Planetary satellites

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

Overview

All planets in the solar system have at least one satellite except Mercury and Venus. There are hundreds of known satellites in the solar system, ranging from about fifteen to more than fifty-two hundred kilometers in diameter. Each year, this number grows as spacecraft and telescopic observations discover more relatively tiny satellites around the gas giants, particularly Jupiter and Saturn. Although the largest satellites of the solar system’s eight planets have been identified, a table of known satellites would become outdated within months as more are constantly discovered. Satellites are essential for the scientific clues they provide concerning their parent planets' origin and the solar system.

Most models of planetary formation start with some variation on the nebular hypothesis devised by German philosopher Immanuel Kant in 1755 and later modified in 1796 by French mathematician Pierre-Simon Laplace. This hypothesis in modern form suggests that the Sun, planets, satellites, and smaller debris in the solar system started as a rotating flattened cloud of gas and dust. This rotating cloud eventually became unstable and broke into rings. Material concentrated in the bulging center of the cloud became compressed, eventually giving birth to the Sun. The gaseous rings eventually cooled, allowing the precipitation of solid crystals that aggregated to form rocky materials. These aggregates, in turn, accreted over geological time 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 or remained freely orbiting the Sun as dwarf planets, asteroids, and comets.

Like the major planets, satellites vary in composition depending on their 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 almost entirely of ice and has been reclassified as either a dwarf planet or plutoid, the first member of a class of objects from the Kuiper Belt called plutinos. This distribution shows that volatile substances (those with low melting and boiling points) are concentrated in the outer solar system. Refractory substances with high melting and boiling points occur in greater abundance within the inner solar system. Satellites in the solar system reveal considerable diversity in terms of surface composition and physical traits.

Earth’s Moon, the nearest satellite to the Sun, is a refractory-rich rocky triaxial ellipsoid approximately thirty-five hundred kilometers in average diameter, more than one-fourth the size of Earth. The Moon is the best known of all other extraterrestrial satellites because of the many spacecraft that have examined it from orbit and landed on its surface. These robotic missions began with the 1959 mission by the Soviet Union to photograph the lunar far side, followed by the National Aeronautics and Space Administration’s (NASA’s) successful Ranger, Surveyor, and Lunar Orbiter projects in the 1960s. The most famous lunar project was NASA’s Apollo program, which landed humans on the Moon. From 1969 to 1972, six Apollo missions landed twelve astronauts on the Moon, who returned numerous rocks and soil samples. The Apollo astronauts also set up experimental equipment for measuring moonquakes and other phenomena. Information from all the lunar missions, whether crewed or robotic, indicate that the Moon is a complex, small satellite with a metallic core, an iron-rich silicate mantle, and a silicon plus aluminum-rich silicate crust.

Nearly two decades transpired before another spacecraft was sent to the Moon after the Russian Luna 24 mission returned a small amount of rock and soil samples to Earth from the Moon’s Sea of Crises in 1976. By this point, NASA had moved on to attempting to land a robotic spacecraft, the Vikings, on Mars. In early 1998, NASA sent the Lunar Prospector into lunar orbit. Outfitted with five different instruments, Lunar Prospector’s objectives were to globally map the lunar resources, determine the Moon’s complex gravity field and minor magnetic field, and look for any outgassing from the surface. The surprising result of Lunar Prospector investigations was the strong indication that between 10 to 100 million metric tons of water ice could be located in permanently shadowed portions of craters near the lunar poles.

The presence of water on the Moon as a resource that could be utilized would greatly enhance the viability of crewed lunar research outposts, especially if those outposts were constructed at or very near the Moon’s poles. This finding was not direct evidence. The spacecraft’s neutron spectrometer picked up the presence of protons; the most reasonable extrapolation of the data was that those protons were bonded in water ice. The potential finding of water on the Moon was not a total surprise. Two years earlier, a Ballistic Missile Defense Organization probe called Clementine used the Moon as a testbed for new sensor technologies, among other things. Some sensor data hinted at the possibility of water near the poles. Lunar Prospector took the search for water to a new level, and the real surprise was the total amount of water that Lunar Prospector seemed to find.

The Moon has no significant 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 Moon’s crust 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 the Italian astronomer Galileo Galilei named maria (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 the 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) lies along the Earth-Moon line, its most stable configuration. Other bodies in the solar system show similar relationships. For example, Mars's satellites, 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 satellite Charon 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 all day and night.

Traveling out from the Sun, the next planet is Mars, with its tiny satellites, Deimos and Phobos. 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 one thousand kilometers in diameter), rock fragments, and dust. Photographed up close by the Viking Orbiter 1 in 1977, Phobos and Deimos 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 satellites were photographed extensively by Voyagers 1 and 2 in 1979 and earlier by Pioneer 10. Jupiter’s four largest satellites are called the Galilean satellites because Galileo discovered them in 1610. Their densities and rock-to-ice ratios decrease with increased distance from Jupiter, a relationship mirrored by the larger solar system. Additionally, geological activity decreases from the closest Galilean moon, volcanically active Io, to the highly cratered outermost satellite, 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 the relative ages of satellite and planetary surfaces. Earth’s Moon, Mercury, and many other bodies are heavily cratered by impacting projectiles and, therefore, are considered to have old surfaces. The Galilean satellites Io and Europa show no craters because their surfaces are renewed 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 sixty satellites were photographed and studied by Pioneer 11, the two Voyager spacecraft, and the Cassini orbiter. Most of its satellites are icy, heavily cratered worlds; one satellite, Titan, appears to have a surface composed of complex hydrocarbon compounds and liquid nitrogen. Still, it is unusual for such a small planet to have an extensive atmosphere consisting primarily of methane (CH₄). Its presence may be caused by frigid temperatures or by the continual production of gases by some type of cryogenic volcanic activity.

Uranus, visited only by Voyager 2 in 1986, has at least twenty-four satellites, of which only five were large enough to have been observed from Earth before Voyager’s visit; many were discovered long after that visit. Of all the satellites 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 separated by huge fault scarps (cliffs) from heavily cratered areas. Proposals to explain Miranda’s bizarre surface features included the breakup and reassembly of the satellite following a catastrophic collision with another body or the upwelling of heated water to form its dark, bulging, grooved terrains.

Neptune was visited in 1989 by Voyager 2, and its two major satellites, Nereid and Triton, were photographed, along with six new ones. One of those, 1989 N1, measures about four hundred kilometers in diameter, replacing Nereid as the second largest Neptunian satellite. The SETI Institute announced in July 2013 that another moon was found orbiting Neptune, bringing the total to fourteen for the planet. Nevertheless, Triton is the most important. It shows a frozen surface with virtually no impact craters. Triton’s landscape includes huge icy lakes of solidified water mixed with ammonia and a so-called cantalouped terrain of intersecting grooves and ridges that may represent fault systems. Frigid Triton has a surface temperature of only thirty-seven kelvins. It has 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 maintained internal heating and resurfacing. It is a remarkable feat for a body in such a cold place.

Knowledge Gained

Satellites are studied primarily for what they tell scientists about the solar system's origin. As intelligent beings who are products of solar system evolution, humans are curious about the solar system and its origin. The mechanisms of satellite formation and eventual capture by larger planets illustrate many dynamic processes that have shaped the solar system since its condensation from the solar nebula perhaps 4.5 billion years ago.

For example, Earth’s Moon poses interesting problems regarding attempts to understand the origin of the Earth. Many ideas have been proposed for the source of the Earth-Moon system. These ideas include the hypothesis that the Moon formed elsewhere in the solar system and was later captured by the Earth or that the Moon “fissioned” off from the early Earth itself, possibly leaving a significant hole known as the Pacific Ocean. Detailed analyses of lunar samples returned by Apollo astronauts (1969–1972) and the Soviet robotic Luna missions (which concluded in 1976) show that lunar rocks are similar in some important respects to terrestrial rocks but also have critical differences. Lunar and earthly rocks have the same ratios of the three oxygen isotopes (nuclei with the same atomic numbers but different mass numbers—hence different amounts of neutrons but the same number of protons): oxygen 15, oxygen 17, and oxygen 18. This indicates that their constituents condensed from the same area in space. Thus, the theory that postulates the Moon was captured after forming elsewhere is no longer held in much esteem.

The fission theory contrasts with the idea that the Moon was formed at some area removed from the Earth. Concentrations of other chemicals in lunar samples than oxygen isotopes display great differences from Earth rocks. For example, the Moon is richer in iron and carbon than Earth. It contains far more refractory elements (such as titanium and calcium) than volatile substances (such as water, sodium, and potassium). These differences indicate that Earth and the Moon could not have formed from the same starting materials as expected if they condensed from a common area of the original solar nebula. Therefore, the fission theory is also no longer considered viable.

An idea to resolve the problem of lunar origin, called the impactor hypothesis, involves a violent collision of the early Earth with a wayward, Mars-sized planet (the impactor). This collision would have blasted out material from Earth to the metallic core and would volatilize or pulverize most of the impactor. A mixture of loose impactor and Earth materials would have orbited the wounded Earth, eventually accreting (accumulating) to form the Moon. Advocates of this hypothesis argue that mixing 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 proportions of proto-Earth and impactor materials, the two bodies’ differences in major chemical components, such as water and iron, are also explained. Fortunately for the future evolution of life, Earth acquired most of the volatiles, including water, which is essential to initiate and sustain life. This theory is preferred in modern science.

The planet and satellite formation complexity is well-illustrated by bodies in the outermost reaches of the solar system; the Neptunian and Pluto-Charon systems are particularly intriguing. Large, icy Triton orbits Neptune in a retrograde direction, clockwise when viewed from “above” the solar system's ecliptic plane. Most planets and satellites rotate and revolve in a counterclockwise direction. Voyager 2 images and measurements show that Triton and Nereid are similar to Pluto-Charon in composition and density. All are methane-rich ice balls resembling the comets originating in the outer solar system. Theorists speculate that Triton, Nereid, and perhaps Pluto may be cometlike objects that Neptune captured early in the solar system's history 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. This event 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, adding significantly to the mass of Neptune’s current thin system of partial arcs.

Although most planetary satellite studies are conducted to learn more about the solar system in general and the possible future of Earth in particular, knowledge of one nearby satellite, Earth’s Moon, will eventually be used to practical advantage. For example, dark lava flows (basalt) on the Moon contain abundant titanium, a valuable component in high-temperature metal alloys (like metal used in rocket bodies). Rocks in the lunar highland areas are mostly anorthosites, composed primarily of the mineral plagioclase feldspar, a potential aluminum source. Some aluminum is extracted from feldspar deposits on Earth, but separating aluminum from the rest of the mineral requires a high-energy input. On Earth, this process is commonly accomplished by locating the aluminum smelter near a source of hydroelectric power. On the Moon, the energy would have to come from the Sun or a small local nuclear power plant.

Earth’s Moon is a potential base for launching spacecraft to other solar system regions. 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. However, wholesale colonization of the Moon to alleviate population pressures on Earth is not feasible. One aspect of the scientific study of the Moon confirms the distinct lack of concentrated water supplies. However, some researchers believe that subsurface water in localized permafrost deposits, such as those found on Mars, will be discovered. Even if they exist, these frozen water deposits would not support large populations of humans.

The United States was not the only nation interested in sending humans back to the Moon. The Chinese had already announced their plans for sending taikonauts (the Chinese word for astronaut) into space and, in due course, to the Moon. The Europeans sent the Small Missions for Advanced Research in Technology 1 (SMART 1) spacecraft to the Moon to produce the highest-resolution mapping of its surface. A Chinese satellite named Chang’e 1 was sent into orbit, the Japanese sent their Kaguya probe to the Moon, and the Indians launched their Chandrayaan spacecraft in 2008, which landed on the Moon's surface that November. For the first time, an international fleet of probes was investigating the Moon. With a government-supported space program, it appeared that Chinese taikonauts might reach the Moon by the second decade of the twenty-first century. A nonbinding deadline for reaching the Moon was set for 2020 but then reset for 2030. In 2023, the first successful Chinese trial test on a main engine capable of taking taikonauts to the Moon was completed.

Context

The five planets known to ancient peoples were called 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, believed that all planets and the Sun revolved around the Earth.

The history of astronomy and science in general was influenced profoundly by Galileo’s discovery of the Jovian satellites. Galileo had constructed a crude telescope and used it to scan the heavens. He discovered the four largest satellites of Jupiter (Io, Europa, Ganymede, and Callisto), known as the Galilean satellites in his honor. In 1610, Galileo published his findings in Sidereus Nuncius (Starry Messenger). Before that, the heliocentric (Sun-centered) model of the solar system proposed by Nicolaus Copernicus was seriously questioned and indeed considered heretical by the Catholic Church. Galileo’s discovery of a “miniature solar system” consisting of satellites around a planet demonstrated that objects could revolve around something other than the Earth, thus displacing the Earth from the center of the universe.

The Dutch astronomer Christiaan Huygens made the subsequent significant discovery—the large satellite of Saturn, Titan—in 1656. By the end of the nineteenth century, eight more satellites had been discovered orbiting Saturn, many of them found by the Italian astronomer Gian Domenico Cassini. Huygens also realized that the “ears” on either side of Saturn first described by Galileo were 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 English astronomer Sir William Herschel, its five largest satellites required an additional 167 years to detect. The last one, tiny Miranda, was discovered by American astronomer Gerard Peter Kuiper in 1948.

Neptune was discovered in 1846 by German astronomer Johann G. Galle, followed in the same year by the discovery of its large satellite Triton by English astronomer William Lassell. Charon, the largest satellite 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.

These later discoveries of other satellite systems, along with continuing discoveries, have proven crucial to understanding the origin of the planets and 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 various combinations. These studies show that, although they share some characteristics, every planet-satellite system formed in some unique way according to conditions prevailing in its particular region of the solar system.

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