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Above: Greek symbol for Saturn
Saturn is the second most massive planet, and also the second largest in size. It is a Gas Giant planet with a rotational period of 10-11 hours (depending on latitude), and an orbital period of 29.5 years. The rapid rotation flattens Saturn at the poles by about 10%, making it the most oblate planet. Its composition is similar to that of Jupiter, being composed mostly of hydrogen and helium. Like Jupiter, it is mostly liquid, with a small rocky core expected, but not directly observed, and like Jupiter, it has an internal heat source (it radiates more energy than it receives)..
Saturn has the lowest density of any planet, 0.7 g/cc, which is less than that of water. Saturn is of such low density that it would float in a (gigantic) bathtub. However, you would not want to put Saturn in a bathtub. Do you know why? Because it would leave a ring!
The interior is probably similar to Jupiter, with metallic hydrogen responsible for the strong magnetic field of Saturn. The concentration of helium relative to hydrogen is somewhat less than for Jupiter. This is thought to be due to the colder temperature of Saturn.
Although the Hubble Space Telescope can now take very good images of Saturn, our best information comes from space probes: Pioneer 11, Mariner 11 and 12, and Voyager I and II.
In Roman mythology, Saturn is the god of agriculture. The associated Greek god, Cronus, was the son of Uranus and Gaia and the father of Zeus (Jupiter). Saturn is the root of the English word "Saturday" (see Appendix 4).
Saturn has been known since prehistoric times. Galileo was the first to observe it with a telescope in 1610; he noted its odd appearance but was confused by it. Early observations of Saturn were complicated by the fact that the Earth passes through the plane of Saturn's rings every few years as Saturn moves in its orbit. A low resolution image of Saturn therefore changes drastically. It was not until 1659 that Christiaan Huygens correctly inferred the geometry of the rings. Saturn's rings remained unique in the known solar system until 1977 when very faint rings were discovered around Uranus (and shortly thereafter around Jupiter and Neptune).
Saturn was first visited by Pioneer 11 in 1979 and later by Voyager 1 and Voyager 2. Cassini, now on its way, will arrive in 2004.
The surface of Saturn bears many similarities with the surface of Jupiter, but the color contrast is generally less. This is thought to be due to Saturn being colder than Jupiter (further from the Sun), so it has different chemical reactions in its atmosphere, leading to different coloration.
There are large anticyclonic cells on the surface, apparently driven by the planet's internal heat source, but none are as large as the Great Red Spot on Jupiter, and they are not as abundant as on Jupiter. The image adjacent left shows a red anticyclonic disturbance about the diameter of the Earth.
Storms on Saturn
The adjacent animation shows a sequence of Hubble Space Telescope images of the Great White Spot, a disturbance discovered in 1990 that can extend completely around the planet. The same storm is illustrated with greater smoothness in this Hubble Space Telescope movie sequence (Ref).
High-Velocity Winds
There are extremely high velocity winds in the atmosphere of Saturn. Unlike the case for Jupiter, the variations in wind speeds are not strongly correlated with the positions of the belts and bands. The wind speeds in the atmosphere of Saturn have been measured to be as high as 1800 km/hr, which is about 4 times the highest speeds in the atmosphere of Jupiter.
The Interior of Saturn
Like Jupiter, Saturn is largely liquid. The slightly higher concentration of helium relative to hydrogen in the atmosphere is thought to be be due to the colder temperature of Saturn. Under these colder conditions, liquid helium does not dissolve in liquid hydrogen and drops of helium sink to the center, depleting the outer regions in helium. Speculation on Saturn's internal heat source is similar to that for Jupiter.
The magnetic field of Saturn is similar to that of Jupiter, but weaker. Electrical currents in liquid metallic hydrogen deep in the interior are assumed to be the cause of the magnetic field. Unlike the case for Jupiter, the magnetic field traps charged particles in "Van Allen belts" rather than in sheets, and there appear to be fewer trapped charged particles than is the case for Jupiter.
The ring system of Saturn is divided into 5 major components: the G, F, A, B, and C rings, listed from outside to inside (but in reality, these major divisions are subdivided into thousands of individual ringlets). The F and G rings are thin and difficult to see, while the A, B, and C rings are broad and easily visible. The large gap between the A ring and and the B ring is called the Cassini division.
The adjacent image is a rare view of Saturn's rings seen just after the Sun has set below the ring plane, taken with the Hubble Space Telescope on Nov. 21, 1995. This perspective is unusual because the Earth is slightly above and the Sun slightly below the rings. Normally we see the rings fully illuminated by the Sun.
Three bright ring features are seen: the F Ring, the Cassini Division, and the C Ring (moving from the outer rings to the inner). The low concentration of material in these rings allows light from the Sun to shine through them. The A and B rings are much denser, which limits the amount of light that penetrates through them. Instead, they are faintly visible because they reflect light from Saturn's disk (Ref).
Ring Structure and Composition
High resolution photographs from the Voyager missions indicate that the rings of Saturn are composed of hundreds of thousands of "ringlets", and that regions like the largest "gap" called the Cassini division, also contain fainter rings (adjacent image). The rings cannot be solid, because they lie inside the Roche limit. They presumably represent either a satellite torn apart by tidal forces, or (more likely) material that was never allowed to condense into moons because of the tidal forces. The evidence indicates that the rings are composed of particles that are mostly ice crystals, with sizes as large as centimeters or meters. The total mass in the rings is about the size of a medium mass moon, and the rings are only about 10 km thick.
Spokes and other Structure
It was expected that collisions between ring particles would tend to make the rings uniform, but Voyager I found changing structures in the radial direction that are termed "spokes". Some of this structure is shown in the adjacent animation. Here is a smoother and longer movie of the same phenomenon. It is thought that gravitational forces alone cannot account for the spoke structure, and it has been proposed that electrostatic repulsion between ring particles may play a role.
The Voyagers found that the rings were not necessarily circular, and even found rings that appeared to be braided (adjacent image). They found further that the outer ring was kept in place by the gravitational interaction of two small "shepherd moons" lying just inside and outside it, and that at least some of the other rings are kept narrow by similar small shepherding satellites. Generally, although we have increased immensely our knowledge of the rings of Saturn over the last 2 decades, we still do not fully understand their structure, dynamics, or origin.
The following image shows a montage of 15 of Saturn's satellites (the current known number is 19). These range in size from Titan, the second largest moon in the Solar System (Ganymede is the largest), to small asteroid-like objects.
Titan, the Largest Moon of Saturn
Titan has an atmosphere. This can be seen faintly in the image on the left as an outline, and more clearly in the following image
taken by Voyager looking back at Titan and showing sunlight scattering in the atmosphere. The atmosphere of Titan has several layers of haze. It has a pressure at the surface of 1.6 times that of Earth, and is made up primarily of nitrogen, with about a 1% concentration of methane. The temperature on the surface is very cold, about -180 degrees Celsius. The atmosphere is extremely opaque because of thick smog that appears to result from sunlight interacting with hydrocarbons, much as smog forms on the Earth.
The clouds are probably composed of liquid nitrogen and methane drops, and it is speculated that Titan may be coverered with hydrocarbon lakes or oceans (specifically, methane and ethane). Although many of the organic chemicals thought to have been the precursors to life on Earth are present on Titan, it appears to be too cold for life as we know it to have evolved there. Here is a movie of infra-red images of Titan made with the Hubble Space Telescape. The structure shown in this animation represents heat variations in the atmosphere and surface of Titan.
Saturn's other Moons
Saturn's other moons have icy surfaces that are very cold so that ice is as rigid as rock and craters form from meteor impacts. The mean densities of these moons are 1.0 to 1.5 g/cc, implying that they are probably mostly ice, though they may have some rocky consituents. Most of Saturns satellites (as for those of Jupiter) are tidally locked and keep the same face turned toward the planet as they orbit.
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Rhea
Iapetus |
The preceding images show the satellite Rhea, which is seen to have many impact craters, and Iapetus, which has one side that is 10 times darker than the other. Other moons of Saturn include Mimas, which has an impact crater 1/4 the diameter of the moon, Enceladus, which may be geologically active because of tidal heating by Saturn, Tethys, which has a large flattened crater that is 1/2 the diameter of the moon and a very large canyon, and Dione, which is heavily cratered with ray structures associated with some craters.
Two small moons have been found to share Tethys's orbit, and one small moon shares Dione's orbit. This sharing of orbits had not been seen before the detailed investigation of Saturns system. Hyperion rotates chaotically because of the influence of Titan's gravity and a highly eccentric orbit. The outermost satellite, Phoebe, may be a captured asteroid.
Unusual Hubble View of Moons and Rings
The adjacent 10 Hubble Space Telescope images capture several small moons orbiting Saturn while the Earth was just above the ring plane and the Sun below it. (The logo used on title line of the pages for these lectures is abstracted from this image.) Moving out from Saturn, the visible rings are: the broad C Ring, the Cassini Division, and the narrow F Ring.
The first pair of images shows Dione, near the upper middle of the frames. Two smaller moons, Pandora (the brighter one closer to Saturn) and Prometheus, appear as if they're touching the F Ring. In the second frame, Mimas emerges from Saturn's shadow and appears to be chasing Prometheus.
In the second image pair, Mimas has moved towards the tip of the F Ring. Rhea, another bright moon, has just emerged from behind Saturn. Prometheus, the closest moon to Saturn, has rounded the F Ring's tip and is approaching the planet. The slightly larger moon Epimetheus has appeared.
The third image pair shows Epimetheus, as a tiny dot just beyond the tip of the F Ring. Prometheus is in the lower right corner. An elongated clump or arc of debris in the F ring is seen as a slight brightening on the far side of this thin ring.
In the fourth image pair, Epimetheus, in the lower right corner, streaks towards Saturn. The long ring arc can be seen in both frames.
The fifth image pair again captures Mimas, beyond the tip of the F Ring. The same ring arc is still visible.
In addition to the satellites, a pair of stars can be seen passing behind the rings, appearing to move towards the lower left due to Saturn's motion across the sky.
The images were taken Nov. 21, 1995, with Wide Field Planetary Camera-2 of the Hubble Space Telescope (Ref).
Shepherd Moons
In various instances now we know that ring structures in the Jovian planets are stabilized by small "shepherd moons" that orbit in or near the rings and stabilize them by their gravitational influences. One such example is the F-ring of Saturn, which is kept narrow by two small shepherd moons, Prometheus and Pandora, as illustrated in the adjacent image (click the button for labels).
We shall find similar examples of shepherd moons for rings associated with other planets. Although such shepherd satellites clearly play a significant role in ring structures, we only partially understand the details of these roles.
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SATURN
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