Liquid Water on Ryugu: A Hidden Water Trail Across Time
A recent study of tiny rock fragments returned from the asteroid Ryugu by JAXA’s Hayabusa2 spacecraft has upended assumptions about when and how water could move through asteroids. The research, involving scientists from the University of Tokyo among others, shows that liquid water once traversed Ryugu’s interior more than a billion years after the body formed. This discovery challenges the idea that water-related processes on asteroids were confined to the very early history of the solar system and suggests a more persistent role for water in shaping these ancient bodies.
The Isotopic Clock That Told the Tale
The team analyzed lutetium–hafnium (Lu–Hf) isotopes, where the decay of 176Lu to 176Hf can serve as a geological clock. In several rock fragments, researchers found an unexpectedly high 176Hf/176Lu ratio, indicating that a late episode of fluid movement had altered the Lu–Hf system. Associate Professor Tsuyoshi Iizuka of the University of Tokyo explained that the results were “a genuine surprise,” forcing scientists to reconsider the chemical history recorded in Ryugu’s rocks.
The most plausible trigger, the researchers say, was an impact on Ryugu’s larger asteroid parent. The collision would have fractured rocks and melted buried ice, allowing liquid water to percolate through the asteroid’s interior. This late fluid activity would have disturbed the Lu–Hf isotopic clock, revealing a geological story that occurred far later than Ryugu’s initial formation.
What This Means for Earth’s Water
One of the most profound implications concerns Earth’s own water. If carbon-rich asteroids like Ryugu can preserve ice for more than a billion years, they could have delivered far more water to a young Earth than previously imagined. Some models suggest that early Earth’s oceans and atmosphere might owe more to these long-lived icy bodies than we thought, potentially tripling earlier estimates of how much water carbon-rich asteroids could have contributed.
As Iizuka notes, Ryugu’s parent body appears to have remained icy long after its formation, challenging the idea that ice was a brief and early feature of such asteroids. The finding invites a broader reconsideration of the initial conditions that gave rise to Earth’s water system and planetary habitability as a whole.
From Tiny Grains to Global Questions
Hayabusa2 delivered only a few grams of material, which meant researchers had to innovate. They developed advanced chemistry and isotope-separation methods to extract meaningful signals from fractions of a grain. The team’s work highlights how modern geochemical analysis can extract a remarkably detailed history from minute samples, pushing the boundaries of what we can learn from asteroid material.
Where the Research Goes Next
Researchers plan to study phosphate veins in Ryugu’s samples to pin down more precise ages for the late fluid flow. They also aim to compare Ryugu’s results with NASA’s OSIRIS-REx samples from asteroid Bennu to determine whether late-water activity was a broader feature of carbon-rich bodies or a Ryugu-specific case. Ultimately, the scientists hope to trace how water was stored, mobilized, and delivered to Earth, offering deeper insight into the planet’s habitability.
As the field moves forward, this discovery encourages a more nuanced view of water in the early solar system and invites ongoing exploration of how icy, water-rich materials shaped the formation and evolution of Earth and its planetary neighbors.
