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by Ian Cameron

    Juniper has 16 fairly well-known moons, and probably a number of minor ones. The four largest are Callisto, Europa, Io and Ganymede. The 12 others, which are much smaller, are Adrastea, Amalthea, Ananke, Carme, Elera, Himalea, Leda, Lysithea, Metis, Pasiphae, Sinope and Thebe. All are named after people who, according to myth, has been seduced by Jupiter, whom the Greeks credited with a startling degree of virility.

    Only the four largest can be seen through a small telescope or binoculars. Eight of the moons, including the four largest, orbit fairly near Jupiter itself, and were probably formed at the same time as Jupiter, from material left over after it had formed (about four-and-a-half billion years ago). The other moons are much further away from Jupiter, and are probably pieces of interplanetary debris that were captured by its gravity at a later time.

    What we know about the four major moons comes mainly from the incredible photographs taken by the Voyager spacecraft which passed near them in the 1970s. Probably the most dramatic sequence of images transmitted by any of the space explorers was of the innermost moon, Io. By coincidence, the spacecraft happened to pass by just as an enormous volcanic eruption took place, throwing a plume of debris hundreds of kilometres above its surface. The reason why volcanos occur on Io is that it is so close to the enormous mass of Jupiter that the tidal forces caused by its gravity keep the whole interior of Io in a molten state.

    Europa, being further away, is largely frozen, but the tidal forces are still large enough to keep the interior partly molten. The result is that the surface of Europa looks like a cracked eggshell, with a complicated system of cracks all over it. The other two large moons are so far out that they are permanently frozen. The surface of Callisto, for example, is covered with huge craters formed by meteorites which crashed into it billions of years ago.

Additions by David Christie

     Galileo discovered and named 4 moons (Io, Europa, Ganymede, Callisto) orbiting Jupiter. (As discoverer he got to pick the names.) These are the ones we can see from Earth with binoculars or ordinary spotting scopes.

    By 1970, eight additional small moons, ranging from 15 to 150 miles in diameter (the biggest of them is Amalthea) had been discovered using observatory telescopes. That has now been increased to 16 by recent space probes. They have also found dust rings around Jupiter.

     The following web page lists 16 known satellites or moons of Jupiter and a lot of other information about the planet: http://seds.lpl.arizona.edu/nineplanets/nineplanets/jupiter.html