Europa is slightly smaller than Io, having a diameter of 3100 km, about the size of Earth's moon. Europa orbits Jupiter at a distance of 671,000 km with a period of 3.6 days. Like Io, Europa is high in density at 3.03 g/cm3 indicating it too has a lot of rocky material in its interior. Europa's surface is covered with a thick mantle of liquid water and a thick icy crust.

     Europa has a complex surface. Much of it is covered with grooves and lines typically 25 km wide and a thousand or so kilometers long. These lines appear to be cracks in the surface where water below has welled up and refrozen. Around these areas there are no craters which indicates a very young surface, one that has covered up whatever craters there were. In a few small areas, there are craters and pitted areas that seem to be very old surface.

     The Galileo spacecraft passed within 692 km (430 miles) of Europa on December 19, 1996. Images and data revealed a world that appeared quite geologically active. The moon seems to show tectonic activity, similar to the Earth. Sections of the crust "float" on an apparent liquid mantle, There are deep grooved areas which seem to have been pulled apart, quite different from the tectonics seen on Ganymede.

     Even more striking was the evidence for ice volcanoes. There were obvious ice flows which point to water volcanoes or geysers. Although no active volcanoes have been found, it would not surprise anyone to find one or more. Because there are no craters around the ice flows, this area is quite young.
 
 

     In this image taken by Galileo, ice flows can be seen which are similar to the ice flows seen around Earth's poles. The layer of water ice is probably about 100 km (about 60 miles) thick and probably some of it is water. Motion of the plates has caused icy water and probably some rocky material, to well up between the boundaries.

This images shows a recent crater that impacted the icy crust. The crater is about 30 km (18.5 miles) in diameter. The whitish material is debris strewn up by the impact. The dark band extending east-west across the image is named Belus Linea. This feature is characterized by a bright stripe down the middle with diffuse outer bands. The curving x pattern indicates fracturing of the icy crust and upwelling of slushy material which then froze.

     We will learn much more about this strange moon as the Galileo spacecraft makes another passby in November of this year.