The king of the planets maintains a lot of attendants at his court. Jupiter has 67 known moons, more than any other planet.
Sixteen of these were known before 1980, some from Earth-based observations and some from the Voyager mission. In the past few years, the revolution in digital imaging for astronomy has revealed many new moons, and many more might not have been found yet.
The four largest moons of Jupiter (from closest in to farthest out) are Io, Europa, Ganymede, and Callisto. They're named for the four lovers of the Greek god Zeus (known to the Romans as Jupiter). All four were discovered by Galileo in 1610 and are known as the Galilean moons. They were independently discovered by the German astronomer Simon Marius, who gave them their names, in 1611.
The smallest of the four Galilean moons, Europa has a very thin oxygen atmosphere and a mass two-thirds that of Earth's moon.
Europa's surface is a bright, smooth layer of frozen water, and fractures in the ice are visible everywhere—just the sort of structures seen in sea ice on Earth.
Jupiter's tidal gravity might heat the interior of Europa, an idea first proposed by John Lewis in the 1970s. So below all the ice might be a deep ocean of liquid water! Many scientists think Europa is one of the bodies in our solar system most likely to support extraterrestrial life.
Europa can be seen from Earth with binoculars or a small telescope, but don't expect to pick out any surface details. As with all of the Galilean moons, Europa can cross the surface of Jupiter—also known as a transit—pass behind the planet, and even cast a visible shadow on the planet's surface.
Io, the innermost of Jupiter's four Galilean moons, orbits the giant planet every 1.8 days and is about the same size as Earth's moon.
Almost no craters are visible on its surface because Io is a hotbed of volcanic activity, flowing with eruptions of sulphur compounds. The same forces that keep Europa's interior liquid keep Io's geology active: tidal heating from the ever-changing distances and angles between Io and Jupiter.
Volcanic cones cover five percent of Io's surface. The Voyager 1 space probe found eight of them to be active during its brief observations alone.
You wouldn't want to smell the place—plumes of sulfur dioxide ejected from the volcanoes can rise 70 to 280 kilometers (45 to 175 miles) above Io's surface. The average surface temperature on Io is minus 152 degrees Celsius (minus 242 Fahrenheit), but can be up to 318 degrees Celsius (604 Fahrenheit) near active volcanoes.
As elsewhere in the solar system, young surfaces are bright surfaces. Were it not outshone by Jupiter, Io would be bright enough to be seen from Earth with the naked eye. As it is, you'll need binoculars or a small telescope, and even with a telescope, no surface details can be seen.
As with all of the Galilean moons, Io can transit the surface of Jupiter, pass behind the planet, and even cast a visible shadow on the planet's surface.
Ganymede is the largest of Jupiter's Galilean moons and the most massive moon in the solar system—twice the mass of Earth's moon. It's even larger than both Mercury and Pluto.
The surface of Ganymede is icy and heavily cratered, with contrasting areas of dark and light terrain. The dark parts are older and have more craters. One theory holds that the light terrain is ice with lots of fractures, which make it more reflective. Ganymede has a very thin atmosphere, but that atmosphere is mainly oxygen.
If not for Jupiter's dazzling glare, you could pick out Ganymede with your naked eyes.
Callisto is the outermost—and second-largest—of the Galilean moons. It has the oldest, most heavily cratered surface of any body in the solar system, meaning it must be a dead world with no geologic activity.
Callisto can be seen with binoculars or a small telescope. It appears featureless, but it's worth the effort to try to observe it.
Although there are many, the other moons of Jupiter are very small. The two largest are Himalia and Amalthea, both of which have diameters just less than 200 kilometers (125 miles).
These smaller satellites are spread out over a wide orbit-scape from close to Jupiter's cloud-tops to almost out of its gravitational reach. The closest is Metis, only 128,000 kilometers (80,000 miles) from Jupiter's center; the most distant is the moon called S/2003 J23, 24 million kilometers (15 million miles) away.
All the recently discovered moons are outside the orbit of Callisto, the outermost Galilean moon. These "new" moons are the smallest known satellites of a major planet, with diameters between two and six kilometers (1.2 and 3.7 miles).
Most of them orbit at steep angles to Jupiter's equatorial plane. Some of these orbit the "wrong way" against the direction of rotation of the bulk of the solar system—these are called retrograde orbits. Astronomers believe these moons are captured asteroids.