Saturn
The sixth planet from the Sun is Saturn. Dusty chunks of ice—some the size of a house, others of a grain of sand—make up its extraordinary rings. The other outer planets also have rings, but Saturn's are much larger and more complex. The planet is a popular target for amateur astronomers, because even a small telescope can reveal the dazzling rings. To the unaided eye, Saturn looks like a bright nontwinkling point of light. It was the most-distant planet known to ancient astronomers.
Four unmanned spacecraft have visited Saturn, obtaining images and data that have greatly increased knowledge about the planet. The first three—Pioneer 11 and Voyagers 1 and 2—were flybys in the late 1970s and early 1980s. The Cassini-Huygens mission arrived in 2004 for a longer study of the planet and its moons and rings.
Basic Planetary Data
Size, Mass, and Density Saturn is the solar system's second largest and second most massive planet, after Jupiter. The diameter at its equator is about 74,898 miles (120,536 kilometers). Since the planet has no solid surface, its diameter is measured at a level where the atmospheric pressure is 1 bar, which is equal to the pressure at sea level on Earth. Saturn's diameter is more than nine times larger than Earth's. The planet is about 95 times as massive as Earth and has more than 750 times its volume. By comparison, Jupiter is about 1.2 times as big as Saturn and 3.2 times as massive.
Saturn has the lowest mean density of any of the planets. With only about 70 percent the density of water on average, the planet would float if it could be placed in water. Earth's density, on the other hand, is about 550 percent that of water, and Jupiter's is about 130 percent.
Orbit and Spin
Since Saturn is not solid, it has no single rotation rate. However, all parts of the planet spin quickly. Clouds in the atmosphere near the equator swirl around fastest, taking about 10 hours, 10 minutes for each rotation. It takes the planet's deep interior roughly 30–40 minutes longer to complete each rotation. A day on a planet is defined by its rotation period, so a day on Saturn is about 10.8 Earth hours, or less than half as long as a day on Earth.
As in other planets, the force of the rotation causes some bulging at the equator and flattening at the poles. Saturn's rapid rotation and low average density make it the least spherical of all the planets. The diameter at its poles is about 10 percent smaller than the diameter at its equator. Jupiter actually spins a bit faster than Saturn, but its shape is less distorted. Jupiter's greater density helps it better resist the force produced by the rapid rotation.
Saturn's rotational axis is tilted about 26.7 degrees relative to the ecliptic, which is an imaginary plane passing through the Sun and Earth's orbit. As Saturn orbits the Sun, first one hemisphere and then the other is tipped closer to the Sun. As a result, Saturn experiences seasons like Earth, which is tilted about 23.5 degrees on its axis. Since each trip around the Sun takes longer for Saturn than Earth, its seasons are longer. Each season on Saturn lasts more than seven years.
Atmosphere
Saturn's composition, like Jupiter's, is very similar to that of the Sun and other stars. Data from the Voyager mission suggest that Saturn's atmosphere is about 91 percent hydrogen and 6 percent helium by mass, making it the most hydrogen-rich atmosphere in the solar system. In comparison, hydrogen makes up about 86 percent of Jupiter's atmosphere and about 71 percent of the Sun. Saturn's overall composition may be more similar to that of Jupiter and the Sun, but more of Saturn's helium may have settled into its interior. Also, some research suggests that the Voyager analysis underestimated the percentage of helium and overestimated the hydrogen.
In spacecraft images of Saturn, the surface one sees is mainly clouds. Its hazy appearance is due to the appreciable atmosphere above the clouds. The highest deck of clouds is made of crystals of frozen ammonia. Farther down there are thought to be clouds made of frozen crystals of ammonium hydrosulfide and, at deeper levels, clouds of water ice crystals and ammonia droplets. All these chemicals are colorless when pure. However, the planet's clouds usually appear golden yellow-brown, perhaps because they also contain phosphorous compounds or some other chemical impurity.
In images captured by the Cassini spacecraft in the early 2000s, the atmosphere appeared blue in the northern hemisphere and yellow-brown in the south. The blue region seemed to be relatively free of the yellow-brown clouds at the highest levels. The cloudless parts of Saturn's sky were likely blue for the same reason that Earth's sky is blue—molecules of gases in the atmosphere scattering sunlight in a way favoring shorter, bluer wavelengths. Scientists are not certain why the higher clouds appeared so much thinner in the north, but it may have been a seasonal effect. Later in the Cassini mission, as spring approached in the northern hemisphere in 2008, the atmosphere there began to turn less blue and more golden colored.
The bands of winds blowing eastward, in the direction of the planet's rotation, are vigorous. They alternate with bands of winds that are barely moving westward. Saturn's strongest winds blow eastward in a band over the equator from 20° N. to 20° S. Maximum wind speeds in this band reach nearly 1,100 miles (1,800 kilometers) per hour. Jupiter has a similar jet of winds near its equator, but Saturn's is twice as wide and its winds blow four times as fast. The fastest winds on Earth occur in tropical cyclones, or hurricanes, and are much slower. Only in extreme cases do they reach sustained speeds of more than 150 miles (240 kilometers) per hour.
Saturn's alternating bands of winds are remarkably symmetrical. Each band north of the equator usually has a counterpart south of the equator with about the same width and wind velocity. This suggests to scientists that the pairs of bands may be connected in some way deeper in the atmosphere.
The pressure increases with depth in Saturn's atmosphere. At the lower levels, where the pressure is very high, the hydrogen is probably crushed into a liquid. The temperature in the atmosphere also varies; at its coldest, it is about − 312° F (− 191° C). As on Earth, the temperature gets colder with altitude in the lowest level of the atmosphere but hotter with altitude in a middle level. In the highest level, the temperature is fairly constant.
Interior The temperatures and pressures in Saturn's interior are very high, and they increase with depth. As in Jupiter, the interior consists largely of hydrogen, which the immense pressure squeezes into a liquid. About halfway down between Saturn's cloud tops and center, the temperature is probably about 10,300° F (5,730° C). The pressure is thought to be some 2 million times greater than at sea level on Earth. Under those conditions, the hydrogen is probably compressed into a liquid metallic state. In this state, the electrons are stripped away from the atomic nuclei, so the hydrogen would conduct electricity like a metal.
Saturn's central, liquid metallic region is denser than Jupiter's. Scientists have determined this by analyzing Saturn's gravity field. The planet's gravity is stronger at the poles than at the equator. The distortions in its gravity field are directly related to the relative amount of mass concentrated in its interior rather than in its atmosphere. This analysis suggests that Saturn's central regions are about half hydrogen by mass and half denser materials. Jupiter's central regions are thought to contain about two thirds hydrogen and only one third denser matter. Some of the denser material must be helium, which may be more concentrated in Saturn's interior than Jupiter's. Saturn's dense core is likely a mixture of rock and ice with about 10 to 20 times the mass of Earth.
Like Jupiter, Saturn radiates nearly twice as much energy as it receives from the Sun, mostly as heat. This means that the planet must generate some of its own heat. Much of this energy is probably left over from when the planet formed some 4.6 billion years ago. Since then, the planet has slowly cooled down, gradually emitting heat. Scientists believe that some of the heat that Saturn produces probably comes from helium settling into its interior. It is thought that the helium separates out of the hydrogen and forms droplets, which sink toward the center. The friction of the droplets pushing against the other matter would create heat. This process is also thought to occur in Jupiter, but to a much lesser degree.
Magnetic Field and Magnetosphere Saturn's magnetic field is much stronger than Earth's but much weaker than Jupiter's. It has two poles, north and south, like a giant bar magnet. As on Jupiter, the orientation of the poles is opposite of that currently found on Earth. This means that a compass on Saturn or Jupiter would point south.
The planet's magnetic field dominates a large region of space called its magnetosphere. This region is shaped like a teardrop. The rounded part extends about 750,000 miles (1,200,000 kilometers) from the planet on the side facing the Sun. There, the magnetosphere holds off the solar wind, a flow of electrically charged particles from the Sun. On the side opposite the Sun, the solar wind pulls the magnetosphere out into a very long tail.
Ring System
The rings occur in groups. The three main groups, from farthest to closest to Saturn, are called the A ring, the B ring, and the C ring. These three rings are visible from Earth through a telescope. The B ring is the broadest, thickest, and brightest ring. Other rings with lower densities of particles lie outside the main groups. The D ring lies between the C ring and Saturn, and the F, G, and E rings extend out from the A ring. Many gaps occur between the rings. The gaps are regions where far fewer particles are found. Some of the major gaps are named after astronomers who studied Saturn. The largest gap, between the A and B rings, is called the Cassini division. It was named after the astronomer Gian Domenico Cassini, who discovered the gap in 1675.
Moons
Spacecraft Exploration
- An Earth-sized storm appears as a light-colored patch just north of Saturn's equator in a composite …
- The Huygens probe captured the first close-up photographs of the surface of Saturn's moon Titan. …
Four unmanned spacecraft have visited Saturn, obtaining images and data that have greatly increased knowledge about the planet. The first three—Pioneer 11 and Voyagers 1 and 2—were flybys in the late 1970s and early 1980s. The Cassini-Huygens mission arrived in 2004 for a longer study of the planet and its moons and rings.
Basic Planetary Data
- A video incorporating photographs and artwork presents an overview of Saturn, the sixth planet from …
Size, Mass, and Density Saturn is the solar system's second largest and second most massive planet, after Jupiter. The diameter at its equator is about 74,898 miles (120,536 kilometers). Since the planet has no solid surface, its diameter is measured at a level where the atmospheric pressure is 1 bar, which is equal to the pressure at sea level on Earth. Saturn's diameter is more than nine times larger than Earth's. The planet is about 95 times as massive as Earth and has more than 750 times its volume. By comparison, Jupiter is about 1.2 times as big as Saturn and 3.2 times as massive.
Saturn has the lowest mean density of any of the planets. With only about 70 percent the density of water on average, the planet would float if it could be placed in water. Earth's density, on the other hand, is about 550 percent that of water, and Jupiter's is about 130 percent.
Orbit and Spin
- A false-color image shows Saturn. Three of its satellites (Tethys, Dione, and Rhea) are visible as …
Since Saturn is not solid, it has no single rotation rate. However, all parts of the planet spin quickly. Clouds in the atmosphere near the equator swirl around fastest, taking about 10 hours, 10 minutes for each rotation. It takes the planet's deep interior roughly 30–40 minutes longer to complete each rotation. A day on a planet is defined by its rotation period, so a day on Saturn is about 10.8 Earth hours, or less than half as long as a day on Earth.
As in other planets, the force of the rotation causes some bulging at the equator and flattening at the poles. Saturn's rapid rotation and low average density make it the least spherical of all the planets. The diameter at its poles is about 10 percent smaller than the diameter at its equator. Jupiter actually spins a bit faster than Saturn, but its shape is less distorted. Jupiter's greater density helps it better resist the force produced by the rapid rotation.
Saturn's rotational axis is tilted about 26.7 degrees relative to the ecliptic, which is an imaginary plane passing through the Sun and Earth's orbit. As Saturn orbits the Sun, first one hemisphere and then the other is tipped closer to the Sun. As a result, Saturn experiences seasons like Earth, which is tilted about 23.5 degrees on its axis. Since each trip around the Sun takes longer for Saturn than Earth, its seasons are longer. Each season on Saturn lasts more than seven years.
- Saturn and its rings are tilted at varying angles toward Earth as the two planets orbit the Sun. …
Atmosphere
- Bands of clouds in Saturn's southern hemisphere appear in an image taken at infrared wavelengths by …
Saturn's composition, like Jupiter's, is very similar to that of the Sun and other stars. Data from the Voyager mission suggest that Saturn's atmosphere is about 91 percent hydrogen and 6 percent helium by mass, making it the most hydrogen-rich atmosphere in the solar system. In comparison, hydrogen makes up about 86 percent of Jupiter's atmosphere and about 71 percent of the Sun. Saturn's overall composition may be more similar to that of Jupiter and the Sun, but more of Saturn's helium may have settled into its interior. Also, some research suggests that the Voyager analysis underestimated the percentage of helium and overestimated the hydrogen.
In spacecraft images of Saturn, the surface one sees is mainly clouds. Its hazy appearance is due to the appreciable atmosphere above the clouds. The highest deck of clouds is made of crystals of frozen ammonia. Farther down there are thought to be clouds made of frozen crystals of ammonium hydrosulfide and, at deeper levels, clouds of water ice crystals and ammonia droplets. All these chemicals are colorless when pure. However, the planet's clouds usually appear golden yellow-brown, perhaps because they also contain phosphorous compounds or some other chemical impurity.
In images captured by the Cassini spacecraft in the early 2000s, the atmosphere appeared blue in the northern hemisphere and yellow-brown in the south. The blue region seemed to be relatively free of the yellow-brown clouds at the highest levels. The cloudless parts of Saturn's sky were likely blue for the same reason that Earth's sky is blue—molecules of gases in the atmosphere scattering sunlight in a way favoring shorter, bluer wavelengths. Scientists are not certain why the higher clouds appeared so much thinner in the north, but it may have been a seasonal effect. Later in the Cassini mission, as spring approached in the northern hemisphere in 2008, the atmosphere there began to turn less blue and more golden colored.
- Saturn is a stormy world. A giant thunderstorm called the Dragon Storm appears as a complex orange …
- A huge hurricane-like storm whirls over Saturn's south pole in a image captured by the Cassini …
The bands of winds blowing eastward, in the direction of the planet's rotation, are vigorous. They alternate with bands of winds that are barely moving westward. Saturn's strongest winds blow eastward in a band over the equator from 20° N. to 20° S. Maximum wind speeds in this band reach nearly 1,100 miles (1,800 kilometers) per hour. Jupiter has a similar jet of winds near its equator, but Saturn's is twice as wide and its winds blow four times as fast. The fastest winds on Earth occur in tropical cyclones, or hurricanes, and are much slower. Only in extreme cases do they reach sustained speeds of more than 150 miles (240 kilometers) per hour.
Saturn's alternating bands of winds are remarkably symmetrical. Each band north of the equator usually has a counterpart south of the equator with about the same width and wind velocity. This suggests to scientists that the pairs of bands may be connected in some way deeper in the atmosphere.
The pressure increases with depth in Saturn's atmosphere. At the lower levels, where the pressure is very high, the hydrogen is probably crushed into a liquid. The temperature in the atmosphere also varies; at its coldest, it is about − 312° F (− 191° C). As on Earth, the temperature gets colder with altitude in the lowest level of the atmosphere but hotter with altitude in a middle level. In the highest level, the temperature is fairly constant.
Interior The temperatures and pressures in Saturn's interior are very high, and they increase with depth. As in Jupiter, the interior consists largely of hydrogen, which the immense pressure squeezes into a liquid. About halfway down between Saturn's cloud tops and center, the temperature is probably about 10,300° F (5,730° C). The pressure is thought to be some 2 million times greater than at sea level on Earth. Under those conditions, the hydrogen is probably compressed into a liquid metallic state. In this state, the electrons are stripped away from the atomic nuclei, so the hydrogen would conduct electricity like a metal.
Saturn's central, liquid metallic region is denser than Jupiter's. Scientists have determined this by analyzing Saturn's gravity field. The planet's gravity is stronger at the poles than at the equator. The distortions in its gravity field are directly related to the relative amount of mass concentrated in its interior rather than in its atmosphere. This analysis suggests that Saturn's central regions are about half hydrogen by mass and half denser materials. Jupiter's central regions are thought to contain about two thirds hydrogen and only one third denser matter. Some of the denser material must be helium, which may be more concentrated in Saturn's interior than Jupiter's. Saturn's dense core is likely a mixture of rock and ice with about 10 to 20 times the mass of Earth.
Like Jupiter, Saturn radiates nearly twice as much energy as it receives from the Sun, mostly as heat. This means that the planet must generate some of its own heat. Much of this energy is probably left over from when the planet formed some 4.6 billion years ago. Since then, the planet has slowly cooled down, gradually emitting heat. Scientists believe that some of the heat that Saturn produces probably comes from helium settling into its interior. It is thought that the helium separates out of the hydrogen and forms droplets, which sink toward the center. The friction of the droplets pushing against the other matter would create heat. This process is also thought to occur in Jupiter, but to a much lesser degree.
Magnetic Field and Magnetosphere Saturn's magnetic field is much stronger than Earth's but much weaker than Jupiter's. It has two poles, north and south, like a giant bar magnet. As on Jupiter, the orientation of the poles is opposite of that currently found on Earth. This means that a compass on Saturn or Jupiter would point south.
The planet's magnetic field dominates a large region of space called its magnetosphere. This region is shaped like a teardrop. The rounded part extends about 750,000 miles (1,200,000 kilometers) from the planet on the side facing the Sun. There, the magnetosphere holds off the solar wind, a flow of electrically charged particles from the Sun. On the side opposite the Sun, the solar wind pulls the magnetosphere out into a very long tail.
- Bright auroras loop around Saturn's south polar region for days in a series of ultraviolet images …
Ring System
- A dramatic backlit view of Saturn reveals its ring system as never before seen. From the vicinity …
- Saturn's three main rings appear in a natural-color composite of six images taken by the Cassini …
The rings occur in groups. The three main groups, from farthest to closest to Saturn, are called the A ring, the B ring, and the C ring. These three rings are visible from Earth through a telescope. The B ring is the broadest, thickest, and brightest ring. Other rings with lower densities of particles lie outside the main groups. The D ring lies between the C ring and Saturn, and the F, G, and E rings extend out from the A ring. Many gaps occur between the rings. The gaps are regions where far fewer particles are found. Some of the major gaps are named after astronomers who studied Saturn. The largest gap, between the A and B rings, is called the Cassini division. It was named after the astronomer Gian Domenico Cassini, who discovered the gap in 1675.
- A diagram shows the full ring system of Saturn, with red lines indicating the orbits of some of the …
- An image taken by the Cassini spacecraft reveals a small moon orbiting within the Keeler gap near …
- A movie shows the moons Pandora, left, and Prometheus, right, orbiting on either side of Saturn's F …
Moons
- The Cassini spacecraft photographed Dione, one of Saturn's major moons, majestically orbiting just …
- Phoebe, Saturn's only irregular major moon, appears in an image captured by the Cassini spacecraft. …
- An edge-on (limb) view of the atmosphere of Titan, Saturn's largest moon, reveals many fine layers …
- Saturn's moon Titan is shrouded in a thick, orangish haze. An image taken by the Cassini spacecraft …
- A dark, branching channel—perhaps carved by a river of liquid methane—appears on …
- Saturn's moon Mimas is partly lit by the Sun in an image taken by the Cassini spacecraft. Visible …
- The icy surface of Saturn's moon Enceladus reflects more light than newly fallen snow. A …
- Jets of water vapor and water ice erupt from Saturn's moon Enceladus in an image taken by the …
- The crater-scarred surface of Hyperion, a major moon of Saturn, appears in an image taken by the …
- The surface of Saturn's moon Iapetus displays unusually large variations in brightness. A mosaic of …
Spacecraft Exploration