Jupiter's Moon Europa
There are several moons in orbit around Jupiter, but Jupiter's moon Europa is one of the most interesting. Of the four largest moons in orbit around Jupiter, Europa is the second closest moon to Jupiter with a distance of 671,000 km. Europa is about the same size as Earth's moon, with a diameter of 3130 km. (For comparison, Earth's moon has a diameter of 3476 km.)
One of the most interesting aspects of Europa is that its surface may be completely covered by frozen water, and that underneath this layer of water ice, there may be a deep ocean of liquid water.
The reason the presence of water is so important is because scientists believe that liquid water is a necessary component of life. This means that if Europa does have water that there could be life there now, or that life might develop there at some point in the future, although scientists are not certain of this.
The closeup of Jupiter's moon Europa was taken by Voyager 2. The cracks in the surface indicate that it has been fractured and refilled with materials from Europas lower layers. Because Europa's surface lacks mountains or craters scientists believe that its surface is covered by a thick ice crust.
Europa also doesn't have many impact craters on its surface. Because recent and ongoing geological activity would erase the evidence of impact craters, their absence suggests that Europa's crust is relatively young. This indicates that Europa is probably still warm a few km below its icy surface, where its liquid water may flow.
Europa's Internal Structure
Although scientists have no way of knowing for sure what the interal structure of Europa is like, they can infer its structure from measurements of the gravity and magnetic fields that were taken by the Galileo spacecraft.
From these measurements it appears that Europa's core (the innermost layer) probably consists of iron and nickel. This metallic core is surrounded by a rocky layer. The layer of rock is surrounded by liquid water, which is surrounded by a thick layer of ice.
The Galileo spacecraft also indicated that Europa has a magnetosphere and a thin atmosphere. In fact, scientists consider Europa important enough that in 2003 when the Galileo spacecraft ran low on fuel, to avoid the possibility of Galileo accidentally crashing into Europa, they purposely crashed the spacecraft into Jupiter, where we are almost certain that no life exists (Gierasch and Nicholson, 2004).
Gierasch, Peter J., and Philip D. Nicholson. "Jupiter." World Book Online Reference Center. 2004. World Book, Inc. (http://www.worldbookonline.com/wb/Article?id=ar293080.)
Photos of Europa: Courtesy of NASA/JPL-Caltech.