The "Rings" Planet:
Saturn is the sixth planet from the Sun and is the second largest in the solar system with an equatorial diameter of 119,300 kilometers. Much of what is known about the planet is due to the Voyager explorations in 1980-81. Its day is 10 hours, 39 minutes long, and at a distance of 9.5 A.U.'s it takes 29.5 Earth years to revolve about the Sun.
Saturn is 95 Earth masses and has a radius of 9.4 Earth radii. The atmosphere is primarily composed of hydrogen (94%) with small amounts of helium (6%) and methane. Notice that this differs slightly from Jupiter, which is richer in helium (10%).
Saturn is the only planet less dense than water (0.7 gm/cc, i.e. it would float). Saturn's hazy yellow hue is marked by broad atmospheric banding similar to, but fainter than, that found on Jupiter.
One of the more obvious features is Saturn's ring system. Inclined at 27 degrees, the rings can be seen at various angles during Saturn's year. The last plane crossing was in May of 1995.
Saturn's features are hazy because its atmosphere is thicker. Jupiters mass is greater than Saturns. Therefore, its gravity is higher and a higher surface gravity compresses the atmosphere to 75 km in thickness. On Saturn, the low mass means less surface gravity and the atmosphere is thicker at 300 km from top to bottom.
The result is that Saturn's atmosphere has more haze and its features (turbulence, cyclones, etc.) are blurred and hard to see. This also causes its colors are muted into a general yellowish hue.
The wind blows at high speeds on Saturn, due to energy emitted from its core like Jupiter (see below). Near the equator, it reaches velocities of 1,100 miles an hour. The wind blows mostly in an easterly direction. The strongest winds are found near the equator and velocity falls off uniformly at higher latitudes. At latitudes greater than 35 degrees, winds alternate east and west as latitude increases.
This movie, taken by the Hubble Space Telescope, shows a rare storm that appears as a white arrowhead-shaped feature near the planet's equator. The storm is generated by an upwelling of warmer air, similar to a terrestrial thunderhead. The east-west extent of this storm is equal to the diameter of the Earth (about 12,700 kilometers). The Hubble images are sharp enough to reveal that Saturn's prevailing winds shape a dark "wedge" that eats into the western (left) side of the bright central cloud.
Saturn's Radiation Output:
As with Jupiter, Saturn radiates more energy than it absorbs from the Sun. In fact, it emits 2.3 times more energy than it receives. Jupiter's remnant heat is leftover energy from the time of formation. But, since Saturn is less massive than Jupiter, it should have less leftover energy yet it radiates more than Jupiter, this is a contradiction.
The answer to this dilemma lies in the missing helium in Saturn's atmosphere. Most of the Jovian worlds have what is called primordial abundances; 90% hydrogen, 9% helium and traces of everything else. This is the same abundance of elements that makes up the whole Universe.
Notice that the inner worlds are very different in abundances due to the changes from being too close to the Sun and too warm (they evolved into their current states). But the Jovian worlds have the same composition now as when they formed, similar to the primordial abundance of the Universe. But Saturn is deficient in helium. Its composition is 94% hydrogen and 6% helium, some helium is missing from the atmosphere.
The process was as follows:
- Saturn cooled faster than Jupiter due to its farther distance from the Sun
- Helium droplets formed when the temperature of the atmosphere dropped below 15 K
- the droplets condensed into a helium rain
- liquid helium is a superfluid and passes through the hydrogen mantle with no resistance
- potential energy of falling helium rain converts into kinetic energy and core heats up
The result is a warmer core and a lack of helium in the upper atmosphereof Saturn.
Saturn is more oblate than Jupiter. From this we deduce that its atmosphere and hydrogen mantle are proportionally larger than Jupiter's. This is not the same as saying that its rocky core is smaller. In fact, the cores of Jupiter and Saturn are similar. Saturn has a much smaller shell of metallic hydrogen, i.e. thinner metallic hydrogen mantle, thicker molecular hydrogen "crust". Therefore, if has more mass concentrated at its center.
Saturn's Magnetic Field:
Saturn's magnetic field is 8,000 times the strength of the Earth's magnetic field. This is quite strong, but less than 1/2 of Jupiter's magnetic field strength even though Jupiter and Saturn have similar rotation rates (the strength of a magnetic field is proportional to the size of the core or mantle and the speed of rotation). This is due to the fact that Saturn's metallic hydrogen shell is smaller than Jupiter's.
Saturn's magnetosphere is smaller and there is no current sheet like Jupiter's. This is due to two reasons;
- the magnetic field is less strong, therefore the magnetosphere is smaller, and
- the rings of Saturn serve to damp out the charged particles that we saw associated with Jupiter's system.
The above image is the first ever taken of bright aurorae at Saturn's northern and southern poles, as seen in far ultraviolet light by the Hubble Space Telescope. The aurora is produced as trapped charged particles precipitating from the magnetosphere collide with atmospheric gases. Hubble resolves a luminous, circular band centered on the north pole, where an enormous auroral curtain rises as far as 2,000 kilometers above the cloudtops. This curtain changed rapidly in brightness and extent over the two hour period of HST observations.
All the Jovian worlds have ring systems due to the massive tidal forces associated with the gas giants.
When a moon or comet approaches within the Roche limit of a planet, the tidal forces overcome the internal forces and disrupt the moon/comet. The broken pieces are distributed into a ring shape. We know that that the rings are not solid or liquid since Doppler measurements show that the rings are made of separate particles moving in circular orbits. High albedo means rings are typically made of ice (captured comets?).
The brightness of the rings is proportional to the size of the particles in the rings. The brightest rings are made of house-sized blocks of rock/ice. The faintest rings are made of icy dust.
Rings are very thin compared to their width. Most are only a few tens of meters to a kilometer in thickness. This is due to the fact that a particle that lies in an orbit above and below the ring must pass through the ring twice each orbit. This leads to collisions which cause the particles to exchange energy and adopt velocities and directions similar to the particles in the rings.
Saturn's rings are the most prominent and were show by Voyager to be composed of hundreds of ringlets. Each ringlet displays a region of high or low number density of particles (note that number density is not the same as particle size). Gaps in the rings are due to orbital resonances with the outer moons.
Orbital resonance occurs when the orbital period of the moon and the orbital period of a ring particle are in a fractional configuration (e.g. 2 to 1 or 3 to 2). Just like pushing someone on a swing, this leads to an extra gravitational pull on the ring particle to accelerate it to a new orbit. The final effect is to "sweep" particles out of the resonance orbits to produces gaps.
Orbital resonance would, after billions of years, eventual sweep all the particles out of a ring. However, the effect of inner moon counteracts the pull from the outer moon. Shepherd moons work in pairs on the inner and outer edge of rings to gravitational push and pull (accelerate and de-accelerate) ring particles. The result is to confine the ring particles to within the shepherd moons orbits.
On thin rings, the interaction of the shepherd moons is to direct particle in a complex path, similar to the flow of traffic on a freeway. The resulting paths can produce changes in density within the ring. Or can result in a twisted ring due to streaming of the orbital paths.
Saturn's rings also display radial spokes of darker regions. These spokes move with the rotation of Saturn as can been seen in this spoke movie. The spokes are thought to be the shadows of smaller particles levitating a few tens of meters above the rings due to electrostatic forces (the "cling" on fabrics fresh out of a dryer).
The F ring, above, resolves into five separate strands in this closeup view. Potato-shaped Prometheus is seen here, connected to the ringlets by a faint strand of material. Imaging scientists are not sure exactly how Prometheus is interacting with the F ring here, but they have speculated that the moon might be gravitationally pulling material away from the ring. The ringlets are disturbed in several other places. In some, discontinuities or "kinks" in the ringlets are seen; in others, gaps in the diffuse inner strands are seen. All these features appear to be due to the influence of Prometheus.
Daphnis, 8 kilometers (5 miles) across, occupies an inclined orbit within the 42-kilometer (26-mile) wide Keeler Gap in Saturn's outer A ring. Recent analyses by imaging scientists illustrate how the moon's gravitational pull perturbs the orbits of the particles forming the gap's edge and sculpts the edge into waves that have both horizontal and vertical components.
Titan is the largest satellite of Saturn, unique in its methane atmosphere:
- Detected to have spectral lines of methane in 1944
- Heavy atmosphere (1.4 times the Earth's) with structure in the form of haze - reddish color from organic molecules
formed by breakdown of CH4 by cosmic rays
- Composition of atmosphere is 90% N2, 7% CH4, 2% Ar
and traces of ethane, acetylene, ethylene, propane and hydrogen cyanide (i.e. smog)
- Titan's surface temperature averages around -178C (-289F)
- The temperature and pressures are near the triple point of
methane (CH4) => methane rain, snow, ice, clouds, rivers, etc.
- Earth's environment is near the triple point of water - water serves as the solvent for organic chemicals to interact = life. Titan is an excellent place to search for new forms of life based on methane as a solvent
Images from the Cassini-Huygens mission:
Above is a moasic of three frames taken by the lander as it entered Titans atmoshere. It shows detail of a high ridge area including the flow down a major river channel (liquid methane river) into what appears to be a "wet" plain.
The above image shows the boundary between lighter-colored uplifted terrain and drainage channels into the darker lower areas. This picture was taken from an altitude of 8 kilometers.
Above is a color image from the surface of Titan. Rocks near the bottom are pebble-sized, objects in the middle of the frame are meters in size. The surface is darker than expected, consisting of a mixture of water and hydrocarbon ice. There is evidence of erosion indicating fluid activity in the recent past.
The principal Saturn's characteristcs are:
- It is visibly flattened (oblate) when viewed through a small telescope; its equatorial and polar diameters vary by almost 10% (120,536 km vs. 108,728 km).
- Saturn is the least dense of the planets; its specific gravity (0.7) is less than that of water.
- Like Jupiter, Saturn is about 75% hydrogen and 25% helium with traces of water, methane, ammonia and "rock"
- Saturn's interior is similar to Jupiter's consisting of a rocky core, a liquid metallic hydrogen layer and a molecular hydrogen layer.
- Saturn's interior is hot (12000 K at the core) and Saturn radiates more energy into space than it receives from the Sun.
- Two prominent rings (A and B) and one faint ring (C) can be seen from the Earth. The gap between the A and B rings is known as the Cassini division. The much fainter gap in the outer part of the A ring is known as the Encke Division (but this is somewhat of a misnomer since it was very likely never seen by Encke).
- Though they look continuous from the Earth, the rings are actually composed of innumerable small particles each in an independent orbit. They range in size from a centimeter or so to several meters. A few kilometer-sized objects are also likely.
- Saturn's rings are extraordinarily thin: though they're 250,000 km or more in diameter they're less than one kilometer thick.
- There are complex tidal resonances between some of Saturn's moons and the ring system: some of the moons, the so-called "shepherding satellites" (i.e. Atlas, Prometheus and Pandora) are clearly important in keeping the rings in place; Mimas seems to be responsible for the paucity of material in the Cassini division.
- Like the other jovian planets, Saturn has a significant magnetic field.
- It has 62 (53 with formal names) confirmed satellites.
Saturn was first visited by NASA's Pioneer 11 in 1979 and later by Voyager 1 and Voyager 2. Cassini (a joint NASA / ESA project) arrived on July 1, 2004. The original mission was scheduled for at least four years, but it still working.
If this is Saturn, where are the rings? When Saturn's "appendages" disappeared in 1612, Galileo did not understand why. Later that century, it became understood that Saturn's unusual protrusions were rings and that when the Earth crosses the ring plane, the edge-on rings will appear to disappear. This is because Saturn's rings are confined to a plane many times thinner, in proportion, than a razor blade. In modern times, the robot Cassini spacecraft orbiting Saturn now also crosses Saturn's ring plane.
A series of plane crossing images from 2005 February was dug out of the vast online Cassini raw image archive by interested Spanish amateur Fernando Garcia Navarro. Pictured above, digitally cropped and set in representative colors, is the striking result. Saturn's thin ring plane appears in blue, bands and clouds in Saturn's upper atmosphere appear in gold. Since Saturn just passed its equinox, today the ring plane is pointed close to the Sun and the rings could not cast the high dark shadows seen across the top of this image, taken back in 2005. Moons appear as bumps in the rings.