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Saturn

Saturn is a gas giant planet, somewhat smaller than Jupiter. Saturn has an extensive ring system, complete with shepherd moons orbiting inside the ring structures and a wide collection of moons orbiting outside the rings.

Perhaps the first thing to notice in comparing Saturn to Jupiter is that the Saturn's surface is much less detailed than Jupiter's. The main reason for this is that Saturn has a considerably lower mass than Jupiter. This means that Saturn's gravitational force is not as strong, to the gases are not compressed as tightly. Saturn's atmosphere is lighter and "fluffier" making atmospheric features less vivid than those of Jupiter.

This graphic shows the relative sizes of Earth, Neptune, Uranus, Saturn and Jupiter, from left to right. Saturn is somewhat smaller than Jupiter but much bigger than any other planet in our Solar system.

As you can see from the values in this table, Saturn is somewhat smaller in radius than Jupiter, but very much less massive. On average, Saturn is roughly half as dense as Jupiter. The rotation periods of the two planets are roughly the same.

The graphs above provide a comparison between the atmospheric structures of Jupiter and Saturn. As you can see, the chemistry of the two planets is very similar, there is a high altitude layer of ammonia ice above a layer of ammonium hydrosulfide ice, above a layer of water ice.

An important point to note in this comparison is that in Jupiter's atmosphere, these layers are spread over less than 100 kilometers of altitude, while in Saturn's atmosphere, the layers are spread over 300 kilometers. Also, Jupiter's radius is larger than Saturn's, so these cloud layers make up a smaller fraction of the planet's volume.

 These different chemicals provide the various colors evident when we look at Jupiter. They are also present in Saturn, but much less visible.

We saw that Jupiter radiates about twice as much energy as it receives from the Sun, and speculate that this energy comes from the heat of Jupiter contracting when it formed, still radiating into space. However, the energy output of Saturn is even higher. Saturn radiates about three times the energy it receives from the Sun. It does not make sense to think that this could also be energy of formation. Since Saturn is smaller, it should have less gravitation energy from formation that could be converted to heat during formation. Also, since Saturn has a smaller surface area than Jupiter, it should have radiated away away energy faster than Jupiter. We think there must be another mechanism present to account for the extra energy radiated by Saturn.

Liquid helium dissolves in liquid hydrogen at the temperatures and pressures found in Jupiter, but inside Saturn, the temperature is lower and helium does not dissolve as easily. It tends to condense into droplets and fall like rain. As the helium falls toward the center of the planet, the strong gravitational force compresses it and heats it up. The heat works its way up through the planet and escapes into space.

This theory is borne out by the fact that less helium is detected in the upper layers of Saturn than is seen in Jupiter.

This image of Saturn in infrared light, taken by the Cassini spacecraft, peers down into the haze to reveal swirling structure to the clouds, like that seen on Jupiter. Saturn is differentially rotating, and has features like belts and zones. Notice the hexagonal shape over the polar region.

Polar aurora like those seen on Jupiter are also evident on Saturn, as shown in this infrared photo.

The Cassini spacecraft captured details in this infrared image of a storm inside the polar hexagon. The eye of the storm measures about 2,000 kilometers wide. The wind speed of clouds at the outer edge of the storm are over 500 kilometers per hour.

In a very simplistic way, we can model the magnetic fields of rotating planets to be like a bar magnet. In this graphic, the relative sizes of the bar magnets corresponds to the relative strengths of the magnetic fields of Jupiter and Saturn. As you can see, Jupiter's magnetic field is considerably stronger than Saturn's. Jupiter's magnetic poles are offset by about ten degrees from its rotation axis, while Saturn's magnetic poles are closely aligned to its rotation axis.

Saturn's rings and moons

All of the Jovian planets in our solar system exhibit ring systems, but the rings seen around Saturn are by far the most prominent. From a tilt, we can see they are very extensive but seen edge-on, they are razor-thin.

This composite image was assembled from photographs taken by the Cassini spacecraft as it crossed the ring plane, showing Saturn's rings edge-on. The main rings are generally about 30 feet thick, though they measure about 175,000 miles across. They are relatively much thinner than a razor blade. This explains why Galileo reported seeing the rings disappear in 1612, though he did not understand why at the time.

This detailed image of Saturn's rings was taken by the Cassini spacecraft, assembled from radio signals sent through the rings back toward Earth. As Cassini moved behind the rings, the signals as they passed through the rings, providing a profile of the material in the rings. Different colors are used to represent different particle size in the rings. Bluish regions indicate the presence of particles less than one centimeter in diameter. Green regions indicate the presence of particles less than five centimeters in diameter. Purple shades indicate a lack of particles less than five centimeters in diameter.

This closeup of Saturn's A ring reveals spiral density wave structure inherent in a 220-kilometer span of the ring, which could be the result of a small moon systematically perturbing the ring, causing the particles to bunch in an oscillatory pattern.

This tiny moon, Pan, has created a gap in the rings of Saturn. Moons like this that orbit within the rings are called shepherd moons, because they "herd" ring particles by gravitationally deflecting them. Pan is a small moon, 17 miles across, and orbits within the Encke gap in Saturn's A ring.

Check out more information about Pan and other shepherd moons in the Cassini Image gallery (see May 2005).

This image shows one of Saturn's shepherd moons, Prometheus interacting with the F-Ring of Saturn. Prometheus orbits Saturn just inside the ring, crossing into the ring's inner edge about every 15 hours, gravitationally pulling ring particles toward it. Since the Prometheus lies closer to Saturn, its orbital speed is a bit faster than that of the ring particles, so it tugs on a different patch of particles each time it moves inside the ring.

The shepherd moon, Pandora, orbits just outside the F-ring. Together with Prometheus, these two moons gravitationally work together to deflect particles and hold them in the F-ring. Pandora is 50 miles across and covered with fine particles of icy dust.

Shepherd moons interact gravitationally with ring particles. The shepherd moons and particles both obey Kepler's second law, so the closer they are to Saturn, the faster their orbital velocity.

This means if a shepherd moon is inside the orbit of a particle, it tugs the particle forward. This deflects the particle outward, since now it is moving too fast for its orbital radius.

If a shepherd moon is outside the orbit of a particle, it tugs the particle backward. This deflects the particle inward, since now it is moving too slow for its orbital radius.

Two shepherd moons close together tend to herd particles into a narrow orbit, as seen above.

The two moons Epimetheus and Janus are close enough to each other that they gravitationally interact strongly, and work to swap orbits with each other.

When Epimetheus is orbiting closer to Saturn, it moves faster, and catches up to Janus from behind. Epimetheus tugs backward on Janus, while Janus tugs forward on Epimetheus.

This causes Janus to fall inward toward Saturn and Epimetheus to move outward. Janus has four times the mass of Epimetheus, so Janus's orbital radius changes less.

Now that Janus is closer to Saturn, it speeds up, while Epimetheus slows down. They both proceed around Saturn, and in four years, Janus catches up to Epimetheus from behind, and they swap orbits again. This orbital dance creating the Janus/Epimetheus ring, was documented by the Cassini spacecraft in 2006.

In general, shepherd moons are small moons that clear gaps in the rings. They are irregularly shaped, unlike large spherical moons. They can exist inside the rings because of their small size. Large spherical moons are held together, dominantly, by the force of gravity. Small moons do not have enough mass for the gravitational force to dominate over the electromagnetic force, so they do not take a spherical form.

Large, spherical moons cannot remain intact inside the Roche limit. Inside the Roche limit, the gravitational force of the planet overwhelms the gravitational force of the moon, and the moon it torn apart. Typically, the Roche limit is about 2.4 times the radius of the planet. This estimate is based on a simple calculation, assuming that the planet and moon have the same constant density. Saturn's Roche limit lies outside it's large A ring.

The E-ring is large and diffuse, created by streams of ice and water vapor emitted from Saturn's moon, Enceladus, orbiting just outside the Roche limit. The E-ring is normally very faint, but is visible here because the Cassini probe was behind Saturn as it took this photograph, so that Saturn was eclipsing the sun.

This image of Saturn's moon, Enceladus shows large surface features that may be evidence of tectonic plate activity. We suspect that a large ocean of liquid water lies underneath the surface ice. Note that impact craters are extremely rare in this image, indicating ongoing resurfacing of the moon. Ice geysers have been seen erupting from the surface of Enceladus. Internal heating is created from the tidal flexing experienced in Saturn's gravitational field.

Saturn has approximately sixty moons, the largest of which is Titan, seen here behind Saturn's rings and the small moon, Epimetheus. Titan's surface features are obscured by its thick nitrogen-rich atmosphere.

Time lapse photography reveals structures beneath the clouds of Titan as it rotates. The dark features are lakes of liquid methane. We believe that Titan experiences weather much like Earth, but at a much colder clime. The surface of Titan is about -180 degrees Celsius. On Titan, the surface is cold enough for methane to exist in liquid form, evaporate and fall like rain. Water geysers act like volcanoes, with water freezing solid on the surface like rock.

This shoreline with deep fjord-like structures marks the edge of a 300 square-mile hydrocarbon lake on Titan. Details in the features along the lake have been seen to change over time, signaling that the lake is active and dynamic.

The Cassini spacecraft dropped a probe, called Huygens, that fell through the methane clouds of Titan for 2 1/2 hours, taking data about the atmosphere as it fell. Huygens successfully landed on the surface, touching down at about 4.5 m/s. This image was taken by the Huygens probe as it sat on the surface of Titan, surviving for some 90 minutes. These surface rocks could be made of water and hydrocarbons on the frigid surface of Titan.

The Cassini spacecraft captured this natural color mosaic image of Saturn eclipsing the Sun. The night side of Saturn is partially lit by light reflecting from the rings. The tiny blue dot below the rings and just to the right of Saturn is the Earth, a billion miles away.

Carolyn Porco, leader of the Cassini spacecraft imaging team in NASA's Jet Propulsion Laboratory, explains how the eclipse image of Saturn and its ring system was created using a mosaic of 33 individual footprints.