Natural Satellite, a body in space that orbits a larger body. The larger body is referred to as the satellite’s primary. Natural satellites that orbit planets are often called moons. Other objects in the solar system that sometimes have satellites include dwarf planets, Kuiper Belt Objects, centaurs, and asteroids. The term satellite is also used to refer to small galaxies that orbit larger galaxies.
Natural satellites are of special interest to astronomers and planetary scientists because these objects provide clues to how the planets and the solar system formed. Studying the orbits of satellites also allows scientists to determine the mass and density of planets and other objects. A number of moons are important to astrobiologists as places where conditions might permit extraterrestrial life to exist. The International Astronomical Union (IAU) gives official names to natural satellites.
II | SOLAR SYSTEM MOONS |
Jupiter and its Moons
Jupiter is the largest of the planets, with a volume more than 1,300 times greater than that of Earth. The massive planet, upper right, is shown here with its four largest satellites: Io, upper left, Ganymede, lower left, Europa, center, and Callisto, lower right.
The best-known natural satellite is Earth’s moon. The Moon is unusually large relative to the size of its primary (Earth) and has a diameter about one-fourth the diameter of the planet. The Moon’s surface, like the surfaces of most of the natural satellites in the solar system, is heavily cratered and geologically inactive.
Neither Mercury nor Venus has any natural satellites, but Mars has two small moons: Phobos and Deimos. Jupiter has more than 60 natural satellites, four of which are quite large: Io, Ganymede, Callisto, and Europa. These large moons were discovered by Galileo with an early telescope in 1610—the first moons detected around another planet. Active volcanoes cover Io, and scientists believe that oceans of water may hide beneath the icy crusts of Ganymede, Callisto, and Europa. All four of these moons are larger than the dwarf planet Pluto, and Ganymede is larger than the planet Mercury, as well. Saturn also has at least 60 natural satellites, the largest of which is Titan. Titan is bigger than Mercury, and is the only moon with a thick atmosphere. Enceladus, one of Saturn’s smaller moons, has active volcanism in the form of geysers that send out vast plumes of liquid water from the south polar region. Uranus has at least 27 moons, none of which is as large as Earth’s moon. Miranda, one of Uranus’s smaller moons, shows signs of terrific upheavals on its surface. Neptune’s largest natural satellite, Triton, is slightly larger than Pluto. Its surface appears to be continually reshaped by the freezing and thawing of nitrogen.
Pluto and Charon
Pluto, left, and its moon, Charon, are so far away that only the most powerful modern telescopes can resolve them as anything more than points of light. The Hubble Space Telescope took this picture.
Neither Mercury nor Venus has any natural satellites, but Mars has two small moons: Phobos and Deimos. Jupiter has more than 60 natural satellites, four of which are quite large: Io, Ganymede, Callisto, and Europa. These large moons were discovered by Galileo with an early telescope in 1610—the first moons detected around another planet. Active volcanoes cover Io, and scientists believe that oceans of water may hide beneath the icy crusts of Ganymede, Callisto, and Europa. All four of these moons are larger than the dwarf planet Pluto, and Ganymede is larger than the planet Mercury, as well. Saturn also has at least 60 natural satellites, the largest of which is Titan. Titan is bigger than Mercury, and is the only moon with a thick atmosphere. Enceladus, one of Saturn’s smaller moons, has active volcanism in the form of geysers that send out vast plumes of liquid water from the south polar region. Uranus has at least 27 moons, none of which is as large as Earth’s moon. Miranda, one of Uranus’s smaller moons, shows signs of terrific upheavals on its surface. Neptune’s largest natural satellite, Triton, is slightly larger than Pluto. Its surface appears to be continually reshaped by the freezing and thawing of nitrogen.
Pluto and Charon
Pluto, left, and its moon, Charon, are so far away that only the most powerful modern telescopes can resolve them as anything more than points of light. The Hubble Space Telescope took this picture.
The dwarf planet Pluto has three moons. Its largest moon, Charon, is half as large as Pluto itself and was discovered in 1978. Some astronomers consider the pair a double dwarf planet. Eris, a dwarf planet larger than Pluto, has a small moon named Dysnomia. The odd football-shaped dwarf planet 2003 EL61 has two small moons.
Astronomers have detected satellites around other types of solar system bodies. At least 100 asteroids are thought to possibly have satellites. Confirmed asteroid moons are given official names and catalog designations by the IAU. The first asteroid moon was discovered in 1993 when the Galileo space probe photographed the asteroid Ida and its moon Dactyl. In the outer solar system a number of centaurs and Kuiper Belt Objects are known to have satellites. To date, satellites have not been confirmed around any comets.
Astronomers have detected satellites around other types of solar system bodies. At least 100 asteroids are thought to possibly have satellites. Confirmed asteroid moons are given official names and catalog designations by the IAU. The first asteroid moon was discovered in 1993 when the Galileo space probe photographed the asteroid Ida and its moon Dactyl. In the outer solar system a number of centaurs and Kuiper Belt Objects are known to have satellites. To date, satellites have not been confirmed around any comets.
III | SATELLITE ORBITS |
Moon Seen from Early Earth
The Moon was much closer to Earth about four billion years ago and raised huge tides. A day lasted less than 15 hours. Over time the Moon has moved farther away from Earth. Its gravity has also slowed the rotation rate of Earth and kept the axis tilt of our planet stable.
The motion of most of the solar system’s natural satellites about their planets is direct: west to east, in the same direction as the rotation of their planets. Such moons are known as regular satellites. Most regular satellites are thought to have formed in place from debris that orbited the particular planet. One large moon (Triton) and many of the small outer moons of the giant planets revolve in the retrograde direction: east to west, opposite the direction of the rotation of their planets. These retrograde satellites tend to orbit far from their primaries and were probably captured by the planets’ gravitational fields some time after the formation of the solar system. Moons that orbit their planets in a retrograde direction are sometimes called irregular satellites.
Most satellites are tidally locked, meaning they keep the same face toward their primary at all times. For such satellites, the period of rotation on their axes matches the period of their orbits. Most satellites around planets also orbit in roughly the same plane as their planet’s equator. However, retrograde satellites, including Triton, often have steeply inclined orbits—another indication that such moons are captured bodies.
Orbiting satellites exert gravitational pulls on their primary and on other orbiting moons or objects. The Moon causes ocean tides to rise and fall on Earth in conjunction with the gravitational pull of the Sun. It also keeps the tilt of Earth on its axis relatively stable. Early in Earth’s history, the Moon helped slow the rapid rotation of our planet, making our periods of night and day more comfortable for complex life forms. The moons that orbit Jupiter and Saturn interact with other moons circling around each planet, creating tidal stresses that heat the interiors of some moons. Saturn’s complex system of rings and moons includes moons that share the same orbit and moons that control the shape of the planet’s rings.
Most satellites are tidally locked, meaning they keep the same face toward their primary at all times. For such satellites, the period of rotation on their axes matches the period of their orbits. Most satellites around planets also orbit in roughly the same plane as their planet’s equator. However, retrograde satellites, including Triton, often have steeply inclined orbits—another indication that such moons are captured bodies.
Orbiting satellites exert gravitational pulls on their primary and on other orbiting moons or objects. The Moon causes ocean tides to rise and fall on Earth in conjunction with the gravitational pull of the Sun. It also keeps the tilt of Earth on its axis relatively stable. Early in Earth’s history, the Moon helped slow the rapid rotation of our planet, making our periods of night and day more comfortable for complex life forms. The moons that orbit Jupiter and Saturn interact with other moons circling around each planet, creating tidal stresses that heat the interiors of some moons. Saturn’s complex system of rings and moons includes moons that share the same orbit and moons that control the shape of the planet’s rings.
IV | SPECIAL FEATURES OF SATELLITES |
Uranus and Its Moons
The planet Uranus (the bright blue object) is surrounded by its five largest satellites clockwise from top left, Ariel, Umbriel, Oberon, Titania, and Miranda, in this collage created from photographs taken by the United States Voyager 2 spacecraft in 1986.
Most of the satellites in the solar system are ancient objects that probably formed shortly after the planets took shape about 4.6 billion years ago. Moons around planets and dwarf planets range in size from planetlike bodies that have a core, a mantle, and a crust to small chunks of debris. Satellites around asteroids, centaurs, and KBOs may be the result of collisions or the breakup of larger objects, or even the gravitational capture of passing objects.
Earth’s moon is made of rocky material thought to have come from a collision between Earth and another object the size of Mars. Except for Triton, most of the major moons that orbit the giant planets Jupiter, Saturn, Uranus, and Neptune most likely condensed from debris left in a disk around each of these planets. Moons in the outer solar system are made of water ice and other frozen material with a mixture of rock. In the cold temperatures from Jupiter outward, ice is hard as rock. Only volcanic Io is completely rocky, likely having lost its original water long ago. Triton is thought to be a large planetlike body from the Kuiper Belt that was captured by Neptune, so its composition may be more like that of the dwarf planet Pluto.
Earth’s moon is made of rocky material thought to have come from a collision between Earth and another object the size of Mars. Except for Triton, most of the major moons that orbit the giant planets Jupiter, Saturn, Uranus, and Neptune most likely condensed from debris left in a disk around each of these planets. Moons in the outer solar system are made of water ice and other frozen material with a mixture of rock. In the cold temperatures from Jupiter outward, ice is hard as rock. Only volcanic Io is completely rocky, likely having lost its original water long ago. Triton is thought to be a large planetlike body from the Kuiper Belt that was captured by Neptune, so its composition may be more like that of the dwarf planet Pluto.
A | Surface Features of Moons |
Surface of Titan
The Huygens probe took this photo of the surface of Titan, Saturn’s largest moon. Titan’s thick atmosphere gives an orange tint to the light of the distant Sun. The rocks scattered on the surface may actually be chunks of water ice.
From about 4 to 3.8 billion years ago large numbers of asteroids and other leftover debris crashed into moons throughout the solar system, covering their surfaces with craters. Since that time, smaller numbers of meteoroids, asteroids, and comets have occasionally struck the moons. Counting impact craters provides astronomers with a way to calculate the age of the surface of a moon. If a moon has few impact craters, it indicates that the moon has a relatively young surface.
View of Titan’s Surface
NASA’s Cassini spacecraft peered through Titan’s thick atmosphere to create this composite image of the surface. The bright areas are highlands that may be composed of water ice, which would be solid as a rock at Titan's frigid temperatures. The dark areas near the moon's equator are lower in elevation. Radar indicates they are covered in part with large dunes made of hydrocarbon particles.
View of Titan’s Surface
NASA’s Cassini spacecraft peered through Titan’s thick atmosphere to create this composite image of the surface. The bright areas are highlands that may be composed of water ice, which would be solid as a rock at Titan's frigid temperatures. The dark areas near the moon's equator are lower in elevation. Radar indicates they are covered in part with large dunes made of hydrocarbon particles.
A young surface can indicate recent geological activity. Volcanic Io and icy Europa around Jupiter both have few impact craters. Gravitational interactions among Jupiter and its major moons are thought to heat the interiors of Io and Europa. This internal heat causes volcanic eruptions on Io and a slushy subsurface layer of ice on Europa. Saturn’s small moon Enceladus has smooth, bright areas on its surface and relatively low numbers of impact craters—it may also be heated by gravitational interactions.
Iapetus’s Two Faces
This view of Saturn's moon Iapetus taken by the Cassini space probe shows part of the satellite's bright trailing hemisphere, lit by the Sun. The opposite hemisphere that faces in the direction that the moon orbits is dark reddish in color. Features include a large impact crater and a puzzling ridge that runs along the moon's equator, visible on the right edge of the image.
Iapetus’s Two Faces
This view of Saturn's moon Iapetus taken by the Cassini space probe shows part of the satellite's bright trailing hemisphere, lit by the Sun. The opposite hemisphere that faces in the direction that the moon orbits is dark reddish in color. Features include a large impact crater and a puzzling ridge that runs along the moon's equator, visible on the right edge of the image.
Titan, Saturn’s largest moon, also has few impact craters. Although icy volcanism from water and ammonia may occur along with some tectonic activity, Titan’s young surface is most likely the result of weather processes. Titan’s dense, cold atmosphere precipitates particles of complex organic molecules that accumulate as dunes and mountains. Methane rain erodes the surface and creates lakes at the moon’s poles. Triton’s young surface also may result from processes in its atmosphere, as well as eruptions of nitrogen geysers from underground pockets.
B | Atmospheres and Auroras on Moons |
Volcano Erupting on Io
Jupiter’s moon Io is the most active volcanic body in the solar system. The eruption seen at the lower left of this photo threw material some 100 km (60 mi) above Io’s surface.
Most moons are too small for their gravity to retain an atmosphere. Nonetheless, cold temperatures or geological activity allow a few moons to maintain detectable atmospheres.
Titan has the densest atmosphere of any moon—60 percent denser than Earth’s atmosphere. Titan’s atmosphere is made up of nitrogen and methane. Triton has a thin nitrogen and methane atmosphere, formed largely from frozen gas on its surface heated by the Sun. Some of Triton’s atmosphere freezes to create polar caps.
Ganymede has a very thin oxygen atmosphere thought to come from charged particles breaking apart molecules of water ice on its surface. Most of the charged particles that strike Ganymede come from volcanoes on the moon Io—the particles are trapped and energized in Jupiter’s giant magnetic field. The charged particles also raise a spray of water molecules that falls back and freezes on the surface of Ganymede, giving its polar regions bright caps of frost. Ganymede also has auroras at its poles, caused by charged particles striking its thin oxygen atmosphere. Little Enceladus has a thin local atmosphere from water vapor released by geysers at one pole.
Io has a thin atmosphere made of sulfur dioxide gas released by volcanic eruptions over many parts of its surface. Some of this atmosphere freezes out as frost on the moon’s nightside, then becomes a gas again when sunlight returns. Auroras occur on the nightside of Io at the moon’s equator.
Titan has the densest atmosphere of any moon—60 percent denser than Earth’s atmosphere. Titan’s atmosphere is made up of nitrogen and methane. Triton has a thin nitrogen and methane atmosphere, formed largely from frozen gas on its surface heated by the Sun. Some of Triton’s atmosphere freezes to create polar caps.
Ganymede has a very thin oxygen atmosphere thought to come from charged particles breaking apart molecules of water ice on its surface. Most of the charged particles that strike Ganymede come from volcanoes on the moon Io—the particles are trapped and energized in Jupiter’s giant magnetic field. The charged particles also raise a spray of water molecules that falls back and freezes on the surface of Ganymede, giving its polar regions bright caps of frost. Ganymede also has auroras at its poles, caused by charged particles striking its thin oxygen atmosphere. Little Enceladus has a thin local atmosphere from water vapor released by geysers at one pole.
Io has a thin atmosphere made of sulfur dioxide gas released by volcanic eruptions over many parts of its surface. Some of this atmosphere freezes out as frost on the moon’s nightside, then becomes a gas again when sunlight returns. Auroras occur on the nightside of Io at the moon’s equator.
C | Magnetic Fields on Moons |
Ganymede is the only moon that has its own intrinsic magnetic field, thought to be generated by a subsurface ocean circulating beneath its icy crust. Europa has no magnetic field of its own. However, Europa orbits inside the giant magnetic field of Jupiter. Measurements taken by the Galileo spacecraft show that Europa has an induced magnetic field that changes as the moon orbits Jupiter. The ability to pick up an outside magnetic field is strong evidence that Europa has a liquid ocean under its surface. A similar fluctuating induced magnetic field was also detected around Callisto, suggesting that Callisto may have a subsurface ocean, as well.
D | Subsurface Oceans on Moons |
Europa’s Ocean Under Ice
The icy crust of the moon Europa likely covers a subsurface ocean. This image taken by the Galileo spacecraft in 1997 shows complex fracture patterns in the ice. The small number of impact craters indicates the icy crust is relatively young in this area of the surface.
The subsurface oceans thought to exist on Jupiter’s moons Europa, Ganymede, and Callisto are likely made of water mixed with salts, allowing the oceans to conduct electricity and have magnetic properties. Such an environment of water and chemicals also might allow life to exist. The hidden ocean on Europa is the best candidate for some kind of life because it is warmer and the moon’s interior may be geologically active.
Scientists are analyzing data from the Cassini space probe to determine if Saturn’s moon Titan might have a subsurface ocean of water mixed with ammonia. Some theoretical works suggests other outer moons may have subsurface oceans containing a mix of water, ammonia, and methane. Moons that are possible candidates include Saturn’s Rhea, Uranus’s Titania and Oberon, and Neptune’s Triton.
Scientists are analyzing data from the Cassini space probe to determine if Saturn’s moon Titan might have a subsurface ocean of water mixed with ammonia. Some theoretical works suggests other outer moons may have subsurface oceans containing a mix of water, ammonia, and methane. Moons that are possible candidates include Saturn’s Rhea, Uranus’s Titania and Oberon, and Neptune’s Triton.
E | Rings and Satellites |
The four giant planets (Jupiter, Saturn, Uranus, and Neptune) are each surrounded by rings in the plane of their equators. Rings are made of rocky or icy material that can range in size from tiny particles to objects as large as houses. Scientists think such rings are likely debris from broken-up satellites or from passing comets, KBOs, or asteroids torn apart by the planet’s gravity. Another possibility is that rings are made of ancient, leftover material that never condensed into satellites.
Complex interactions can occur between ring material and existing satellites. Small so-called shepherd moons orbit along the edges of rings or within gaps between rings, keeping the ring material in place. Small satellites may form out of ring material or be shattered by impacts or collisions to form new rings. In some cases, rings may be made of material thrown into space off the surface of satellites by impacts or by geological activity.
Complex interactions can occur between ring material and existing satellites. Small so-called shepherd moons orbit along the edges of rings or within gaps between rings, keeping the ring material in place. Small satellites may form out of ring material or be shattered by impacts or collisions to form new rings. In some cases, rings may be made of material thrown into space off the surface of satellites by impacts or by geological activity.
V | SEARCH FOR EXTRATERRESTRIAL LIFE ON MOONS |
Geysers on Enceladus
Plumes of icy material, left, extend from the south polar region of Enceladus, a moon of Saturn, in an image taken February 17, 2005, by the Cassini spacecraft. A color-coded image, right, shows a much more extended plume, reaching as far as 418 km (260 mi) into space. Planetary scientists concluded that the plumes represent liquid-water geysers, and they theorized that this active volcanism was caused by tidal forces that created friction and heat within the interior of Enceladus. The detection of carbon molecules on the moon’s surface, along with the presence of heat and liquid water, means that Enceladus might be able to support life.
Beginning in the 1970s, space probes such as Voyager, Galileo, and Cassini have provided scientists with a wealth of information about moons in the outer solar system. During the same period, researchers discovered microorganisms and even complex life on Earth that can survive under extreme conditions once thought to be too hostile for life. Such so-called extremophile organisms are found in superhot seafloor volcanic vents or in geyser pools, in rocks deep underground or under ice, and even in toxic or corrosive chemical environments. Some of these organisms thrive without oxygen or light, or in freezing cold or boiling heat.
Astrobiologists (scientists who study the prospects for life elsewhere in the universe) have raised the possibility that life might exist on distant moons such as Europa, Enceladus, or Titan. Underground liquid water and organic chemicals, combined with energy from geological heat, gravitational stresses, or charged particles, might create the chemistry and processes needed for life. Europa is seen as one of the most promising places to hunt for extraterrestrial life—it has a subsurface ocean and signs of complex chemistry and active geology. Space missions to send probes that can penetrate its icy crust and explore the hidden ocean beneath are under consideration.
In the mid-1990s scientists began finding planets around other stars—discoveries that greatly improve the odds that life might exist beyond our solar system. Many of the extrasolar planets detected so far are extremely large and often have orbits very near their suns. Although conditions on such planets appear hostile to life, these planets may have large satellites where life might be possible. However, scientists may need many decades to develop technology that can detect and study satellites orbiting such distant planets.
Astrobiologists (scientists who study the prospects for life elsewhere in the universe) have raised the possibility that life might exist on distant moons such as Europa, Enceladus, or Titan. Underground liquid water and organic chemicals, combined with energy from geological heat, gravitational stresses, or charged particles, might create the chemistry and processes needed for life. Europa is seen as one of the most promising places to hunt for extraterrestrial life—it has a subsurface ocean and signs of complex chemistry and active geology. Space missions to send probes that can penetrate its icy crust and explore the hidden ocean beneath are under consideration.
In the mid-1990s scientists began finding planets around other stars—discoveries that greatly improve the odds that life might exist beyond our solar system. Many of the extrasolar planets detected so far are extremely large and often have orbits very near their suns. Although conditions on such planets appear hostile to life, these planets may have large satellites where life might be possible. However, scientists may need many decades to develop technology that can detect and study satellites orbiting such distant planets.
VI | MORE INFORMATION ABOUT SATELLITES |
Encarta has separate articles on most of the natural satellites that have been studied in detail. These include the moons of Mars: Deimos and Phobos; the moons of Jupiter: Adrastea, Amalthea, Ananke, Callisto, Carme, Elara, Europa, Ganymede, Himalia, Io, Leda, Lysithea, Metis, Pasiphae, Sinope, and Thebe; the moons of Saturn: Atlas, Calypso, Dione, Enceladus, Epimetheus, Helene, Hyperion, Iapetus, Janus, Mimas, Pan, Pandora, Phoebe, Prometheus, Rhea, Telesto, Tethys, and Titan; the moons of Uranus: Ariel, Belinda, Bianca, Caliban, Cordelia, Cressida, Desdemona, Despina, Juliet, Miranda, Oberon, Ophelia, Portia, Puck, Rosalind, Sycorax, Titania, and Umbriel; and the moons of Neptune: Galatea, Larissa, Naiad, Nereid, Proteus, Thalassa, and Triton. In addition, overviews of planets’ systems of moons and rings appear in the planet articles Jupiter, Saturn (moons and rings), Uranus, and Neptune.
Largest Natural Planetary Satellites
Astronomical radius
|
Average distance from planet
| |||||
| Planet | Satellite | km | mi | km | mi | Orbital period (days) |
| Jupiter | Ganymede |
2,634
|
1,637
|
1,070,000
|
664,900
|
7.155
|
| Saturn | Titan |
2,575
|
1,600
|
1,222,000
|
759,200
|
15.95
|
| Jupiter | Callisto |
2,403
|
1,493
|
1,883,000
|
1,170,000
|
16.69
|
| Jupiter | Io |
1,830
|
1,137
|
422,000
|
262,200
|
1.769
|
| Earth | Moon |
1,737
|
1,080
|
384,400
|
238,900
|
27.32
|
| Jupiter | Europa |
1,565
|
972
|
671,000
|
416,900
|
3.551
|
| Neptune | Triton |
1,353
|
841
|
354,800
|
220,400
|
5.877
|
| Source: U.S. Naval Observatory. | ||||||
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