Europa's diameter is about 3000 kilometers. It probably has an iron core, and an atmosphere that's mostly oxygen. The surface is icy and very smooth. There are not a lot of craters, but there are some cracks and lines. Because the surface is so young and smooth, scientists believe that there is a liquid ocean under the surface, a so-called subsurface (below the surface) ocean, and that it is kept warm by tidal heating. In other words, Jupiter's strong gravitational pull on Europa makes it warm.