The magnetosphere of Jupiter encounters the solar wind at about a million kilometers from the planet. The bow shock from this boundary reaches beyond the orbit of Saturn.

Saturn:

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 4.6 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 Atmosphere:

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 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:

The result is a warmer core and a lack of helium in the upper atmosphere of Saturn.

Saturn's Interior:

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; 1) the magnetic field is less strong, therefore the magnetosphere is smaller, and 2) 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.

Ring systems:

All the Jovian worlds have ring systems due to the massive tidal forces associated with the gas giants.

Jupiter

Saturn

Uranus

Neptune

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.

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.

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).

Uranus:

Uranus is the seventh planet from the Sun and is the third largest in the solar system. It was discovered by William Herschel in 1781. It has an equatorial diameter of 51,800 kilometers (1.9 Earth radii) and orbits the Sun once every 84.01 Earth years.

Uranus has a mean distance from the Sun of 19.1 A.U.'s. The length of a day on Uranus is 17 hours 14 minutes. Uranus masses 14.5 Earth masses.

Uranus is distinguished by the fact that it is tipped on its side with a rotational axis that is inclined to the orbit by 98 degrees.

Its unusual position is thought to be the result of a collision with a planet-sized body early in the solar system's history (also note the moons show evidence of a violent event in the past).

Note that this unusual axial tilt leads to a peculiar seasonal and diurnal motion as viewed from the ``surface''. For example, during summer in the northern hemisphere an observer would see the Sun making circles in the sky every 17 hours. As summer wanes, the Sun would gradually move south. Eventually, the Sun would rise and set to the autumnal equinox of equal day and night 21 years after the summer solstice. Then the nights would grow longer until one day the Sun would not rise and a long 21 year night would begin.

Uranus's atmosphere:

The atmosphere of Uranus is composed of 83% hydrogen, 15% helium, 2% methane and small amounts of acetylene and other hydrocarbons. Methane in the upper atmosphere absorbs red light, giving Uranus its blue-green color. In the inner Jovian worlds (Jupiter and Saturn), ammonium hydrosulfide dominates the coloration of atmospheres with its red's and yellows. But as the temperature drops below 70 K, ammonia gas freezes into ice crystals and drops out of the atmosphere. Methane becomes more dominate and, being a blue gas, the outer Jovian worlds (Uranus and Neptune) go from blue-green to deep blue in their coloration. Note also that methane, CH₄ is a greenhouse gas.

The atmosphere of Uranus is arranged into clouds running at constant latitudes, similar to the orientation of the more vivid latitudinal bands seen on Jupiter and Saturn, although these clouds are only visible in the infrared. Winds at mid-latitudes on Uranus move in the direction of the planet's rotation. These winds blow at velocities between 90 and 360 miles per hour.

Uranus lacks an internal energy source such as Jupiter and Saturn, and thus its atmosphere energy system is much less active, resulting in fewer features (i.e. storms, eddies, etc.). Cloud patterns are only seen at the warmer, lower levels deep below the atmospheric haze. In addition, the tilted axis of Uranus produces uneven warming in the two hemisphere which produce long-term North-South flows across the latitude zones. The combination of these effects means that the atmospheric features are washed out much like Saturn.

Neptune:

Neptune is the outermost planet of the gas giants. It has an equatorial radius of 24,900 kilometers (about 1.4 Earth radii). If Neptune were hollow, it could contain nearly 60 Earths. Neptune orbits the Sun every 165 years at a distance of 30 A.U.'s. It masses 17 Earth masses and has a mean density of 1.7 gm/cc.

Neptune has eight moons, six of which were found by Voyager. A day on Neptune is 16 hours and 6.7 minutes long. Neptune was discovered on September 23, 1846 by Johann Gottfried Galle, of the Berlin Observatory, and Louis d'Arrest, an astronomy student, through mathematical predictions made by Urbain Jean Joseph Le Verrier.

Neptune's atmosphere:

Unlike Uranus with its lack of atmospheric features, Neptune is a dynamic planet with several large, dark spots reminiscent of Jupiter's hurricane-like storms. The largest spot, known as the Great Dark Spot, is about the size of the earth and is similar to the Great Red Spot on Jupiter.

Other dark spots display cyclone-like structure in their centers.

Just like the storms on Jupiter, the dark spots on Neptune ``tumble'' along the zones absorbing smaller storms to power themselves. The most surprising thing about these storms is that, unlike Jupiter, they are short-lived. Recent HST do not show the Great Dark Spot.

Long bright clouds, similar to cirrus clouds on Earth, were seen high in Neptune's atmosphere. At low northern latitudes, Voyager captured images of cloud streaks casting their shadows on cloud decks below.

The strongest winds on any planet were measured on Neptune. Most of the winds there blow westward, opposite to the rotation of the planet. Near the Great Dark Spot, winds blow up to 1,200 miles an hour.

Neptune emits 2.7 times more energy than it receives from the Sun. This access energy powers the atmosphere to produce the storms that are not seen on its twin planet Uranus. The source of internal energy should not be leftover energy from formation (i.e. Jupiter) since Neptune is too small. Nor is it due to an unusual chemical change, such as the helium rain for Saturn. Rather, it is due to the fact that methane is highly abundant in Neptune's atmosphere, and methane is an excellent insulator of heat (i.e. the greenhouse effect). Neptune has a sub-zero type greenhouse effect that is trapping formation heat that should have been radiated billions of years ago like Uranus.

Uranus/Neptune interior:

The interiors of Uranus and Neptune are almost identical, due to the fact they are similar in mass and size. Both have rocky cores like Jupiter and Saturn. But at that point the similarity ends. The pressures are never sufficient to convert molecular hydrogen to metallic hydrogen in the interiors of Uranus and Neptune. Instead, a large mantle of icy water and ammonia forms about 20,000 km below the surface.

Uranus/Neptune magnetic field:

The magnetic fields for both Uranus and Neptune are unusual and are not well understood at this time. As the diagram below shows, the magnetic fields of the strongest three worlds, Jupiter, Saturn and the Earth, are all roughly aligned with the rotational axis of the planets. The generation of these magnetic fields occurs in liquid mantles around solid cores (liquid rock for the Earth, metallic hydrogen for Jupiter and Saturn).

Uranus and Neptune, on the other hand, have radically different magnetic fields. Not only are they not aligned with the rotational axis of the planet, but neither are they located at the center of the planet either. The magnetic fields are probably be generated by local events in the icy mantles of both planets and may be unstable.

Pluto/Charon:

Pluto/Charon was discovered on February 18, 1930, making it the last planet found in our Solar System. Originally thought to be a single planet, it has since been shown that Pluto and Charon are a binary planet system, two worlds in orbit around each other.

Pluto/Charon is usually farther from the Sun then any of the nine planets with a mean distance of 39.4 A.U.'s; however, due to the eccentricity of its orbit, it is closer than Neptune for 20 years out of its 249-year orbit. Pluto/Charon made its closest approach during 1989 and will remain within the orbit of Neptune until March 14, 1999. Pluto/Charon'orbit is also highly inclined by 17 degrees to the orbital plane of the other planets. Observations also show that Pluto/Charon's spin axis is tipped by 122 degrees.

Pluto/Charon is an enigma for its orbital irregularites and how it could have formed as a binary planet. But mostly for the fact that it is a Terrestrial world out beyond the Jovian worlds, difficult to explain in the context of the protoplanet hypothesis for the origin of the solar system.

Because Pluto has not yet been visited by any spacecraft, it remains a mysterious planet. Due to its great distance from the sun, Pluto's surface is believed to reach temperatures as low as -240C (-400F). From Pluto's surface, the Sun appears as only a very bright star. Ground-based observations indicate that Pluto's surface is covered with methane ice and that there is a thin atmosphere that might freeze and fall to the surface as the planet moves away from the Sun.

Charon (in the upper left) is bluer than Pluto and appears to be covered with water-ice rather than methane ice. Its orbit is gravitationally locked with Pluto, so both bodies always keep the same hemisphere facing each other. Pluto's and Charon's rotational periods and Charon's orbital period are all 6.4 Earth days.

The above Hubble Space Telescope picture, which was made in blue light, show that Pluto is an unusually complex object, with more large-scale contrast than any planet, except Earth. Pluto probably shows even more contrast and perhaps sharper boundaries between light and dark areas than is shown here, but Hubble's resolution (just like early telescopic views of Mars) tends to blur edges and blend together small features sitting inside larger ones.

Pluto/Charon Surface Features:

The brightness variations in this movie may be due to topographic features such as basins and fresh impact craters. However, most of the surface features are likely produced by the complex distribution of frosts that migrate across Pluto's surface with its orbital and seasonal cycles and chemical byproducts deposited out of Pluto's nitrogen-methane atmosphere.

The surface temperature of Pluto/Charon is near the freezing point of methane. As Pluto/Charon approaches perihelion a haze forms from evaporating methane. The Pluto/Charon moves away from the Sun, the methane haze ``snows'' onto the surface. The surface of Pluto/Charon should be dark due to radiation darkening where cosmic ray (high energy particle from outside the Solar System) break down molecules which leads to hydrocarbon growth (i.e. soot). However, the ``yearly'' methane snow regenerates the surface and produces the geography we see today.

Pluto/Charon Interiors:

The interiors of Pluto and Charon are mostly a mystery with models based on their mean densities. Pluto is so cold that there should be no liquid mantle of any element. A rocky core surrounded by ice is the most likely configuration. Charon is a mix of rocky and ice similar to the outer moons of Jupiter.