Icy asteroids?

Stony meteorites known as carbonaceous chondrites show signs of liquid water no more than a million years ago, scientists say.

This is a startling discovery, because carbonaceous chondrites are believed to be chips blown off the surfaces of ancient asteroids whose composition hasn’t altered since the dawn of the Solar System, 4.5 billion years ago.

And while planetary scientists have long suspected that these asteroids once contained water, they didn’t know whether that water still persists in the form of ice or was long ago lost to space, says Simon Turner, an isotope geochemist at Macquarie University in Australia. 

To test this, Turner and colleagues in Australia, the US and France ran a long-shot experiment. 

They obtained samples from nine carbonaceous chondrites that had been collected within a couple days of falling to Earth—quickly enough that the Earth’s humid environment didn’t have time to overwhelm the subtle chemical signatures the researchers hoped to find. 

Then, in a study just published in Science, they measured the various isotopes of uranium and thorium within the samples. 

They focused on these elements, Turner says, because uranium is highly soluble in water, whereas thorium isn’t. “Any fluid flow [through the rock] should separate these two elements from each other,” he explains.

Most importantly, it would separate uranium-234 (half-life 246,000 years) from its decay product thorium-230 (half-life 75,400 years).

“That would set a clock ticking that would continue to tick for a couple hundred thousand or a million years,” Turner says. 

After that, the thorium will have decayed to a point where it can no longer be adequately measured.

Amazingly, all of the carbonaceous chondrites Turner’s team tested contained isotope ratios that indicated that they had seen liquid water flowing through them recently enough for the method to detect. That means the asteroids from which they originated are “frozen mudballs, rather than solid rocks”. 

When exactly that occurred is open to debate. It’s possible it happened when the meteorites hit Earth’s atmosphere and their surfaces were heated into the incandescent streaks we see as meteors. But according to Turner, it was more likely a consequence of the impact that blasted them off their parent asteroids and sent them hurtling into space, eventually to fall to Earth.

However it happened, it means these parent asteroids still contain ice. 

“That could be of interest for mining asteroids,” Turner says – and it also adds to the likelihood that asteroids were a major part of how the infant Earth received its water.

Humberto Campins, a planetary scientist from the University of Central Florida, US, who was not a member of the study team, calls it a significant discovery. 

Not that he’s surprised to find that asteroids still contain water. Recent asteroid space missions by Japan and the US, he says, have found spectroscopic signals of hydrated silicates on the surfaces of their respective asteroids. That’s not ice per se, but Campins says, “it’s still water, which could be mobilised during impacts”.

What makes the new finding really interesting, he says, is what it reveals about how impacts may affect the evolution of materials on asteroids. “The collisional history of asteroids is more complex than we thought,” he says.