Saturn: Diameter, 120,000 km; Mass, 95.2 Earth masses; Density, 0.7 (density of water is 1.0); Rotation Period, 10 hours, 14 minutes; Axis Inclination, 26 44 ; Oblateness, 0.1; Surface Gravity, 1.15 (Earth = 1.0)
Today, we will mainly discuss Saturn. Its orbit, and its place in our solar system, are shown on the next two slides. It is a little more than 9.5 times further from the sun than is the earth, and almost twice as distant from the sun as Jupiter. Its orbit, like the earth s, lies pretty much in the ecliptic plane. Saturn rotates in the same sense as its orbital motion, as does the earth, and its rotational axis is inclined relative to the plane of its orbit, much like the earth s. The inclined rotation axis of Saturn means that Saturn has seasons, like we do on earth, although it is amusing to think how the seasons on Saturn might be affected by the presence of Saturn s ring system.
The following tables, repeated from last time, give comparisons of some of the physical properties of the planets. The most important columns of these tables to see for the moment are those that compare the sizes and material compositions of the planets. These data show that Saturn, like Jupiter, is a giant gaseous planet entirely different from the earth.
Sizes of the Giant Gas Planets
Saturn, like Jupiter, rotates very rapidly. For this reason, it is visibly oblate. Saturn has just about the same radius and rotation period as Jupiter, but it is less than a third as massive. Therefore Saturn exerts much less gravitational force than Jupiter does near its surface. Consequently, Saturn has less gravity than Jupiter to counterbalance the large centrifugal force generated near its equatorial surface by its rapid rotation. The result is that Saturn is visibly much more oblate than Jupiter. Remember that this situation, where Saturn is much less massive, yet nevertheless not very much smaller than Jupiter, arises because both planets are made out of gases rather than rocks or ice. If you were to add more mass to Jupiter, its gases would become more compressed by the increased gravity, and the planet might not become much larger. If you added enough mass, Jupiter might even become smaller!
Figure 11.3 (a) Gravity alone makes a planet spherical, but rapid rotation flattens out the spherical shape by flinging material near the equator outward. (b) Saturn is clearly not spherical.
The rapid rotation of Saturn, just as on Jupiter, inhibits circulation of any parcel of gas in the atmosphere over a wide range of latitudes. If a parcel of gas in the atmosphere near the equator were to travel very far toward either pole, the principle of conservation of its angular momentum about the planet s rotation axis would force it to spin up to a tremendous velocity. The gas therefore prefers to organize its circulation into bands, so that no part of the atmosphere travels over too wide a range of latitudes. The atmospheric circulation therefore breaks up into a series of belts and zones, horizontal bands in which prevailing wind velocities alternate. As on Jupiter, these motions are driven by convection. Convection carries heat outward from the interior to the surface, where it is radiated into space.
Despite the similarities of Saturn and Jupiter just mentioned, Saturn does not exhibit the garish atmospheric features that are so striking on Jupiter. Because of its overall lower surface gravity, Saturn s is thicker in its atmosphere than that of Jupiter. As a result, a layer of tan haze overlies the clouds and washes out Saturn s appearance.
However, the Cassini spacecraft has allowed us to look below this obscuring layer of haze to reveal a diverse array of clouds in the depths of Saturn. This new view on the right side on the next slide shows clouds at an altitude of about 30 kilometers (19 miles) underneath the clouds usually observed on Saturn. This is distinctly different from the typical view of Saturn in reflected sunlight, shown on the left. The left view is characterized by broad expanses of clouds near the 1-bar (1 Earth atmosphere of pressure) level, such as the white cloud seen circling the equator, with little hint of the discrete cloud complexes lying underneath. This new view was obtained at the far infrared wavelength, using Saturn s own thermal radiation emanating from Saturn s interior as the light source instead of the Sun. Clouds block this upwelling radiation to varying degrees, depending on the thickness of the cloud, and would appear as dark areas.
To render them visible as white clouds in the image shown on the right, the original view was photographically inverted. This technique enables clouds to be imaged across the globe, under both daytime and nighttime conditions. The images are also unaffected by shadowing from Saturn s immense ring system, unlike the left-hand image, which has much of the northern hemisphere covered by ring shadow.
The Cassini spacecraft discovered this hexagonal jet stream pattern, which has apparently been persistent for decades, at Saturn s north pole.
The Cassini spacecraft discovered this hexagonal jet stream pattern, which has apparently been persistent for decades, at Saturn s north pole.
View of Saturn from the Cassini Spacecrft, 2005. Here colors are pretty much as you would see them.
The Rings Of course, the most striking feature of this planet is its magnificent ring system. With the assistance of spacecraft, we have discovered that all the Jovian planets have Rings, but none as spectacular as Saturn's. All the Jovian worlds have ring systems due to the massive tidal forces associated with the gas giants. Saturn s rings orbit the planet inside what is called the Roche zone. This is the region so close to the planet (within 2 or 3 Saturn radii) that the tidal forces caused by the planet prevent the accumulation of sizable bodies as a result of their selfgravity.
Natural color mosaic of Saturn s rings at high resolution taken by the Cassini spacecraft in 2004. Gaps, gravitational resonances and wave patterns are all present, and the delicate color variations across the system are clearly visible. This mosaic of six images covers a distance of approximately 62,000 kilometers along the ring plane, from a radius of 74,565 kilometers to 136,780 kilometers (46,333 to 84,991 miles) from the planet's center. This view is from Cassini's vantage point beneath the ring plane. The rings are tilted away from Cassini at an angle of about 4 degrees. Images taken using red, green and blue spectral filters were used to create this natural color mosaic. The images were acquired using the Cassini spacecraft narrow angle camera on Dec. 12, 2004, at a distance of approximately 1.8 million kilometers (1.1 million miles). The image scale is 10.5 kilometers (6.5 miles) per pixel.
Rings are very thin compared to their width. Most are only a few tens of meters to a kilometer in thickness, compared to the almost 300,000 km diameter. This means that relatively speaking they are thinner than a CD. 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 (the collisions are inelastic, i.e. the particles do not bounce off each other perfectly like billiard balls) and adopt velocities and directions similar to the particles in the rings. Inelastic collisions are very important in the formation of the solar system. They are likely to have produced a thin disk of dusty/icy material particles orbiting the early sun, from which the planets themselves began to accumulate.
Taken by Cassini less than a degree above the ring plane
Here is an image from the Cassini probe that shows how thin the rings are, even when viewed from close up.
An artist s representation of particles in Saturn s A ring
Saturn s rings look very different when viewed in reflected light or transmitted light. Tiny dust particles scatter light, but mostly do not change its direction very much. (Just think about how dust on your car s windshield scatters sunlight.) Therefore when the sun is behind Saturn s rings, we see mostly light scattered by small dust-sized particles. Marble- or boulder-sized particles mostly scatter light back in the general direction that it came from. Therefore when we view Saturn s rings lit by sunlight which is reflected back toward us, we see the regions populated by these larger particles as the brightest, and the regions populated mainly by smaller particles appear dark.
Appearance of Saturn s rings under different lighting conditions (from Sky & Telescope Jan., 1981, p. 10)
Voyager-1 view of Saturn in its crescent phase
Voyager-2 view of Saturn in its crescent phase
Saturn s rings orbit the planet inside what is called the Roche zone. This is the region so close to the planet (within 2 or 3 Saturn radii) that the tidal forces caused by the planet prevent the accumulation of sizable bodies as a result of their selfgravity. Within the Roche zone, the tidal forces tending to pull an object apart are comparable to the gravitational forces tending to hold it together. Therefore only relatively small bodies, especially bodies held together by non-gravitational forces, as your body is, can survive inside the Roche zone.
Within the Roche zone, the tidal forces tending to pull an object apart are comparable to the gravitational forces tending to hold it together. Therefore only relatively small bodies, especially bodies held together by nongravitational forces, as your body is, can survive inside the Roche zone. When a moon or comet approaches within the Roche zone 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 in reflected sunlight 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.
Saturn s rings may have formed according to one of the following 2 scenarios: They are remnants of the original nebula from which Saturn formed. They were prevented from accumulating into a single moon by the tidal forces of gravity. They are the result of the tidal disruption of one or more objects that wandered into the Roche zone. If the object did not approach Saturn closely enough to be completely disrupted, its disintegration may have been aided by cometary or meteoroid impacts. One can object to the first scenario by arguing that over a period of perhaps 100 million years (a relatively short period compared to the age of the solar system) the particles now in Saturn s rings will be ground into dust by collisions or spiral into the planet. However, these particles may be continually resupplied from the progressive disintegration of small moons.
Artist s concept of Saturn s rings and major icy moons. The outer rings, labelled G and E, are diffuse. The E ring, the largest in the solar system, extends from the orbit of Mimas to that of Titan, a distance of 1 million km. The main rings (A,B,C) are less than 100 meters thick. They are perhaps only a few hundred million years old.
Saturn s rings show a wealth of internal structure. Many of the phenomena that cause this structure are complex, and we will not discuss them. However, some of the processes are easy to understand. The rings are exceedingly thin, as can be observed on the occasions when the earth (or a spacecraft) crosses the ring plane. The ring particles are composed mainly of water ice, and when these particles collide with one another, the collisions are not elastic. Such particles tend to chip each other rather than to bounce like superballs (or like the atoms in a gas). The inelastic collisions of the ring particles have caused the extremely low degree of random velocities compared to the size of the orbital velocities. The small random velocities make the rings so thin.
One of the most striking features of the rings are the prominent gaps within them. The most prominent of these is the Cassini Division.
Here we see Enceladus hovering past the B Ring, with 4 faint bands visible within the Cassini Division below and to the right of this moon.
The Cassini Division is due to an orbital resonance with the moon Mimas. Orbital resonance occurs when the orbital period of the moon and the orbital period of a ring particle are in a fractional relationship (e.g. 2 to 1 or 3 to 2). Just like pushing someone on a swing repeatedly at precisely the same point in the swing cycle, this fractional relationship of the orbital periods of the moon and the ring particle leads to an extra gravitational pull on the ring particle each time it comes to the same point in its orbital motion. This periodic extra force, always in the same direction, accelerates the ring particle into a new orbit. The final effect is to sweep particles out of the circular resonance orbits and into elliptical orbits that cause collisions of the ring particles and therefore produce gaps in the rings.
The Cassini Division is due to an orbital resonance with the moon Mimas. The Cassini Division is a 4800 km gap that occurs at 120,000 km from Saturn s center. At this location, a ring particle orbits Saturn in exactly half the time that it takes the moon Mimas to orbit. Thus every two orbits such a particle receives the same gravitational nudge from Mimas, which tends to make the ring particle s orbit eccentric (elliptical rather than circular). The growing eccentricity of the ring particle s orbit leads it to collide with other ring particles, and this process clears out a gap in the rings the Cassini Division.
Saturn s complex rings are both an intriguing scientific puzzle and a supreme natural wonder. This view shows, from upper right to lower left, the thin C ring, multi-toned B ring, the dark Cassini Division, the A ring and narrow F ring. At the bottom, Saturn s moon Mimas (398 kilometers, or 247 miles across) orbits about 45,000 kilometers (28,000 miles) beyond the bright core of the F ring. The little moon is heavily cratered and is thought to be largely composed of water ice. The bright speck just outside of (below) the F ring is the shepherd moon Pandora (84 kilometers, or 52 miles across). The image was taken in visible light with the Cassini spacecraft narrow angle camera on Jan. 19, 2005, at a distance of 1.8 million kilometers (1.1 million miles) from Saturn. The image scale is 11 kilometers (7 miles) per pixel. Pandora was brightened by a factor of seven to aid visibility.
M i m a s & P a n d o r a
Mimas as viewed by the Cassini spacecraft.
In the following image, the Enke gap in the outer A ring is clearly visible. It has a different cause than the Cassini Division.
Solid body rotation compared with motion due to gravity.
The inner moon shown orbits faster than the ring particles outside it. Therefore its gravitational attraction tends to speed up the orbital motion of these ring particles. As the ring particles begin to orbit faster, they experience a greater centrifugal force away from the planet, and they therefore move to more distant orbits. The outer moon shown orbits slower than the ring particles inside its orbit. Therefore its gravity tends to slow down the orbital motion of these particles. As these ring particles begin to orbit more slowly, they experience a reduced centrifugal force. As a result, Saturn s gravity pulls them closer to it, and they move to orbits closer to the planet. These effects of a moon on the ring particles inside and outside its own orbit explain the clearing of the Encke gap.
Pair of satellites near Saturn s F ring -- Sky & Telescope, 1/81, p. 12
The Enke gap in the outer A ring is 320 km wide. It is believed that it is caused by the gravitational disturbances of the 20-km-diameter satellite Pan orbiting within it. The gravitational tug of Pan on nearby ring particles pushes them into orbits with greater separation from Pan, thus clearing out the Enke gap in the ring. This counter-intuitive behavior of the ring particles is a result of the special nature of Keplerian motion, which is illustrated on the following slides.
Keeler Gap Moon
Cassini confirms that a small moon is orbiting within the narrow Keeler gap, near the outer edge of Saturn s A ring.
Saturn s moon Prometheus (63 miles across), followed by Pandora, is seen here making new diagonal gores in the tenuous material inside Saturn s F ring.
The small moon Prometheus and Saturn s F ring.
Saturn s moon Prometheus (63 miles across) is seen here making a new diagonal gore in the tenuous material inside Saturn s F ring. Prometheus creates a new gore each time it comes closest to the F ring, and the memory of these is preserved from previous passes.
Prometheus (63 miles across) and Atlas (12 miles across) orbit between Saturn s A and F rings. (view from Cassini spacecraft)
Prometheus (63 miles across) and Saturn s F ring. (view from Cassini spacecraft)
Pandora, shepherd moon orbiting just outside Saturn s F ring. (view from Cassini spacecraft) Recent computer simulations in Japan suggest that the Fring may have been formed as the result of a collision between Prometheus and Pandora that produced partial disintegration but left these two fragments with dense cores intact.