Small fragments of rock can reveal a lot when they’re analyzed with powerful laboratory instruments. New research into tiny fragments of the asteroid Ryugu sampled by JAXA’s Hayabusa2 mission showed that water flowed through it more than one billion years after it formed. This new insight overturns the previous understanding that asteroids only experienced water activity in the very earliest stages of solar system formation.
When JAXA’s Hayabusa2 mission returned samples from asteroid Ryugu, researchers eagerly awaited access to them. After JAXA performed an initial analysis, they made samples available, and researchers around the world submitted research proposals. The work is paying off as different research continues to appear in scientific journals.
Ryugu is a rubble pile asteroid, and up to 50% of its volume is empty space. New research in Nature shows that fluid flowed through the carbonaceous asteroid more than one billion years after formation, likely due to an impact that melted ice and opened up channels for the melt water to flow through. The research is titled “Late fluid flow in a primitive asteroid revealed by Lu–Hf isotopes in Ryugu,” and the lead author is Tsuyoshi Iizuka, an Associate Professor in the Department of Earth and Planetary Science at the University of Tokyo.
Sample rock from asteroid Ryugu. Hayabusa2 returned a total of 5.4 grams of material, over 50 times more than anticipated. Image Credit: JAXA
For astronomers, asteroids provide critical clues about how the Solar System formed. They’re the basic building blocks from which planets form, and unlike bodies like Mars and Earth, they haven’t been heavily modified by things like differentiation, heating, and geological processes. The study of asteroids has helped scientists understand the original materials present at the beginning of the Solar System.
One thing asteroids have helped researchers understand is how Earth got its water. Carbonaceous (C-type) asteroids like Ryugu contain organic compounds and water-bearing minerals that they delivered to Earth when the planet and Solar System were both young. But while scientists have a basic understanding of how the Solar System formed, including how Earth got its water, there are big voids in the picture.
Scientists have spent a great deal of time and energy trying to understand Earth’s water. There are two broad pathways that likely both contributed. One involves oxygen outgassing from magma during Earth’s magma ocean phase, combining with hydrogen in the atmosphere, and forming water. The other involves delivery by asteroids and comets, which also delivered life’s building blocks.
This artist’s illustration shows a molten planet with asteroids nearby. Earth likely got its water from two sources: off-gassing during the magma ocean phase, and delivery by asteroids and comets. Image Credit: University of Sheffield/Mark Garlick.
One of the issues with the water-delivery-by-asteroid understanding is that evidence showed it must have happened relatively quickly. Maybe too quickly to explain how Earth got its water. But by showing that water flowed through Ryugu more than one billion years after the Solar System formed, the new research may have filled the void in our understanding.
“We found that Ryugu preserved a pristine record of water activity, evidence that fluids moved through its rocks far later than we expected,” said lead author Iizuka in a press release. “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.” If it persisted in asteroids longer than thought, then they could’ve delivered it to Earth for longer than thought.
The result is based on Lutetium–hafnium dating. It’s a common tool in geochronological dating that’s based on the decay of lutetium-176 (¹⁷⁶Lu) to hafnium-176 (¹⁷⁶Hf). Each element has different chemical behaviours, and their extremely long half lives—longer than the age of the Universe—lead to different ratios in different rock, even rock billions of years old.
Researchers expected to find predictable ratios of the two isotopes based on the asteroid’s age, and based on what’s been found on other asteroids. But they didn’t. The ratio of ¹⁷⁶Lu to ¹⁷⁶Hf was much greater than anticipated. Both elements remain relatively immobile during most geological processes, but they have different ionic charges and sizes. Hafnium’s higher charge density makes it less soluble in aqueous solutions than lutetium is.
Based on that, the researchers determined that a fluid was washing lutetium out of the rocks.
“We thought that Ryugu’s chemical record would resemble certain meteorites already studied on Earth,” said Iizuka. “But the results were completely different. This meant we had to carefully rule out other possible explanations and eventually concluded that the Lu-Hf system was disturbed by late fluid flow. The most likely trigger was an impact on a larger asteroid parent of Ryugu, which fractured the rock and melted buried ice, allowing liquid water to percolate through the body. It was a genuine surprise! This impact event may be also responsible for the disruption of the parent body to form Ryugu.”
This figure shows how water flowed on Ryugu. (1) The Ryugu parent body accreted from ice and dust in the outer protosolar disk at about 2 Myr after the Solar System formed. (2) Ice melting by short-lived radioactive heating induced early aqueous alteration under water-saturated and isochemical (constant chemical composition) conditions at ≤7 Myr. (3) The saturated water refroze when cooled, forming interstitial ice. (4) More than 1 Gyr later, an impact generated heat that melted interstitial ice and crated rock fractures for fluid pathways, resulting in a limited escape of fluid. (5) Ryugu migrated from the main belt to the near-Earth orbit about 5 Ma and has significantly degassed water through ice sublimation and vapour diffusion since then. Image Credit: Iizuka et al. 2025. Nature
The results suggest that carbonaceous asteroids may have delivered water to Earth in greater quantities and later than thought. If Ryugu’s parent body maintained ice for one billion years, then other similar bodies that bombarded the young Earth could have, too. Most asteroid impacts with Earth occurred within the planet’s first one to two billion years, and this research shows that water delivery could’ve occurred during more of that time span than thought.
“The idea that Ryugu-like objects held on to ice for so long is remarkable,” said Iizuka. “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. Though 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.”
This figure shows the proportion of lost Lu (p) needed to account for the apparent 176Hf excess in the pristine Ryugu sample (after correction for nucleosynthetic effects), depending on the time interval from parent-body accretion to Lu loss (Δt). The solid line and grey band represent the mean and 95% confidence interval, respectively. The light blue area indicates the period of early aqueous alteration and the horizontal dashed line denotes the upper limit of p for the sample. Image Credit: Iizuka et al. 2025. Nature
When Hayabusa2 was at Ryugu, some of the first data it sent back showed that the asteroid was extremely dry. This highlights the value of sample return missions where sampled can be subjected to more rigorous interrogation. Even though each research sample was tiny, requiring new, sophisticated methods of study, the samples have revealed a lot that would remain hidden without them.
“Our small sample size was a huge challenge,” recalled Iizuka. “We had to design new chemistry methods that minimized elemental loss while still isolating multiple elements from the same fragment. Without this, we could never have detected such subtle signs of late fluid activity.”
Phosphate veins play a role in this research too, but need more work to understand. The researchers intend to study them in greater detail to narrow down the ages of the fluid flow with more accuracy.
“Na–Mg phosphates occur as veins that cross-cut brecciated fabrics of altered lithology, demonstrating their formation after early alteration and impact events,” the authors write in their research. Vaporization during formation can sometimes create voids in the veins, and phosphates precipitated there should show Lu enrichment and Hf deficits that complement what they found in their samples.
They also want to compare their findings with the results from the asteroid Bennu, which NASA’s OSIRIS-REx delivered samples from in 2023. If those samples show similar water activity as the Ryugu sample, then sample return missions will have shown how valuable they are, by re-writing our understanding of the Solar System, Earth, water, and even life.