High-powered cannon reveals how Earth may have got its oceans

Scientists have shed light on how water ended up on Earth by firing marbles at speeds of 11,000mph into a makeshift planet.

Exactly how water was transported in the early solar system is one of planetary science’s major mysteries. Previous research has pointed to icy comets, which would have crashed into a bone-dry Earth and delivered what became our planet’s oceans. Yet this hypothesis doesn’t match with isotopic measurements, which suggest the composition of Earth’s water is instead similar to that bound up in asteroids.

If asteroids were responsible for delivering water to planets, moons and other asteroids, the specific mechanism has remained unclear. How, for example, do you deliver water without it simply boiling off on impact?

A study at Brown University, published in Science Advances, attempts to elucidate this question – with the help of the high-powered Vertical Gun Range at NASA’s Ames Research Center. The researchers first made marble-sized projectiles that were designed to imitate so-called carbonaceous chondrite meteorites, derived from ancient asteroids. They then fired these marbles into dry pumice powder at speeds of 5km/s and analysed the post-impact debris for signs of water.

(Footage from the Hypervelocity impact experiments. Credit: Terik Daly)

They discovered that up to 30% of the “meteorite’s” water was trapped in the debris, particularly in rock melted by the heat of the impact, and rock made of impact debris that has been welded together, known as breccias. These melts and breccias form inside a vapour plume, caused by the heat of the collision.

“Previous studies concluded that the high energy of the impact would vaporise all the water,” Peter Schultz, co-author of the paper and a professor in Brown’s Department of Earth, Environmental and Planetary Sciences, told Alphr. “But these experiments demonstrated that, even at high speeds, water can be trapped inside impact glasses and breccias, as well as small fragments of water-bearing pieces of the impactor as well.

“This means that while Earth was forming, objects containing water need not lose their water during the collision. Instead, it gets trapped in the byproducts of the collision and builds up as the Earth accreted.”

As well as hinting at how water found its way to Earth, this mechanism could hold the key to understanding the process for water being transported to other planets, and even our own moon. This could have significant implications for modeling the development of our solar system.

“We know Mars has water,” Schultz explains. “So, this process may explain how Mars collected its water very early on. In addition, this process could explain some of the water (or hydroxyls) found on the moon and asteroids, such as Vesta.  It’s not that we would find carbonaceous chondrites; rather, the water that’s inside could be trapped in impact materials.”

The team will next look at various places on the moon where this process could explain the presence of water trapped in debris, known as ejecta, and will also test the process at different scales – such as tiny impacts. Whether or not all of this is an excuse to fire more things out of a high-powered cannon, it could help us understand the origins of water on our world.

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