The Outer Planets


Separated from the four inner planets by the Asteroid Belt, the four outer planets dwarf the inner four. They do not have rocky surfaces the way Mercury, Venus, Earth and Mars do. Instead, Jupiter, Saturn, Uranus and Neptune are referred to as the four gas giants.

outer planets

of course, compared to our sun, they are not so giant!

Gas giants and sun

Another major difference between the outer planets and the inner planets involves moons and rings. There are only three moons involved with the inner planets -- one for Earth and two for Mars. On the other hand, each of the outer four planets have multiple moons. None of the inner planets has a ring around it. All four of the outer planets do, with Saturn's being the largest and most famous. Let's look at the outer planets and some of their moons. Jupiter is on this page with its moons and the other three have their own pages you can link to.



Jupiter is the largest planet in our solar system, famous for its giant "red spot" and the lines across it.


Jupiter is 5.2 AU from the sun. Like Mercury, it has almost no axis tilt -- only 3 degrees. Its diameter is 89,000 miles -- eleven times that of Earth. Being that large, we might assume it would take a pretty long time to turn on its own axis, but that is not true. It whirls around at an enormous speed, completing a Jupiter day in a little less than ten hours. It takes 11.86 Earth years to go around the sun once, for one of its own years.

Jupiter from the Inside Out

Jupiter has a massive metal and rock core which is 13 times the mass of the entire Earth.

Jupiter structure

Around the core is a 'shell' of water, methane, and ammonia ice.

Above this shell is a mantle of liquid metallic hydrogen and helium (protons in a sea of free electrons). This is surrounded by more liquid hydrogen and helium and, then, above that, is the atmosphere. The atmosphere is composed of mostly hydrogen (75%) and helium (24%). The last 1% is a combination of methane,water vapor, and ammonia. What we see through telescopes in the outside of the atmosphere, the cloud layers.

Jupiter radiates more energy than it gets from the sun.

Jupiter's Cloud Layers

Jupiter cloud layers


As you can see, Jupiter has both colored belts and white ones, as well as that famous 'red spot.' The dark belts are warm, rising gas from beneath, which brings up brown, red and orange clouds. The colors are caused by the sulfur and phosphorus compounds of hydrogen which are in the lower layers. The white zones show where ammonia crystals are descending. The temperature at the surface of the clouds has been measured by orbiting spacecraft at -162F. In addition, there are strong winds going in opposite directions along these belts and zones.

Jupiter jet streams

The result is massive turbulence between the belts and zones. Wind speeds can reach 375 miles per hour. The cross currents of winds cause strong cyclonic storms. The great red spot is the largest and longest lasting of these. It is partly the result of this turbulence combined with a large updraft at that point, at the edge of a white zone. It is probably caused by a permanent or semi-permanent updraft of heat from the radioactive core. An old photograph shows us that the red spot used to be much larger -- we know it has been around for at least 350 years, as Galileo saw it as early as 1610:

red spot

The photograph on the right shows us a lot of the turbulence surrounding the red spot. This "spot" is a static feature; its position only varying slightly as it rotates around Jupiter. This shows that it must have a relationship with the planet's core or "mantle" area. Is the red spot small now? That depends on your point of view. Here it is compared to Earth:

red spot vs. earth


Above Jupiter

Although Saturn is the planet famous for its ring system, all four outer planets have rings. Jupiter's was found in 1979. It goes from 60,000 to 135,000 miles from the center of Jupiter (the tops of the clouds are so variable it is not accurate to measure from them). The rings are about 75,000 miles wide. We have seen from our spacecrafts what these rings are made up of: tiny particles blasted out of Jupiter's four inner moons when they are hit by micro-meteorites (VERY small meteorites). The particles in the rings are angular and have not been eroded into rounded shapes. This indicates that they are quite young.

The picture below gives you some idea of why Jupiter's rings were not seen until we had spacecraft:

Jupiter rings

If you look very carefully, you will see a somewhat brighter line close in to Jupiter. This is the "main ring." The shadowing rings outside it are called "gossamer rings."

Jupiter moon, ring system

The four inner moons which are responsible for the rings are small. Amalthea is the largest (about a hundred miles across), following by Thebe, Metis, and then little Adrastea. The four moons which are much better known are much larger -- they are the moons Galileo saw and so they are called the "Galilean moons:" Io, Europe, Ganymede and Callisto. But Jupiter actually has 67 moons! They are divided into three groups.

1. The four innermost, which create the rings
2. The four major Galilean moons
3. 59 small moons orbiting beyond Callisto's orbit. They are small. Many of their orbits are highly inclined and not on the plane of the eight closer moons (on Jupiter's equator). Many have retrograde orbits. That means they do not circle Jupiter in a counter-clockwise way (as seen from Jupiter's north pole), but rather circle Jupiter the other way, in a clockwise direction. Their inclined, and often retrograde orbits mean these 59 moons are captured moons -- they were swung into orbit around Jupiter because of Jupiter's gravitational attraction as they came swinging in from other parts of the solar system.

The Galilean Moons



Io is slightly larger than our moon. It is 2270 miles in diameter (across). Like our moon, its rate of turning on its own axis is so slow that one side always faces Jupiter. But there the similarity ends.

First, Io cuts across Jupiter's magnetic field lines. This generates electric currents. As the charge builds up on Io's surface, Io and Jupiter end up exchanging lightning bolts, which evens the charge differential. The sound of these lightning bolts was picked up by radiotelescopes in the early 1950's. Astronomers wondered what that noise was. We could see nothing through our optical telescopes. But when the spacecraft Voyager circled Jupiter, it found the source of that noise -- giant lightning bolts being exchanged between Io and Jupiter. The result of the interaction with Jupiter are currents which may carry more than one trillion watts. .Do these lightning bolts affect the four small inner moons? It is very possible that a number of the 'micro-meteorite' hits that have resulted in Jupiter's rings were actually the result of the lightning bolts which would have ejected material from those little moons.

Io lightning


Io also does something no other moon does: it has created its own plasma ring, called a torus:

Io torus

Io is so close to Jupiter that it responds to Jupiter's gravity with volcanic activity. Even though Io is relatively small, Jupiter's pull keeps it hot inside so that there are active volcanoes. This effect is magnified by the fact that Io's orbit is somewhat elliptical. Tidal friction generated by this increases the interior heat and therefore the volcanic activity as well. The material from the volcanoes is swept up by Jupiter's rapidly rotating magnetic field. As the material comes out of the interior of Io, it is already ionized because of the heat. This is why the torus is a plasma.

What astronomers expected from Io was not what they got, and they were quite shocked at what they saw. They had expected the moons to be lifeless. What they saw were volcanoes. A lot of them. They were shooting material up to 190 miles above the surface of the moon.

Io volcano 1 Io volcano2
Io eruption map Io Named Volcanoes

The top two pictures are photographs of volcanoes erupting. The bottom left is a heat photograph, showing where some volcanoes are and the bottom right shows five named volcanoes of Io's over 400 volcanoes.

The photo below is of an area in Ethopia, but the landscape here is considered to resemble closely what Io's landscape looks like, but without the water:


Today, salt mining takes place in the vicinity of the Dallol Volcano, ... the immediate area of the volcano is, of course, uninhabitable. Fascinatingly, scientists believe the alien-like landscape of the volcano bears a striking resemblance to the surface of Io, the violently volcanic moon of the planet Jupiter.(Bright Side)



Europa is smaller than our moon, at only 1960 miles in diameter. It has the smoothest surface in the solar system. That is because it has a thin outer layer of water ice. The difference between its highest and lowest elevations is only about one hundred yards. The streaks we see are probably ice cracks where the water has come up and re-frozen. The features on the surface are only differences in color, not vertical relief. We are not sure why there are differences in color. It may be due to chemicals or due to materials deposited on the surface.

The icy layer resembles sea ice on earth. For this reason, astronomers are guessing there is a signicant layer of liquid water underneath. Their guess is that it is about 35 miles deep. If so, Europa is the only place in the solar system that has liquid water apart from Earth. Like Io, Europa's orbit is slightly elliptical and the tidal friction this produces would keep any interior water liquid. It is because of the possibility of liquid water that NASA is spending quite a bit of money to try to get a space probe to land on Europa.

There are only three significant craters on Europa (about 3.5 miles across). This indicates a continual resurfacing of the moon by the ice.



Ganymede is the largest moon in the solar system. It is 3300 miles across. It is larger than Mercury (3100 miles across). It has a thin oxygen atmosphere, an icy crust which flows slowly, and two types of terrain: the dark and the light areas. The lighter areas are covered with extensive grooves and ridges. The dark areas are smoother, and probably somewhat younger (they have fewer craters). Both areas are extensively covered with craters. Inspection of the craters shows they are very old, which means they are probably primarily the result of the Late Heavy Bombardment. The grooves and ridges of the lighter areas have craters both on top and below them.

We do see, in the bright spots in the photograph above, some relatively young impact craters with rays extending out from them. These younger craters probably came from the breakup of the Asteroid Planet and its moon.




Callisto is the farthest out of the four Galilean moons of Jupiter. It is 3000 miles wide, just a little smaller than Mercury. It has the most cratered surface in the solar system. It is entirely covered with craters. It couldn't escape anything: it got caught by pieces of the Asteroid Planet and its moon that exploded as well as being hit before that by the Late Heavy Bombardment. It had no atmosphere to slow down the incoming debris or burn anything up. It's surface is a firm, steady, and thick water ice. The oldest craters show signs of aging as the ice rims have flattened a bit. The largest ring structure on Callisto is Valhalla. It is about 1800 miles across. It is an impact crater.


Valhalla is larger than the Hellas Basin on Mars, and larger than the Caloris Basin on Mercury. This makes it the largest ring structure in the solar system.

* * * * * *

In 1994, comet Shoemaker-Levy took aim at Jupiter and, breaking up before it hit, smashed into it several times. We got some excellent photographs of the entire sequence of events:

Here is the comet as it was nearing Jupiter. The tidal forces of Jupiter's gravitational pull were breaking it up.


It became more separated as it neared Jupiter:

Shoemaker-Levy breakup

As a result, when it hit Jupiter, it hit in a number of places, circling the south pole as Jupiter rotated:

S-L hit

What we see above are the bright spots where the pieces were burning up going through Jupiter's atmosphere. Dark clouds of debris arose from the impact sites. This can be seen in the photograph below. The dark spot further up is one of the moons.

Jupiter comet clouds

Jupiter, as the largest planet, also has the largest magnetosphere (plasma sphere):

Jupiter magnetosphere

outer planet plasma spheres