Firing rocks from a cannon hints at how water reached Earth

By firing marble-sized rocks from a giant cannon, scientists have found a way in which the infant Earth might have received much of the water that today makes life possible.

The source of this water is one of biggest mysteries in understanding, not only the Earth’s evolution, but also that of the Moon, Mars, and even the asteroids, says Terik Daly, a planetary scientist now at Johns Hopkins University, in Baltimore, US. 

One concern is that as these bodies formed stepwise from repeated collisions, a great deal of heat was released by each impact. And because they were smaller than the present Earth, these bodies had less gravity, which scientists thought would cause any water that reached them via such collisions to boil off into space.

But we know that the Earth has water, as do asteroids. Even the Moon appears to have water trapped within its mantle. 

To see how water might have been retained under such conditions, Daley went to NASA’s Ames Research Centre in California, where there is a giant gun that can fire .30-calibre pellets at speeds up to five kilometres per second (or 18,000 kilometres an hour). That’s slower than asteroids hit the Earth today, but about right for conditions in the early solar system – and similar to those that still pertain in the asteroid belt. 

To simulate the impacts, he used pellets of serpentine, a hydrated mineral that is about 12% water. “The same mineral carries water in meteorites,” he says. 

The pellets were fired at a target composed of bone-dry volcanic rock. Debris from the collisions was then collected and analysed.

The researchers found that despite the heat, nearly 30% of the water was retained. Some was in millimetre-scale fragments of the original rock that survived the collision, holding the water originally locked within their minerals. But more came from water vapour that became trapped in new materials created by the impact — rocks known as impact melts and breccias. 

“These are like sponges that soak up water that was vaporised, and keep it on the planet,” Daley says.

“Nature has a tendency to be more interesting than our models, which is why we need to do experiments,” adds co-author, Pete Schultz.

That said, the experiment is by no means a perfect analogue of the early solar system. 

“There is always an issue when lab experiments are scaled up to cosmic impacts,” says Kevin Zahnle, a planetary scientist at NASA Ames who was not part of the study team. 

Daly agrees. A similar question, he adds, is what happens at higher impact speeds. But one effect, he suggests, is that there would be more melt, which should mean that much of the water would still be captured and retained. 

“We’ve thought carefully about how to extrapolate to higher speeds and meter or kilometre objects,” he says. And he adds that by experimenting on a small scale, it’s possible to start to understand the dynamics of what would happen at larger scales. 

Humberto Campins, an asteroid specialist from the University Central Florida, Orlando, adds that he’d love to see the experiment re-done with materials analogous to the complex mix of materials in an actual meteorite, rather than just a single mineral. 

Still, he says, it’s a useful find because it shows that the impact melts are capturing more water than would have been anticipated. “That is an interesting experimental result that has relevance to the delivery of water to the Earth, the asteroids, and the Moon,” he says.

Daly and Schultz’s study was published earlier this is published in the journal Science Advances.