A Billion-Year Secret Uncovered
A remarkable discovery about the asteroid that gave rise to Ryugu challenges long‑standing ideas about when and where water could move within rocky bodies. Scientists analyzing tiny rock fragments returned by JAXA’s Hayabusa2 mission have found evidence that liquid water once percolated through Ryugu’s parent body, and that this activity occurred more than a billion years after the asteroid originally formed. The work, published in Nature, suggests that water processes on carbon‑rich asteroids persisted far longer than previously thought and may have played a larger role in supplying Earth with oceans.
Hayabusa2, which touched down on Ryugu in 2018 and delivered samples to Earth, provided researchers with a rare, pristine look at a primitive asteroid. The new results come from examining isotopes of lutetium (Lu) and hafnium (Hf) in these samples. The decay of 176Lu to 176Hf can act as a geological clock, but the researchers found a Lu–Hf ratio that was higher than expected. This anomaly pointed to a process that had washed lutetium out of the rocks—evidence for late, liquid movement through Ryugu’s interior.
“We thought Ryugu’s chemical record would resemble certain meteorites already studied on Earth,” said Associate Professor Tsuyoshi Iizuka of the University of Tokyo. “But the results were completely different.” He added that the team had to rule out other explanations, and ultimately concluded that late fluid flow disturbed the Lu–Hf system. The most plausible trigger, they proposed, was an impact on a larger asteroid that created Ryugu’s parent body, fractured rocks, melted buried ice, and allowed liquid water to percolate through the body. “It was a genuine surprise!” Iizuka remarked. The same impact event may have helped break apart the parent body to form Ryugu itself.
These findings imply that water‑rich materials in the early solar system may have retained ice and remained active far longer than scientists imagined. The presence of late‑stage fluids in Ryugu’s history reshapes how researchers model the delivery of water to Earth and the conditions that made our planet habitable.
Implications for Earth’s Water
The most striking consequence is a revised view of where Earth’s water could originate. If carbon‑rich asteroids like Ryugu carried substantial ice long after their formation, then the early flux of icy bodies to the inner solar system could have delivered more water than current models allow. The analysis suggests these parent bodies could have held ice for more than a billion years, potentially delivering two to three times more water to a young Earth than previously estimated. Such a change would influence theories about the timing and extent of Earth’s oceans and atmosphere.
“The idea that Ryugu‑like objects held on to ice for so long is remarkable,” Iizuka noted. “It suggests that the building blocks of Earth were far wetter than we imagined. This forces us to rethink the starting conditions for our planet’s water system.”
What Comes Next
The researchers emphasize that analyses of Ryugu remain limited by the tiny amount of sample material—Hayabusa2 returned only grams, and many experiments could use only milligrams. To maximize information gain, the team developed refined chemical methods to separate elements and measure isotopes with unprecedented precision. They plan to study phosphate veins in Ryugu’s rocks to pin down more precise ages for the late fluid flow and to compare these results with NASA’s OSIRIS‑REx samples from Bennu, testing whether similar water activity occurred there as well.
Ultimately, the researchers hope to trace how water was stored, mobilized, and delivered to Earth, offering a crucial piece in the broader puzzle of planetary habitability. The discovery underscores how modern analytical techniques can extract groundbreaking stories from tiny samples and reminds us that the history of water in the solar system is more dynamic than once thought.
Bottom Line
Ryugu’s legacy continues to reshape our understanding of water’s journey through the solar system—and perhaps the very story of how Earth became habitable.
