Scientists find evidence of flowing water on Ryugu’s ancient parent asteroid. ‘It was a genuine surprise!’

Liquid water flowed across the surface of the asteroid that birthed the near-Earth object (NEO) Ryugu much later than scientists had thought possible, a new study finds.

The discovery that water existed in liquid form a billion years after the parent body of Ryugu formed came from the study of rock samples collected from the NEO by Japan’s Hayabusa2 probe between 2018 and 2019, and returned to Earth on Dec. 5, 2020.

Carbonaceous asteroids like the spinning-top-shaped Ryugu have long been known to form from ice and dust in the outer solar system as the planets were forming around the infant sun around 4.6 billion years ago. Thus, objects like Ryugu are thought to contain a “fossil record” of unspoiled material from the dawn of our planetary system. However, before this research, scientists had thought that asteroid water activity only lasted for the earliest moments of solar system history.

Thus, the new discovery could change how we think about planet formation around 4.6 billion years ago, as well as further solidifying the idea that asteroids pelted the primordial Earth and delivered much of our planet’s water.

“We found that Ryugu preserved a pristine record of water activity, evidence that fluids moved through its rocks far later than we expected,” research team member Tsuyoshi Iizuka, a scientist at the University of Tokyo, said in a statement. “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.”

Ryugu’s chemical imbalance

Iizuka and colleagues arrived at their conclusion when they examined radioactive isotopes of the elements lutetium and hafnium in Ryugu rock samples. This is useful because the radioactive decay of these isotopes can be used as a natural clock for geological processes.

The concentration of these isotopes can, therefore, be correlated to the age of an asteroid. The Ryugu samples hauled home by Hayabusa2 contained larger quantities of hafnium isotopes compared to lutetium isotopes than were expected. This indicated that some fluid was washing out lutetium from rocks on the asteroid.

“We thought that Ryugu’s chemical record would resemble certain meteorites already studied on Earth,” Iizuka explained. “But the results were completely different. This meant we had to carefully rule out other possible explanations and eventually concluded that the lutetium-hafnium 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 also be responsible for the disruption of the parent body to form Ryugu.”

If Ryugu’s parent body did indeed contain water for over one billion years, one of the main takeaways is the implication that carbon-rich asteroids may have contained much more water than previously thought. That means they may have delivered much more water to Earth by striking the surface of our primordial planet than previously estimated. This would have had a significant effect on Earth’s early oceans and its atmosphere.

“The idea that Ryugu-like objects held on to ice for so long is remarkable,” Iizuka explained. “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.”

What is remarkable about this discovery is the fact that the team was able to conduct their study with samples of Ryugu equivalent to a fraction of a grain of rice. This required the development of new and sophisticated element-separating techniques and improved methods to analyze isotopes with incredible precision.

“Our small sample size was a huge challenge,” Iizuka said. “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 next step for the team will be to investigate veins of phosphate within the Ryugu samples. This should allow the scientists to ascertain a more precise age for the flow of water across the parent body that birthed Ryugu. The researchers will also compare their results to analyses of samples of the asteroid Bennu, which were returned to Earth by the NASA mission OSIRIS-REx in September 2023.

This could reveal if the late water flow on Ryugu’s asteroid parent were unique to this body or if similar water activity has been preserved on other asteroids.

The team’s research was published on Wednesday (Sept. 10) in the journal Nature.