Water once persisted on Ryugu parent asteroid long after formation

Water once persisted on Ryugu parent asteroid long after formation
by Riko Seibo
Tokyo, Japan (SPX) Sep 22, 2025

A research team led by the University of Tokyo has discovered that liquid water flowed on the ancient parent body of near-Earth asteroid Ryugu more than a billion years after its birth. The finding, based on Hayabusa2-returned rock fragments, challenges long-held beliefs that asteroid water activity ended shortly after solar system formation.

Carbonaceous asteroids are thought to have supplied Earth with water. Ryugu, visited by JAXA's Hayabusa2 spacecraft in 2018, preserved an unaltered record of fluid activity. "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 of the University of Tokyo. "This changes how we think about the long-term fate of water in asteroids."

The team analyzed isotopes of lutetium and hafnium, using their decay relationship as a geochemical clock. Unexpectedly high hafnium-to-lutetium ratios indicated that liquid water once leached lutetium from the minerals. The likely cause was an asteroid impact, fracturing Ryugu's parent body, melting buried ice, and allowing water to percolate through the rock.

This suggests carbon-rich asteroids may have retained ice for far longer than previously assumed. If such objects struck early Earth, they could have delivered two to three times more water than current models predict, influencing the planet's oceans and 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."

Due to the limited sample size-just a few grams divided among global researchers-the team developed new chemistry methods to minimize elemental loss and achieve highly precise isotopic measurements. This innovation enabled them to detect subtle evidence of late-stage water activity.

Future studies will examine phosphate veins in Ryugu samples for more accurate dating and compare results with asteroid Bennu material from NASA's OSIRIS-REx mission. These efforts aim to reveal whether such long-lived water reservoirs were common or unique to Ryugu, deepening our understanding of Earth's early habitability.

Research Report:Late fluid flow in a primitive asteroid revealed by Lu-Hf isotopes in Ryugu