Sodium Carbonates Reveal Ryugu’s Saline Past

By Dr Silpaja Chandrasekar, PhDReviewed by Laura ThomsonFeb 21 2025

In a paper published in the journal Nature Astronomy, researchers reported the presence of sodium carbonates, chlorides, and sulfates in Ryugu samples, indicating past interaction with alkaline, salt-rich water. They suggested that highly concentrated brine formed through evaporation or freezing during the final stages of aqueous alteration.

Although Ryugu is not a wet body, these findings suggest similar processes may have occurred in other carbonaceous asteroids. Terrestrial weathering of meteorites could obscure salt precipitation evidence. Sodium salts may help compare water evolution in carbonaceous bodies with alkaline subsurface oceans on Ceres and the moons of Jupiter and Saturn.

Aqueous History of Ryugu

Previous studies showed that Ryugu samples resemble ivuna-type carbonaceous chondrites, among the most primitive and hydrated asteroidal materials. The parent body likely formed beyond Jupiter’s orbit and underwent significant aqueous alteration. Sodium-rich phases were detected, though some may have decomposed during analysis. Ryugu samples closely resemble ivuna-type carbonaceous chondrites, indicating a chemically primitive and water-rich origin. The parent body likely formed beyond Jupiter’s orbit and experienced extensive aqueous alteration.

Ryugu Sample Analysis

The Ryugu samples from Hayabusa2’s second touchdown site were preserved in nitrogen-filled chambers at JAXA before being allocated for study. A coarse grain (C0071) measuring 1.54 mm and an aggregate of fine grains (C0369) under 1 mm were analyzed. Optical microscopy was performed on C0071 in a nitrogen-filled glovebox at JAXA, followed by field-emission scanning electron microscopy (FE-SEM), synchrotron radiation X-ray diffraction (SR-XRD), and computed tomography (CT) at SPring-8, ensuring minimal air exposure.

Further studies were conducted at Kyoto University and UVSOR using scanning transmission electron microscopy (STEM) and synchrotron radiation-based scanning transmission X-ray microscopy (STXM). Samples were transported in nitrogen-filled containers, though some procedures required brief air exposure of under two minutes.

SR-CT and SR-XRD were conducted at SPring-8’s BL20XU beamline, where C0071 was enclosed in a polyimide capsule to prevent air exposure. X-ray energy was set to 37.7 keV, and CT imaging achieved a pixel size of 0.494 µm. FE-SEM analysis was performed at JAXA using a Hitachi SU6600, with additional observations of C0071 and C0369 (~200 grains) at Kyoto University using a JEOL JSM-7001F. During exchange procedures, samples were briefly exposed to air (under 10 seconds).

Electron-transparent sections for STEM and STXM were prepared using a focused ion beam (FIB) system at Kyoto University. Before FIB sectioning, a platinum layer was applied for conductivity, and thin sections (50–200 nm) were mounted on transmission electron microscopy (TEM) grids.

FE-TEM analysis at Kyoto University utilized a JEOL JEM 2100F, which performed imaging, selected area electron diffraction (SAED), and STEM-EDX mapping. Samples were exposed to air for less than a minute during mounting. STXM and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy were carried out at the UVSOR Synchrotron Facility under controlled atmospheric conditions. A Fresnel zone plate focused the X-ray beam to 50 × 50 nm², enabling high-resolution carbon and sulfur spectral feature analysis.

Signs of Ancient Brine on Ryugu

Ryugu grain C0071 was analyzed using scanning electron microscopy, synchrotron radiation-based X-ray computed tomography, and X-ray diffraction. It mainly contained phyllosilicates, iron- and nickel-bearing sulfides, magnetite, and carbonates. A sodium-rich vein, 6–20 µm wide and up to 500 µm long, was identified, suggesting pre-existing sodium-rich surfaces before fracturing. The vein was not a product of terrestrial contamination and showed signs of space weathering. Researchers found sodium-rich patches with chlorine on the surface.

Transmission electron microscopy and scanning transmission X-ray microscopy revealed that the sodium-rich veins extended about 1 µm deep and contained anhydrous sodium carbonate (natrite) and hydrous sodium carbonate (thermonatrite). The sodium carbonates had a polycrystalline structure and were accompanied by minor sulfur and fluorine content. Sulfur was identified as sodium sulfate, and the elemental ratios closely matched natrite. Slight variations in composition were noted due to analytical precision and potential electron-beam damage. Sections from sodium-rich patches also confirmed the presence of sodium carbonates.

STEM-energy dispersive X-ray spectrometry showed that sodium carbonates in grain C0071 contained irregular-shaped voids. Chlorine-rich spots, less than 200 nm, were found on carbonate surfaces and void walls. These were identified as crystalline halite (NaCl). The presence of halite suggests interactions involving chlorine-bearing fluids. The structural and compositional findings provide insights into the mineralogical history of Ryugu's grains.

Conclusion

Researchers found that Ryugu's parent body once contained alkaline, salt-rich water, leaving behind sodium carbonates, chlorides, and sulfates. They suggest that brine concentration resulted from evaporation or freezing during the final stages of aqueous alteration. Similar processes may have influenced other carbonaceous asteroids, though terrestrial weathering can obscure evidence. Comparing these sodium salts with those in Ceres and icy moons could offer new insights into the evolution of water in the Solar System.

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