Jupiter's Moons

Europa and Io, I believe, hold promise for the future of sustaining life.

The following is taken from this website:

http://filer.case.edu/sjr16/jupiter_moons.html

These are the four Galilean satellites. They are named so because they were the only four moons that Galileo was able to see. They are also the largest of Jupiter's moons. From top-left in a clock-wise direction, they are Ganymede, Callisto, Europa, and Io.

If Ganymede were not bound to Jupiter, it would be considered a planet in its own right. It is actually bigger than Mercury, being 390 km (234 miles) larger in diameter. It has heavily cratered dark regions, with lighter expanses in-between. Geologists think that it used to have plates, like the Earth, but they froze together soon after Ganymede's birth.

Callisto, the outermost of the Galilean moons, is almost an exact twin of Mercury in size and appearance. Every square mile is covered with craters or other signs of bombardment. Other than that, there are no distinct characteristics.

Europa, closer to Jupiter than Ganymede, is the smoothest natural body in the solar system. It resembles a billiard ball until seen very close-up. At that distance you can start to see dark, deep, and narrow cracks. In scale, though, the relief is no bigger than a line on a billiard ball made with a felt-tipped marker. Geologists think that Europa has liquid water underneath the icy surface - and possibly life.

Io, closer yet, is commonly compared to a pizza. Its volcanoes make it the most active world in the solar system. They spew out the sulfuric acid that gives Io its many colors. They also make Io one of the only three moons with an atmosphere in the solar system. Saturn's Titan and Neptune's Triton are the other two moons. Io is similar in size and composition to our moon. Io is caught in the middle of a tug-of-war between Jupiter and the other moons. Tension has melted the interior and raised the surface temperatures so high that scientists calculated that it generates the most heat for its size of any body in the solar system, except for the sun.