News Column

Protoplanet Vesta ID'd as Source of Vital Asteroids

Sept. 21, 2012

Tom Beal

Dawn spacecraft. Image courtesy of NASA.

The Dawn spacecraft's recent visit to the giant "proto-planet" Vesta verified that it is the source of many hydrogen-rich asteroids that fell to Earth.

Using an instrument designed and operated for the NASA mission by Tucson-based Planetary Science Institute, scientists deduced that the oldest surfaces of the giant asteroid are covered with carbonaceous chondrites, the type of rocks that may have salted the Earth with the building blocks of life.

GRaND, the Gamma Ray and Neutron Detector, was used by a team of scientists led by Tom Prettyman, of the Planetary Science Institute, to provide an elemental map of Vesta during Dawn's 13-month orbit.

Their results were published Thursday in Science Express, the online journal of AAAS, the science society.

The instrument, which can peer up to a meter below the surface of the giant space rock, found the most hydrogen in a wide band around its equator.

Scientists knew hydrogen was present on the giant space rock (330 miles in diameter) but previously held differing theories about where it came from.

Hydrogen is present, for instance on Earth's moon, in small amounts on the surface from solar wind and trapped in shaded craters from comet impacts.

Vesta, though, has no sheltered spots on its poles, said Prettyman, and the hydrogen-abundant band is in a location where ice would quickly sublimate.

Hydrogen is also present on Mars beneath the polar surface in the form of water-ice.

Vesta is different, Prettyman said in an interview. Its hydrogen signature comes from hydrated minerals on its equatorial surface.

Scientists have long identified Vesta as the source of about 6 percent of the meteorites that fall to Earth.

"The new findings solidly confirm the connection between Vesta and a class of meteorites found on Earth called the Howardite, Eucrite and Diogenite (HED) meteorites," said a statement from NASA's Jet Propulsion Laboratory.

Prettyman said "this result will stimulate a lot of additional work in the area of meteorite studies." The relation between Vesta and the HED meteorites had long been known but this more precise measurement of the ratios of hydrogen, iron, silicon and oxygen "clinches the link," he said.

The study found little hydrogen in the massive craters on Vesta, where the hydrated minerals were scattered or ejected in collisions 1 and 2 billion years ago.

Early this month, the Dawn spacecraft left Vesta for a 2015 rendezvous with an even bigger space rock -- the dwarf planet Ceres, an icy rock about 590 miles in diameter.

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Source: (c)2012 The Arizona Daily Star (Tucson, Ariz.) Distributed by MCT Information Services

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