How a giant impact formed asteroid Vesta's 'belt'

 

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A massive collision: A high-speed camera recorded a laboratory simulation of colliding heavenly bodies. An analysis of shock propagation suggests what may have caused the tilted canyon-like grooves around the equator of the asteroid Vesta.

Collisions of heavenly bodies generate almost unimaginable levels of energy. Researchers at Brown used NASA's ultra-high-speed cannon and computer models to simulate such a collision on Vesta, the second-largest object in the asteroid belt. Their analysis of the images -- taken at a million frames per second -- shows how Vesta may have gotten the deep grooves that encircle its midsection.

When NASA's Dawn spacecraft visited the asteroid Vesta in 2011, it showed that deep grooves that circle the asteroid's equator like a cosmic belt were probably caused by a massive impact on Vesta's south pole. Now, using a super high-speed cannon at NASA's Ames Research Center, Brown University researchers have shed new light on the violent chain of events deep in Vesta's interior that formed those surface grooves, some of which are wider than the Grand Canyon.

"Vesta got hammered," said Peter Schultz, professor of earth, environmental, and planetary sciences at Brown and the paper's senior author. "The whole interior was reverberating, and what we see on the surface is the manifestation of what happened in the interior."

The research suggests that the Rheasilvia basin on Vesta's south pole was created by an impactor that came in at an angle, rather than straight on. But that glancing blow still did an almost unimaginable amount of damage. The study shows that just seconds after the collision, rocks deep inside the asteroid began to crack and crumble under the stress. Within two minutes major faults reached near the surface, forming deep the canyons seen today near Vesta's equator, far from the impact point.

The research, led by Angela Stickle, a former graduate student at Brown and now a researcher at the Johns Hopkins University Applied Physics Laboratory, will appear in the February issue of the journal Icarus and is now available online.

"As soon as Pete and I saw the images coming down from the Dawn mission at Vesta, we were really excited," Stickle said. "The large fractures looked just like things we saw in our experiments. So we decided to look into them in more detail, and run the models, and we found really interesting relationships."

For the study, the researchers used the Ames Vertical Gun Range, a cannon with a 14-foot barrel used to simulate collisions on celestial bodies. The gun uses gunpowder and compressed hydrogen gas to launch projectiles at blinding speed, up to 16,000 miles per hour. For this latest research, Schultz and his colleagues launched small projectiles at softball-sized spheres made of an acrylic material called PMMA. When struck, the normally clear material turns opaque at points of high stress. By watching the impact with high-speed cameras that take a million shots per second, the researchers can see how these stresses propagate through the material.

The experiments showed that that damage from the impact starts where one would expect: at the impact point. But shortly after, failure patterns begin to form inside the sphere, opposite the point of impact. Those failures grow inward toward the sphere's center and then propagate outward toward the edges of the sphere like a blooming flower.

Using numerical models to scale the lab collision up to the size of Vesta, the second-largest object in the asteroid belt, the researchers showed that the outward-blooming "rosette" of damage extending to the surface is responsible for the troughs that form a belt around Vesta's equator.

The results answer some questions about Vesta's belt that had long been puzzling. Chief among them is the orientation of the belt with respect to the crater. The belt's angle isn't exactly what would be expected if it were caused by the Rheasilvia impact.

"The belt is askew," Schultz said, "as if Vesta were making a fashion statement."

These new experiments suggest that the crooked belt is the result of the angle of impact. An oblique impact causes the damage plane to be tilted with respect the crater. The orientation of Vesta's belt sheds light on the nature of the impact. The researchers conclude that the object that created Rheasilvia came in at an angle less than 40 degrees, traveling at about 11,000 miles per hour.

"Vesta was lucky," Schultz said. "If this collision had been straight on, there would have been one less large asteroid and only a family of fragments left behind."

The research shows that even a glancing blow can have tremendous consequences.

"When big things happen to small bodies," Schultz said, "it shakes them to the core."

Astrophysicists to probe how early universe made chemical elements

 

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ASU astrophysicists have received an important research grant to study how massive stars, such as Eta Carinae, depicted here, evolve and eventually seed the universe with heavy elements created by nuclear reactions inside them.

In the beginning, all was hydrogen -- and helium, plus a bit of lithium. Three elements in all. Today's universe, however, has nearly a hundred naturally occurring elements, with thousands of variants (isotopes), and more likely to come.

Figuring out how the universe got from its starting batch of three elements to the menagerie found today is the focus of a new Physics Frontiers Center research grant to Arizona State University's School of Earth and Space Exploration (SESE). The grant is from the National Science Foundation's Joint Institute for Nuclear Astrophysics -- Center for the Evolution of the Elements. Of the full $11.4 million NSF grant, about $1 million will come to ASU over five years.

SESE astrophysicist Frank Timmes is the lead scientist for ASU's part of the Physics Frontiers Center research project. Timmes, ASU's director of advanced computing, focuses his astrophysical research on supernovae, cosmic chemical evolution, their impacts on astrobiology and high-performance computing. He is also a scientific editor of The Astrophysical Journal.

The evolution of elements project also includes Michigan State University in Lansing (the lead institution), the University of Notre Dame in South Bend, Indiana, and the University of Washington in Seattle.

Joining Timmes on the project will be astrophysicists Patrick Young, Evan Scannapieco and Sumner Starrfield, also from the School of Earth and Space Exploration In addition, the award will fund two postdoctoral researchers to collaborate on the effort.

Take it from the top

Time started 13.7 billion years ago with the Big Bang, which produced the basic three elements. Yet by the time the Bang was a billion years old, essentially all the other chemical elements we know had formed. How did this happen?

"It takes place inside stars," says Timmes. "They're the element-factories of the universe. They take light stuff, such as hydrogen and helium, process it in nuclear reactions, and then crank out carbon, nitrogen, oxygen and all those good things that make you and me."

While the broad outline is clear, details are a lot murkier, he says, and that's where ASU's researchers enter the picture.

"ASU's contribution is to provide the glue between experimental low-energy nuclear astrophysics measurements and astronomical observations of stars," Timmes says.

Ancient stars were fundamentally different from those today, he notes, because they started off with a different collection of initial ingredients -- no heavy elements. But those first-generation stars are gone.

As Timmes explains, "The stars that began back then went through their life cycles and died, so we naturally don't directly see them today. But when they died, they exploded and threw out little bits of carbon, oxygen and nitrogen, which ended up in the next generation of stars."

Round and round in cycles

In a process that still continues today, massive stars create more and more complex elements, then explode as supernovas and scatter the newly created elements into space for another generation of stars to use. Cycle after stellar cycle, stars became steadily richer in heavier and more complex elements.

The sun, its planets and moons all formed about 4.5 billion years ago. Most of the elements they contain didn't exist when the universe was young, so what generation does the sun belong to?

Timmes explains, "A typical massive star, in round numbers, lives about a million years. The Big Bang occurred about 7 billion years before the sun formed. I need a thousand generations of massive stars to get us to a billion years, so I need on the order of 10,000 generations of massive stars to get one with the sun's composition.

"We are the product of many, many, many previous generations of stars."

The researchers at the School of Earth and Space Exploration plan to develop computer models of stars of all sizes, masses and chemical compositions, then set them on their life courses. It's building stars in computers and comparing them to observations of stars to see how the universe builds them for real.

"The toughest theoretical problem we have to work on is how stars explode," says Timmes. "In a loose, hand-waving sense, we know that stars explode, of course, but exactly how it happens isn't well-known or understood."

The new research project fits well with the expertise of the school's astrophysicists. And there's another plus as well. With this project, ASU is joining a small group of research centers that deal with "Frontiers Physics." The entire country has only about ten such centers, Timmes explains. Highly competitive and highly sought-after, they cover subjects such as biological physics and theoretical physics.