Recent research has uncovered that liquid water flowed across the surface of the asteroid that birthed the near-Earth object (NEO) Ryugu much later than scientists had previously believed. This groundbreaking discovery was made possible by studying rock samples collected from Ryugu by Japan's Hayabusa2 probe between 2018 and 2019, which returned to Earth on December 5, 2020.
Carbonaceous asteroids like Ryugu, which has a distinctive spinning-top shape, are known to have formed from ice and dust in the outer reaches of the solar system as the planets coalesced around the young sun approximately 4.6 billion years ago. These celestial bodies are thought to hold a fossil record of untouched material from the early solar system. However, prior to this study, scientists believed that water activity on asteroids was limited to the initial phases of solar system history. The new findings challenge this long-held belief and could reshape our understanding of planet formation during that period.
According to Tsuyoshi Iizuka, a member of the research team and a scientist at the University of Tokyo, "We found that Ryugu preserved a pristine record of water activity, evidence that fluids moved through its rocks far later than we expected." This suggests that water on asteroids like Ryugu remained for a much longer duration than previously assumed, indicating a more complex history of water retention in asteroids.
The research team reached their conclusion by examining radioactive isotopes of the elements lutetium and hafnium in the rock samples from Ryugu. The radioactive decay of these isotopes serves as a natural clock for geological processes, allowing scientists to correlate their concentrations with the asteroid's age. Surprisingly, the Ryugu samples contained higher levels of hafnium isotopes compared to lutetium isotopes than anticipated, suggesting that some fluid was leaching lutetium from the rocks.
Iizuka elaborated, "We thought that Ryugu’s chemical record would resemble certain meteorites already studied on Earth, but the results were completely different." This unexpected finding led the team to rule out alternative explanations and ultimately conclude that the lutetium-hafnium system had been disrupted by late fluid movement.
The likely cause of this fluid activity was an impact on a larger parent asteroid of Ryugu, which fractured the rock and melted buried ice, allowing liquid water to percolate through. This impact event may also explain how Ryugu was formed from the remnants of its parent body. If Ryugu's parent body retained water for over one billion years, this implies that carbon-rich asteroids may have harbored significantly more water than previously estimated. Consequently, these asteroids could have delivered greater quantities of water to early Earth than believed, potentially influencing the planet's early oceans and atmosphere.
Iizuka remarked, "The idea that Ryugu-like objects held on to ice for so long is remarkable. It suggests that the building blocks of Earth were far wetter than we imagined." This revelation necessitates a reevaluation of the initial conditions that contributed to Earth's hydrosphere. While it's premature to draw definitive conclusions, Iizuka and his team hope to further elucidate how and when Earth became habitable through ongoing research.
One remarkable aspect of this study is that the team managed to conduct their research using Ryugu samples equivalent to a fraction of a grain of rice. This achievement required the development of advanced element-separating techniques and improved methods for analyzing isotopes with exceptional precision. Iizuka noted, "Our small sample size was a huge challenge. We had to design new chemistry methods that minimized elemental loss while still isolating multiple elements from the same fragment." Without these innovations, detecting subtle signs of late fluid activity would have been impossible.
The next phase for the research team will involve investigating veins of phosphate within the Ryugu samples to determine a more precise timeline for the water flow on its parent body. Additionally, the researchers plan to compare their findings with analyses of samples from the asteroid Bennu, which were returned to Earth by NASA's OSIRIS-REx mission in September 2023. This comparison could reveal whether the late water activity observed on Ryugu's parent body is unique or if similar processes have been preserved in other asteroids.
