Although Europa is an icy moon, evidence suggests that its composition is much like the rocky planets – an iron core surrounded by a silicate rock mantle.
The first evidence of a liquid ocean came from the Galileo spacecraft, which revealed that the moon has an induced magnetic field. In order for this to happen, there has to be a conductive layer under the surface and a salt water ocean is the most likely explanation. Although this part has been commonly accepted, there are two different possibilities for the ice layer: the thin ice model and the thick ice model.
In the thin ice model, the crust would be just a few kilometres thick and float atop the liquid ocean layer. Heat from the mantle would rise through the water and crack the crust, causing some of the striking surface formations such as the jumbled chaos terrain. More researchers agree with the thick ice model, in which the crust is somewhere between 10 and 30 kilometres (6 to 19 miles) thick. Rising heat creates a softer, warmer layer of ice underneath. These can function as glaciers, floating around and causing fractures in the hard surface. The remaining craters on Europa give some indication that the ice is thick instead of thin. They are flat on the bottom and appear to contain flat, fresh ice, which would be less likely to happen if the ocean was directly reacting with the crust.