All the planets in the solar system (except Mercury and Venus) have satellites. At least one asteroid, 532 Herculina, has also been found to have a companion orbiting it. Altogether, there are at least 61 principal planetary satellites, some of which are bigger than the planet Pluto. Most satellites orbit in the same direction as the parent body’s axial spin; those that do not were probably captured by the planet after its original satellite system formed. The first planetary satellite to be observed was our moon, but it was not until Galileo’s observations in the early seventeenth century that astronomers realized that the earth is not unique in having a companion satellite.
The satellites of Mars
The two Martian satellites were found in 1877 by the American astronomer Asaph Hall. Named Phobos and Deimos, after Mars’s mythological companions Fear and Terror, they were known only as faint points of light until the 1970’s, when the American Mariner probes took measurements of them. The largest dimension of Phobos is 14 miles (23 kilometers), whereas that of Deimos, which is also ellipsoidal, is only about 6 miles (10 kilometers). Both are irregular in shape and deeply pitted with craters. Observations by the Viking orbiter missions of 1975/1976 showed both had undergone considerable bombardment over their lifetimes; the largest crater on Phobos, Stickney, is 6 miles (10 kilometers) across. Their densities are similar, at twice that of water. The irregularity in shape of these two satellites, together with their small size and density, suggests that both are captured asteroids. Phobos is the only satellite in the solar system whose orbital period is less than the rotation period of its parent planet.
The satellites of Jupiter
In 1610, the four largest Jovian satellites were discovered independently by Galileo and the German astronomer Simon Marius; they are now collectively called the Galilean satellites. In order outward from the planet they are lo, Europa, Ganymede, and Callisto. They range in diameter from 1,942 miles (3,126 kilometers)— the size of Europa, which is slightly smaller than our moon—to 3,278 miles (5,275 kilometers), the diameter of Ganymede, which is larger than Mercury and probably the largest satellite in the solar system.
lo has been described as the nearest one can get to hell in the solar system. When the American probe Voyager 1 passed by Jupiter in 1979, it discovered that lo’s surface is covered in sulfur. The satellite is volcanically active and sends plumes of sulfurous material into space, which forms a torus around Jupiter. The absence of cratering suggests that the surface is regenerated by the volcanic deposition of sulfur over a period of only a million years.
The activity of lo probably results from heat caused by the tidal forces acting on lo generated by Jupiter, Europa, and Ganymede. The whole surface of the satellite heaves up and down by as much as 330 feet (100 meters) every 1 j days, lo is thought to have a molten silicate core, and it is from this central region that plumes of material are shot hundreds of miles above the satellite’s surface. The surface is also continually bombarded and eroded by the high-energy particles within Jupiter’s magnetosphere.
All the Galilean satellites are embedded in the magnetosphere. By passing through the lines of Jupiter’s intense magnetic field, an electric current of 5 million amperes is created. This flows between lo and the planet along a flux tube, which may be responsible for auroral displays on Jupiter.
The next satellite out from Jupiter, Europa, is completely different from the other Galilean satellites and is one of the smoothest objects in the solar system. It has a bright surface covered by a brownish network of fissures, with few craters of more than 30 miles (50 kilometers) across (which implies a relatively young surface). The fissures indicate an expansion of the ice crust that makes up Europa’s surface, and it is believed that beneath this crust lies a silicate mantle.
The surface of Ganymede, the third of the Galileans out from Jupiter, appears to be much older, with far more craters in its thick ice crust, which is itself split into tectonic plates. These surface features suggest that the satellite’s interior was once more active than now, with considerably more heat in its mantle. The ice crust is frozen to a depth of about 60 miles (100 kilometers), and beneath it may be a layer of convecting water or soft ice surrounding a silicate core.
The outermost of the Galileans is Callisto, which is slightly smaller than Ganymede, being 2,995 miles (4,820 meters) across. It or
bits at a distance of 1,170,000 miles (1,883,000 kilometers) from Jupiter. Callisto’s surface consists of a dark slurry that probably froze on formation, and is now churned up only by micrometeoroid impacts. Callisto seems to have been severely bombarded when it was quite young—one impact has left a crater ring system that is 1,900 miles (3,000 kilometers) in diameter. Since those early impacts, little else has occurred to alter the appearance of the rocky ice crust, which extends to a depth of several hundred miles, and, like Ganymede’s, probably covers a convecting water mantle.
The next Jovian satellites were discovered in 1892. Amalthea, Jupiter’s innermost moon (excluding smaller bodies found by the Voyager missions), was also photographed by the Voyager craft and found to be a dark red, rocky object resembling an asteroid, with a large crater in the surface. Its red color may be due to the emanations of Io. All the other satellites are comparatively minor objects, up to 106 miles (171 kilometers) across. The outermost four orbit Jupiter in a retrograde direction on complex paths greatly affected by the sun. They are all probably captured asteroids. In addition to these major satellites, there are many smaller bodies orbiting Jupiter.
The satellites of Saturn
The largest member of Saturn’s family of 18 satellites, Titan, is 3,190 miles (5,140 kilometers) in diameter and orbits 759,000 miles (1,221,000 kilometers) from the planet. It is the only satellite in the solar system to possess a substantial atmosphere, consisting almost entirely of nitrogen. Because of its distance from the sun, Titan’s surface is cold—about —274° F. ( — 170° C)—and may be covered by oceans of liquid methane. It is also believed that Titan has an icy crust and a mantle that extends to a depth of 530 miles (850 kilometers), which covers a core of low-density rock.
Of Saturn’s other major satellites, the innermost is Mimas, which is 240 miles (390 kilometers) across and heavily cratered. One impact has left a huge depression in it—almost one-third of its diameter. The next satellite out is Enceladus, which appears as a smooth sphere 310 miles (500 kilometers) across. Tethys appears to have a large gouge across its surface. It is 650 miles (1,050 kilometers) across and seems to be covered in ice mixed with some other, perhaps stony, material.
Dione and Rhea are two of Saturn’s satellites that are fairly comparable; Dione is 700 miles (1,130 kilometers) across, and Rhea’s diameter is 950 miles (1,530 kilometers). The Voyager probes found them both to be heavily cratered, and both have icy crusts. Rhea’s surface is crisscrossed with light streaks of material produced by impact-generated fractures, and one hemisphere of Dione shows similar features.
Of the outer three satellites—Hyperion, Iapetus, and Phoebe—Iapetus is the most interesting because its brightness varies by a factor of six as it orbits Saturn. The fluctuation results because one hemisphere is covered by much darker material than the other, whose surface is reflective ice. The outermost satellite, Phoebe, orbits a very eccentric path in the opposite direction of Saturn’s rotation; it is probably a captured asteroid. Saturn also has nine other satellites that are much smaller.
The satellites of Uranus
Voyager 2 circled around Uranus during January 1986, and the planet is now known to have 15 satellites. Most of these are quite small and are believed to orbit around the planet’s equatorial plane at a high inclination to the ecliptic (98°). The satellites are named for characters from Shakespeare’s plays: Miranda, Ariel, Um-briel, Titania, Oberon, Puck, Portia, Juliet, Cressida, Rosalind, Belinda, Desdemona, Cordelia, Ophelia, and Bianca. Miranda is only 300 miles (480 kilometers) in diameter and therefore very faint (magnitude 16.5). Miranda probably has a density 1.3 times that of water, and consists of a crust of water-ice with a rocky core. Such characteristics are common to Ariel, Umbriel, Titania, and Oberon. The last is at a distance of 362,600 miles (583,500 kilometers) from Uranus. Titania is the largest member, with a diameter of 990 miles (1,590 kilometers). None of these satellites has an appreciable atmosphere, because their masses are too low, and the temperature at that distance from the sun is about -333° F.(-203° C).
The satellites of Neptune
Uranus has a satellite system that is highly regular, but Neptune’s system shows signs of a catastrophe having taken place at some stage in its history.
By far the largest of its eight satellites, Triton has a highly inclined, retrograde orbit that is unstable. The satellite is due to break up as it approaches Neptune closely in a few hundred million years from now. It is about 1,700 miles (2,720 kilometers) across, has a density about twice that of water, and orbits at a distance of 221,750 miles (354,800 kilometers) from Neptune. Triton appears to be the coldest place in the solar system; its surface temperature is about —400 F. ( — 206 C). Voyager 2s images revealed a bizarre landscape of canyons, peaks, and craters, resembling the skin of a cantaloupe. Some scientists believe this strange geology may be due to violent eruptions of ice volcanoes at Triton’s south polar cap. Triton is the only moon in the solar system to have auroras.
Another of Neptune’s satellites, Nereid, is an exceptionally faint (19th magnitude) aster-oidal body, which was discovered by Kuiper in 1949. It follows the most elongated path of any satellite. Nereid is about 3.4 million miles (5.5 million kilometers) from its parent planet.
For many years, the physical characteristics of Pluto were largely unknown, because of its distance from earth. The discovery in 1978 of a satellite, Charon, enabled some of the questions to be answered. It appears that Pluto and Charon form the most closely-matched planetsatellite pair in the solar system, with Charon’s mass being about one-sixteenth that of Pluto, and its diameter about one-third. Charon has a magnitude of 17. It orbits just 12,500 miles (20,100 kilometers) from Pluto in a retrograde orbit, and its orbital period is the same as the rotational period of Pluto, so the same hemisphere of Pluto always faces Charon. Tidal interactions are also likely to have locked Charon in such a way that it keeps the same face toward Pluto at all times. Pluto and Charon thus form the only known completely synchronous pair in the solar system. Apart from its orbital characteristics, very little is known about the satellite.