While we live on an impressively wet planet today, the Earth probably didn’t start out with our lush collections of rivers, lakes and oceans. With no obvious way for the planet’s iron and other minerals to have spontaneously transmuted into H2O billions of years ago, scientists have long suspected that our water was instead delivered from space via icy objects like comets. Further research found that our water looked more like the water found on icy asteroids like Ceres, which was clearly abundant in the asteroid belt, but it still posed a problem. Asteroids tend to burn and explode a lot when they get too close to the Earth, so how would any of their water survived long enough to help soak our planet?
With no asteroids or spare planets at their disposal, researchers from Brown University turned to the Vertical Gun Range at the NASA Ames Research Center to simulate icy impactors. Marble-sized projectiles were fabricated to match the composition of carbonaceous chondrites, or meteorites suspected of being formed in ancient, icy asteroids. The miniature proxy-meteors were then fired at a chunk of dry pumice powder, which served as their stand-in for the once-parched surface of the Earth.
The small impactors hit their targets at more than 11,000 miles-per-hour, releasing heat and an impressive amount of debris in all directions. As with real asteroid impacts, enough heat was generated in these collisions to outright destroy some material, including some of the water ice. However, some of the mineral content also melted, which weirdly enough was key to some of the water’s survival. Because the rock melted and re-cooled so quickly, it could capture some of the water inside the resulting glass, keeping it “safe” from vaporizing. Additional water was similarly captured by flying breccias— random debris thrown and heated by the impact that were hot enough to become “welded” together.
Making sense of previous predictions
This experiment not only helps explain some of the water on Earth, but also some of the confusing H2O around the solar system. Since previous estimates found that asteroid impacts should vaporize water, researchers had a hard time explaining the presence of water in impact craters on places like the Moon and Mercury. The physical test proved that those estimates hadn’t captured the full complexity of such an impact, and that delivering water via screamingly-fast hunks of icy rock is apparently more practical than you might think.
My five-year-old asked: What does the Vertical Gun Range look like?
“Gun” may be a misleading term here, because the equipment in question doesn’t really look much like pistol, rifle or cannon, at least outside science fiction. A large barrel launches a projectile into an enclosed, reinforced chamber. That chamber is outfitted with a number of sensors and cameras so that researchers can learn more details about the behavior of whatever collision is being studied. NASA has more on the AVGR in this handy PDF.
Source: Projectile cannon experiments show how asteroids can deliver water by Brown University, Phys.org