Satellites Of Planets

Type of physical science: Astronomy; Astrophysics

Field of study: Planetary systems

All but two of the planets in the solar system have one or more smaller bodies in orbit around them. These satellites are important for what they tell scientists about the origins of the main planets and the evolution of the solar system.

Overview

Of the nine major planets in the solar system, all but two (Mercury and Venus) have at least one satellite. There are more than fifty-five satellites in the solar system, satellites ranging in size from about 15 kilometers to more than 5,200 kilometers in diameter (see table). Their surface composition varies from dense and rocky, such as Earth's moon, to thin and icy, such as the moons of Saturn and other outer planets. Satellites are important scientifically for the clues they provide toward understanding the origin of their parent planets and the solar system in general.

Most models of planetary formation start with some variation on the "nebular hypothesis" devised by the German philosopher Immanuel Kant in 1755 and later modified by Pierre-Simon Laplace, a French mathematician, in 1796. This hypothesis in modern form allows that the sun, planets, satellites, and smaller debris in the solar system started as a rotating flattened cloud of gas. This rotating cloud became unstable eventually and broke into rings, as material concentrated in the bulging center of the cloud became compressed, eventually igniting as the sun. The gaseous rings cooled eventually, allowing precipitation of solid crystals that aggregated to form rocks. These rocks accreted eventually into larger and larger bodies. The largest of these bodies became the major planets, with smaller bodies generally falling into the planets adding to their mass. Under certain circumstances, small bodies were captured as satellites.

Like the major planets, satellites vary in composition depending upon their relative distance from the sun. Satellite composition can be expressed in terms of two major components: ice and rocky silicate material. Satellites in the inner solar system (Earth's moon, for example) are composed almost exclusively of rocky material, whereas those orbiting Jupiter and beyond are mixtures of ice and rocky material. Thus, satellite density decreases in the outer solar system.

The distribution of ice and rock among satellites parallels the preponderance of rocky planets in the inner solar system (Mercury through Mars) and gas-rich planets in the outer solar system (Jupiter through Neptune; Pluto is composed of nearly all ice). From this distribution, it can be noted that "volatile" substances (those with low melting and boiling points) are concentrated in the outer solar system, while "refractory" substances (those with high melting and boiling points) occur in greater abundance in the inner solar system. Satellites in the solar system reveal a considerable diversity in terms of surface composition and physical characteristics. Earth's moon, the nearest satellite to the sun, is a refractory-rich rocky sphere almost 3,500 kilometers in diameter (more than one-fourth the size of Earth; see table). The Moon is the best-known of all other extraterrestrial bodies because of the many space probes that have examined it from orbit and on its surface. These unmanned missions included the 1959 mission by the Soviet Union to photograph the lunar "far side," followed by successful United States projects in the 1960's--Ranger, Surveyor, and Lunar Orbiter. The most famous lunar mission was Apollo, the manned exploration of the Moon. Six missions (from 1969 to 1972) actually landed astronauts on the Moon, who returned numerous rocks and "soil" samples and set up experimental equipment for measuring "moonquakes" and other phenomena. Information from combined lunar missions indicate that the Moon is a complex, small satellite with a metallic core, iron-rich silicate mantle, and silicon plus aluminum-rich silicate crust.

The Moon has no atmosphere, a trait it shares with most other small bodies in the solar system (Saturn's satellite Titan and Neptune's satellite Triton are exceptions). Also, the crust of the Moon is far less complex than Earth's. It consists of densely cratered highland areas composed of a feldspar-rich (calcium-aluminum silicate) rock called "anorthosite" (light-colored areas), with dark basalt lava flows (iron-rich silicate rock) filling huge impact craters that Galileo named "mare" (Latin for seas). The Moon contains no real granite rocks such as those that compose much of Earth's continents.

The Moon always shows the same face to Earth as it orbits, because the Moon's rotational rate is equal to its orbital rate. This situation is an example of "synchronous rotation," in which the rotation rate of a body has some precise mathematical relationship to orbital period (time required to complete one orbit). The Moon's 1:1 ratio of orbital-to-rotational period results from the Earth-side of the Moon bulging out because of gravitational tidal forces between Earth and the Moon. Eventually, this bulge (the side facing Earth) comes to lie along the Earth-Moon line, its most stable configuration. Other bodies in the solar system show similar relationships.

For example, the moons of Mars--Phobos and Deimos--rotate so that the same side always faces Mars. Many satellites of Jupiter and Saturn also show this relationship. Pluto and its moon, Charon, both revolve and rotate at the same rate. This means that an observer on Pluto would always see Charon in precisely the same place in the sky, day and night.

Traveling out from the sun, the next planet is Mars, with its tiny moons, Deimos and Phobos (see table). These oddly shaped, rocky bodies (neither is spherical) are most likely escapees from the nearby asteroid belt, a zone between Mars and Jupiter that contains thousands of small planetoids (up to about 1,000 kilometers in diameter), rock fragments, and dust.

Photographed up close by Viking Orbiter 1 in 1977, they appear to be composed of the same materials that occur in certain meteorites.

At Jupiter, a miniature solar system is found. Jupiter and most of its moons were photographed extensively by Voyagers 1 and 2 in 1979 and earlier by Pioneer 10. Jupiter's four largest satellites (shown in table) are called the "Galilean satellites" because they were discovered by the famous Italian scientist Galileo in 1610. Their densities and rock-to-ice ratios decrease with increased distance from Jupiter, a relationship that is mirrored by the larger solar system. In addition, geological activity decreases from the closest Galilean moon, volcanically active Io, to the highly cratered outermost moon, Callisto. The high crater density on Callisto's surface shows that its surface is very old and has not become "resurfaced" by high-energy processes such as volcanic activity or erosion. This same reasoning is used elsewhere in the solar system to deduce relative ages of satellite and planetary surfaces. Earth's moon, Mercury, and many other bodies are highly cratered by impacting projectiles and, thus, are considered to have old surfaces. The Galilean satellites Io and Europa show no craters because their surfaces are resurfaced constantly by molten sulfurous compounds on Io and liquid water that freezes to ice on the surface of Europa. The heat source to produce this volcanic activity results from tidal forces originating from massive Jupiter.

Saturn and most of its twenty moons were photographed and studied by Pioneer 11 and the Voyager space probes. Most of its satellites are icy, highly cratered worlds; one satellite, Titan, is the second largest satellite in the solar system (Jupiter's Ganymede is larger; see table).

Titan appears to have a surface composed of complex hydrocarbon compounds and liquid nitrogen, but it is unusual for such a small "planet" to have an extensive atmosphere composed mostly of methane (CH₄). Its presence may be caused by the extremely cold temperatures or by the continual production of gases by volcanic activity.

Uranus, visited by Voyager 2 in 1986, has fifteen moons, of which only five were large enough to have been observed from Earth prior to the Voyager space probe. Of all the moons photographed by Voyager 2, the most surprising by far is Miranda. Miranda has nearly crater-free dark areas with concentric grooves and chevron-shaped features that are separated by huge fault scarps (cliffs) from highly cratered areas. Proposals to explain Miranda's bizarre surface features included the breakup and reassembly of the moon following a catastrophic collision with another body or the upwelling of heated water to form the dark, bulging grooved terrains.

Neptune was visited in 1989 by Voyager 2 and its two major moons, Nereid and Triton, were photographed, along with six new moons. One of them, 1989 N1, measures about 400 kilometers in diameter, replacing Nereid as the second largest Neptunian satellite (see table).

Nevertheless, Triton is the most important. It shows a frozen surface with virtually no impact craters. Triton's landscape includes huge frozen "lakes" of solidified water mixed with ammonia, along with a so-called cantalouped terrain of intersecting grooves and ridges that may represent fault systems. Frigid Triton (surface temperature equals -236 degrees Celsius) also shows scattered, dark linear streaks on its surface that may represent nitrogen-powered geysers spewing dark organic matter out on the surface. Winds blowing in Triton's thin methane atmosphere align these geyser streaks and blow thin, ice crystal clouds around this miniature planet. Triton is a unique world that has somehow maintained internal heating and recent resurfacing, truly remarkable feats for a body in such a cold place, which is visited frequently by comets.

Applications

The study of planetary satellites provides mostly scientific information bearing on the origin of the solar system. Nevertheless, knowledge of one nearby satellite, the Moon, can be used for practical advantage. For example, the dark lava flows (basalt) on the Moon contain abundant titanium, a valuable component in high-temperature metal alloys (like metal used in rocket bodies). The rocks of the lunar highland areas are mostly anorthosites, composed primarily of the mineral plagioclase feldspar, which is a potential source of aluminum. Some aluminum is extracted from feldspar deposits on Earth, but the process to separate aluminum from the rest of the mineral requires a high-energy input. On Earth, this process is accomplished commonly by locating the aluminum smelter near a source of hydroelectric power; on the Moon, the energy would have to come from the sun.

The Moon is a potential base for launching spacecraft to other regions of the solar system. This endeavor would require the construction of a lunar base where the Moon's low gravity would greatly facilitate spacecraft launches. Far less energy must be expended on the Moon to escape its gravity field than is required on Earth. Wholesale colonization of the Moon to alleviate population pressures on Earth, however, are not feasible. One aspect of the scientific study of the Moon confirms the distinct lack of concentrated water supplies on the Moon, although some researchers believe that subsurface water in localized "permafrost" deposits will be discovered in the future, such as those found on Mars. Even if they exist, these frozen water deposits would not support large populations of humans.

Satellites are studied primarily for what they tell scientists about the origin of the solar system. As intelligent beings who are products of solar system evolution, humans have a natural curiosity about its origin. The mechanisms of satellite formation and eventual capture by larger planets illustrate many of the dynamic processes that have shaped the solar system since its condensation from the solar nebula perhaps 5 billion years ago. As an example, Earth's satellite poses interesting problems toward interpreting the origins of Earth. Many ideas have been proposed for the origin of the Earth-Moon system. These ideas include the hypothesis that the Moon formed elsewhere in the solar system and was captured later by the earth, or that the Moon "fissioned" off from the earth, possibly leaving a major hole now known as the Pacific Ocean.

Detailed analyses of lunar materials returned by Apollo astronauts and the Soviet Luna missions (surface sampling robots that returned to Earth) show that lunar rocks are similar in some important aspects to Earth's rocks, but they also show critical differences. Lunar and terrestrial rocks have the same ratios of the three oxygen isotopes--oxygen 16, oxygen 17, and oxygen 18--which indicates that their constituents condensed from the same area in space. This is in contrast with the idea that the Moon was formed at some area removed from the earth. Other chemical abundances, however, display great differences between the earth and the Moon. For example, the Moon is richer in iron and carbon compared to Earth and contains far more refractory elements (such as titanium and calcium) compared to volatile substances (such as water, sodium, and potassium). These differences indicate that Earth and the Moon could not have formed from exactly the same starting materials as would be expected if they condensed from a common area of the original solar nebula.

An idea to resolve the problem of lunar origin, called the "impactor hypothesis," involves a violent collision of the early Earth with a foreign, Mars-sized planet (the "impactor").

This collision would blast out material from Earth all the way to the metallic core and would volatize or pulverize most of the impactor. A mixture of loose impactor and Earth materials would orbit the wounded Earth, eventually accreting (accumulating) to form the Moon.

Advocates of this hypothesis argue that mixing of impactor and Earth materials to form both the Moon and Earth would explain the similar oxygen isotope ratios. Yet, because the Moon and Earth would be composed of different ratios of proto-Earth and impactor materials, the two bodies differ in major chemical components such as water and iron. Fortunately for the future evolution of life, Earth acquired most of the volatiles, with water being essential to initiate and sustain life.

The complexity of planet and satellite formation is well illustrated by bodies in the outermost reaches of the solar system; the Neptunian system and Pluto-Charon are particularly intriguing. The large icy Triton orbits Neptune in a "retrograde" direction, clockwise when viewed from "above" the solar system plane. Most planets and satellites rotate and revolve in a counterclockwise direction. Voyager 2 images and measurements show that Triton and Nereid are very similar to Pluto-Charon in composition and density. They are all methane-rich ice balls that resemble the comets that originated in the outer solar system. Theorists speculate that Triton, Nereid, and perhaps Pluto may be cometlike objects that were captured by Neptune early in the history of the solar system and revolved around Neptune in the normal direction. Triton's retrograde orbit may have resulted from a collision or close encounter with a large passing planetoid, an event that may also have caused Nereid to assume its strange, elongated elliptical orbit. In this model, Pluto was split in two (to make Charon) by the encounter and ejected into the outer solar system. Regardless of whether this hypothesis is correct, the Neptunian system is known to be unstable, indicating that something has disturbed it since the original formation of the solar system. In about 10 to 100 million years, Triton will spiral close enough to Neptune to be torn to pieces by tidal forces. This will add greatly to the mass of Neptune's current thin ring system.

Context

The five planets known to ancient peoples were known as the "wandering stars" to distinguish them from the "fixed stars." At least by the time of the ancient Greeks, the Moon was recognized as a satellite of the earth. Most Greek philosophers, such as Plato and Aristotle, who were the scientists of their day, believed that all planets and the sun revolved around the earth.

The first satellites, besides the Moon, were discovered in 1610 by Galileo. He had constructed a crude telescope and used it to scan the heavens. He discovered the four largest moons of Jupiter (Io, Europa, Ganymede, and Callisto) now known as the "Galilean satellites" in his honor. In 1610, Galileo published his findings in SIDEREUS NUNCIUS (starry messenger).

The next major discovery--the large satellite of Saturn, Titan--was made by the Dutch astronomer Christiaan Huygens in 1656. By the end of the nineteenth century, eight more moons were discovered orbiting Saturn, many of them discovered by the Italian astronomer Giovanni Domenico Cassini. Huygens also realized that the "ears" on either side of Saturn described by Galileo were actually rings, a feature attributed to individual particles (tiny satellites) by James Clerk Maxwell in 1857.

Although the distant planet Uranus was discovered in 1781 by the English astronomer Sir William Herschel, its five largest moons were discovered over a 167-year period. The last one, tiny Miranda (see table), was discovered by the American astronomer Gerard Peter Kuiper in 1948. Neptune was discovered in 1846 by the German astronomer Johann G. Galle, followed in the same year by the discovery of its large moon Triton by the English astronomer William Lassel.

Charon, the moon of Pluto, was discovered in 1978 by James W. Christy of the United States Observatory after he noticed that a photographic image he had taken of Pluto showed a lump on one side. This lump was shown later to move relative to Pluto, confirming the existence of a satellite.

The history of astronomy and of science in general was influenced profoundly by Galileo's discovery of the Jovian moons. Before that, the heliocentric (sun-centered) model of the solar system was seriously questioned. Galileo's discovery of a miniature solar system showed that objects could revolve around something other than the earth, thus displacing the earth from the center of the universe.

The later discovery of other satellite systems has proven crucial to the understanding of the origin of the planets, as well as the satellites themselves. Scientific study of satellites shows that the solar system evolved amid violent collisions, gravitational perturbations of orbits, and heating and cooling of surfaces and interiors, in a bewildering variety of combinations. These studies show that although they share some common characteristics, every planet-satellite system formed in some unique way according to conditions prevailing in its particular region of the solar system.

Principal terms

MOON: a term equivalent to "satellite" but also applied specifically to Earth's natural satellite

PERIOD: the time required to complete one revolution (orbit) or rotation around an axis (rotational period)

PERTURBATION: the act of altering the orbital course (direction or speed) of satellite or planetary orbits usually initiated by gravitational effects from or collision with another object

REFRACTORY: refers to substances that melt or boil at relatively high temperatures and, conversely, condense from liquids or gas at high temperature

REVOLVE: to move along a path or "orbit" around a point or body

ROTATE: to spin around an axis

SATELLITE: any natural or artificial body that orbits another body

TIDAL FORCE: the gravitational attraction exerted between two bodies that may potentially distort their shape and cause internal deformation and heating

VOLATILE: refers to substances that melt or boil at relatively low temperatures and, conversely, condense from liquids or gas at low temperature

Bibliography

Chapman, Clark R. PLANETS OF ROCK AND ICE FROM MERCURY TO THE MOONS OF SATURN. New York: Charles Scribner's Sons, 1982. This book by one of the premier planetary scientists gives a detailed account of the characteristics of planets and satellites. Describes the processes involved in their formation and evolution. Illustrated with black-and-white photographs. Includes a detailed index. Contains essential information for planetary enthusiasts.

Editors of Time-Life Books. VOYAGE THROUGH THE UNIVERSE. Alexandria, Va.: Time-Life Books, 1988-1991. Two volumes of this series of books on astronomy discuss the solar system: THE FAR PLANETS (1988) and THE NEAR PLANETS (1989). Excellent full-color illustrations, diagrams, and tables. Gives a historical treatment, emphasizing the excitement and drama of discovery. Each volume gives a timely and complete treatment for the nonspecialist, covering Voyager missions up to and including the encounter with Neptune and its moons. Extensive glossary, bibliography, and index.

Hartmann, William K. MOONS AND PLANETS. Belmont, Calif.: Wadsworth, 1983. A college-level textbook written in a style accessible to nonscience majors. New terms are written in boldface type and defined in the text. Lavishly illustrated with well-executed drawings and black-and-white photographs. Hartmann is a distinguished planetary scientist and an accomplished artist. His paintings of remote planetary scenes have fostered a greater understanding of the variety of features displayed by planetary surfaces. Includes appendices, with extensive planetary vital statistics, a chapter-by-chapter bibliography, and detailed index.

La Cotardiere, Philippe de. ASTRONOMY. New York: Facts on File, 1987. A beautifully illustrated book for the nonspecialist but detailed enough for college students and professionals. Most illustrations and photographs are in full color, with information tables throughout. The section on the solar system includes extensive discussion of satellites. Good bibliography and index.

Miller, Ron, and William K. Hartmann. THE GRAND TOUR: A TRAVELER'S GUIDE TO THE SOLAR SYSTEM. New York: Workman, 1981. This book is designed to introduce the nonspecialist to the intriguing and beautiful world of the solar system. The informative text complements the expertly executed color paintings of planetary landscapes rendered by Hartmann. Color and black-and-white photographs from Voyager and other space probes are also included. This book is a visual delight, sure to pique the interest of the most jaded student. Includes an illustrated glossary, information tables, and an extensive index.

Couplings and Resonances in Planetary Orbits

Ring Systems of Planets