Jupiter has 67 known moons, although the count could go higher. One issue with deciding how many moons there are orbiting Jupiter is the question of how big an object must be to be considered a moon. Some moons are large and spherical, others are smaller and resemble asteroids. Is a ten meter long asteroid a moon? Is a ten centimeter long object a moon? There is not yet a good definition about exactly how big an object must be, to be considered a moon.
Jupiter's Galilean moons
Jupiter's four largest moons are called the Galilean moons, since they were discovered by Galileo Galilei in 1610. They were the first objects discovered in our solar system that did not directly orbit the Sun. This was an important discovery at the time, in that it proved that the Sun could not be considered the center of the universe.
The Galilean moons, as pictured above, are shown left to right increasing in orbital distance. They are Io, Europa, Ganymede and Callisto, respectively. As you can see, there is a great deal of difference in the surface appearances of these moons. The differences have to do with the tidal effects arising from the gravitational interactions between the moons and Jupiter.
Since the force due to gravity decreases as the square of the distance between the two objects, you would expect gravitational effects to be stronger for closer moons.
In some ways, Jupiter's Galilean moon system can be considered like a miniature star and planetary system. We believe that these moons formed at the same time as Jupiter.
As this animated gif shows, Io, Europa and Ganymede exhibit orbital resonance. Io completes two orbits around Jupiter in the same amount of time that Europa completes one orbit, giving these moons a 2:1 orbital resonance. Europa orbits twice for each of Ganymede's orbits, giving a 2:1 orbital resonance between Europa and Ganymede and a 4:1 resonance between Io and Ganymede, or a 4:2:1 resonance for the three moons. Callisto, the farthest out of the Galilean moons, is not in resonance with the other three.
The craters visible on the surface of Io are volcanic in origin. There are practically no impact craters to be seen. The yellow coloring of the surface is due to the composition: sulfur and molten silicate rock. Io is volcanic because of the tremendous amount of tidal flexing caused by its interaction with Jupiter.
The video above explains and illustrates the concept of tidal flexing, involving the Jupiter-Io system.
Io's orbital resonance with Europa and Ganymede gives it a periodic tug, pulling its orbit into an eccentric ellipse, with Jupiter at one focus of the ellipse. Io's rotation on its axis remains constant, but its orbital speed varies, moving faster when it is closer to Jupiter and slower when it is farther away. This had the effect of kneading the moon, greatly heating it inside, enough to support extensive volcanic activity. When impacts do happen on Io, the impact craters are quickly filled in by volcanic flows. Io is continuously resurfaced from the effects of tidal flexing.
This montage photograph of Jupiter and Io from NASA's New Horizons spacecraft highlight a volcanic plume above Io, with lava glowing red against the surface of the moon.
This image of Jupiter's second moon, Europa, shows a moon vastly different from Io. Europa is similar to Io in that it obviously undergoes constant resurfacing, as evidenced by the marked lack of impact craters. One impact crater is seen in the lower left in the photo, otherwise, Europa's surface is covered with frozen ice, criss-crossed with great cracks. We believe tidal flexing is also responsible for the resurfacing of Europa, generating heat that keeps water in a liquid state under the frozen surface. The flexing causes cracks to form, which refreeze.
This close-up image of the surface of Europa shows an area of 350 by 750 kilometers. The brown material evident in the cracks probably means that the liquid water extends down to the rocky core of the moon, where volcanic activity sends plumes upward to the surface.
Since liquid water is necessary to support life as we know it, scientists speculate that Europa is a good candidate for finding extrasolar life in our solar system.
NASA's proposed mission to Europa is scheduled to launch sometime after 2020, sending a probe on a close flyby trajectory near Jupiter's moon. Instruments onboard the probe will include an ice penetrating radar capable of determining the thickness of the surface ice, and possibly revealing subsurface lakes like Lake Vostok in Antarctica. The properties of Europa's magnetic field will be directly measured, providing clues about the extent and salinity of the ocean. A surface dust analyzer will determine the composition of small particles ejected from Europa's surface. Several imagers and spectrometers will be onboard to provide pictures in various wavelengths and analysis of the chemistry.
Liquid water is the main reason for wanting to explore Europa. Where liquid water exists, the possibility of life as we know it also exists. However, Europa lies in a region of intense radiation, which can be harmful for instruments. The planned mission to Europa would orbit Jupiter, making flybys to Europa, then fly back toward Jupiter where the radiation is less intense.
The Hubble Space Telescope has recent images showing what appear to be transient plumes of water vapor being expelled from the surface of Europa, rising about 120 miles above the surface. The images were taken in UV light when Europa was in transit, passing in front of Jupiter. This is further evidence of liquid water beneath the frozen surface of Europa. The Europa mission probe will possibly fly through these plumes, taking direct measurements of the water ice particles. Europa is known to have a thin atmosphere of mostly molecular oxygen, probably due to UV radiation from Jupiter splitting water into oxygen and hydrogen.
What comes next? NASA scientists are considering the feasibility of actually landing on the surface of Europa. One of the issues involved in making a decision like this involve the possibility of cross-contamination, unintentionally introducing microbes from Earth onto Europa.
Ganymede and Callisto
Ganymede (left) and Callisto (right) are the next two moons of Jupiter. As you can see from these photographs, Ganymede has more impact craters than Io or Europa, and Callisto is very heavily covered with impact craters. This is a direct result of the increasing distance from Jupiter, and the decreasing amount of tidal flexing and resurfacing. The surface of Ganymede shows cracking on a large scale, and we speculate that there could be liquid water beneath the surface, as in Europa.
This artist's representation of cutaway views of the Galilean moons are based on measurements of the gravitational and magnetic fields of the moons, taken by the Galileo spacecraft. We believe that all of the moons, except for Callisto, have metallic cores made of iron and nickel, surrounded by shells of rock. Io's rock layer extends to the surface, while Europa and Ganymede have layers of liquid water and water ice. Callisto has the lowest density of the moon system, and may not be differentiated inside.
This graphic displays the relative sizes of the Galilean moons. Ganymede is the largest moon in our solar system, and is not much smaller than the planet Mars.
Jupiter does have rings, as seen in this photograph taken by the Voyager 1 spacecraft, but they are nowhere near as prominent and extensive as the rings of Saturn. It is believed that the rings are a result of dust blown off of inner moons from meteor impacts.
Jupiter's rings lie inside the orbits of the Galilean moons - closer to Jupiter. Within these rings there are small moons: Metis, Adrastea, Amalthea, and Thebe. These tiny moons resemble captured asteroids.
This image of Metis, Adrastea, Amalthea and Thebe, left to right, is a composite of images taken by NASA's Galileo spacecraft. They are shown with the correct relative sizes, with Amalthea about 154 miles long.