The largest of the planets in our Solar System, the name Jupiter was an accident since the ancient astronomers did not know Jupiter's real size. Its radius is 11.3 Earth radii, its mass is 317 Earth masses. It is composed mostly of hydrogen (90%) and helium (9%) and traces of everything else. Jupiter's mean density is 1.3 gm/cc, close to that of water.

As we discussed before, the mean density of the Jovian worlds is near the value for water, 1 gm/cc, versus the terrestrial worlds which have average densities near the value of rocks, 3 to 5 gm/cc. This was due to the fact that temperatures in the outer Solar System are low because of the large distance from the warm Sun. So volatile compounds, such as ices like H2O, CO2, NH3, CH4, which tended to evaporate in the inner Solar System (although not all of them since there was plenty leftover to form secondary atmospheres) are abundent in the outer Solar System and make-up most of the comets, moons and rings around the Jovian worlds.

Note that H2O (water), CO2 (carbon dioxide), NH3 (ammonia) and CH4 (methane) are the simplest molecules you can make with hydrogen (H), carbon (C), oxygen (O) and nitrogen (N) = often called the HCNO compounds. Jupiter is also rich in NH4SH = ammonium hydrosulfide.

Jupiter's Formation:

The formation of Jupiter (and the other Jovian worlds) starts with the accretion (build-up) of ice-covered dust in the outer, cold solar nebula

Note that, due to gravity, the heavier elements sink to the core of proto-Jupiter, thus, we expect the core region to be rocky. While the lighter elements (H and He) remain in the atmosphere.

Jupiter's Atmospheric Features:

The exterior of Jupiter is noted by its brightly colored latitudinal zones, dark belts and thin bands dotted with numerous storms and eddies. Due to differential rotation, the equatorial zones and belts rotate faster than the higher latitudes and poles as seen in this Jupiter movie.

The zones and belts are zonal jet streams moving with velocities up to 400 miles/hr. Wind direction alternates between adjacent zones and belts. The light colored zones are regions of upward moving convective currents. The darker belts are made of downward sinking material. The two are therefore always found next to each other. The boundaries of the zones and belts (called bands) display complex turbulence and vortex phenomenon.

Note: Upward moving gases in Jupiter's atmosphere bring white clouds of ammonia/water ice from lower layers. Downward moving gases sink and allow us to view the lower, darker layers.

The most obvious feature on Jupiter is the Great Red Spot which is actually the top of a large cyclone twice the size of the Earth.

Redspot movie

Jupiter's Atmosphere:

Gas planets do not have solid surfaces, but rather build-up in pressure and density as one goes deeper towards the core. Different colors represent different depths into Jupiter's atmosphere. The colors (reds, browns, yellows, oranges) are due to subtle chemical reactions involving sulfur. Whites and blues are due to CO2 and H2O ices.

The detailed structure in Jupiter's atmosphere is dominated by physics known as fluid mechanics. Note that the atmosphere of Jupiter is so dense and cold that it behaves as a fluid rather than a gas. At the point were we see atmospheric features the pressure is 5 to 10 times that of the Earth's atmospheric pressure at sea level.

The simplest theories in fluid mechanics predict two types of patterns. One pattern occurs when a fluid slips by a second fluid of a different density. Such an event is known as a viscous flow and produces wave-like features at the boundary of the two fluids. A second pattern is produced by a stream of fluid in a constant medium, called turbulent flow. The stream breaks up into individual elements, called eddies. These eddies can develop into cyclones.

Cyclones develop due to the Coriolis effect where the lower latitudes travel faster than the higher latitudes producing a net spin on a pressure zone. The cyclones on Jupiter are regions of local high or low pressure spun in such a fashion. Note that the direction of the spin differs in the two hemispheres where clockwise spin is in the North and counter-clockwise spin is in the South.

Brown ovals are low pressure cyclones/storms in the North. White ovals are high pressure cyclones/storms in the South. Both can last on the order of tens of years. The Great Red Spot is a large high pressure storm that has lasted over 600 years.

On the Earth, the energy to power our storm systems comes from sunlight. Jupiter is too far from the Sun and receives very little energy. The energy needed to power all the turbulence in Jupiter's atmosphere comes from heat released from the planet's core.

Jupiter's interior:

Jupiter is highly oblate (flattened). Plus, Jupiter has a very high rotation rate (once every 9.8 hours). These two facts combine to tell us that Jupiter has a very small solid core.

Jupiter's interior consists mostly of hydrogen and helium. These elements are gaseous at the top of Jupiter's atmosphere down to several thousand kilometers. At this point, the pressures and temperatures compress these gases into a liquid state.

The liquid hydrogen, in molecular form at these levels (H2), continues to be compressed further reaching a metallic state. This occurs in a transition zone located 20,000 km below the atmosphere. Notice that at no time is there any real ``surface'' as one drops into Jupiter's interior.

At the very center of Jupiter is a small (15 Earth masses) rocky core, leftover from the icy dust particles that originally collected in the early solar nebula.

A planet absorbs energy from the Sun in the form of light and converts the energy into heat. The heat is then reradiated back into space (mostly from the nightside of the planet). Based on how much energy Jupiter absorbs from the Sun, then its mean temperature should be 105 K (about -280 F). However, IR and radio measurements of Jupiter show that it has a mean temperature of 125 K, or 20 degrees too warm. In other words, Jupiter radiates about twice as much energy as it receives. Conservation of energy requires that this heat come from someplace and the only reservoir is the core of Jupiter. Thus, this extra heat is leftover energy from the time of Jupiter's formation.

Many textbooks refer to Jupiter as a ``failed star''. This is due to the fact that if Jupiter were slightly more massive the temperatures in its core would have reached the ignition point for thermonuclear fusion. This is the process where stars turn hydrogen into helium and release energy (i.e. the star shines). If Jupiter were 100 times more massive, our Solar System would have had two stars.

Jupiter's radiation output:

IR and radio measurements revealed two components to Jupiter's radiation output; a thermal and non-thermal component.

A thermal component is associated with the leftover heat of formation (see above). The non-thermal component is associated with radiation that does not follow a Planck curve but follows what is know as a power-law spectrum. A spectrum that associated with synchrotron radiation.

Jupiter's magnetic field:

The magnetic field of Jupiter is 19,000 times stronger than the Earth's magnetic field. Even with a large rocky core and high rotation rate, the magnetic field is too strong. The origin of Jupiter (and other Jovian planets) strong magnetic field is the metallic hydrogen shell that surrounds Jupiter's rocky core. Metal is an excellent conductor of electric current and supplies the energy for the generation of an intense and large magnetic field.

A strong magnetic field can capture charged particles from the solar wind (i.e. high speed protons and electrons) and particles ejected from the inner moon, Io. These particles are trapped in the inner magnetic belts and are reflected back and forth between the north and south magnetic poles.

A visible result of this interaction is aurora or northern lights on Jupiter.

The interaction of Jupiter's strong magnetic field and nearby space produces a region known as Jupiter's magnetosphere. The magnetosphere has several features:

  • North and South magnetic poles
  • current sheet along the magnetic equator
  • plasma torus associated with Io's orbit
  • magnetopause - the boundary where the magnetosphere encounters the solar wind

    Magnetosphere movie

    The rapid rotation of Jupiter spews charged particles into a current sheet around the magnetic equator of Jupiter. Inside this current sheet orbits the moon Io. The current sheet sweeps up ejected ions from Io's geysers to make a plasma torus. The region around this plasma torus and the inner moon system is intensely radioactive with levels around 1000 times the radioactive levels of the Earth's surface. This region of space is uninhabitable by man or machine without heavy shielding.

    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 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 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 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 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.





    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.