SUMMARY OF KEY IDEAS

• Jupiter is by far the largest and most massive planet in the solar system.

• Jupiter and Saturn have rocky cores surrounded by a thin shell of ices, then a thick shell of liquid metallic hydrogen and an outer layer of ordinary liquid hydrogen and helium. Jupiter’s core may be dissolving. Both planets have an overall chemical composition very similar to that of the Sun.

• The visible features of Jupiter exist in the outermost 100 km (62 mi) of its atmosphere. Saturn has similar features, but they are much fainter.

• Three cloud layers exist in the upper atmospheres of both Jupiter and Saturn. Because Saturn’s cloud layers extend through a greater range of altitudes, the colors of the Saturnian atmosphere appear muted.

• The colored ovals visible in the Jovian atmosphere are gigantic storms, some of which (such as the Great Red Spot) are stable and persist for years or even centuries.

• Jupiter and Saturn have strong magnetic fields created by electric currents in their metallic hydrogen layers.

• Four large satellites orbit Jupiter. The two inner Galilean moons, Io and Europa, are roughly the same size as our Moon. The two outer moons, Ganymede and Callisto, are approximately the size of Mercury.

• Io is covered with a colorful layer of sulfur compounds deposited by frequent explosive eruptions from volcanic vents. Europa is covered with a smooth layer of frozen water crisscrossed by an intricate pattern of long cracks.

• The heavily cratered surface of Ganymede is composed of frozen water with large polygons of dark, ancient crust separated by regions of heavily grooved, lighter-colored, younger terrain. Callisto has a heavily cratered ancient crust of frozen water.

• Both Jupiter and Saturn have rings. Saturn is circled by a system of thin, broad rings lying in the plane of the planet’s equator. Each major ring is composed of a great many narrow ringlets that consist of numerous fragments of ice and ice-coated rock. Jupiter has a much less substantial ring system.

• Saturn’s moon Titan has a thick atmosphere of nitrogen, methane, and other gases, as well as lakes of methane and ethane.

• Saturn’s moon Enceladus has areas with very different surface features: an older, heavily cratered region and a newer, nearly crater-free surface created by tectonic activity.

• Uranus and Neptune are quite similar to each other in appearance, mass, size, and chemical composition. Each has a rocky core surrounded by a thick, watery mantle topped by a layer rich in normal hydrogen and helium; the axes of their magnetic fields are steeply inclined to their axes of rotation; and both planets are surrounded by systems of thin, dark rings.

• Uranus is unique in that its axis of rotation lies near the plane of its orbit, producing greatly exaggerated seasons on the planet.

• Uranus has five moderate-sized satellites, the most bizarre of which is Miranda.

• Triton, the largest satellite of Neptune, is an icy world with a tenuous nitrogen atmosphere. Triton moves in a retrograde orbit that suggests it was captured into orbit by Neptune’s gravity. It is spiraling down toward Neptune and will eventually break up and form a ring system.