Near-Earth asteroid Ryugu was previously covered in flowing water, say researchers examining samples returned by the Japan Aerospace Exploration Agency's (JAXA) Hayabusa2 mission, with implications for scientists' understanding of space impacts like those that formed Earth.

The manner in which Earth gained such an abundance of water has long been a blind spot in understanding the formation of our solar system. While researchers have long believed that asteroids composed of ice and dust, like Ryugu, brought the liquid to Earth, many details of the events are uncertain.

In 2018, JAXA's Hayabusa2 spacecraft landed rovers on Ryugu's surface to supplement remote sensing data with direct rock samples. Some tiny pieces were eventually returned to Earth in 2020, giving scientists access to previously missing information about near-Earth objects.

"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 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."

Two isotopes are present in the samples, lutetium and hafnium, to act as a radioactive clock, allowing scientists to measure geological processes by their radioactive decay. Based on previous work studying Ryugu, the researchers were fairly certain about the concentrations of the two isotopes that they would find in their analysis.

However, the measurements surprised them, with a much greater ratio of hafnium to lutetium than expected, suggesting something was washing the lutetium from 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."

That disturbance most likely occurred on Ryugu's larger parent asteroid, the researchers say. Such an event would have fractured rocks and melted ice, allowing for liquid water to begin flowing throughout, and possibly led to the discharge of Ryugu itself from the larger body. Such a finding indicated that carbonaceous asteroids like Ryugu may have contained much more water than previously suspected by two to threefold.

"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."

Unfortunately, the researchers were only able to work with a sample smaller than a grain of rice, as the few grams of material returned from Ryugu have been divided up between many researchers. To mitigate the limitations of such a tiny amount of material, the team devised innovative techniques for separating the elements and increasing analytical precision, maximizing what could be gleaned.

"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."

Next, the team is focusing on phosphate veins, which appear in the Ryugu samples, which will provide further context for the age of any liquid flow. Additionally, they seek to compare the findings with other asteroid samples, such as those NASA's OSIRIS-REx collected from Bennu, to determine how unique Ryugu's conditions may be. Continued research will provide further insight into how our planet acquired its life-giving water supply.

The paper, "Late Fluid Flow in a Primitive Asteroid Revealed by Lu-Hf Isotopes in Ryugu," appeared in Nature on September 11, 2025.