The Galilean Moons
Io
Diameter: 3,650km
The tremendous gravitational pull of Jupiter on this innermost of the four Galilean moons, together with its closeness to the planet, means Io whizzes round Jupiter in just 1.75 Earth days. This fast orbital speed is easily seen in a small telescope: it visibly shifts position in just a few hours. Physically, Io is the most volcanic place in the entire Solar System. The whole world is covered in sulphurous lava flows and volcanoes erupting in plumes more than 500km high.
Eurpopa
Diameter: 3,140km
The second Galilean moon out from Jupiter, Europa, should theoretically be visible with the naked eye since it shines at magnitude +5.3. But Jupiter’s overwhelming brightness makes it difficult to separate the moon from the planet. Europa’s brightness is due to its surface being smooth and icy. Scientists suspect that underneath is a liquid water ocean, leaving open the possibility that life may lurk in the depths.
Ganymede
Diameter: 5,260km
The third major moon out from the planet is not only Jupiter’s biggest, but it is also the largest moon in the entire Solar System. This is a world with a cold ice surface, a large warm ice (possibly water) mantle, a rocky interior and a liquid iron core. It measures a tremendous 5,260km across, which is bigger than Mercury. Indeed, if Ganymede was released into space, it would be classed as a planet.
Callisto
Diameter: 4,820km
The last of the four giant Galilean satellites is Callisto. It is the third largest of the Solar System, after Titan, the biggest of Saturn’s moons. Callisto ranks as one of the most cratered worlds known – its entire icy, ancient surface is covered with impact craters that date right back to the time of the early Solar System, when the moon formed. Like Europa, it is thought that beneath the surface may lie a watery ocean.