Astronomy Picture of the Day

Explanation: What mysteries might be solved by peering into this crystal ball? This crystal ball is quite unusual because it is actually a moon of Jupiter, the crystals are ice-crystals, and the ball is not only dirty and opaque but cracked beyond repair. Nevertheless, speculation is rampant that oceans exist under these tortured ice-plains that could support life. Europa, the smallest of Jupiter's Galilean moons, was photographed last month in natural colour by the robot spacecraft Galileo, now in orbit around Jupiter. The brown patches are what one might think: dirt -- tainting an otherwise white ice-crust. Europa, nearly the same size as Earth's Moon, similarly keeps one face toward its home planet. The hemisphere of Europa shown above is the one that always trails. Why is Europa's surface the smoothest in the Solar System? Where are Europa's craters?