This water flow occurred on an ancient asteroid more than one billion years after its formation and was probably triggered by an impact that generated heat for ice melting and opened rock fractures for water migration, according to an analysis of material returned by JAXA’s Hayabusa-2 spacecraft.
This image of the asteroid Ryugu was captured by the Optical Navigation Camera – Telescopic (ONC-T) on JAXA’s Hayabusa-2 spacecraft on June 26, 2018, from a distance of 13.7 miles (22 km). Image credit: JAXA / University of Tokyo / Kochi University / Rikkyo University / Nagoya University / Chiba Institute of Technology / Meiji University / Aizu University / AIST.
Ryugu is a near-Earth Cg-type asteroid belonging to the Polana collisional family.
Also known as 1999 JU3, the diamond-shaped object was discovered in May 1999 by astronomers with the Lincoln Near-Earth Asteroid Research.
It measures approximately 900 m (0.56 miles) in diameter and orbits the Sun at a distance of 0.96-1.41 astronomical units (AU) once every 474 days.
“We have a relatively good understanding of how the Solar System formed, but of course there are many gaps,” said University of Tokyo researcher Tsuyoshi Iizuka and colleagues.
“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.”
“We found that Ryugu preserved a pristine record of water activity, evidence that fluids moved through its rocks far later than we expected,” Dr. Iizuka added.
“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.”
In the study, the authors analyzed isotopes of lutetium (Lu) and hafnium (Hf), because radioactive decay from lutetium-176 to hafnium-176 can serve as a sort of clock for measuring geological processes.
Their presence in certain quantities in the samples studied was expected to relate to the age of the asteroid in a fairly predictable way.
But the ratio of hafnium-176 to lutetium-176 was far higher than anticipated.
This strongly implied to the researchers that a fluid was essentially washing out lutetium from the rocks containing it.
“We thought that Ryugu’s chemical record would resemble certain meteorites already studied on Earth,” Dr. Iizuka said.
“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.”
One of the most important implications is that carbon-rich asteroids may have contained and delivered much more water to Earth than previously thought.
It seems Ryugu’s parent body retained ice for over a billion years, meaning similar bodies striking a young Earth could have carried an estimated two to three times more water than standard models account for, significantly affecting our planet’s early oceans and atmosphere.
“The idea that Ryugu-like objects held on to ice for so long is remarkable,” Dr. Iizuka said.
“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.”
The results appear in the journal Nature.
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T. Iizuka et al. Late fluid flow in a primitive asteroid revealed by Lu-Hf isotopes in Ryugu. Nature, published online September 10, 2025; doi: 10.1038/s41586-025-09483-0
