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

Data from the orbiter DAWN confirm theories about the history of Vesta, which dates to the early days of planet formation. The protoplanet is also home to the solar system's second largest mountain.

By Staff writer / May 10, 2012

NASA's Dawn probe took this image of Vesta last year. The asteroid has now been determined to be a "protoplanet", the only survivor of a class of rocky bodies most similar to the full-sized planets Mercury, Venus, Earth and Mars.

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The one-time asteroid – now “protoplanet” – Vesta is living up to its billing as a planet wannabe with a case of arrested development.

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Data from NASA's DAWN mission to Vesta, and later to another protoplanet, Ceres, have essentially confirmed the broad outlines of Vesta's history and structure, initially inferred from meteorites thought to be chips off the old protoplanet.

But the details are providing surprises – from the unexpectedly young age of its largest impact basin to the height of the mountain that sits at the center of the basin. Measurements by the orbiter DAWN mark it as the second highest mountain, base to peak, in the solar system after Mars' Olympus Mons volcano.

“Vesta's history seems to be more similar to the rocky, terrestrial planets than to its larger sibling, the dwarf planet Ceres,” said Carol Raymond, DAWN's deputy lead scientists and a co-author of one of six research papers detailing the results in Friday's issue of the journal Science.

Formed within the first 2 million years after the sun's disk of dust and gas began to form solid clumps, Vesta is “a key witness to the events at the very beginning of the solar system,” she said at a NASA briefing Thursday. “We believe Vesta is the only intact member of a family of similar bodies that have since perished.”

The others were broken up in collisions that helped formed the asteroid belt, a broad expanse of space rubble that orbits the sun between Mars and Jupiter. Vesta and Ceres both orbit the sun among these “main belt” asteroids.

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.

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