Jupiter and Saturn


Jupiter and Saturn are the two largest planets. In these planets, the great pressure caused by the planets' gravity squashes the hydrogen to form an ocean of liquid under the atmosphere. Beneath the liquid hydrogen, the pressure squashes the hydrogen so hard that it turns into a solid, like a metal.


Neptune and Uranus


In the two other gas giants, Neptune and Uranus, there is an ocean of water, methane and ammonia beneath the atmosphere. This ocean surrounds the rocky core.

Can all the gas giants be seen from Earth?
No. Only Jupiter and Saturn can be seen regularly with the naked eye. Occasionally Uranus may be seen, when it is reflecting a large amount from the Sun. Neptune can only be seen with a powerful telescope. Jupiter can be seen for a few months each year. It appears as a bright light. If it is observed with binoculars, four moons can be seen around it and if it is observed by a small telescope its clouds can be seen. Saturn is dimmer but its rings can be seen with a small telescope.


What do things weigh on Jupiter?
If you were in a spacecraft in the cloud tops of Jupiter you would find the force of gravity was about two and half times that on Earth. You can get a sense of this by using a force meter to weigh some familiar objects, but pull on each one gently until it seems to weigh two and a half times more.

Have space probes been sent to the gas giants?
Yes. The first space probe to reach Jupiter was Pioneer 10 in 1972. It passed by the planet and has now left the Solar System. Voyager 2, launched in 1977, has visited all the gas giants on its way out of the Solar System. There is often a long time between the launch of a probe and its visit to a gas giant. Voyager 1, for example, was launched on 5 September 1977 but did not reach Jupiter until 5 March 1979.



The moons of gas giants


Do all the gas giants have moons?
Yes they do. Jupiter has 16 moons, Saturn has 18 moons, Uranus has 15 moons and Neptune has 8 moons. As more studies are made on the gas giants, more moons may be discovered. You could check on the Internet for the most recent news about the moons of the gas giants.

Why are gas giant moons being studied?
Because they have been found to be exciting and interesting places. Io is a moon of Jupiter which has volcanoes. Large volcanoes shoot out sulphur which gives the moon a red coat. Smaller volcanoes shoot out sulphur dioxide gas which turns to a solid and falls as acid snow. Triton is a moon of Neptune. It is a little smaller than our Moon but has an atmosphere containing nitrogen, like the Earth. Parts of its surface are covered in pink ice.

Which is the largest moon in the Solar System?
Ganymede. It is a moon of Jupiter. Ganymede is 5,276 kilometres in diameter. You can see how it compares with our Moon by making three Plasticine balls – one 12 cm in diameter for the Earth, one 3.5 cm in diameter for our Moon and a Plasticine ball 5.2 centimetres in diameter for Ganymede. Ganymede also has many craters like our Moon but its surface is covered in ice.

Could there be life on any of the moons?
There may be. Europa is a moon of Jupiter which has an icy surface. Underneath, scientists believe that there is an ocean of water. On the ocean floor there could be hot water springs which contain a chemical called hydrogen sulphide. These springs exist at the bottom of oceans on the Earth and strange life forms live there. Perhaps similar life forms may live at the bottom of the oceans of Europa.

How could we find out if there was life on Europa?
We could send a space probe that could burrow down through the ice and sink onto the ocean floor to look for hot springs. The cameras on board could send back pictures of what the probe found.