How did Earth get its water?
For many years, planetary scientists thought that large comets were the likely source of water for Earth’s oceans, lakes, and rivers. But some researchers have essentially ruled out these icy snowballs for bringing water to Earth after it was discovered that some comets were far richer in heavy hydrogen than the water on our planet. The next possible suspect has been asteroids, but scientists have questioned whether any water could survive the fiery delivery of an asteroid impact to a planetary body.
But new experiments using a high-powered projectile cannon have yielded evidence that, depending on their impact angle and speed, water-rich asteroids can provide more water to planetary bodies than was originally estimated. The experimental results were published in the journal Science Advances.
"The origin and transportation of water and volatiles is one of the big questions in planetary science," said Terik Daly, a postdoctoral researcher at Johns Hopkins University who led the research while completing his Ph.D. at Brown University. "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."
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Earth's surface is seventy percent liquid water. Based on our current understanding of how the solar system formed, primordial Earth couldn’t have held on to the water that may have been part of its original makeup. Any water molecules present as the planet was forming would have evaporated or been blown off into space, as our young, unstable sun had a tendency to unleash fierce solar storms. Scientists think the surface water on our planet today must have come much later — perhaps hundreds of millions of years later.
One likely source for Earth's water is carbonaceous asteroids and a type of meteorite called carbonaceous chondrite. Isotopic measurements have shown that Earth's water is similar to water bound up in these water-rich space rocks.
Daly and Brown planetary scientist Pete Schultz used marble-sized projectiles with a composition similar to carbonaceous chondrites. Using the Vertical Gun Range at the NASA Ames Research Center, a projectile cannon blasted the material at an extremely dry target made of pumice powder. The cannon shot the projectiles at speeds around 5 kilometers per second (more than 11,000 miles per hour). The researchers then analyzed the post-impact debris with several different devices, looking for signs of any trapped water.
The team found that as much as 30 percent of the water present in the impactors could be transferred to the post-impact debris, especially when the collisions took place at speeds and angles that are known to happen frequently in our solar system.
In their paper, the researchers said that most of that water was trapped in impact melt, which is rock that has been melted by the heat of the impact and then solidifies as it cools. Water was also found in impact breccias, which are rocks made of the debris from both the impactor and the planetary body, welded together by the heat of the impact.
"The impact melt and breccias are forming inside that plume," Schultz said in a statement. "What we're suggesting is that the water vapor gets ingested into the melts and breccias as they form. So even though the impactor loses its water, some of it is recaptured as the melt rapidly quenches."
The researchers said their experiments provide some clues about the mechanism through which the water was retained. As parts of the impactor are destroyed by the heat of the collision, a vapor plume forms that includes water that was inside the impactor.
Schultz told Seeker that it would be difficult to estimate if the amount of asteroid impacts early Earth endured would account for the amount of water on Earth today because one would have to account for both input and output.
“In other words, water may be delivered by one impact but can be vaporized by another,” he said
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Schultz, who teaches geoscience at Brown University, has worked at the Vertical Gun Range for 33 years and became its principal investigator in 2012.
The facility was built in 1965 and helped scientists prepare for the Apollo moon missions. It has been used to simulate planetary collisions from all sorts of impactors — simulated asteroids, comets, and even rocket parts.
Schultz used the Vertical Gun Range to help understand data from the Lunar Crater Observation and Sensing Satellite, or LCROSS. The Centaur upper stage from the launch vehicle for LCROSS and the Lunar Reconnaissance Orbiter impacted the moon’s south pole, and instruments aboard LCROSS confirmed the presence of water in a lunar crater named Cabeus.
Asked if the projectile tests were similar to the ones used for LCROSS, Schultz said no … and yes.
“These were very different experiments designed to assess how much water that could be trapped in the products of an impact,” he said. “But our results are relevant to the LCROSS experiment because they provide a process by which water can be trapped in lunar materials (impact melt and breccias) and then later released. The released water (or OH) molecules will bounce around until trapped in the cold traps at the lunar poles.”
According to Schultz and Daly, new research has suggested that lunar water may have an asteroid origin as well, and asteroid impacts could explain other water-rich planetary bodies like the giant asteroid Vesta, which is thought to have a vast supply of internal water ice.
But Schultz said he can’t rule out comets as another water source — at least not yet.
“There are actually two comets that have the same heavy water ratio as on Earth,” he said. “The problem is that scientists haven't been able to measure enough comets and to understand the connection between heavy hydrogen and where they are in the solar system.”
He added, “But this new study demonstrates that water can be delivered by water-bearing meteoroids to asteroidal surfaces like Vesta and could have contributed to the Earth's budget.”