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September 20, 2012 - PASADENA, Calif. -- This perspective view of Marcia crater on the giant asteroid Vesta shows the most spectacularly preserved example of "pitted terrain," an unexpected discovery in data returned by NASA's Dawn mission. This view is a mosaic of images from Dawn's framing camera, overlain on a digital terrain model with five times vertical exaggeration.

At right is a close-up of the floor of Marcia, which contains the largest concentration of pits on Vesta. Marcia is one of the youngest craters on Vesta, with a diameter of about 40 miles (70 kilometers).

Scientists think low-speed collisions with carbon-rich meteorites left hydrated minerals on Vesta's surface. It is thought that heat generated during later, high-speed collisions with asteroid belt rocks released water that was previously bound within the hydrated minerals. This water is thought to have explosively degassed into space, leaving behind pothole-like depressions as it escaped.