Asteroid Vesta's hydrogen suggests water-delivery role

It's no aqua-world, but the giant asteroid Vesta is surprisingly rich in one watery ingredient ? hydrogen. The discovery, combined with its oddly pitted terrain, suggests that water arrived on young planets ? including early Earth ? during an intense round of meteor impacts.

The 530-kilometre-wide Vesta is unusual among asteroids because it's thought to be the seed of a terrestrial planet that didn't finish forming.

"Vesta is an example of such a world ? as Earth once was ? frozen in an embryonic state," says Mark Sykes, an astronomer at the Planetary Science Institute (PSI) in Tucson, Arizona, who wasn't involved in the new studies.

The object therefore offers clues to the earliest stages of planet formation in our solar system.

Using data from NASA's Dawn spacecraft, PSI's Thomas Prettyman and colleagues compared the chemical composition of Vesta's surface with that of howardite-eucrite-diogenite, or HED, meteorites.

Dawn's spectral maps show more hydrogen in regolith, near the asteroid's equator, and less in relatively young impact basins, including the large crater Rheasilvia, from which many HED meteorites probably originated.

Hits and pits

"Vesta, like our moon, was thought to be bone dry, and yet we find this material that has been distributed all over Vesta's surface," says Prettyman. The HED meteorites, meanwhile, contain traces of carbon-rich chondrites, some of which hold significant amounts of water-bearing minerals.

The team thinks hydrogen was delivered to Vesta though a swarm of carbonaceous chondrite meteors. These rocks hit at slow enough speeds that their hydrous content was preserved on Vesta's surface. Later, high-speed impacts ejected some of the hydrogen-rich surface material, leaving behind the erratic pattern.

The idea is supported by 30- to 500-metre-wide pits seen in Dawn's images of some smaller craters, according to a second study led by Brett Denevi of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland.

Denevi's team thinks the pits formed when some of the impacts caused volatile compounds ? also found in carbonaceous chondrites ? to degas due to the high temperatures.

Journal reference: Science, DOI: 10.1126/science.1225354 and 10.1126/science.1225374

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