Vesta, the solar system's brightest asteroid, used to be round until it got pounded by another space rock. The collision not only carved out a huge crater and left Vesta somewhat flattened, but it also cracked opened deep rifts around the asteroid's equator, some of them wider than the Grand Canyon.
To understand how Vesta's belt of fractures formed, scientists tried to recreate this violent impact in a lab and watch it slow motion, by shooting beads at softball-sized spheres with a high-speed cannon.
Scientists recently got a close look at the craggy surface of Vesta, the second most massive body in the asteroid belt, thanks to NASA's Dawn probe. Between 2011 and 2012, the spacecraft orbited the 330-mile-wide (530 kilometers) protoplanet. [Photos: Asteroid Vesta and NASA's Dawn Spacecraft]
"Vesta got hammered," Peter Schultz, a professor of earth, environmental, and planetary sciences at Brown University in Rhode Island, said in a statement. "The whole interior was reverberating, and what we see on the surface is the manifestation of what happened in the interior."
To get a better idea of how that reverberation played out, Schultz and colleagues turned to a massive cannon at the NASA's Ames Research Center in Moffett Field, California.
Dubbed the Ames Vertical Gun Range, the cannon has 14-foot (4.2 meters) barrel that's used to mimic celestial collisions, launching projectiles up to 16,000 mph (25,750 km/h) in vacuum chamber. For these experiments, Schultz's team shot tiny Pyrex beads at larger spheres that were about 4 inches (10 centimeters) in diameter and made of PMMA, a brittle, transparent acrylic material. PMMA is a good substitute for rock because it has mechanical properties similar to that of Earth's upper crust and it becomes opaque under stress, which allows researchers to track the damage.
Images taken with superfast cameras that capture a million shots per second show that the damage starts at the impact point. But then cracks start to form in the sphere's interior, opposite the impact point, the researchers said. These cracks extend to the sphere's center and grow outward toward the sphere's surface, in a pattern that looks like a blooming "rosette," according to the study. The edges of these "petals" are responsible for Vesta's deep canyons.
The experiments also shed light on the orientation of those canyons, which circle Vesta like a belt.
"The belt is askew, as if Vesta were making a fashion statement," Schultz said. That flashy placement suggests the impact object struck at an angle less than 40 degrees, at about 11,000 mph (17,700 km/h).
"Vesta was lucky," Schultz added. "If this collision had been straight on, there would have been one less large asteroid and only a family of fragments left behind."
The study was led Angela Stickle, a former graduate student at Brown, who is now a researcher at the Johns Hopkins University Applied Physics Laboratory. The results have already been published online and will appear in the February 2015 issue of the journal Icarus.