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, and that it is kept warm by tidal heating. in other words, jupiter's strong gravitational pull on europa makes it warm.