Above: a gas giant planet. Gas giants are large planets comprised almost entirely of 'gas' though most of this
gas is not gas in the ordinary sense! A typical gas giant is ten times the radius of ten Earth (radius about 70
000 km). Examples include Jupiter (equatorial diameter 142 985 km) and Saturn (equatorial diameter 120 537
km). The atmosphere of Jupiter is 86% hydrogen, 13% helium and traces of methane, ammonia and water. The
colours of the atmospheric is due to sulphur and sulphur compounds. Clouds of ammonia ice particles also
occur in Jupiter's atmosphere. Red clouds are made of ammonia ice and form the topmost cloud layer. The
middle cloud layer contains whitish ammonium hydrogen sulphide ice crystals and the lowest brown cloud deck
contains water ice crystals.
The upper atmosphere of a gas giant like Jupiter can be divided into darker horizontal bands called belts and
lighter (whitish) bands called zones. The gas flows in alternating directions around the planet as zones
alternate with belts (see the diagram below). The darker belts contain cooler gas which is sinking, whilst the
lighter zones contain warmer gas that is rising. This illustrates the process of convection occurring inside the
atmosphere. Convection is the process whereby less dense warm gas rises and denser cooler gas sinks.
These flows of gas are illustrated by the arrows in the diagram below. Turbulence arises at the boundaries
between zones and belts, which may give rise to vortex storms, for example, the Great Red Spot on Jupiter is
a long-lasting storm that is larger than the diameter of the Earth! Equatorial wind speeds on Jupiter are from
about 400 to 600 kph.
Above: bands of gas in the upper atmosphere of a gas giant like Jupiter. The whiter bands,
called zones contain rising warmer gas, whilst the darker belts contain sinking cooler gas. The
gas in alternating zones and belts also flows in opposite directions around the planet as shown
by the straight arrows.
The diagram above shows the internal structure of a typical gas giant planet. The atmosphere beneath the
cloud tops is shown as the outer blue layer and is composed mostly of hydrogen and helium. At a depth of
about 1000 km the increasing pressure causes the hydrogen gas to liquefy (shown as the green layer). At a
depth of about 7000 km the hydrogen changes state again, turning into fluid metallic hydrogen, shown in
light blue. Metallic liquid hydrogen is a form of hydrogen that one would not encounter at ambient conditions on
the Earth, but it can be produced in laboratories by the application of tremendous pressure. At the centre there
is thought to be a core, consisting of ice around a hot rocky and/or metallic core. This core is possibly not
solid, because of the high temperatures, but may be liquid and slushy (with some materials solid and others
liquid) - it is sure to be unlike any familiar material!
Jupiter radiates twice as much heat as it receives from the Sun (and Saturn 80% more). Some of this heat
comes from contraction - Jupiter is slowly contracting by 1 mm per year, converting gravitational potential
energy into heat. Possibly helium rains down inside the fluid metallic hydrogen layer, which would also
generate heat. Much heat would be produced by the decay of radioactive materials inside the planet, and
residual heat left over from the planet's formation is also still slowly escaping into space.
The graphs below show how the pressure (on the left) and temperature (on the right) are predicted to increase
with depth below the cloud tops of Jupiter. Note that the pressure scale on the vertical axis of the left hand
graph is given in scientific units, for example, 5.00E+07 means 5 followed by 7 zeros, or 50 million (50 000 000)
atmospheres of pressure (1 atm is the typical pressure at sea level on the Earth). The temperature in the
right-hand graph is given in degrees kelvin, K. (To convert from degrees Kelvin to degrees centigrade subtract
273.15, which makes little difference to such large numbers as we are dealing with here). The point to note is
how immense the pressure becomes deep inside Jupiter and how high the temperature is. Matter behaves very
strangely indeed under such extreme conditions.
Click on the thumbnails above to view additional images of Saturn (left) and Jupiter. Jupiter map courtesy of