Asteroid Ryugu still had large amounts of liquid water inside over one billion years after it formed

Large amounts of liquid water once flowed inside asteroid Ryugu. A new study shows that this did not happen only in the first few million years of the solar system, but more than a billion years later.

Japan’s Hayabusa2 mission brought pieces of Ryugu back to Earth in December 2020. The reentry capsule was recovered in Australia, and work began to open it and distribute samples to labs.

Ryugu’s water record

The team studied the ratio of isotopes in the returned grains and found a clear fingerprint of liquid water moving through rock long after the asteroid formed.

That late flow was strong enough to carry certain elements out of the rock and into fractures.

This work was led by Tsuyoshi Iizuka at the University of Tokyo. The group reports that Ryugu’s parent body likely held between 20 and 30 percent water by mass.

In addition, the contribution of such bodies to Earth’s water may need to be revised upward by a factor of two to three.

Hayabusa2 collected a total of 5.4 grams from two sites, including material excavated from about 3.3 feet below the surface. The mission project confirms the returned mass and notes that the goal had been only 0.1 gram.

“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 Iizuka.

Why the clock ran fast

The team used radiometric dating based on the decay of one element (lutetium 176) into another (hafnium 176) over time to figure out when key events happened inside the asteroid.

Those clocks are reliable when elements stay put, but they can be thrown off if fluids move them around.

Some tiny grains looked older than the solar system, which is impossible.

The only sensible reason is that liquid water flowed through parts of the rock and carried away a small amount of lutetium, which made the age calculation appear too old.

The method is powerful because lutetium and hafnium behave differently during water rock interactions. Lutetium dissolves more readily under certain alkaline conditions, while hafnium tends to stay locked in solid phases.

That contrast let the researchers map the movement of water, not by seeing the water itself but by measuring the chemical wake it left behind.

They found evidence for at least several percent loss of lutetium in affected spots, which is enough to skew the apparent ages.

How water flowed through Ryugu

Early in solar system history, short-lived radioactive heat can melt ices inside small worlds. Later on, that heat is gone, so you need another source to melt ice and set fluids moving.

The study points to impacts that warmed the interior and cracked open pathways for liquid to travel.

Sunlight alone cannot do the job deep below the surface, since it can only warm material down to about 16 inches, while some of the analyzed material came from about 3.3 feet.

These late fluids were not a steady flood. The mineral record shows limited flow along fractures, followed by loss of water as vapor that escaped to space through pores near the surface.

That pattern fits the chemistry that the team measured. Certain minerals show corrosion where water passed by, and complementary products formed when the water later cooled and minerals precipitated.

Ryugu’s impact on Earth

If Ryugu’s parent body retained a large store of ice and liquid for so long, similar carbonaceous asteroids could have delivered more water to the inner planets than many models assumed. That matters for the story of Earth’s oceans.

CI type meteorites, known as CI chondrites, have long been used as a benchmark for primitive, water rich material.

Ryugu’s chemistry is close to that group, and the new data suggest that past estimates of how much water such bodies can deliver were too low.

The authors note that early water rock interaction on these bodies happened at mild temperatures and alkaline pH, then slowed as the rock cooled.

Later impacts briefly restarted the system, sending pulses of fluid through the interior.

Independent analysis of Ryugu carbonates supports this picture of early low temperature alteration within the first few million years. The new work extends the timeline and shows that water stayed available much later.

Ryugu lessons for worlds

Hayabusa2 did not just scrape the surface. It created an artificial crater, collected subsurface material, and returned a finely curated set of grains to Earth for careful study.

Those grains let scientists test ideas about aqueous alteration without guessing from telescope data alone. They also help calibrate the isotope clocks that date rocks around the solar system.

The results hint that small bodies can hold on to ice and liquid longer than expected. That increases the odds that collisions early in Earth’s history could deliver large water loads.

Finally, the work underscores the value of sample return missions. A few grams of unweathered material can answer questions that remote sensing might never settle.

The study is published in Nature.

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