What Formed the Strange Troughs on Vesta?

New research suggests that the humble asteroid Vesta may have cracked open like an egg.

NASA’s Dawn probe was launched in September 2007 to study the two largest members of the asteroid belt, Ceres and Vesta. But it didn’t have to work hard to get there.

“When we think of asteroid belts, we probably picture Han Solo maneuvering the millennium falcon through a dense set of irregularly shaped gray rocks in space,” according to Christian Klimczak, associate professor in the Franklin College of Arts and Sciences department of geology. “While most rocks are indeed irregularly shaped and gray, they are far apart and NASA’s Dawn spacecraft did not have to maneuver around any other asteroids to reach and explore Vesta.”

Vesta is particularly intriguing among the asteroids because it’s large enough to have an iron core, a mantle, and a crust, not unlike the Earth. That’s why astronomers consider Vesta to be a planetesimal: an almost-planet. “Vesta was on the way to becoming an Earth-like planet, too, but planet formation stopped along the way there early in the history of our solar system,” Klimczak said. “Therefore, studying Vesta helps us understand the very early days of our planetary neighborhood and how our own planet formed.”

Except things didn’t go as awesomely for Vesta as it did for the Earth. For one, the little asteroid was struck by two other large asteroids that managed to leave impact craters so big that they cover almost its entire southern half. By studying Vesta closely, researchers like Klimczak try to understand the evolution of the planetesimal.

“Rock properties are influenced by environmental conditions like surrounding stresses and the presence of water,” said Jupiter Cheng, doctoral candidate in the department of geography and a co-author on a new study of Vesta along with Klimczak. “Since Vesta is much smaller than Earth, or even the moon, it has a weaker gravity, and rock would deform differently near the surface than what we see on Earth.”

Cheng and Klimczak are especially interested in two large troughs which surround the impact basins. Planetary scientists had long assumed that the troughs were created by the impacts, but that relationship had never been firmly established.

“Our work used crater counting methods to explore the relative age of the basins and troughs,” Cheng said. The fewer the craters a region has, the younger it is, and so by comparing the number of craters between the trough and the basins, the researchers can explore their relative age.

“Consequently, counting the number of craters of various sizes in a given area allows us to determine how long they have accumulated and, consequently, how long ago the surface formed,” she said. “Our result shows that the troughs and basins have a similar number of the crater of various sizes, indicating they share a similar age. However, the uncertainties associated with the crater counts allow for the troughs to have formed well after the impacts.”

But how exactly did the troughs form? “The leading hypothesis suggests that these troughs are fault-bounded valleys with a distinct scarp on each side that together mark the down-drop (sliding) of a block of rock. However, rock can also crack apart and form such troughs, an origin that has not been considered before,” said Cheng.

“Our calculations also show that Vesta’s gravity is not enough to induce surrounding stresses favorable for sliding to occur at shallow depths, instead, the physics shows that rocks there are favored to crack apart,” she said. “Therefore, the formation of these troughs must involve the opening of cracks, which is inconsistent with the leading hypothesis in the scientific community. Taken all together, the overall project provides alternatives to the previously proposed trough origin and geological history of Vesta, results that are also important for understanding similar landforms on other small planetary bodies elsewhere in the solar system.”

Whatever happened to little Vesta, it was downright nasty.