It鈥檚 no aqua-world, but the giant asteroid Vesta is surprisingly rich in one watery ingredient 鈥� hydrogen. The discovery, combined with its oddly pitted terrain, suggests that water arrived on young planets 鈥� including early Earth 鈥� during an intense round of meteor impacts.
The 530-kilometre-wide Vesta is unusual among asteroids because it鈥檚 thought to be the seed of a terrestrial planet that didn鈥檛 finish forming.
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鈥淰esta is an example of such a world 鈥� as Earth once was 鈥� frozen in an embryonic state,鈥� says , an astronomer at the Planetary Science Institute (PSI) in Tucson, Arizona, who wasn鈥檛 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鈥檚 Dawn spacecraft, PSI鈥檚 and colleagues compared the chemical composition of Vesta鈥檚 surface with that of howardite-eucrite-diogenite, or HED, meteorites.
Dawn鈥檚 spectral maps show more hydrogen in regolith, near the asteroid鈥檚 equator, and less in relatively young impact basins, including the large crater Rheasilvia, from which many HED meteorites probably originated.
Hits and pits
鈥淰esta, like our moon, was thought to be bone dry, and yet we find this material that has been distributed all over Vesta鈥檚 surface,鈥� 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鈥檚 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鈥檚 images of some smaller craters, according to a second study led by of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland.
Denevi鈥檚 team thinks the pits formed when some of the impacts caused volatile compounds 鈥� also found in carbonaceous chondrites 鈥� to degas due to the high temperatures.
Journal reference: Science, DOI: and
