Fig1. False-colored electron microscope image of sodium carbonate on Ryugu grain.(©Kyoto University)
Background: The Hayabusa2 spacecraft from the Japan Aerospace Exploration Agency (JAXA) explored the asteroid Ryugu and returned surface grains to Earth. It was anticipated that the grains brought back directly from the asteroid might contain previously undiscovered substances not found in meteorites. Among these could be highly water-soluble materials, which readily react with moisture in Earth's atmosphere and are therefore difficult to detect unless examined in their pristine state as preserved in space.
Research Methods and Findings: The research group that included Dr. Yada Toru from JAXA, carefully preserved the Ryugu samples in a state completely untouched by the atmosphere through the use of glove boxes, sealed sample containers, and airlocks for the electron microscopes at the curation facilities at JAXA's Institute of Space and Astronautical Science. Under these conditions, the team observed the surface of the sample grains using an optical microscope and a scanning electron microscope (SEM). This analysis revealed small white mineral veins that had developed on the surface of the grains (Figures 1 and 2).
Fig.2. Optical microscope image of the Ryugu grain. The sodium carbonate vein is indicated by an arrow.（©Kyoto University）
Using a transmission electron microscope (TEM), which allows observation of structures at the nanometer scale, they identified sodium carbonates (Na₂CO₃ and a hydrate phase), halite crystals (NaCl: sodium chloride), and sodium sulfate (Na₂SO₄) (Figure 3).
Fig.3. Transmission electron microscope image of sodium carbonate vein (false color image).（©Kyoto University）
Ryugu is currently about 900 meters in diameter, but the asteroid would have originated from a parent body of tens of kilometers in size, which existed around 4.5 billion years ago during the early days of the Solar System (Figure 4). The interior of this parent body may have been heated by the decay of radioactive elements and filled with hot water below 100°C. Aqueous alteration, the interaction of liquid water with minerals, occurred in the parent body of Ryugu, and created hydrated minerals such as phyllosilicates that have previously been seen in the returned samples. The salt crystals found in this study may have precipitated within brines of this parent body.
The salt crystals are all highly water-soluble. The fact that they dissolve easily in water suggests that their crystallization could only occur if the liquid was extremely limited and had a very high salt concentration. Therefore, the research group hypothesized that the salt crystals formed during the disappearance of the liquid water after the major aqueous minerals found in the Ryugu samples precipitated in the parent body (Figure 4).
Fig.4. Evolution of alkaline brines through the history of Ryugu samples.（©Kyoto University）
One possible explanation for the loss of liquid is the evaporation of saltwater. If large-scale fractures formed and connected the interior of the parent body to the outer vacuum environment, the liquid body could have undergone depressurization and evaporation. On Earth, when lakes dry up, highly concentrated saltwater forms, leading to the precipitation of minerals such as sodium carbonate and halite. The remnants of the vaporization are referred to as "evaporites". Similar processes may have occurred in Ryugu's parent body.
Another possibility is the freezing of liquid water. After the Ryugu's parent body reached its peak temperature, it cooled due to the exhaustion of the radioactive heat. The remaining alkaline brines probably concentrated as H2O ices formed. As a result, the sodium salts would have formed at subzero Celsius temperatures. The frozen ice could have sublimated into space over time.
Currently, Ryugu shows no signs of large amounts of liquid, nor do its surface grains appear wet. Until now, it was unclear how the liquid water in the parent body was lost. This study has revealed that the loss of liquid water in Ryugu's parent body occurred through evaporation or freezing.
Implications: The detection of sodium carbonates in Ryugu samples is unexpected because they have never been discovered in meteorites that reach Earth. On the other hand, sodium carbonates and halite are also expected in surface deposits on the dwarf planet Ceres, in water plumes from Saturn's satellite Enceladus, and on the surfaces of Jupiter's moons, Europa and Ganymede. These icy bodies are thought to harbor subsurface oceans or liquid reservoirs. Since the salt production is closely linked to geological settings and brine chemistry in the aqueous bodies, the discovery of sodium salts in Ryugu samples will provide new insights for comparing the evolution of water in carbonaceous bodies and alkaline subsurface oceans in the icy bodies.
Paper Information:
Paper Title：Sodium carbonates on Ryugu as evidence of highly saline water in the outer Solar System.
Authors：Toru Matsumoto1,2*, Takaaki Noguchi2, Akira Miyake2, Yohei Igami2, Megumi Matsumoto3, Toru Yada4, Masayuki Uesugi5, Masahiro Yasutake5, Kentaro Uesugi5, Akihisa Takeuchi5, Hayato Yuzawa6, Takuji Ohigashi7, Tohru Araki6.
1 The Hakubi Center for Advanced Research, Kyoto University; Kitashirakawaoiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan.
2 Division of Earth and Planetary Sciences, Kyoto University; Kitashirakawaoiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan.
3 Department of Earth Science, Graduate School of Science, Tohoku University; 6-3 Aoba, Aramaki, Aoba-ku, Sendai 980-8578, Japan.
4 Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency; 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan.
5 Japan Synchrotron Radiation Research Institute; 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan.
6 UVSOR Synchrotron Facility, Institute for Molecular Science, 38 Nishigo-Naka, Myodaiji, Okazaki, Aichi, 444-8585, Japan.
7 Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki, 305-0801 Japan.
Journal： Nature Astronomy
DOI：10.1038/s41550-024-02418-1
