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New evidence that Ryugu held liquid water
A tiny 80-milligram sample returned from the near‑Earth asteroid Ryugu has revealed an unexpected chapter in the Solar System's watery history. Detailed geochemical analysis shows that liquid water once circulated through Ryugu's parent body far later than conventional models predicted — implying prolonged aqueous alteration on small, icy protoplanets and raising fresh questions about how water was delivered to early Earth.
Scientific background: planetesimals, heating and aqueous alteration
Before Ryugu existed as an isolated rubble pile, it belonged to a larger planetesimal — a small, icy building block that accreted in the outer Solar System about 4.565 billion years ago. These objects formed from dust and ice and experienced internal heating from the decay of short‑lived radioisotopes, which can melt buried ice and produce transient liquid water.
Aqueous activity on Ryugu and its parent body. (1) The Ryugu parent body accreted from ice and dust. (2) Ice melting due to short-lived radioactive heating. (3) The saturated water refroze upon cooling, forming interstitial ice. (4) More than 1 billion years later, an impact generated heat, resulting in a limited escape of fluid. (5) Ryugu migrated from the main belt to the near-Earth orbit about 5 million years ago and has significantly degassed water since then. (Iizuka et al., Nature 2025)
The new study indicates that Ryugu's parent body did not simply freeze and remain inert. Instead, liquid water re‑mobilized within its interior much later — roughly a billion years after initial formation — likely triggered by an impact that fractured and heated the body. Those warm, short‑lived fluid episodes appear to have migrated through porous rock without fully evaporating or causing large‑scale mineral overprinting.
Methods and key discovery: lutetium-hafnium chemistry
Researchers based their interpretation on the lutetium‑176 (176Lu) to hafnium‑176 (176Hf) isotopic system preserved in the returned sample. Radioactive decay of 176Lu to 176Hf follows well‑characterized pathways in dry, closed systems, but interaction with liquid water can perturb that system by redistributing elements and altering isotopic ratios.
Analyses of the 80‑milligram sample showed a 176Lu/176Hf ratio that differs sharply from ratios measured in typical meteorites that fell to Earth. After excluding alternative explanations, the authors concluded that late, low‑temperature fluid flow disturbed the Lu‑Hf isotopic system — direct chemical evidence that aqueous fluids circulated inside the parent body long after formation.
"It was a genuine surprise!" says geochemist Tsuyoshi Iizuka of the University of Tokyo. "We found that Ryugu preserved a pristine record of water activity, evidence that fluids moved through its rocks far later than we expected."
Implications for planetary science and Earth's water budget
If small planetesimals like Ryugu's parent could retain and then briefly remobilize liquid water billions of years after formation, the implications are twofold. First, asteroid fragments reaching the inner Solar System may have carried more liquid water than previously assumed. Second, impacts by wet asteroids during Earth's accretional period could have delivered significantly more water to the young planet than standard dry‑rock models predict — potentially two to three times more in some scenarios.
This finding helps address a long‑standing problem in planetary science: the apparent lack of moisture in the inner early Solar System relative to the volumes needed to seed Earth's oceans and atmosphere. Ryugu joins a growing list of small bodies whose chemistry suggests that icy and hydrated materials were more resilient than models had allowed.
Mission context and current state of Ryugu
Hayabusa2, the Japan Aerospace Exploration Agency (JAXA) sample‑return mission, collected and delivered Ryugu material to Earth, enabling the high‑precision laboratory work behind these conclusions. Today Ryugu itself is dehydrated compared with its ancient state, having migrated from the main asteroid belt to a near‑Earth orbit around 5 million years ago and degassed much of its water since then. The study was published in Nature.
Expert Insight
Dr. Elena Morales, planetary scientist at the European Planetary Institute (fictional), comments: "These results remind us that small bodies can host surprisingly complex thermal and chemical histories. Finding direct isotopic evidence of late fluid flow in Ryugu's material suggests similar processes may have been common among planetesimals — and that the raw ingredients for Earth's oceans may have been abundant and mobile when the time came. Future sample returns and targeted meteoritic studies will help quantify how much water these bodies actually delivered."
Conclusion
Ryugu's returned sample provides compelling chemical evidence that liquid water circulated within its parent body much later than previously believed. By disturbing the lutetium‑hafnium isotopic system, late‑stage fluids left a durable record that revises our understanding of water retention and transport in small icy bodies. The discovery strengthens the idea that asteroids could have been a more important source of Earth's primordial water than standard models allow, and it underlines the value of returned samples for reading the Solar System's early chemical history.
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