following the initial description of the particles recovered from the sample containment chamber in the clean chamber system, detailed analysis of the samples by the HAYABUSA2 project and the Extraterrestrial Materials Group was conducted. Sampler Team and the Extraterrestrial Material Analysis Group of the Institute of Space and Astronautical Science (ISAS).

As a result, it was found that the Q particles and the Ryuguu sample are similar in various aspects, including the type, shape, chemical composition, and rock structure of the minerals that make up the Q particles. On the other hand, the Ryuguu sample was pointed out to be similar to a certain meteorite that already exists on Earth (Ivuna-type carbonaceous chondrite ^{(Note 6)} ), and it was necessary to show that the Q particle was different from an Ivuna-type carbonaceous chondrite. In response, the team focused on the chemical composition of the fine-grained hydrous silicate minerals that make up the majority of the Q grains, which are described as a mixture of hydrous silicates and iron sulfides when averaged over a range of 1 micrometer, which is similar to the hydrous silicates of the Ryugu sample with This fits well with the fact that iron sulfide particles are abundant in the hydrous silicates of the Ryuguu samples. On the other hand, analysis of ibna-type carbonaceous chondrites using the same method revealed that the average elemental composition of the hydrous silicate minerals in the 1 micrometer range is not a mixture of hydrous silicate and iron sulfide, but a combination of iron-rich and sulfur- or calcium-rich components in the hydrous silicate These components are not a mixture of hydrous silicate and iron sulfide. These components are thought to correspond to hydrous iron oxides (iron-rich components) and sulfates (sulfur- and calcium-rich components) that are present in ibna-type carbonaceous chondrites but absent in the Ryugu sample. The analysis of the Ryuguu samples indicates that hydrous iron oxide and sulfate are weathering minerals that were formed on Earth after the Ivuna-type carbonaceous chondrites fell to the ground.

The Q particles are thought to be particles that were ejected with the sample catcher when the spacecraft left Ryuguu and pulled out the sample introduction to the sample catcher for container storage before returning to Earth. It was concluded that the particles were caught in the gap between the container body and the sample catcher when the container lid was closed, and returned to Earth. Since the Hayabusa2 sampler worked as scheduled on the surface of Ryuguu and collected a total of 5 grams of centimeter- to millimeter-sized particles, there may have been a lot of particles that spilled out. Such particles may have affected the sealing of the container sealing surface, and the HAYABUSA2 sampler team believes that the results can be used to interpret the recently published results of gas composition analysis inside the sample storage container. The findings will also be utilized in the design of the sample collection mechanism, including the sealing performance of the sample storage container, for the sample return mission to be considered in the future.

This study has been published in Geochemical Journal, an international journal published by the Geochemical Society of Japan (early online publication).
This research was conducted by the following members.

Aiko Nakafuji	Research Scientist, Institute of Space and Astronautical Science, JAXA
Shiori Inada	Department of Earth and Planetary Environmental Science, Faculty of Science, The University of Tokyo
Shizumoe Furuya	Project Specialist, Graduate School of Science, The University of Tokyo
Seihiro Nishimura	Senior Staff, Institute of Space and Astronautical Science, JAXA
Tatsu YADA	Senior Staff, Institute of Space and Astronautical Science, JAXA
Masamasa Abe	Associate Professor, Institute of Space and Astronautical Science, JAXA
Hirohiro Usui	Professor, Institute of Space and Astronautical Science, JAXA
Hideto Yoshida	Project Specialist, Graduate School of Science, The University of Tokyo
Takeshi Mikouchi	Professor, The University of Tokyo, University Museum
Kanako Sakamoto	Senior Staff, Institute of Space and Astronautical Science, JAXA
Hajime Yano	Assistant Professor, Institute of Space and Astronautical Science, JAXA
Yayoi Miura	Assistant Professor, Earthquake Research Institute, The University of Tokyo
Yoshisaki Takano	Senior Researcher, Japan Agency for Marine-Earth Science and Technology
Shinji Yamanouchi	Technical Staff, Graduate School of Science, Kyushu University
Takashi Okazaki	Associate Professor, Graduate School of Science, Kyushu University
Hirotaka Sawada	Senior Staff, International Space Exploration Center, JAXA
Shogo Tachibana	Professor, UTokyo Organization for Planetary and Space Science, Graduate School of Science, The University of Tokyo / Project Professor, Institute of Space and Astronautical Science, JAXA

Journal

Journal name	Geochemical Journal
Title of paper	Ryugu particles found outside the Hayabusa2 sample container
Author(s)	Aiko Nakato, Shiori Inada, Shizuho Furuya, Masahiro Nishimura, Toru Yada, Masanao Abe, Tomohiro Usui, Hideto Yoshida, Takashi Mikouchi, Kanako Sakamoto, Hajime Yano, Yayoi N. Miura, Yoshinori Takano, Shinji Yamanouchi, Ryuji Okazaki, Hirotaka Sawada, and Shogo Tachibana*.
DOI	doi.org/10.2343/geochemj.GJ22017

Terminology

1 Hayabusa2

JAXA's spacecraft that successfully explored the vicinity of the asteroid Ryuguu, collected samples by landing twice, and returned samples to Earth. ↑up

Note 2 Asteroid Ry uguu

A C-type asteroid with a near-Earth orbit. It is about 1 km in diameter and shaped like an abacus ball. It is thought to be a rubble-pile object with many rocky accumulations. ↑ (up)

Note 3 Sampler

Name of the sampler onboard Hayabusa2. When the approximately 1-meter-long cylindrical structure (sampler horn) touches the asteroid's surface, a bullet is fired from inside the horn to collect samples that have flown up from the surface. ↑up

Note 4 Sample storage container

A container to seal the recovered Ryuguu sample and bring it back to Earth. The container itself was stored inside the re-entry capsule. ↑up

Note 5 Sample catcher

Container installed at the top of the sampler horn to contain samples that ascend the sampler horn; it has a three-room structure to separate and store particles recovered at different points. ↑up

Note 6: Ivuna-type carbonaceous chondrite

A meteorite with an elemental abundance very similar to that of the sun, except for volatile elements such as hydrogen and noble gases. It is often used in astronomy and earth sciences as a chemical reference for the solar system. On the other hand, they experience intense chemical reactions with water in celestial bodies. It is also a very rare meteorite with only 9 meteorites classified as this type on earth. ↑up