Key Ideas

Nature of the Galilean Satellites: The four Galilean satellites orbit Jupiter in the plane of its equator. All are in synchronous rotation.

• The orbital periods of the three innermost Galilean satellites, Io, Europa, and Ganymede, are in the ratio 1:2:4. This forms an orbital resonance.
• The two innermost Galilean satellites, Io and Europa, have roughly the same size and density as our Moon. They are composed principally of rocky material. The two outermost Galilean satellites, Ganymede and Callisto, are roughly the size of Mercury. Lower in density than either the Moon or Mercury, they are made of roughly equal parts ice and rock.
• The Galilean satellites probably formed in a similar fashion to planets in our solar system but on a smaller scale.

Io: Io is covered with a colorful layer of sulfur compounds deposited by frequent explosive eruptions from volcanic vents. These eruptions resemble terrestrial geysers.

• The energy to heat Io’s interior and produce the satellite’s volcanic activity comes from tidal forces that flex the satellite. This tidal flexing is aided by an orbital resonance: the 1:2:4 ratio of orbital periods among the inner three Galilean satellites.
• The Io torus is a ring of electrically charged particles circling Jupiter at the distance of Io’s orbit.
• When Jupiter’s magnetic field sweeps over the Galilean satellites, it induces electric currents, which in turn, create an induced magnetic field.

Europa: While composed primarily of rock, Europa is covered with a smooth layer of water-ice.

• The surface has hardly any craters, indicating a geologically active history. Other indications are a worldwide network of long cracks and ice rafts that indicate a subsurface layer of liquid water or soft ice. As with Io, tidal heating and an orbital resonance is responsible for Europa’s internal heat.
• There is probably an ocean beneath Europa’s frozen surface. Minerals dissolved in this ocean may explain Europa’s induced magnetic field.

Ganymede: Two types of terrain are found on the icy surface of Ganymede: areas of dark, ancient, heavily cratered surface and regions of heavily grooved, lighter-colored, younger terrain.

• Ganymede is highly differentiated and probably has a metallic core. It has a surprisingly strong magnetic field and a magnetosphere of its own.
• While there is at present little tidal heating of Ganymede, it may have been heated in this fashion in the past. An induced magnetic field suggests that it, too, has a layer of liquid water beneath the surface.

Callisto: Callisto has a heavily cratered crust of water-ice. The surface shows little sign of geologic activity, because there was never any significant tidal heating of Callisto. However, some unknown processes have erased the smallest craters and blanketed the surface with a dark, dusty substance.

• Magnetic field data seem to suggest that Callisto has a shallow subsurface ocean.

Titan: The largest Saturnian satellite, Titan, is a terrestrial world with a dense nitrogen atmosphere and an important 5 percent methane. Temperatures and pressures on Titan are near the point where methane can be found in a solid, liquid, or gas phase.

• Titan shows evidence for flooding expected from methane rain, and many lakes near its north pole. Methane appears to work in a cycle similar to water in Earth’s hydrological cycle.

Other Satellites: As of 2012, Jupiter has a total of 67 confirmed satellites and Saturn has a total of 62.

• Beyond the Galilean satellites, Jupiter has many small satellites that move in much larger orbits that are noticeably inclined to the plane of Jupiter’s equator. Many of these orbit in the direction opposite to Jupiter’s rotation. These do not appear to have formed with Jupiter, and they are thought to be captured asteroids.
• In addition to Titan, six moderate-sized moons circle Saturn in regular orbits: Mimas, Enceladus, Tethys, Dione, Rhea, and Iapetus. They are probably composed largely of ice, but their surface features and histories vary significantly. The other, smaller moons include shepherd satellites that control the shapes of Saturn’s rings and captured asteroids in large retrograde orbits.
• Enceladus ejects plumes of water from a region near its south pole. Minerals detected in these plumes indicate a subsurface ocean in contact with a rocky mantle.