Discovered by Galileo in 1610, bright, yellow Europa is the smallest of Jupiter’s moons. Its surface is very smooth. The dark lines in this picture—taken by the Voyager spacecraft from a distance of 19 million kilometers—are superficial cracks. Although Europa’s crust is thought to be icy, its interior is hot. One theory suggests that Europa’s interior used to be hot enough to melt the crust, but over time the surface cooled and became hard and smooth.