Findings from Ryugu samples indicate that materials brought to Earth five years ago contain traces of past water flowing through rocks. These data prompt a reevaluation of previous ideas about the source of water and shed light on the processes involved in harboring water during the early stages of the Solar System.
Approximately 900 meters wide, 162173 Ryugu is described as an asteroid with a tumbling shape, sometimes intersecting Earth’s orbit. The team that visited this celestial body during JAXA’s Hayabusa2 mission in 2018-2019 successfully returned samples to Earth in December 2020. Led by geochemist Tsuyoshi Iizuka from the University of Tokyo, the research published in Nature revealed chemical anomalies in Ryugu’s samples that could only be explained by liquid water. Iizuka stated, “Ryugu preserves an undisturbed record of the effects of water. We found evidence of fluids passing through rocks; it was truly a surprise,” he said.
The research team analyzed isotopes of lutetium-176 (Lu-176) and hafnium-176 (Hf-176) formed by the decay of this isotope in the samples. Usually, these isotope ratios are used to determine the age of rocks; however, significantly higher amounts of Hf-176 were found in Ryugu’s samples than expected. The researchers explained that this difference could be the result of ancient water flow transporting or washing out Lu-176. According to Iizuka, this event could have occurred due to a collision between Ryugu and a larger asteroid. The collision might have cracked the rocks, melted the ice in the water, and allowed liquid water to move through the asteroid.
The Significance of Long-Term Water and Earth’s Origin This discovery suggests that water may have persisted in asteroids longer than expected, even for billions of years after planet formation. Many scientists believed that water could not be stored for such a long time. Iizuka explains, “This changes our thinking about the long-term behavior of water in asteroids. Water is depleting more slowly than we thought.”
Today, a widely held theory suggests that Earth’s water sources largely come from asteroids and comets. The new findings from Ryugu indicate that the role of asteroids in this process might be larger and more surprising, and even that their potential to deliver water to our planet could be three times greater than previously estimated. However, the absence of liquid water traces in samples from the Bennu asteroid raises the possibility that these two celestial bodies may have different origins. The team’s plans include pinpointing the exact timing of water flow in the phosphate veins of Ryugu and conducting similar analyses on samples from Bennu. These studies are important steps toward understanding the universe’s water journey.
