New Evidence That Liquid Water Traveled Through Ryugu
A storm of new findings from a team including researchers at the University of Tokyo suggests that liquid water once moved through the body of the asteroid that became the near-Earth Ryugu. What makes the discovery remarkable is the timing: this activity appears to have persisted more than a billion years after Ryugu’s own formation. The results come from studying tiny rock fragments returned by JAXA’s Hayabusa2 mission, which in 2018 performed a landmark close-up of Ryugu and delivered samples to Earth for analysis.
Traditionally, scientists expected water-related processes on asteroids to fade early in the solar system’s history. The latest work upends that view by showing a pristine record of late-stage fluid activity preserved in Ryugu’s rocks. If fluids could move through Ryugu long after its birth, then carbon-rich asteroids—ancient building blocks of the outer solar system—might have remained wetter for far longer than previously thought. This has profound implications for models of how Earth acquired its oceans and atmosphere.
How Lu–Hf Isotopes Tell a Fluid Story
The heart of the study lies in lutetium–hafnium (Lu–Hf) isotopes. Radioactive 176Lu decays to 176Hf with a known clock slower or faster depending on the geological context. In rocks that record a straightforward, closed system, the ratio of 176Hf to 176Lu grows predictably as time passes. But the researchers found a Lu–Hf ratio in Ryugu samples that was far higher than expected, pointing to a disturbance in the isotope system.
Lead author Tsuyoshi Iizuka describes the inference: fluids moving through the rocks would effectively wash lutetium out, altering the Lu–Hf clock. The team concluded that late-stage fluid flow occurred after the asteroid parent body of Ryugu fractured and buried ice melted, creating a transient, watery medium that percolated through the rock long after Ryugu’s formation. The finding was so unexpected that the scientists describe it as a genuine surprise, reshaping how we understand water’s longevity in small solar-system bodies.
Implications for Earth’s Water Reservoirs
Perhaps the most consequential implication is for Earth’s own oceans. If Ryugu-like bodies retained ice for more than a billion years, they could have delivered far more water to a young, volatile Earth than current models suggest. The study raises the possibility that carbon-rich asteroids contributed two to three times more water to the early Earth than previously estimated, potentially altering how we reconstruct the planet’s early oceans and atmosphere.
“The idea that Ryugu-like objects held on to ice for so long is remarkable,” Iizuka said. “It forces us to rethink the starting conditions for Earth’s water system and may influence how we model planetary habitability.” While it is early to draw sweeping conclusions, the finding opens new avenues for understanding how the solar system’s water inventory evolved and how much of Earth’s life-supporting environment could be tied to long-lived icy reservoirs in asteroids.
Methods, Challenges, and the Road Ahead
Hayabusa2 returned only a few grams of material, and researchers typically work with tens of milligrams per experiment. To extract meaningful signals from such tiny samples, the team developed advanced chemistry and isotope-separation techniques that minimize elemental loss while enabling multiple elements to be analyzed from a single fragment. Those innovations were essential to detecting subtle Lu–Hf disturbances caused by late-fluid activity.
Looking ahead, researchers plan to examine phosphate veins within Ryugu’s material to pin down more precise ages for the late fluid flow. Comparative work with NASA’s OSIRIS-REx samples from asteroid Bennu will test whether this late water activity was common among carbon-rich bodies or a Ryugu-specific event. Ultimately, the goal is to map how water was stored, mobilized, and eventually delivered to Earth, a narrative that continues to shape our understanding of planetary habitability.
Why This Matters for Our View of the Solar System
Taken together, the results suggest that the early solar system was a messier, wetter place than once thought. If liquids traveled through asteroids long after their birth, these bodies may have served as extended water reservoirs, gradually seeding terrestrial worlds. The findings urge scientists to revisit long-standing assumptions about the timing of water delivery and the range of environments that could support liquid water in the early solar system. As more samples are studied and new missions explore carbon-rich asteroids, our picture of how Earth became habitable will continue to evolve.
