Jupiter has at least 16 moons. In 1610, Galileo and Marius
independantly discovered the first four moons: Io, Europa, Ganymede, and
Callisto. This discovery was the first evidence of another body besides
Earth being the center of motion in a planetary system.
Marius named the moons after various figures in Greek mythology. Io was a beautiful maiden whom Zeus had an affair with. So was Europa. So was Callisto. Zeus was quite the lady's god. Ganymede was a handsome young Trojan boy whom Zeus made a cupbearer to the gods.
Much of what we know about the Galilean moons comes from the Voyager1, Voyager2, Pioneer10, Pioneer11, and Ulysses missions, as well as the Galileo spacecraft which is currently exploring Jupiter and its moons. Also the Hubble Space Telescope has been used to gather data on the Galilean sattelites.
Io is the innermost Galilean moon. It is slightly larger than
Earth's moon. Io is similar to the terrestrial planets in
composition as it appears to be made up of silicate rock with an
iron core. The Galileo spacecraft determined that Io's core is about
half its diameter in length. Patches of sulphur and sulphur dioxide frost
have been discovered on Io's surface.
Io is very young and active. We know Io is young because it has very few craters. Instead of craters, Io is covered with calderas which are craters formed by eruptions or the collapse of volcanic vents. This picture shows some calderas which exist on Io's south pole. Io is very active because of the gravitational forces exerted upon it by Jupiter, Europa, and Ganymede. Io is constantly stretched and contorted by these tidal forces causing it to generate heat. Although Io is much smaller than Earth, it generates twice as much heat.
Io's orbit cuts across Jupiter's magnetic field lines, generating an electrical current of around 3 million amps. Jupiter's magnetic field also strips away material from Io as the moon oribts the planet. This material forms a torus of intense radiation around Jupiter. Particles escaping from this torus are one reason why Jupiter has such a large magnetosphere. Recent data from the Galillleo spacecraft suggests that Io may have its own magnetic field. If so, Io would be the only satellite in the solar system to have its own magnetic field. No one is sure yet what might be the cause of Io's magnetic field. The Galileo space probe lead to the discovery of this possible magnetic field by measuring a large drop in Jupiter's magnetic field strength near Io.
Io has a very thin atmosphere made up of sulphur dioxide and possibly some other gases as well.
All of the Galilean moons are believed to have a water ice surface except for Io. The most likely explanation for Io's lack of water ice is that Io is closer to Jupiter than all the other Galilean moons. Thus billions of years ago when Jupiter was still cooling and raditing energy, any water ice that may have been on Io was blown away. The other Galilean moons retained their water ice surfaces because they were far enough away from Jupiter to not be affected by Jupiter's radiation.
Europa is the second innermost of the Galilean moons. It is
slightly smaller than Earth's Moon. It is similar in composition
to the terrestrial planets but less dense, which probably due to a
lack of an iron core. Europa's surface is
extremely smooth - unlike anything else in the solar system. It has
very few craters which leads researchers to believe that like Io,
Europa is very young. No one is exactly sure how old Europa's
surface is, however.
Europa appears to have a surface made of water ice. Many researchers believe that below Europa's surface there may be a deep sea of liquid water. If so it would be one of the few places in the solar system where liquid water exists.
One of the striking features of Europa's surface is the dark streaks which criss-cross the planet. One possible cause of these streaks is the expansion and contraction as water beneath the surface freezes and thaws, pushing and pulling against the ice surface of the planet. This process couldn't account for all the surface streaks that are observed, however.
Europa has a very thin oxygen atmosphere (about 1e-11 bar). This atmosphere is not caused by organic processes. Rather it appears to be caused by sunlight and charged particles hitting the liquid ice surface of the satellite. This produces water vapor which is further broken down into oxygen and hydrogen. The hydrogen escapes leaving only oxygen.
Ganymede is the largest of Jupiter's moons. It is third
innermost of the Galilean moons. It is also the largest satellite
in our solar system. It is larger than Mercury but only half as
massive. It is believed to consist of a rocky core with a mantle of
water or ice and an ice surface.
Ganymede has two distinct surface types. seems older, darker, and highly cratered. The southern hemisphere has younger, lighter regions with a lot of grooves and ridges. These grooves and ridges are of a tectonic nature but their origin is unknown. Craters are seen on both types of terrain but they are far more prevalent to the north. The craters seem to be between 3 and 3.5 billion years old as are the grooves and ridges seen on the southern hemisphere.
The Hubble Space Telescope has observed that there is a thin ozone atmosphere on Ganymede. The origin of this ozone layer is uncertain but it is probably caused by high energy ions hitting the water ice surface of Ganymede and somehow forming ozone.
Callisto is the outermost of the Galillean moons. It is the
second largest of Jupiter's moons, being slightly smaller than
Mercury and only one-third its mass. Callisto is probably composed
of a rocky core surrounded by a large mantle of water or water ice
and a water ice surface. Its
is covered entirely with craters and there is no evidence of tectonic
Some of Callisto's larger craters are surrounded by concentric rings which probably were once huge cracks that have been smooothed over by millions of years of slow ice flow. These types of craters are named multi-ring basins. The largest one of these on Callisto is called Valhalla.
Another interesting surface feature of Callisto is . Gipul Catena is a long line of impact craters probably caused by an object that was broken apart by Jupiter's gravitational field before it impacted with Callisto.