Liquid Water Persisted on asteroid Ryugu’s Parent Body for Billions of years, New Analysis Reveals
TOKYO – An international team led by University of Tokyo researcher Tsuyoshi Iizuka has discovered evidence that liquid water flowed through the parent asteroid of Ryugu for a surprisingly long period – over a billion years – challenging previous assumptions about the delivery of water to Earth. The findings, published September 10, 2025, in the journal Nature, suggest carbon-rich asteroids may have contributed substantially more water to our planet than previously estimated.
Researchers analyzed isotopes of lutetium (Lu) and hafnium (Hf) in samples returned from the asteroid Ryugu by the Hayabusa2 mission. The ratio of hafnium-176 to lutetium-176 was unexpectedly high, indicating that lutetium had been washed out of the rocks by a fluid.
“We found that Ryugu preserved a pristine record of water activity, evidence that fluids moved through its rocks far later than we expected,” Dr.Iizuka said. “This changes how we think about the long-term fate of water in asteroids. The water hung around for a long time and was not exhausted so quickly as thought.”
The team believes a large impact on Ryugu’s parent body fractured the rock and melted buried ice, allowing liquid water to percolate through the asteroid’s interior.This impact event may have also been responsible for the breakup of the parent body, ultimately forming Ryugu.
The prolonged presence of water on Ryugu’s parent body has significant implications for understanding Earth’s early oceans and atmosphere. Scientists estimate that similar bodies impacting a young Earth could have delivered two to three times more water than current models predict.
“It seems Ryugu’s parent body retained ice for over a billion years, meaning similar bodies striking a young Earth could have carried an estimated two to three times more water than standard models account for,” Dr. Iizuka explained. “The idea that Ryugu-like objects held on to ice for so long is remarkable. It suggests that the building blocks of Earth were far wetter than we imagined.”
The research prompts a reevaluation of the conditions necessary for Earth to become habitable. “This forces us to rethink the starting conditions for our planet’s water system,” Dr. iizuka stated. “Tho it’s too early to say for sure, my team and others might build on this research to clarify things, including how and when our Earth became habitable.”
It’s long been understood that carbonaceous asteroids like Ryugu formed in the outer Solar System and supplied water to Earth. This new study provides crucial insight into the longevity of that water source.