Scientists analyzing samples from asteroid Ryugu have discovered evidence of liquid water activity that persisted over a billion years after its formation. This finding, based on Japan's Hayabusa2 mission, challenges assumptions about water processes on asteroids and could reshape theories on how Earth acquired its oceans. The research highlights the role of carbon-rich asteroids in delivering water to our planet.
The discovery comes from microscopic rock samples collected by the Japan Aerospace Exploration Agency's (JAXA) Hayabusa2 spacecraft during its 2018 mission to the near-Earth asteroid Ryugu. Hayabusa2 returned small bits of rock and dust to Earth, providing direct evidence of the asteroid's history.
Researchers, led by Associate Professor Tsuyoshi Iizuka from the University of Tokyo's Department of Earth and Planetary Science, examined isotopes of lutetium (Lu) and hafnium (Hf) in the samples. They found unusually high levels of 176Hf compared to 176Lu, indicating that liquid water had seeped through the rocks long after Ryugu formed. "We found that Ryugu preserved a pristine record of water activity, evidence that fluids moved through its rocks far later than we expected," Iizuka said. This activity occurred more than a billion years after the asteroid's initial formation, contradicting the view that water-related processes on asteroids were limited to the solar system's earliest stages.
The team attributes the fluid flow to an ancient impact on Ryugu's larger parent asteroid, which fractured the rock and melted buried ice, allowing water to percolate. "The most likely trigger was an impact on a larger asteroid parent of Ryugu, which fractured the rock and melted buried ice," Iizuka explained. Despite the limited sample size—only a few grams total, with experiments using mere milligrams—the researchers developed precise methods to detect these subtle signs.
These findings suggest that carbon-rich asteroids like Ryugu retained frozen water for over a billion years, potentially delivering two to three times more water to the young Earth than previously estimated. Such impacts may have significantly contributed to the formation of Earth's oceans and atmosphere. "The idea that Ryugu-like objects held on to ice for so long is remarkable," Iizuka noted, emphasizing the need to rethink planetary water systems.
Future work includes studying phosphate veins in the samples for more precise timelines and comparing results with NASA's OSIRIS-REx samples from asteroid Bennu. The study was published in Nature and supported by Japan Society for the Promotion of Science grants.