Saturn.

THE INTERIOR OF SATURN

Of all nine planets, Saturn has the lowest density (690 kg m-3) . It is less than a fifth the density of the inner terrestrial planets, Mercury, Venus, and Earth.

Composition

Like Jupiter, Saturn is made up of light elements, comprising mostly hydrogen and helium. The great size and mass of the gas giants (Saturn is 120,500 kilometres across) makes them gravitationally powerful, preventing the light elements from escaping. The relative proportions of hydrogen and helium in Saturn and Jupiter are slightly different though, as inferred from the amounts detected in the atmosphere. Saturn has 11% helium in its upper atmosphere, whereas Jupiter has 19%. Deeper within Saturn the percentage rises to about 50%, whilst hydrogen comprises 67% of the material near Jupiter's centre.

The interior structure of Saturn is thought to be quite similar to Jupiter's interior. But differences are expected between the two gas giants, related to their mass and thus gravity as well as variation in the ratio of hydrogen to helium. Compositionally they are broadly similar, both have a small rocky inner core, which is encased in a very thick layer of liquid metallic hydrogen.

Cross-section of Saturn.

Metallic hydrogen

Within Saturn at high pressure, hydrogen atoms are broken apart. Their electrons are freed leaving bare protons. The resulting matter is called "liquid metallic hydrogen". Metallic hydrogen is not a metal but a liquid with unusual electrical characteristics, a liquid with the conductive properties of a metal. The metallic hydrogen layer is in turn mantled by liquid hydrogen and an outer envelope of gaseous hydrogen.

Hydrogen makes a transition from a gas to a liquid at a depth of 1000 kilometres in Saturn - the same transition occurs at the same depth in Jupiter. However, because Saturn has a lower mass and a smaller gravitational pull, the liquid hydrogen layer is far thicker than Jupiter's because on Jupiter the gas is more highly compressed. On Saturn the metallic hydrogen core is encountered at greater depth within the planet. The transition from liquid to metallic hydrogen occurs on Saturn 32,000 kilometres down, where the temperature is 9000 K, on Jupiter however the boundary is just 25,000 kilometres below the surface.

The core

It is possible that sandwiched between the rocky inner core and the metallic hydrogen is a layer of very compacted ice about 12,000 kilometres deep. The icy part of the core could have a mass of about 20 to 30 times that of Earth's.

Saturn's inner core is probably small and rocky. It is estimated to be about the same size as the Earth and to have a mass three times that of our planet. Calculations indicate that the pressure at the core of Saturn is 18,000 atmospheres and has a temperature of 13,000 K.

Helium rain

Saturn radiates nearly twice as much energy into space as it receives from the Sun. The planet emits 20 x 1016 watts, mostly at infrared wavelengths. Some energy was available within Saturn when it formed from the gaseous solar nebula, but this does not explain its ability to radiate heat presently. Despite its large size it ought to have expended its internal energy after about 2.5 thousand million years.

One idea how Saturn is able to continue radiating heat, is that at a certain depth within the planet, hydrogen and helium are immiscible. This means that, like oil and water, they don't mix. At a depth where hydrogen changes into its metallic state, helium is separated out and moves as droplets to a deeper level; this is called "raining-out". This does not happen on Jupiter because it is hotter (11,000 K at the liquid/metallic hydrogen boundary) and the hydrogen and helium are able to intermingle. On Saturn, however, the droplets of helium are retarded by friction as they fall. This generates the heat which is transferred through the upper layers and radiated into space.