Isotopic analysis determines that water once flowed on asteroid Ryugu

Summary

How the evolution of Ryugu may have played out over a billion years. Credit: Nature (2025). DOI: 10.1038/s41586-025-09483-0 A team of researchers, including those at the University of Tokyo, discovered that liquid water once flowed on the asteroid that spawned near-Earth asteroid Ryugu more than a billion years after it first formed. The finding, based on tiny rock fragments returned by the Hayabusa2 spacecraft of the Japan Aerospace Exploration Agency (JAXA), overturns long-held assumptions that water activity on asteroids only occurred in the earliest moments of solar system history. This could impact current models, including those describing the formation of Earth. We have a relatively good understanding of how the solar system formed, but of course there are many gaps. One such gap in our knowledge is how Earth came to possess so much water. It's long been known that so-called carbonaceous asteroids like Ryugu formed from ice and dust in the outer solar system supplied water to Earth. Ryugu was famously visited by the Hayabusa2 spacecraft in 2018, the first visit of its kind, where not only were in-situ data collected, but small samples of material were brought back to Earth too. And it's thanks to this endeavor that researchers can help fill in some missing details in the picture of our creation. "We found that Ryugu preserved a pristine record of water activity, evidence that fluids moved through its rocks far later than we expected," said Associate Professor Tsuyoshi Iizuka from the Department of Earth and Planetary Science at the University of Tokyo. "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 study is now published in the journal Nature. The heart of the discovery comes from the analysis of isotopes of lutetium (Lu) and hafnium (Hf), whose radioactive decay from 176Lu to 176Hf can serve as a sort of clock for measuring geological processes...