4.5  Floating, sinking and planetary structure

One everyday effect related to density is whether something floats or sinks. Polystyrene packing beads float on water because they are less dense than water. Pebbles sink in water because they are more dense than water. Another effect can be observed if you make a traditional salad dressing from vinegar and oil: the denser liquid, vinegar, will sink and the less dense liquid, oil, will float on top of it. In the interiors of Earth and Jupiter, the denser material in each case sinks to the centre (Figure 11).

Figure 11  Cutaways showing the interiors of Jupiter and Earth. The core of the Earth is much denser than the rocks at the Earth’s surface. Astronomers aren’t yet certain what is at the core of Jupiter or other giant planets, and it is possible that they have rocky cores.

Show description|Hide description

A diagram of Jupiter and Earth, with a section cut out of each of them, showing the interiors. For Jupiter, the outer layer is labelled gaseous hydrogen, the second layer is labelled liquid hydrogen, and the third layer is labelled metallic hydrogen. The centre is labelled, with a question mark, core of rock, metals, and hydrogen compounds. For Earth, the outer layer is labelled atmosphere, the second layer is labelled crust, and the third layer is labelled mantle. The centre is labelled core of iron.

Figure 11  Cutaways showing the interiors of Jupiter and Earth. The core of the Earth is much denser than the rocks at the Earth’s ...

Generally, both planets and stars behave a bit like salad dressing: the denser material sinks and the less dense material floats. So if a planet is composed of several different materials, they will generally be arranged with the densest material at the centre and the least dense at the surface. Clearly it is not straightforward to make a direct measurement of the composition of the material at the centre of any planet. Scientists are fairly sure from indirect measurements that the Earth has an iron core. This sinking and floating would have taken place when the interior was molten early in Earth’s formation.