Freckles on Europa
In this image from NASA's Galileo spacecraft, several brown spots can be seen on the icy surface of Jupiter's moon Europa. The spots visible in this image are about 6 miles (10 km) across. They are called "lenticulae," which is the Latin term for freckles. The similar size and shape of these spots suggests that warmer ice may be welling up from the bottom of the ice shell while colder ice sinks. This process would be similar to that in a lava lamp. The long lines in the image represent cracks and fissures in the ice, possibly cause by the motion of liquid water under the ice.
NASA / JPL Image