Asteroid Ryugu’s Waters: Origins of Earth’s Oceans
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Ryugu Asteroid Reveals Evidence of Long-Lasting Liquid Water
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New analysis of samples from the asteroid Ryugu suggests water persisted within the space rock for billions of years,challenging previous assumptions about asteroid evolution and potentially informing theories about Earth’s water origins.
Background: The Hayabusa2 Mission and Asteroid Ryugu
A group of scientists, including researchers from the University of Tokyo, has found evidence that liquid water once moved through the asteroid that eventually gave rise to the near-Earth asteroid Ryugu. Remarkably, this activity took place more than a billion years after the asteroid first formed. The revelation, which relies on microscopic rock samples collected by the Japan Aerospace Exploration Agency (JAXA) Hayabusa2 spacecraft, challenges the long-standing belief that water-related processes on asteroids only occurred in the earliest stages of the solar system’s evolution.These findings could influence scientific models that describe how Earth and its oceans developed.
Although scientists have a solid general picture of how the solar system came together,many details remain uncertain. One of the biggest questions concerns how Earth ended up with so much water. It has long been thought that carbon-rich asteroids, like Ryugu, which formed from ice and dust in the outer solar system, were key suppliers of water to our planet. Hayabusa2’s 2018 mission to Ryugu marked the first time such an asteroid was both observed up close and sampled directly. The mission returned small bits of rock and dust to Earth,giving researchers a rare chance to fill in missing pieces of our planet’s early story.
The discovery: Isotopes as a Time Capsule
“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 from the Department of Earth and Planetary Science at the University of Tokyo.”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 weary so quickly as thought.”
The key to the discovery lies in the isotopes of lutetium (Lu) and hafnium (Hf), elements that form a natural radioactive clock through the decay of 176Lu into 176Hf.By analyzing their ratios in Ryugu’s samples,researchers expected to determine when the asteroid’s parent body-the larger asteroid Ryugu originated from-last experienced melting. Rather, they found evidence of fluid alteration much later in the asteroid’s history. The research team published their findings in the journal Science on June 8, 2023 (“Late accretion and the survival of water in Ryugu”).
The Lu-Hf isotopic system revealed that aqueous alteration – the process by which water chemically changes the composition of rocks - occurred on Ryugu’s parent body approximately 89 to 144 million years after the formation of the solar system, a period previously thought to be too late for significant water activity on asteroids. This suggests that the parent body was heated internally, potentially by the decay of radioactive elements, allowing water to circulate and react with the rock for an extended period.
Implications for Earth’s Water and Asteroid Evolution
This discovery has significant implications for understanding the origin of Earth’s water. If water persisted for longer periods within asteroids like Ryugu, it increases the likelihood that these asteroids could have delivered substantial amounts of