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Planetary Pretender: Asteroid Vesta Has Planet-Like Features

NASA's Dawn spacecraft is getting up close and personal with the giant asteroid, revealing rift valleys, mountainous uplifts and a belt of grooves near its equator



NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

NANTES, France—Asteroids are often considered debris, the scraps and odd lumps that went unused in the forming of the planets. But when it comes to Vesta, one of the largest asteroids in our solar system, Chris Russell hardly considers the rock a mere castoff. "I've started calling it the smallest terrestrial planet," said Russell, the principal investigator for NASA's Dawn mission, which sent a spacecraft into orbit at Vesta in July.

Russell and his colleagues gave a sense Monday of why they hold Vesta in such high regard. In a press conference here at a joint meeting of the American Astronomical Society Division for Planetary Sciences and the European Planetary Science Congress, researchers working on the Dawn mission announced some of the findings that the spacecraft has collected since entering orbit—findings that make Vesta look considerably more like a world unto itself than a mere leftover. "We found that Vesta is a really interesting alien, small world," said the mission's deputy principal investigator, Carol Raymond of the NASA Jet Propulsion Laboratory in Pasadena, Calif.

Vesta is an irregular ellipsoid just 560 kilometers in diameter—too small and odd-shaped to qualify as a dwarf planet. But it seems to have a large metallic core and a basaltic crust, just like Earth. And it bears scars from past geologic shake-ups, most likely triggered by large impacts, including rift valleys, mountainous uplifts and an intriguing belt of grooves near its equator. "It has tectonic features, like on Earth," Russell said.

The asteroid made a particularly interesting target for Dawn because the space rock carries evidence of the history of much of the solar system as well. Vesta-derived meteorites that landed on Earth allowed planetary scientists to measure the asteroid's age in the laboratory, showing that it is one of the oldest large bodies in the solar system. (Because it is small relative to the terrestrial planets, it would have cooled and solidified much more quickly than those bodies.) As such, Vesta provides a valuable lab for studying the materials and processes that formed the planets during the first millions of years in the early solar system.

It now appears that the past four billion years have been quite an interesting time in Vesta's evolution as well. In fact, the asteroid boasts a mountain that Earth cannot match in terms of altitude. A giant peak at the asteroid's south pole, which is currently unnamed, rises roughly 20 kilometers from its base to its summit, about twice the height of Mauna Kea in Hawaii, the tallest mountain—from its base on the ocean floor to its peak—on Earth.

"The south polar mountain on Vesta is very, very large—almost as large as the largest mountain in the solar system, Olympus Mons," Russell said, referring to the towering, roughly 25-kilometer volcanic peak on Mars. "We have not figured out the tectonic cause of the south polar mountain, so that's work ahead of us."

The mountain rises from the center of a large impact basin, but Raymond cautioned that it was too soon to definitively say that the peak is simply a crater with an impact-uplifted center. A large impact in the south billions of years ago did leave one clearly identifiable mark, blanketing the southern hemisphere in ejecta and filling nearby craters. A preliminary chronology analysis based on crater counts across the asteroid shows that something reset the clock on the southern hemisphere long ago. "Ages from some areas in the south appear to be much younger, as much as a billion years younger, than the north," Raymond said.

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