A groundbreaking discovery by a team of researchers, including experts from the University of Tokyo, has revealed that liquid water once flowed on the asteroid that eventually gave rise to the near-Earth asteroid Ryugu. This significant finding, derived from tiny rock fragments returned by the Japan Aerospace Exploration Agency’s (JAXA) Hayabusa2 spacecraft, challenges the long-held belief that water activity on asteroids was limited to the earliest moments of the solar system’s history. The implications of this discovery could profoundly impact existing models of Earth’s formation.
The research, which utilized samples collected during Hayabusa2’s historic 2018 mission to Ryugu, suggests that the presence of water on such asteroids persisted far longer than previously thought. According to Associate Professor Tsuyoshi Iizuka from the University of Tokyo’s Department of Earth and Planetary Science, “We found that Ryugu preserved a pristine record of water activity, evidence that fluids moved through its rocks far later than we expected.” This revelation necessitates a reevaluation of how water was retained and distributed within asteroids over time.
Unraveling the Mysteries of the Solar System
Our understanding of the solar system’s formation is relatively comprehensive, yet numerous gaps remain. One such gap pertains to how Earth acquired its abundant water supply. It has long been understood that carbonaceous asteroids like Ryugu, which formed from ice and dust in the outer solar system, delivered water to Earth. The Hayabusa2 mission, the first of its kind to return samples from an asteroid, has provided researchers with the opportunity to fill in some of these missing details.
The heart of this discovery lies in the analysis of isotopes of lutetium (Lu) and hafnium (Hf). The radioactive decay from 176Lu to 176Hf serves as a geological clock, allowing scientists to measure processes over time. Surprisingly, the ratio of 176Hf to 176Lu in the samples was significantly higher than anticipated, strongly suggesting that fluid activity had altered the chemical composition of the rocks.
Implications for Earth’s Water Origin
This unexpected finding suggests that carbon-rich asteroids may have delivered far more water to Earth than previously estimated. Ryugu’s parent body appears to have retained ice for over a billion years, indicating that similar bodies impacting a young Earth could have contributed two to three times more water than standard models account for. This insight could significantly alter our understanding of the early development of Earth’s oceans and atmosphere.
“The idea that Ryugu-like objects held on to ice for so long is remarkable,” Iizuka noted. “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.”
Challenges and Future Research
Despite the small sample size returned by Hayabusa2—only a few grams of material—researchers have developed sophisticated methods to maximize the information gained. Each experiment could only use a few tens of milligrams, requiring the team to innovate new techniques for separating elements and analyzing isotopes with extraordinary precision.
“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.”
Looking ahead, the researchers plan to study phosphate veins within Ryugu samples to determine more precise ages of the late fluid flow. They aim to compare their findings with samples collected from asteroid Bennu by NASA’s OSIRIS-REx spacecraft, exploring whether similar water activity occurred there or if it was unique to Ryugu. Ultimately, Iizuka and his colleagues hope to trace the journey of water from storage and mobilization within asteroids to its eventual delivery to Earth, continuing to shape our understanding of planetary habitability.
