Galileo discovered these four satellites of Jupiter in 1610, which revolutionized the study of astronomy and our view of ourselves, since it confirmed that the entire Universe didn't revolve around Earth. Furthermore, the Galilean Moons have captured the public imagination when it was discovered that they contain liquid water, and possible extra-terrestrial life. Read on...
Distance from Jupiter: 262,000 miles
Orbit period: 1.77 days
It's hard to imagine anyplace being more poisonous or inhospitable to life than Io. Unlike most satellites in the Solar System, Io's surface is made of silicate rock (it is not covered in a sheet of ice). Covered in over 400 volcano systems, sulfur geysers blast plumes of poisonous smoke miles into the air. Its surface is covered with flows of molten lava, several longer than 500 km. This satellite is so close to Jupiter that they interfere with eachother's magnetic fields. Beginning in 1973, Io has been visited five times by unmanned space probes.
Distance from Jupiter: 417,000 miles
Orbit period: 3.55 days
Covered in ice and probably some liquid water, Europa's surface is the smoothest in the Solar System. It also has a nearly circular orbit, with an eccentricity of only 0.009. Its thin oxygen atmosphere and water oceans have led to conjecture that it may host microscopic sea life, ensuring a high profile for the moon's exploration. It is believed Europa may have a sub-surface ocean kept warm by tidal heat. Plans to visit, as part of NASA's Jupiter Icy Moons Project, were cancelled due to budget cuts in 2005.
Distance from Jupiter: 1,170,000 miles
Orbit period: 16.69 days
The surface is heavily cratered, leading scientists to believe this satellite is extremely old. Callisto hosts craters over 100 km in diameter, and its Valhalla Region contains a chain of craters over 1500 km long. It has never been warm enough to melt its ice component, but beneath the crust lies a salt ocean. Callisto has a strange interior that is not entirely uniform.
Distance from Jupiter: 665,000 miles
Orbit period: 7.15 days
Ganymede is the largest natural satellite in the Solar System, and one of the only visible with the naked eye other than our own moon. Like most satellites, it is composed of rock and water ice. Its craters are quite flat, lacking the peaked rings as on the moon. In fact, its suface appears to be wrinkly. Recently, the Hubble telescope discovered ozone (the gas that protects our planet from Solar radiation) in its atmosphere.
Want to check out more photos from the Hubble telescope? Click here!