Jupiter and its moons

1.3 The interior

David A. Rothery Teach Yourself Planets, Chapter 9, pp. 107-39, Hodder Education, 2000, 2003.

Copyright © David Rothery

Impressive as was the descent of the Galileo entry probe, it penetrated less than a third of a per cent of the way to Jupiter's centre. Our picture of the planet's interior is therefore based on theoretical models of how a large mass of what we understand to be Jupiter's composition should behave, supplemented by insights from the planet's gravity and magnetic fields. As Figure 2.5 shows [figure not included in this course], the outer layer, composed dominantly of molecular hydrogen (mixed with helium), extends below the cloud tops to a depth of about 10,000 km. It would be an oversimplification to refer to the whole layer as 'the atmosphere', because in its lower part (where the pressure exceeds a hundred thousand times the Earth's atmospheric pressure) this hydrogen would behave more like a liquid than a gas.

However, the first clear break in properties is believed to occur at about 10,000 km depth, where pressure is about a million times that at sea-level on Earth and temperature is about 6000°C. Under these conditions the bonds holding together the hydrogen molecules (each consisting of two hydrogen atoms) must break. In this situation, the hydrogen atoms are unbound and the electrons are free to wander through the spaces between the atoms, rather than forming chemical bonds as they would under lower pressure. These are the properties of a molten metal, and hence this layer is described as metallic hydrogen in Figure 2.5. Below this layer Jupiter must have a shell of high pressure ice (probably mostly water in composition), surrounding a core of rock and possibly an inner core of iron. The pressure in the core is probably about 40 million times Earth's atmospheric pressure and the temperature about 17,000°C. The state of the rocky material in Jupiter's core is unknown. It is probably molten, but the tremendous pressure (20 times that occurring within the Earth) could compress it into a solid. If so, the minerals would be high-density varieties rather than those familiar on Earth. Jupiter's core may look small at the scale of Figure 2.5, but probably contains 10-20 times the mass of the Earth.

Jupiter has by far the strongest magnetic field of all the planets, 20,000 times the strength of the Earth's. It is tilted at 9.6° relative to the planet's rotational axis, and appears to be generated by convection within Jupiter's thick metallic hydrogen layer. This powerful magnetic field holds in place a belt of ionized sulfur centred about the orbit of Jupiter's innermost large satellite, Io, and channels high energy electrons towards Jupiter's poles, where they produce spectacular auroral displays (Figure 9.3 [ Figure 6 is a higher quality version of this image]).

Figure 9.3: Visible evidence of Jupiter's internally generated magnetic field: an aurora (glow from ionized atoms) seen by the spaceprobe Galileo on Jupiter's night-side[.] Lines of latitude and longitude have been superimposed[.]
NASA NASA

A notable property of Jupiter, which it shares with Saturn and Neptune though not Uranus, is that it radiates to space about twice as much energy as it receives from the Sun. This rate of heat loss, about seven watts per square metre, is 1000 times the feasible rate of radiogenic heat production in Jupiter's rocky core. Instead, it probably indicates that Jupiter is still contracting in size, converting gravitational potential energy into heat. If Jupiter had been about thirteen times more massive, this process would have been sufficient to raise its internal pressure and temperature to the point where nuclear fusion of hydrogen begins, and Jupiter would have become more like a star than a planet.