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NASA mission confirms: Ex-asteroid Vesta is a planet that almost was

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Astronomers have matched telescopic data on Vesta's mineral composition to that of a class of chemically similar meteorites known by its initials HED and a group of asteroids known as Vestoids. DAWN's close-in analysis confirms the Vesta-HED meteorite link, notes Harry McSween, a researcher at the University of Tennessee who heads the mission's surface-composition working group.

Detailed gravity measurments show Vesta as a layered body with an iron core, a mantle made of silicate rocks, and a crust largely consisting of volcanic rock known as basalts – essentially the same structure as the four inner planets – Mercury, Venus, Earth, and Mars – as well as the moon. The iron core is about 140 miles across and accounts for about 18 percent of Vesta's mass.

These measurements confirm modeling estimates for Vesta based on analyses of the HED meteorites. This means scientists can be more confident about the inferences they make on conditions in the early solar system based on the meteorite studies, Dr. Raymond says.

Although Vesta has emerged intact from the relentless collisions that formed the asteroid belt, it still bears the geological scars from collisions. Indeed, Vesta is serving as a record book detailing the collision history within the solar system, says David O'Brien, a researcher at the Planetary Science Institute in Tucson, Ariz., and another member of the DAWN team.

Researchers have built a catalog of some 2,000 craters wider than 2-1/2 miles, including seven large impact basins – hard to spot because the surface has been so worked-over by lesser, but more recent impactors.

Detailed maps built from DAWN's data show two enormous impact basins – Rheasilvia and Venenian – scarring a large swath of Vesta's southern hemisphere. Rheasilvia's basin measures about 300 miles across, with the Venenian basin spanning some 200 miles. All on a protoplanet only about 325 miles in diameter. By counting craters within the basins, researchers estimate that Rheasilvia is only about 1 billion years old. Venenian, which partially underlies Rheasilvia, is at least 2 billion years old, Dr. O'Brien says.

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