Liquid water was present on the asteroid that formed Ryugu a billion years longer than scientists previously thought. This surprising finding comes from rock samples collected by Japan’s Hayabusa2 probe between 2018 and 2019 and returned to Earth in December 2020.
Ryugu is a carbonaceous asteroid, shaped like a spinning top, formed from ice and dust in the outer solar system around 4.6 billion years ago. Scientists long believed that water activity on asteroids lasted only during the early solar system. However, this new research changes that perspective.
According to Tsuyoshi Iizuka from the University of Tokyo, “We found that Ryugu preserved a pristine record of water activity, showing that fluids moved through its rocks far later than we expected.” This suggests that asteroids may have played a more crucial role in delivering water to Earth.
The study revealed unexpected chemical imbalances in the Ryugu samples. Researchers examined radioactive isotopes of lutetium and hafnium, using them as a natural clock to gauge geological processes. They found a higher concentration of hafnium than expected, indicating that fluids were washing out lutetium from Ryugu’s rocks.
Interestingly, the study indicates that an impact on a larger asteroid could have fractured the rock and melted buried ice, allowing liquid water to flow through Ryugu for a significant time. Iizuka noted, “This impact event may also be responsible for the disruption of the parent body to form Ryugu.”
The implications of these findings are profound. If Ryugu’s parent body contained water for such an extended period, it suggests that many carbon-rich asteroids might also have held more water than previously thought. This could mean they delivered much larger amounts of water to Earth than we estimated, greatly influencing the planet’s early oceans and atmosphere.
There’s a growing interest in how this research impacts our understanding of Earth’s habitability. Iizuka emphasizes, “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.”
Despite working with samples no larger than a grain of rice, researchers used innovative techniques to analyze these smaller pieces. Iizuka shared, “Our small sample size was a huge challenge. We had to develop new methods that minimized elemental loss while isolating multiple elements.” This ingenuity allowed them to detect subtle signs of late fluid activity.
Looking ahead, the research team plans to investigate phosphate veins within the Ryugu samples to better pinpoint the age of the water flow. They will also compare their results with samples from the asteroid Bennu, which were returned by NASA’s OSIRIS-REx mission in September 2023. This could reveal whether the late water flow experienced by Ryugu’s parent body is unique or shared by other asteroids.
This study, published in the journal Nature on September 10, 2025, opens up new avenues in our understanding of asteroids and their roles in shaping the Earth we know today.
