The Jovian Planets

 

Part I

Jupiter

 

The Jovian planets represent a fundamentally different type of planet than the Terrestrial group.  First of all they are large and massive.  Some statistics on Jupiter:

 

 

The colorful belts and zones you see are cloud features stretched out parallel to the equator of the planet by the rapid rotation - 9h 56m.  A storm feature called the Great Red Spot is similar to a hurricane, but is three times the size of the Earth and it has persisted ever since we have had telescopes to look at it.

 

All of the Jovian planets have rings, but only Saturn's are visually striking and permanent.  The others may be transient and have a consistency of smoke.

 

The interior model of Jupiter is devised using a wide range of information, physics, and chemistry.  Below the clouds the atmosphere (mainly composed of molecular hydrogen) is clear and grows denser as we penetrate further into the body of the planet.  The density increases to the point where the hydrogen liquefies and, eventually, takes on the properties of a metal (liquid metallic hydrogen).  Here we have the interior fluid required to produce the powerful magnetic field.  At the very center of Jupiter, the models allow for a small (diameter of the Earth) solid core.

 

The moon system of Jupiter resembles a miniature solar system.  Some of the moons are very small and irregularly shaped.  Galileo discovered the four large moons (Io, Europa, Ganymede, and Callisto) we call the Galilean satellites.  Since the beginning of radio astronomy, we have known that Io was special.  Io seems to control the sporadic radio emissions coming from Jupiter.  These emissions only appear when Io is in certain places in its orbit.  Io traps a piece of the powerful Jovian magnetic field.  This part of the field, called the Io Flux Tube, moves around Jupiter with Io, rather than with the rest of the field.  As the flux tube moves, it comes over a source region in the atmosphere of Jupiter, which causes the radio emissions.  Later a ring of sodium and potassium atoms was discovered scattered around the orbit of Io.  From the Voyager flybys we see a very odd-looking surface.  During the Voyager mission we discovered that Io has active volcanoes that send sulfurous lava flows over the entire surface, wiping out any craters that form.  The moon is kept hot on the interior by tidal heating from Jupiter.

 

Next in line is Europa, where tidal heating is less that at Io.  Europa is covered in ice and there is growing evidence that a global ocean exists under the ice.  If impactors strike and break through the ice, liquid geysers develop which "heal" the crater.

 

Still farther away from Jupiter is Ganymede, the largest moon in the solar system and bigger than Mercury.  Tidal heating is still less effective here, the interior is likely cold, and we see many craters on the surface.  Recall that a cratered surface is older than a smooth surface.

 

Part II

 

A close-up look at Ganymede shows that the surface has been contoured with many parallel, deep furrows caused by ancient ice flows.

 

Callisto is the most distant of the Galilean satellites and shows the most heavily cratered (oldest) surface.

 

Saturn

 

Some of the visual features we see from Earth base on Saturn include:

 

 

The interior model is much like that of Jupiter - liquid molecular hydrogen, metallic hydrogen, solid core as we go from outside to inside.  The main difference in the two planets is that Saturn has only 95 MÅ compared to 318 MÅ for Jupiter.  Saturn cannot raise the pressure as rapidly as Jupiter.  Thus the liquid hydrogen zone is bigger and the metallic hydrogen zone is smaller than for Jupiter.

 

The rings of Saturn are very broad (150,000 km) but extremely thin (perhaps less than 1 km).  The rings orbit about the equator of the planet and the rotational axis of Saturn is inclined some 24°.  As Saturn orbits the Sun we sometimes see the rings broadly displayed and other times the rings are edge on.  Because the rings are so thin, they disappear when seen edge on.

 

The existence of the rings is related to a gravitational boundary called the Roche Lobe.  Within the Roche lobe (or limit) tidal forces are great enough to disrupt any object of substantial size.  Two possibilities exist for the rings of Saturn:

 

 

We do note that all planetary rings are found inside of the Roche Lobe of the planet and all moons are outside.

 

The gap in the ring structure called the Cassini Division can be explained using Gravitational Resonances.  Since the ring particles act individually and obey Kepler's Laws, the distance of the particle from the center of Saturn determines its orbital period.  A particle at the distance of the Cassini Division has a period exactly half the orbital period of the moon Mimas.  Quite often the moon, the ring particle, and Saturn will align.  When such alignments occur, the combined gravitational forces of Mimas and Saturn will pull the ring particle out of its location.  Thus the Cassini Division is swept free of particles by a gravitational resonance between Saturn and Mimas.  The whole number ratio between the orbital periods does not have to be two.  Any whole number ratio should produce similar results.  We can predict that Saturn should have a great number of gaps in the rings structure induced by its many moons.  The Cassini Division is simply the most obvious.

 

Amoung the surprises of the Voyager flybys of Saturn were:

 

 

The moon systems also had some new features.  Most of the smaller moons are saturated in craters again, indicating a very old surface.  In the case of these moons, however, we are looking at ice craters.  Many of the moons of the outer planets are composed mostly of water ice.

 

Part III

 

The largest moon of the Saturnian system is Titan.  Titan has long been known to have an atmosphere.  The chemical mix is mostly nitrogen with methane providing many of the same functions that water does on the Earth.  We can not see through a permanent haze or smog layer in the atmosphere so a return mission to Titan is underway (Cassini).

 

Uranus

 

Three years and ten AU after the passage of Saturn Voyager II flew by Uranus.  Weather patterns in the atmosphere are almost nonexistent due to the great distance away from the Sun.  Uranus must also not be generating much internal heat.  The characteristic blue-green color is due to absorption of sunlight by methane.  Methane absorption occurs mainly in the red part of the spectrum.  White light with the reds subtracted away leaves blue-green (cyan).

 

The interior models are modified by the lesser pressure at Uranus compared to Jupiter or Saturn.  There is insufficient pressure to produce the liquefied hydrogen we saw in the larger planets.  Instead a region of molecular hydrogen gas eventually converts in water zone before reaching the solid core.

 

Thin rings exist around Uranus that resemble the F-ring of Saturn.  We saw that the F-ring was kept thin by the action of shepherd satellites and we again find this at Uranus.

 

One interesting moon of Uranus is Miranda.  This small ice moon has a very tortured surface that suggests that it may have been fractured and reassembled in the past.  We again see that catastrophe has played a role in the formation of the solar system.

 

Neptune

 

Three more years are required to reach Neptune.  The weather here is surprisingly active.  Since the planet is 30 AU from the Sun, the weather of Neptune must be driven by internal energy.  The great dark spot in the atmosphere resembles the great red spot of Jupiter.  The interior of this world probably resembled the interior of Uranus.

 

The moon system for Neptune is strange - the orbits are not in the equatorial plane of the planet and the orbits are quite eccentric.  These strange orbits may be the result of catastrophe.

 

The moon Triton has a very thin atmosphere and a surface covered in methane and nitrogen ice.  As meteors strike the surface and penetrate the ice covering, liquid methane geysers may results.