Around 500 million years ago, something awful seems to have happened on Venus. Maybe in spurts or maybe all at once, a fury of volcanism paved over nearly the entire surface. Some scientists think Earth's planetary sister could have supported life for billions of years, yet scarcely a trace now remains of that lost world. To fathom why a planet would have done such a thing to itself, researchers need to know its inner torment.
"Unraveling the mystery of why terrestrial planets evolve the way they do really requires that we understand the interior structure of Venus," says planetary scientist Ellen Stofan of Proxemy Research, an institute based in Laytonsville, Md. "This question links strongly back to the whole issue of why Earth is habitable and Venus apparently not."