Asteroid Ryugu’s hidden waters could explain how Earth got its oceans

Although scientists have a solid general picture of how the solar system came together, many details remain uncertain. One of the biggest questions concerns how Earth ended up with so much water. It has long been thought that carbon-rich asteroids, like Ryugu, which formed from ice and dust in the outer solar system, were key suppliers of water to our planet. Hayabusa2's 2018 mission to Ryugu marked the first time such an asteroid was both observed up close and sampled directly. The mission returned small bits of rock and dust to Earth, giving researchers a rare opportunity to fill in missing pieces of our planet's early story.

"We found that Ryugu preserved a pristine record of water activity, evidence that fluids moved through its rocks far later than we expected," said Associate Professor Tsuyoshi Iizuka from the Department of Earth and Planetary Science at the University of Tokyo. "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."

The key to the discovery lies in the isotopes of lutetium (Lu) and hafnium (Hf), elements that form a natural radioactive clock through the decay of ¹⁷⁶Lu into ¹⁷⁶Hf. By analyzing their ratios in Ryugu's samples, researchers expected to determine the asteroid's age in a straightforward way. Instead, they found much higher levels of ¹⁷⁶Hf compared with ¹⁷⁶Lu than anticipated. This unusual imbalance suggested that liquid water had once seeped through the rocks, effectively leaching lutetium out of them.

"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."

The study's implications are far-reaching. It suggests that carbon-rich asteroids could have stored and delivered much more water to Earth than scientists previously assumed. Ryugu's parent asteroid appears to have retained frozen water for over a billion years, meaning similar bodies colliding with the young Earth might have delivered two to three times more water than current models estimate. Such impacts could have played a major role in shaping the early oceans and atmosphere.

"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."

Hayabusa2 only brought back a few grams of material. With many researchers wanting to run tests on it, each experiment could only use a few tens of milligrams, fractions of a grain of rice. To maximize the information gained, the team developed sophisticated methods for separating elements and analyzing isotopes with extraordinary precision, realizing the full potential of current geochemical analytical techniques.

"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."

The researchers also plan to study phosphate veins within Ryugu samples to pin down more precise ages of the late fluid flow. They will also compare their results with NASA's samples collected from asteroid Bennu by the OSIRIS-REx spacecraft, to test whether similar water activity might have happened there too, or whether it was unique to Ryugu. Eventually, Iizuka and colleagues hope to trace how water was stored, mobilized and finally delivered to Earth, a story that continues to shape our understanding of planetary habitability.

Funding: This work was supported by Japan Society for the Promotion of Science KAKENHI grants (21KK0057, 22H00170).