Earth is, obviously, a very wet planet, but what's less obvious is where all that water originally came from. One suggested scenario says that asteroids delivered water to a bone-dry early Earth, but whether that would work in practice, or if the water would boil away in the heat of the impact, remained unanswered. Now, scientists at Brown University have tested the theory using a high-powered cannon, and found that a surprising amount of water is transferred in the process.

For a while, icy comets seemed like the most likely source of water on Earth, but recent studies found evidence to the contrary. On closer inspection of 11 comets including 67P/Churyumov–Gerasimenko, the deuterium-to-hydrogen ratios were very different to that of Earth's oceans.

Water-rich carbonaceous asteroids, on the other hand, were a much clearer match. But that introduces a new problem – in models, it seemed that water and other volatile materials wouldn't survive the crash.

"Impact models tell us that impactors should completely devolatilize at many of the impact speeds common in the solar system, meaning all the water they contain just boils off in the heat of the impact," says Pete Schultz, co-author of the Brown study. "But nature has a tendency to be more interesting than our models, which is why we need to do experiments."

So, the researchers set up physical experiments to test how much water, if any, could be deposited via asteroid impacts. For the study, the team used marble-sized projectiles that were made of similar stuff to carbonaceous chondrites, the water-rich meteorites that are the most likely culprits. The stand-in for the parched early Earth was a material made of pumice powder.

The scientists used the Vertical Gun Range at the NASA Ames Research Center to fire the projectiles at the target material, at speeds of about 18,000 km/h (11,000 mph). After the impact, the team then analyzed the debris to see how much water was transferred to the target.

The team found that the heat of the impact does indeed boil the water away, but rather than lose the vapor to space, it becomes trapped in the rocky debris left behind. In the intense heat of the collision, rock on both the struck surface and the meteorite melt, then cool and resolidify, creating new formations known as the impact melt and breccias. These formations were found to contain up to 30 percent of the space rock's original water content.

"What we're suggesting is that the water vapor gets ingested into the melts and breccias as they form," says Schultz. "So even though the impactor loses its water, some of it is recaptured as the melt rapidly quenches."

In the early days of the solar system, collisions between planets, protoplanets and asteroids were a far more frequent occurrence, and the Brown experiment suggests that this could explain how Earth became so watery. This might also account for water found elsewhere in the solar system, including the Moon's mantle and surface, and even in hostile environments like the poles of Mercury.

"The origin and transportation of water and volatiles is one of the big questions in planetary science," says Terik Daly, lead researcher on the study. "These experiments reveal a mechanism by which asteroids could deliver water to moons, planets and other asteroids. It's a process that started while the solar system was forming and continues to operate today."

The research was published in the journal Science Advances.

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