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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.

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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.

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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.

“The relatively young ages for these basins stand in stark contrast to what we find on the moon,” he says. All of the impact basins there are at least 3.5 billion years old. This indicates that the collision environment each inhabits “is surprisingly different." The story the difference tells is complicated, he says, one “we're only beginning to unravel.”

Sitting in the middle of the Rheasilvia Basin is a central mound that rises roughly 16 miles above the basin floor. Measurements of the mountain show it take up one-third of the crater's diameter.

“At first this was very surprising,” O'Brien says. Large craters on the moon or on other bodies don't look like Rheasilvia. But craters on moons in the outer solar system can have similar structures. In essence, he says, such craters form when the size of the crater is almost as large as the body it sculpts. He estimates that the impact excavated enough material to fill the Grand Canyon 1,000 times over – or roughly 250,000 cubic miles of debris.

Much of that fell back to repave Vesta's southern hemisphere, where no crater beyond Venenian's is older than 1 billion years.

That leaves Vesta's northern hemisphere as the repository for information on its geological history reaching farther back in time. In late April, DAWN scientists received approval from NASA to remain at Vesta for an extra 40 days before heading for a rendezvous with Ceres in February 2015. Up to now, sunlight has illuminated the southern half of Vesta. With the extension, the sun will increasingly shine on the northern hemisphere, giving the team a chance to extend their data to the once-dark side of the protoplanet.

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