Asteroid Ryugu. Credit: ©2025 Jaxa, UTokyo & collaborators (CC-BY-ND)
Scientists have uncovered evidence that liquid water flowed on the asteroid Ryugu more than a billion years after it formed.
The results challenge how scientists model the formation of the Earth, as water activity on asteroids was previously thought to have occurred only in the earliest moments of the solar system’s formation.
“We found that Ryugu preserved a pristine record of water activity, evidence that fluids moved through its rocks far later than we expected,” says Associate Professor Tsuyoshi Iizuka from the University of Tokyo, Japan.
“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.”
About 71% of the Earth’s surface is covered by water, which has been crucial in the origin and evolution of life. However, the source of Earth’s vast amounts of water remains shrouded in mystery.
One of the leading hypotheses suggests that while hydrogen and oxygen already existed on the planet, water also came from carbonaceous asteroids that crashed into Earth and brought water in the form of ice.
Samples collected from Ryugu in 2018 by Japanese Aerospace Exploration Agency’s (JAXA) Hayabusa2 spacecraft have helped researchers take another step toward filling in the gaps of life on Earth’s origin story.
Lutetium-176 radioactively decays to hafnium-176 with a half-life of 37.1 billion years, so measuring the ratio of one isotope to the other can help scientists measure geological processes.
When the researchers examined the asteroid samples, they discovered the ratio of hafnium-176 to lutetium-176 was higher than expected.
“It was a genuine surprise!” says Iizuka.
“We thought that Ryugu’s chemical record would resemble certain meteorites already studied on Earth. But the results were completely different.”
The researchers think the higher ratio of hafnium indicates that there may have been fluid flowing through the rock, clearing out the lutetium.
Diagram showing how the researchers think the evolution of Ryugu played out over at least a billion years. Credit: ©2025 Iizuka et al. (CC-BY-ND)
“We had to carefully rule out other possible explanations and eventually concluded that the Lu-Hf system was disturbed by late fluid flow,” says Iizuka.
“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.”
According to the researchers, this unusual ratio implies that the asteroids that collided with Earth billions of years ago may have been carrying even more water than scientists previously thought.
They suggest these rocky bodies could have been carrying up to 2 or 3 times more water than current models predict, which would have significantly shifted the Earth’s oceans and atmosphere.
“It suggests that the building blocks of Earth were far wetter than we imagined,” says Iizuka.
“This forces us to rethink the starting conditions for our planet’s water system.”
The researchers now plan to compare these results with samples of the asteroid Bennu, collected by NASA’s OSIRIS-REx spacecraft.
“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,” says Iizuka.
The results from this study are available to read in Nature.