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A team of researchers, including those at the University of Tokyo, discovered that liquid water once flowed on the asteroid that spawned near-Earth asteroid Ryugu more than a billion years after it first formed. The finding, based on tiny rock fragments returned by the Hayabusa2 spacecraft of the Japan Aerospace Exploration Agency (JAXA), overturns long-held assumptions that water activity on asteroids only occurred in the earliest moments of solar system history. This could impact current models, including those describing the formation of Earth.

We have a relatively good understanding of how the solar system formed, but of course there are many gaps. 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.

Ryugu was famously visited by the Hayabusa2 spacecraft in 2018, the first visit of its kind, where not only were in-situ data collected, but small samples of material were brought back to Earth too. And it's thanks to this endeavor that researchers can help fill in some missing details in the picture of our creation.

"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 study is now in the journal Nature.

The heart of the discovery comes from the analysis of isotopes of lutetium (Lu) and hafnium (Hf), whose radioactive decay from ¹⁷⁶Lu to ¹⁷⁶Hf 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 ¹⁷⁶Hf to ¹⁷⁶Lu was far higher than anticipated. This strongly implied to the researchers that a fluid was essentially washing out the lutetium from the rocks containing it.

"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."

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 more than 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," 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.