An analysis of rock samples brought back by Japan's probe from the asteroid Ryugu found that water flowed even 1 billion years after the asteroid formed. Until now, scientists thought water existed only for several million years after an asteroid formed. If water flowed on the asteroid for a long time, the amount could be greater than expected. Scientists said the findings confirm the possibility that Earth's water came via asteroids like Ryugu.
An international team led by the University of Tokyo, with researchers from more than 10 countries including Korea, the United States, the United Kingdom, France and Germany, said it "found evidence that water moved even after more than 1 billion years had passed since the parent body that became Ryugu formed," and published the results on the 11th in the journal Nature. Korean researchers, including Choi Byun-gak, a professor of earth science education at Seoul National University, and Park Chang-geun, principal researcher at the Korea Polar Research Institute (KOPRI), took part in the study.
Asteroids are small celestial bodies that orbit the sun in long ellipses and are thought to be remnants of planets formed in the early solar system. They are similar to comets but differ in that they do not have volatile tails. Ryugu is an asteroid rich in carbon.
The Japan Aerospace Exploration Agency (JAXA) launched the unmanned probe Hayabusa2 in 2014 to explore Ryugu. Hayabusa2 arrived at Ryugu in 2018, collected 5.4 grams of rock samples, and returned to Earth in Dec. 2020. JAXA organized a global joint research team in 2019 to study these samples and has published more than 20 papers.
Asteroids rich in carbon and water are commonly found in the outer part of the solar system's asteroid belt. They formed from the gathering of dust and ice, and likely delivered water and organic matter to planets like Earth. Earlier studies identified evidence that water and rock interacted only for several million years immediately after Ryugu's parent body formed, but whether water existed for a long period afterward was unknown.
Through sample analysis, the team confirmed evidence that fluids flowed even hundreds of millions of years after the parent body formed. They used the process by which the metal lutetium transforms into hafnium in watery environments within rock. By looking at the rate at which lutetium turns into hafnium, they can tell when water moved through the rock and how long it flowed.
Tsuyoshi Iizuka, a professor at the University of Tokyo who led the study, said, "Until now, water inside asteroids was thought to exist mostly in the form of hydrated minerals, in which water is contained within minerals, but this study confirms the possibility that water in an icy state existed inside asteroids for a long period," adding, "The amount of water that came to Earth from asteroids may have been 2 to 3 times greater than previously expected."
Choi Byun-gak, a professor of earth science education at Seoul National University who has been involved in the study since 2019, said, "This study is the first to show the possibility that water in an icy state came to Earth via asteroids even in the planet's early days," and explained, "A similar phenomenon likely occurred commonly in carbonaceous asteroids of a similar type."
References
Nature (2025), DOI: https://doi.org/10.1038/s41586-025-09483-0