Vesta is an asteroid. So it is pretty small relatively. Enormous troughs that wrap around the giant asteroid Vesta may actually be dropped blocks of terrain bounded by fault lines, suggesting a geologic complexity beyond that of most asteroids. Since the discovery of the troughs last year in data from NASA's Dawn spacecraft, scientists have been working to determine the story behind these unusual features. The research reinforces the claim that Vesta has a core, mantle and crust, a structure normally reserved for larger bodies, such as planets and large moons. So for an asteroid it has some interesting full planetary features.
Vesta is the second-most-massive body in the asteroid belt, though only 28% as massive as Ceres. The surface area is approximately the same as that of Pakistan. Vesta orbits in the inner asteroid belt interior to the Kirkwood gap at 2.50 AU. It has a differentiated interior, and is similar to 2 Pallas in volume but about 25% more massive.
An extensive system of troughs encircles Vesta's equatorial region. The biggest of those troughs, named Divalia Fossa, surpasses the size of the Grand Canyon. It spans 289 miles (465 kilometers) in length, 13.6 miles (22 kilometers) in width and 3 miles (5 kilometers) in depth. The complexity of the troughs' morphology can't be explained by small collisions. New measurements from Dawn indicate that a large collision could have created the asteroid's troughs, said Debra Buczkowski, a Dawn participating scientist based at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., who is the lead author of a new paper in Geophysical Research Letters, a journal of the American Geophysical Union. The crustal layer at the surface appeared to stretch to the breaking point and large portions of the crust dropped down along two faults on either side of the downward-moving block, leaving the giant troughs we see today.
The scale of the fracturing would only have been possible if the asteroid is differentiated - meaning that it has a core, mantle and crust. "By saying it's differentiated," said Buczkowski, "we're basically saying Vesta was a little planet trying to happen."
Most asteroids are pretty simple. "They’re just like giant rocks in space," said Buczkowski. But previous research has found signs of igneous rock on Vesta, indicating that rock on Vesta’s surface was once molten, a sign of differentiation. If the troughs are made possible by differentiation, then the cracks aren’t just troughs, they’re graben. A graben is a dip in the surface that forms when two faults move apart from each other and the ground sinks into the widening gap, such as in Death Valley in California. Scientists have also observed graben on the Moon and planets such as Mars.
The images from the Dawn mission show that Vesta’s troughs have many of the qualities of graben, said Buczkowski. For example, the walls of troughs on simpler asteroids such as Eros and Lutetia are shaped like the letter V. But Vesta’s troughs have floors that are flat or curved and have distinct walls on either side, like the letter U — a signature of a fault moving apart, instead of simple cracking on the surface.
There are other qualities of Vesta that could be clues to how the troughs formed. For example, unlike the larger asteroid Ceres, Vesta is not classified as a dwarf planet because the large collision at its south pole knocked it out of its spherical shape, said Buczkowski. It’s now more squat, like a walnut. But if Vesta has a mantle and core, that would mean it has qualities often reserved for planets, dwarf planets and moons -- regardless of its shape.
So what is Vesta? A would be planet or dwarf planet. An asteroid? A big rock?
For further information see Troughs.
Vesta image via NASA.