The parent body of Ryugu, a carbon-rich C-type asteroid, experienced what planetary scientists call aqueous alteration. Roughly 4.5 billion years ago, heat released by the radioactive decay of aluminum-26 melted the ice that was mixed into the asteroid’s rocky material. That liquid water reacted with the original minerals, transforming them into the phyllosilicates, carbonates, pyrrhotite, magnetite, and organic matter that dominate Ryugu samples today. But the precise chemical character of that water — its acidity, its saltiness, its oxidizing or reducing power — has remained frustratingly uncertain, because liquid water itself is almost never preserved in returned samples or meteorites.

Previous attempts to reconstruct Ryugu’s ancient fluids relied on what researchers call forward models: computer simulations that assume an initial mineral composition and water-to-rock ratio, then calculate what chemistry should have resulted. The problem is that those assumptions may not match reality. A team led by Takahiro Kawai and Yoshio Takahashi of the University of Tokyo, working with colleagues at Kanazawa University, JAMSTEC, and several Japanese synchrotron facilities, took a fundamentally different approach. Instead of guessing the starting conditions, they measured the actual chemical species locked inside the returned Ryugu grains, then used those measurements as hard constraints on their equilibrium calculations.

The centerpiece of the study is a technique called X-ray absorption near-edge structure spectroscopy, or XANES. By tuning synchrotron X-rays to the characteristic absorption energies of specific elements, the team could determine exactly which chemical compounds hosted sodium, potassium, calcium, aluminum, and chlorine within the samples. The measurements were performed at the Photon Factory in Tsukuba, SPring-8 in Hyogo, and the new NanoTerasu facility in Sendai, using beam spots roughly one square millimeter — large enough that each spectrum averaged over thousands of mineral grains, ensuring the results reflected bulk chemistry rather than local quirks.

The results were striking. For decades, researchers had assumed that the water inside Ryugu’s parent body was essentially a sodium chloride solution — terrestrial-style seawater, in other words, with Na+ cations balanced by Cl− anions. But the XANES spectra told a different story. The sodium in the Ryugu samples was almost entirely adsorbed onto saponite, a magnesium-rich clay mineral, with its spectral signature matching Na-adsorbed saponite rather than the sharp double peaks characteristic of halite. Linear combination fitting showed that NaCl contributed less than 10 percent of the total sodium. Potassium told the same tale: it, too, was overwhelmingly bound to saponite interlayers rather than dissolved as KCl.

Chlorine proved even more surprising. Rather than sitting in evaporite salts, roughly 90 percent of the chlorine in the samples was incorporated directly into the crystal structure of lizardite, a serpentine-group mineral, where Cl atoms substitute for hydroxyl groups. This kind of structural chlorine is known from serpentinites on Earth’s ocean floor, where seawater reacting with olivine creates similar minerals under intensely reducing conditions. The team confirmed the finding with scanning transmission X-ray microscopy on focused-ion-beam-prepared sections, showing that calcium, too, was adsorbed onto saponite in the phyllosilicate regions rather than existing only in carbonates.

With the actual chemical species pinned down, the researchers turned to chemical equilibrium modeling using the Geochemist’s Workbench software. They assumed the minerals observed in the samples — serpentine, magnetite, dolomite, siderite, pyrrhotite — had reached solubility equilibrium with the water, and that the cations adsorbed in saponite’s interlayers were in exchange equilibrium with dissolved ions. The calculations produced a portrait of Ryugu’s water that is dramatically different from previous estimates. The pH was strongly alkaline, between 10.3 and 11.5. The redox potential, or Eh, was between −0.5 and −0.8 volts — so reducing that it approaches the boundary where water itself begins to decompose and release hydrogen gas.

The salinity finding may be the most consequential. The total dissolved ion concentration worked out to roughly 7 milliequivalents per kilogram — about one-hundredth the saltiness of Earth’s seawater, and ten to one hundred times more dilute than earlier models had predicted for similar bodies. The discrepancy arises because earlier forward models never accounted for the adsorption of sodium and chloride onto phyllosilicate surfaces. When those sinks are included, the brine essentially vanishes. This has a cascading implication: because chlorides never concentrated to saturation, Ryugu’s water was likely lost not through the evaporation of liquid water — the process that leaves behind salt crusts — but through the slow sublimation of ice in the vacuum of space.

The team argues this points to a specific alteration environment: beneath a frozen surface ice layer. The extremely high hydrogen fugacity implied by the reducing conditions, combined with the absence of evaporites, suggests the parent body’s interior was sealed under ice, allowing hydrogen gas to accumulate to pressures exceeding one bar. Only later, after the body migrated into the inner solar system, did the ice sublimate away. This scenario contrasts sharply with what OSIRIS-REx found at asteroid Bennu, where a full sequence of evaporite minerals records the evaporation of ancient brines. Despite their similar origins, Ryugu and Bennu apparently lost their water in fundamentally different ways — and that difference matters for how much water each type of body could have delivered to the early Earth.

The alkaline, reducing chemistry also has tantalizing implications for prebiotic chemistry. The synthesis of nucleobases like uracil — which has already been detected in Ryugu samples — is favored under alkaline conditions, as is the formose reaction that generates sugars from formaldehyde. The strongly reducing environment would have suppressed the oxidative degradation of formaldehyde, extending its lifetime and keeping it available as a feedstock for building more complex molecules. Intriguingly, the same alkaline conditions may explain why Ryugu contains far less ammonia than Bennu: above pH 10.3, ammonium ions convert to volatile ammonia that cannot be retained by clay minerals. The team notes that if ammonium were locked into phosphates instead, the picture could shift, and further speciation work is needed. But if their reconstruction holds, Ryugu’s waters were not a benign, neutral bath — they were a caustic, hydrogen-saturated, alkaline reactor, one whose peculiar chemistry may have quietly set the stage for the organic molecules that eventually seeded our planet.

Subject of Research: Reconstruction of the aqueous alteration chemistry of the Ryugu parent body using XANES speciation and chemical equilibrium modeling of returned samples

Article Title: Reconstructing the aqueous environment of the Ryugu parent body through the speciation of multiple elements

Article References: Reconstructing the aqueous environment of the Ryugu parent body through the speciation of multiple elements. (n.d.). https://doi.org/10.1038/s41467-026-78348-5

Image Credits: AI Generated

DOI: 10.1038/s41467-026-78348-5

Keywords: Ryugu, Hayabusa2, aqueous alteration, XANES spectroscopy, phyllosilicates, saponite, lizardite, chlorine speciation, prebiotic chemistry, carbonaceous asteroids, astrobiology, solar system