Vesta shows new type of weathering

Posted by Kate Taylor

The highly-patterned surface of the giant asteroid Vesta is caused by a long history of strikes from hundreds of other, smaller asteroids.

Data from NASA's Dawn mission show that a form of weathering that occurs on the moon and other airless bodies in the inner solar system doesn't alter Vesta's outermost layer in the same way.

Meanwhile, carbon-rich asteroids have also been splattering dark material on Vesta's surface over a long span of the body's history.

"Dawn's data allow us to decipher how Vesta records fundamental processes that have also affected Earth and other solar system bodies," says Carol Raymond, Dawn deputy principal investigator at NASA's Jet Propulsion Laboratory.

"No object in our solar system is an island. Throughout solar system history, materials have exchanged and interacted."

Over time, soils on Earth's moon and asteroids such as Itokawa have undergone extensive weathering, with an accumulation of tiny metallic particles containing iron, which dulls the fluffy outer layer.

But Dawn's visible and infrared mapping spectrometer (VIR) and framing camera haven't found these on Vesta, leaving it bright and pristine.

However, the bright rays of the youngest features on Vesta degrade rapidly and disappear into the soil, as frequent, small impacts continually mix the fluffy outer layer of broken debris.

Vesta also has a much steeper topography than other large bodies in the inner solar system, leading to landslides that further mix surface material.

Early pictures of Vesta showed a variety of unexpected dramatic light and dark splotches on Vesta's surface, which make it one of the brightest rocky bodies in our solar system.

An initial theory was that the dark patches were geological in origin. But they're widely distributed, and don't show any relation to the underlying geography. The most likely source now seemsto be the carbon-rich material in meteoroids, which are also believed to have deposited hydrated minerals from other asteroids on Vesta.

To get the amount of darkening we now see on Vesta, scientists on the Dawn team estimate about 300 dark asteroids with diameters between 0.6 to 6 miles likely hit Vesta during the last 3.5 billion years. This would have been enough to wrap the proto-planet in a blanket of mixed material about three to seven feet thick.

"This perpetual contamination of Vesta with material native to elsewhere in the solar system is a dramatic example of an apparently common process that changes many solar system objects," says Vesta team member Tom McCord. "Earth likely got the ingredients for life - organics and water - this way."

Launched in 2007, Dawn spent more than a year investigating Vesta. It left this September, and is now on its way to the dwarf planet Ceres.