News in Science

Vesta reveals its watery secrets

Asteroid discovery Astronomers say they're surprised to discover the surface of the giant asteroid Vesta is covered in hydrogen.

The findings, reported in two papers in the journal Science, are painting a new picture of the distribution of water through the early solar system.

The unexpected discovery was made by NASA's Dawn spacecraft, which is studying Vesta and Ceres, the two most massive objects in the asteroid belt between Mars and Jupiter.

The lead author of one paper, Dr Thomas Prettyman from the Planetary Science Institute in Tucson Arizona, says the findings indicate Vesta was bombarded by meteors called carbonaceous chondrites.

Prettyman and colleagues believe these water-rich meteors are the most likely way for hydrogen to reach the asteroid.

"We were really excited to see the results, which were a complete surprise," says Prettyman.

"The hydrogen was found on some of the asteroids oldest surfaces."

"Younger regions such as the Rheasilvia basin had the lowest concentrations, indicating it had been buried or blown away by the impact, which formed the basin."

Prettyman and colleagues say the observations are consistent with the gradual accumulation of hydrogen caused by carbonaceous chondrites.

Serendipity

The hydrogen was discovered while Prettyman and colleagues were gathering data on another class of meteorite linked to Vesta, known as Howardite, Eucrite and Diogenite or HED meteorites.

"Our measurements confirmed a link between Vesta and HED meteorites which are thought to originate from the asteroid," says Prettyman.

"The eucrites would have flowed out onto Vesta's surface, the diogenites would have been in the lower crust and the howardites were surface dirt and rocks, melted and ejected by other meteorite impacts."

"This means laboratory studies of these meteorites will give us a complete picture of how Vesta evolved through differentiation to form a core, mantel and crust."

Pitted Terrain

The second paper in Science reports the discovery of irregular rimless depressions in and around impact craters on Vesta, which are also being linked to the hydrogen discovery.

Scientists found the pitted terrain in high resolution images taken by Dawn during a 210 kilometre high, low altitude mapping orbit.

They believe the unusual terrain was caused by hydrated minerals that vaporised soon after a meteor impact on the surface.

Water-rich carbonaceous chondrites may be the source of these hydrated minerals.

Prettyman says the findings are great news.

"I'm really excited to see the results after years of development of the instruments and spacecraft," says Prettyman.

"We launched in 2007, now it's 2012 and the investigation of the first target is complete."

Dawn began its five billion kilometre odyssey on 27 September 2007 arriving at Vesta in July 2011.

After departing Vesta earlier this month, it is now on a two and a half year journey to the dwarf planet Ceres.