The famous asteroid, Ryugu, is drip-feeding scientists crucial information about its watery past.
Using a tiny, 80-milligram sample of the near-Earth asteroid, researchers have discovered evidence that liquid water was sloshing about within the rock much more recently than scientists believed was possible.
The findings suggest that Ryugu's parent asteroid hosted liquid water for an astonishingly long time, without evaporating, degassing, or chemically reacting with minerals.
"It was a genuine surprise!" says geochemist Tsuyoshi Iizuka from 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."
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Before it was born, Ryugu was part of a 'planetesimal' – the seed of a planet – that was forming on the outer reaches of our Solar System about 4.565 billion years ago.
Made from accumulating ice and dust, the frozen protoplanet seems to have thawed a billion years after its formation.
It's possible that the thaw occurred after a collision, which fractured and heated the planetesimal, melting its buried ice and allowing water to flow.
That very collision, or perhaps one after, could have burst the protoplanet like a water balloon, spraying liquid-filled asteroids into the inner Solar System.
If that's true, then similar rocky bodies that struck young Earth billions of years ago may have delivered two to three times more water than standard models account for.
The apparent paucity of moisture in the inner early Solar System has long been a problem for the hypothesis that asteroids first delivered water to Earth, seeding our planet's oceans and atmospheres.
Ryugu could be the missing key. Nor is it the only asteroid that seems to defy our understanding of how water copes on rocky objects without atmospheres to protect them.
"The idea that Ryugu-like objects held on to ice for so long is remarkable," says Iizuka.
"It suggests that the building blocks of Earth were far wetter than we imagined. This forces us to rethink the starting conditions for our planet's water system."
The chemical record from Ryugu is based on the radioactive decay of lutetium-176 (176Lu) to hafnium-176 (176Hf).
When liquid water is present, it interrupts the way this decay occurs. In Ryugu samples, the ratio of 176Lu to 176Hf was completely different from that of Earthly meteorites (asteroids that have actually crashed into our planet).
"This meant we had to carefully rule out other possible explanations and eventually concluded that the Lu-Hf system was disturbed by late fluid flow," says Iizuka.
Today, Ryugu is all dried out, but its chemical composition speaks volumes about the conditions in our early Solar System.
The asteroid hypothesis for Earth may hold water after all.
The study was published in Nature.