The four Galilean moons of Jupiter are Io, Europa, Ganymede, and Callisto. Although Jupiter now has 63 known moons, these four are the largest and best known, having been first discovered by Galileo Galilei in 1610. These are among the largest and heaviest moons in the solar system, and some are larger than both the dwarf planet Pluto and the true planet Mercury. Their discovery represented a final blow to the Ptolemaic system of geocentrism: clearly not all celestial objects could revolve around the Earth, if some revolved around other planets.
- Callisto -
Callisto is the second-largest of the Galilean moons, smaller only than Ganymede. It is also almost exactly the same size as the planet Mercury. Unlike the other three Galilean moons, Callisto has a distant orbit from Jupiter, circling the planet once for every 17 days on Earth, in contrast to Io, which makes the trip in less than two days. Like all of the Galilean moons, Callisto is tidally locked with Jupiter: that is, Jupiter's gravitational force means that the same surface of Callisto is always pointing toward Jupiter's surface below. The same process of tidal locking has occurred with Earth's own moon: we always see the same side of the Moon's surface.
Like the other moons, Callisto is made up of a mix of water ice (with the possibility of a small, underground liquid ocean somewhere) and a rocky core. Unlike Europa or Ganymede, it is not considered a strong candidate for the evolution of life in such an ocean. However, because it is much farther from Jupiter, radiation on Callisto is far lower than on the other Galilean moons. For this reason, if manned spacecraft ever tried to reach this part of the solar system, they would likely attempt to land on Callisto first. Both the Voyager probes and the Galileo probe were able to extensively map the surface of Callisto, something the earlier Pioneer probes failed to accomplish.
- Europa -
At 1900 miles along the equator, Europa is the smallest of the Galilean moons, and slightly smaller than our own Moon. It makes its trip around Jupiter once every 3.5 Earth-days. Along with Saturn's moon Titan, Europa has quickly become one of the most intriguing and attractive objects in the solar system in the search for life. This is because space probe missions have confirmed that Europa's "surface" is actually a worn, fissured layer of water ice, below which is a massive internal ocean probably stretching over the entire moon. Only below this is the iron core characteristic of the rocky planets and the Galilean moons.
It is unclear what, if any, life would develop in a massive subterranean ocean. However, scientists have speculated that if life could evolve around the deep-sea vents on the bottom of Earth's ocean, it might do the same under comparable conditions on Europa. Although the Galileo spacecraft studied Europa's surface in some detail, NASA has been attempting for years to arrange the funding for a dedicated mission to study the conditions on this mysterious moon. The Europa Jupiter System Mission, if it is eventually launched (current plans are to do so in about a decade), would accomplish this mission.
- Ganymede -
Ganymede is the largest moon in the solar system: at nearly 3300 miles in diameter, it is almost the size of the planet Mars. It completes one orbit in roughly one Earth-week, and, like all the Galilean moons, is tidally locked with Jupiter. It is made up of a combination of a large layer of ice surrounding a much smaller iron core. Unique among moons, Ganymede's iron core means that it possesses its own magnetic field, which interacts with Jupiter's far stronger field in bizarre and unpredictable ways.
When the Galileo spacecraft passed by Ganymede, scientists were startled to find that the probe's studies of the magnetic field implied the existence of a a subterranean ocean. Unlike that of Europa, Ganymede's is far deeper and smaller. Nevertheless, studying the interior composition of the moon would be an important goal for a future mission to Jupiter.
- Io -
Io, the fourth-largest moon in the solar system, is the most volcanic object in our solar system. Hundreds of volcanoes dot the surface, many higher than any mountain on Earth. In part due to the tidal effects of orbiting nearby Jupiter, Io has been heated up to the point that sulfur plumes regularly explode from its surface, climbing hundreds of miles into space as a result of the moon's weak gravity. (Earth's much stronger gravity acts to hold down volcanic plumes.) Every time one of these massive eruptions occurs, a small amount of this material actually escapes Io's gravity, pulled away by Jupiter. For this reason, Io is perpetually surrounded by a vast cloud of sulfur and other particles which have been pulled away from Io and are gradually floating away under gravitational pressure from Jupiter.
Io travels very quickly, thanks to its close proximity to Jupiter's immense gravity well. Whereas Earth's moon makes a rotation in one month, Io speeds around the far larger Jupiter in just over 42 hours - less than two days. Like all the other Galilean moons, it is tidally locked with Jupiter. All of the major space probes which have studied Jupiter - the Pioneer and Voyager twin probes, and Galileo - have also studied Io. The last of these, Galileo, was able to confirm that the moon has a molten iron core, similar to that of Earth.