Probe
Country of origin
Launched
Comments
Pioneer 11
USA
1973
Fly by in 1979
Voyager 1
USA
1977
Fly by in 1980
Voyager 2
USA
1977
Fly by in 1981
Cassini
USA, ESA, ASI
1997
In orbit from 2004
Position and Orbit
Saturn is the second largest planetary member of the solar system and is the sixth planet from the Sun. Its orbit is slightly elliptical and lies between Jupiter and Uranus. Saturn has a mean distance from the Sun of just over 1430 million km, placing it about 9.5 times farther from the Sun than Earth. It travels around the Sun once every 29.46 years and it rotates on its axis with a period of 10 h 40 min. As with Jupiter, the short rotational period has resulted in Saturn becoming flattened or oblate. The equatorial diameter is 120,536 km, which is 10 % more than its polar diameter of 108,728 km. This shape suggests the interior is a liquid rather than a solid or gas.
Density and Composition
Saturn’s is the least dense of all the planets in the solar system, mainly because of its composition of light gases. Like Jupiter, Saturn is a gaseous planet composed of about 75 % hydrogen and 25 % helium with traces of water, methane, ammonia and ‘rock’ similar to the composition of the primordial solar nebula from which the solar system was formed. Saturn’s mass is about 95 times greater than Earth’s but it has 800 times the volume.
The interior of Saturn is also similar to Jupiter in that is contains a rocky core, a liquid metallic hydrogen mantle and a liquid outer layer of molecular hydrogen. Traces of various ices are also present. With increasing height, the outer layer of liquid hydrogen becomes gaseous.
Because of its smaller mass and size, Saturn’s interior is less compressed than Jupiter’s. The core is about 25,000 km in diameter, while the mantle is about 12,000 km thick. Saturn’s core contains around 26 % of the total mass of the planet, as opposed to around 4 % for Jupiter. The temperature, pressure and density inside the planet all rise steadily toward the core, which, in the deeper layers of the planet, cause hydrogen to transition into a metal (Fig. 10.5).
Fig. 10.4Radar image taken by Cassini of the hydrocarbon lakes on Titan, Saturn’s largest moon (Credit: NASA, ESA).
Fig. 10.5Internal structure of Saturn.
As with Jupiter, Saturn’s interior is hot, about 12,000 °C at the core, and the planet radiates 2.5 times more energy into space than it receives from the Sun.
The strength of gravity on Saturn is only slightly more than Earth’s gravity (10.4 compared to 9.8 N/kg). This means that a 75 kg person weighs 735 N on Earth, but on Saturn would weigh 780 N.
The Surface
Saturn’s surface and interior are similar to those of Jupiter. Saturn’s mantle is surrounded by ordinary liquid hydrogen, so there is no solid surface layer. When observing Saturn, we are looking at its cloud tops. These cloud tops lack the colours visible on Jupiter. Photographs taken by Voyager 1 show faint bands across Saturn’s surface but these are nowhere near as prominent as those on Jupiter. This is mainly because, the gravitational pull of Saturn is much weaker than Jupiter’s, and hence the layers of gases are more weakly held together. The banded appearance of the cloud layer is thought to be caused by differences in the temperature and altitude of the atmospheric gas masses.
Saturn does have some long-lived spots like the Great Red Spot on Jupiter. In 1990, the Hubble Space Telescope observed an enormous white cloud near Saturn’s equator that was not present during the Voyager encounters. In November 1994, another spot was observed near Saturn’s equator. This storm-like spot was 12,700 km across which is about the same size as Earth.
The Atmosphere
Saturn’s atmosphere contains mostly hydrogen (96 %) and helium (3 %). Trace amounts of ammonia, acetylene, ethane, propane, phosphine and methane have been detected in Saturn’s atmosphere. Ultraviolet radiation from the Sun causes methane photolysis in the upper atmosphere, leading to a series of hydrocarbon chemical reactions with the resulting products being carried downward by eddies and diffusion. This photochemical cycle is modulated by Saturn’s annual seasonal cycle.
The atmosphere consists of a banded pattern, similar to Jupiter’s, but the bands are fainter and wider near the equator. There are three layers of clouds. The lower layer of clouds contains water ice crystals. The middle layer contains clouds of ammonium hydrosulfide, while the uppermost layer contains ammonia ice crystals. The clouds generally rotate with Saturn with a period of 10 h 14 min at the equator to 10 h 40 min at high latitudes. The clouds appear yellow in colour and move in zones parallel to the equator, with winds that alternate from east to west between zones. Wind speeds are generally higher than those on Jupiter. This high-velocity wind of 1800 km/h has remained fairly constant over decades. Saturn also has storms like those seen on Jupiter, but they are less visible and less frequent, although they last longer.
Because Saturn is 9.53 times further from the Sun than Earth, its atmosphere receives only 1 % of the solar energy Earth receives. It radiates more than twice this amount from its interior. Gases being heated by the interior and Saturn’s fast rotation generate circulation patterns in the atmosphere.
A strange hexagonal wave pattern around the north polar vortex in the atmosphere of Saturn was first noted in the Voyager images. The sides of the hexagon are each about 13,800 km long, which is longer than the diameter of the Earth. The structure rotates with a period of 10 h 40 min (the same period as that of the planet’s radio emissions) which is assumed to be equal to the period of rotation of Saturn’s interior. The hexagonal feature does not shift in longitude like the other clouds in the visible atmosphere. Most astronomers believe the pattern was caused by some standing-wave pattern in the atmosphere.
Imaging of the south polar region of Saturn by the Hubble Space telescope indicates the presence of a jet stream, but no strong polar vortex nor any hexagonal standing wave.
The Rings
The most prominent feature of Saturn is its ring system, which encircles the planet around its equator. The rings do not touch Saturn. As Saturn orbits the Sun, the rings tilt at the same angle as the equator. Sometimes we see the rings edge on from Earth and sometimes they are nearly upright. The Voyager space probes showed much more detail about the rings than could be seen from Earth, and four additional rings were discovered, bringing the total to seven.
Closer examination of the rings by space probes has revealed that the seven rings are actually composed of hundreds of narrow, closely spaced ‘ringlets’.
The closest ring to Saturn is the faint D ring. The inner edge of this ring lies about 6700 km from the cloud tops. The C ring begins at about 14,200 km altitude. The densest of the rings is the B ring that begins at about 31,700 km above the cloud tops. The Cassini Division, which is a gap about 4700 km in width, is at an altitude of 57,200 km. The gap probably formed as a result of the gravitational pull between ring particles and Saturn’s moon Mimas. Beyond this gap, the A ring begins at 76,500 km altitude, followed by the narrow, faint F ring. Beyond the F ring is the tenuous G ring, discovered by the Voyager space probe, and the even more tenuous E ring (the outermost ring). The bulk of the ring system spans about 275,000 km but is only about 1.5 km thick.
Saturn’s rings, unlike the rings of other planets, are very bright because they reflect light. The rings consist of countless small particles, ranging in size from a centimeter to 10 m across. In order to measure the size of the particles in Saturn’s rings, scientists measured the brightness of the rings from many angles as the spacecraft flew around the planet. They also measured changes in radio signals received as the craft passed behind the rings. If all the particles were compressed to form a single body, the body would be only about 100 km across. Data from the Voyager spacecraft co
nfirmed that the particles consist of ice and ice-coated rocks.
Voyager also identified two tiny satellites, Prometheus and Pandora, each measuring about 50 km across, orbiting Saturn on either side of the F ring. They help to keep the icy particles into a well defined, narrow band about 100 km wide. The F ring also contains ringlets that are sometimes braided and sometimes separate. Dark radial ‘spokes’ that appear and disappear in the B ring are thought to be caused by Saturn’s magnetic field.
Observations made by the Cassini probe in 2006 showed that the D ring isn’t flat like the other rings. It appears to have corrugations like a tin roof. These corrugations are thought to have been caused by an impact as recently as 1984.
Saturn’s rings are thought to have formed from a cloud of particles that came from the breakup of a moon or from material that did not combine to form a moon. Moons that orbit within the rings act as shepherd satellites to create sharp edges and gaps between the rings.
Temperature and Seasons
At the cloud tops and in the rings, the temperature is about −185 °C. Frozen water is in no danger of melting or evaporating at these cold temperatures. Temperature and pressure increases with depth below the cloud tops. In the outer core, the temperature reaches about 12,000 °C and the pressure about 12 million times the pressure on Earth’s surface.
Because Saturn is titled on its axis and it takes 29.5 years to orbit the Sun, any season on Saturn would last more than 7 Earth years.
Magnetic Field
Saturn’s magnetic field was first detected with the fly-by of NASA’s Pioneer 11 spacecraft in 1979. Convection currents in the mantle of liquid metallic hydrogen generate the strong magnetic field. Saturn’s field is about 36 times less powerful than the field of Jupiter but 570 times more powerful than Earth’s field. Because the magnetic field is less powerful than Jupiter’s fewer charged particles are trapped in Saturn’s magnetic field. The rings and moons also absorb some charged particles.
Saturn’s magnetosphere is intermediate in size between Earth and Jupiter’s, but it extends beyond the orbit of Saturn’s moon Titan. Data from space probes show that Saturn’s magnetosphere contains radiation belts similar to those of Earth. Variations in the magnetic field are thought to be responsible for the presence of dark spokes seen moving in Saturn’s rings.
On Earth, the magnetic polar axis and the rotational axis vary by about 11°, but on Saturn the two axes are within 1° of each other (see Fig. 10.6).
The Cassini spacecraft has detected auroral emissions around both poles of Saturn. The blue-ultraviolet emissions are thought to be caused by hydrogen gas being excited by electron bombardment. The images showed that the auroral light responded rapidly to changes in the solar wind strength. In 2013, astronomers using the Hubble Space Telescope captured new images of the dancing auroral lights at Saturn’s north pole. The ultraviolet images capture moments when Saturn’s magnetic field is affected by bursts of particles streaming from the Sun. It appears that when particles from the Sun hit Saturn, the planet’s magnetotail collapses and later reconfigures itself, an event that is reflected in the dynamics of its auroras.
Fig. 10.6Saturn’s magnetic field. Shown are the magnetic axis and rotational axis.
Moons of Saturn
Saturn has 62 moons, nearly as many as Jupiter. The moons fall into three groups—there are 21 moons between 133,000 km and 527,000 km from the planet, three between 1,221,000 km and 3,560,000 km, and the rest between 11,294,000 km and 24,500,000 km.
The moons range in diameter from about 3 km to 5150 km but the smaller ones are more asteroid-like and may not be true moons. Some astronomers call them ‘moonlets’. Most of these moons are icy worlds heavily covered with craters caused by impacts very long ago. Some astronomers believe that the moons may have condensed from a series of gas rings cast off from Saturn about 4.5 billion years ago.
There were 18 known moons orbiting Saturn when the Cassini spacecraft began its historic journey to the planet in 1997. During Cassini’s 7-year journey to Saturn, Earth-based telescopes discovered 13 additional moons. When Cassini reached Saturn in 2004, three more moons were discovered (Methone, Pallene and Polydeuces). On 1 May 2005, Cassini found another moon, hidden in a gap in Saturn’s outer A ring.
In 2006, astronomers using the 8.2 m Subaru telescope in Hawaii detected nine more moons orbiting Saturn. These moons or satellites are about 6–8 km in size and they travel on highly eccentric, retrograde orbits (opposite to the planet’s rotation). These objects were probably captured by Saturn’s gravity.
As of mid-2008, a total of 62 moons or satellites have been detected around Saturn.
Many of the moons have been officially named, the rest have been given temporary numbers until they are fully confirmed.
Only seven of the known moons of Saturn are massive enough to have collapsed into a spherical shape (see Table 10.3). The rest are irregular in shape suggesting they are captured asteroids. Unlike Jupiter, Saturn has only one planet-sized moon; this is called Titan. Titan is second in size only to Ganymede among the moons in the solar system, and it is also larger than the planet Mercury. It can be seen fairly easily through telescopes from Earth.Table 10.3Largest moons of Saturn (in order of increasing distance from Saturn)
Name
Distance from Saturn (km)
Period (days)
Diameter (km)
Year discovered
Mimas
185,520
0.94
397
1789
Enceladus
238,000
1.37
500
1789
Tethys
294,000
1.89
1060
1684
Dione
377,400
2.74
1120
1684
Rhea
527,100
4.52
1530
1672
Titan
1,221,850
15.95
5150
1655
Iapetus
3,560,800
79.33
1440
1671
Christiaan Huygens discovered Titan in 1655, the same year he discovered the rings. It has a diameter of 5150 km and it orbits Saturn at a distance of about 1.2 million km. Titan takes about 16 days to orbit Saturn and it also takes this time to rotate once on its axis. Thus this moon always has the same side facing Saturn.
Titan is thought to be made of half water ice and half rock or silicates. A mantle of ice and an icy crust that may contain some liquid water surrounds its rocky core. Titan is the only moon in the solar system to have a dense atmosphere. Brown-orange clouds in the atmosphere completely obscure its surface and little sunlight reaches the surface. Voyager data showed most of the atmosphere is nitrogen gas (94 %) with the rest mainly methane (5 %). The nitrogen gas may have originally been in the form of ammonia, which broke up into hydrogen and nitrogen. Hydrogen, being light, may have escaped Titan’s weak gravity. There are traces of hydrocarbons such as, methane, acetylene, ethane, ethylene, and propane. All the oxygen is present as water ice. The atmosphere is four times as dense as Earth’s but because of the weak gravity, atmospheric pressure is only 1.6 times greater than Earth’s (Fig. 10.7).
Fig. 10.7Saturn’s largest moon is Titan—it is the only moon with a substantial atmosphere. In 2004 the Huygens probe took this photo of the surface of Titan while descending through Titan’s atmosphere (Credit: NASA).
The temperature on Titan is about −178 °C, which is below the freezing point of water, but near the freezing point of methane. Thus it is expected that the surface contain lakes of hydrocarbon liquids like ethane. Nitrogen reacts with these hydrocarbons to produce other compounds, some of which are the building blocks of organic molecules essential for life.
Pictures taken by the Cassini spacecraft in July 2004, showed a murky landscape with a variety of features, s
uch as giant equatorial sand dunes, polar lakes, and methane-soaked mud flats. So far, only one mountain ridge has been detected. There is evidence of volcanoes, flows and calderas. The northern region contains well-defined lakes, channels and islands. The first infrared pictures revealed water ice as dark patches and masses of clouds in the southern hemisphere.
Cassini also mapped interaction between the huge magnetosphere that surrounds the Saturn system, and Titan’s dynamic atmosphere. The 80,000 km wide gas cloud that follows Titan as it orbits the planet is evidence that its atmosphere is breaking up.
On 14 January 2005, the Huygens probe (released from Cassini) entered Titan’s atmosphere at about 6 km/s, and its heat shield reached 8000 °C. Three parachutes were used to slow the probe down and it landed on the surface with a ‘splat’ in what appeared to be mud. The images of the surface showed a pale orange, eroded landscape of rocks and ice blocks, together with what looked like drainage channels. A thin crust gives the surface a squishy consistency. Titan also has large plains containing longitudinal hydrocarbon sand dunes that run for hundreds of kilometres. The dunes are up to a kilometre wide and as high as 300 m.
On 23 June 2014, NASA reported that it had strong evidence that nitrogen in the atmosphere of Titan came from materials associated with comets in the Oort cloud, and not from the materials that formed Saturn in earlier times.
Scientists now believe that Titan has a subsurface ocean made of water mixed with ammonia. On 2 July 2014, NASA reported the ocean inside Titan maybe as salty as the Dead Sea on Earth.
The Solar System in Close-Up Page 18