11. Uranus and Neptune

Interiors and magnetic fields

Water worlds

The atmosphere above the cloud tops of Uranus and Neptune consists mainly of molecular and atomic hydrogen, warmed by the Sunís ultraviolet rays. The tenuous gas forms an extensive hydrogen corona around Uranus, but is held closer to the cloud tops above Neptune. The overwhelming abundance of hydrogen in the outer atmospheres of Uranus and Neptune resembles that in Jupiter, Saturn and the Sun.

Fig. .. 

Unlike Jupiter and Saturn, however, Uranus and Neptune cannot consist mostly of the lightest element hydrogen, or they would have a lower mean mass density then observed. For their size, Uranus and Neptune are too massive for hydrogen to be their main ingredient, and their bulk must instead be composed of heavier abundant elements. To put it another way, both planets are too small for their mass to be mainly composed of hydrogen and helium, and must consist mainly of heavier material.

Fig. .. 

Since Uranus and Neptune have similar mass, size, composition and rotation, their interiors are also expected to be alike. But they must be quite different from Jupiter and Saturn inside. The hydrogen in Uranus and Neptune is confined within a thin atmosphere and liquid molecular shell that do not extend to great depths and contribute only about 15 percent of the planetary mass. These two planets do not have enough hydrogen, or sufficient mass and internal pressure, to squeeze the hydrogen into a metallic state. So there is no internal shell of liquid metallic hydrogen inside Uranus and Neptune.

Most of their interior probably consists of a vast internal ocean of water, H2O, methane, CH4, and ammonia, NH3. Although customarily denoted as ices, since they would be frozen at the cloud tops of these planets, these substances are kept liquid by the high temperatures, up to 8,000 degrees kelvin, deep in the planetary interiors. These molecules will form from atoms of hydrogen, H, oxygen, O, carbon, C, and nitrogen, N, the most abundant heavy elements in the material from which the Sun and giant planets originated.

Uranus and Neptune are not unlike the cores of Jupiter and Saturn, which similarly contain 10 to 20 Earth masses of melted ice and molten rock. But Uranus and Neptune are almost all core, without the deep envelope of hydrogen and helium that make up most of the mass of Jupiter and Saturn. The differences between these four planets apparently derive primarily from the amounts of hydrogen and helium that they were able to attract and hold as they formed.

Tilted magnetic fields

Like the Earth, Jupiter and Saturn, both Uranus and Neptune have strong magnetic fields. But the resemblance ends there. Here on Earth our magnetic pole is very near our geographic pole, which is very useful for navigation with a compass. The magnetic and rotational axes of Jupiter and Saturn are also closely aligned. But they are way off kilter on both Uranus and Neptune.

It is almost certain that the same dynamo process as that responsible for Earthís magnetic field generates the magnetic fields of Uranus and Neptune. In this mechanism, swirling currents in a fluid conduct electricity, generating and sustaining a planetís magnetism. This happens in Earthís molten metallic core, and it occurs within the liquid metallic hydrogen inside Jupiter and Saturn. Unlike these two giants, there is no shell of liquid metallic hydrogen inside Uranus and Neptune, but electrical currents within their vast internal oceans might generate the magnetic fields. It is probable that the electrical conductivity within Uranus and Neptune is provided by water-rich material that has a conductivity that is about two orders of magnitude less than that of metals. It is also likely that this conductivity comes from protons, not electrons, within the ionized waters.

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Copyright 2010, Professor Kenneth R. Lang, Tufts University