Chapter 10: The Giant Planets

10.1 Exploring the Outer Planets

There are five outer planets: Jupiter, Saturn, Uranus, Neptune, and Pluto. The first four of these are known as giant or jovian planets. Pluto is discussed in chapter 11.

Composition and Chemistry

The giant planets are composed primarily of hydrogen and helium, also the primary constituents of the Sun. Take Jupiter for example. It is basically a big ball of hydrogen and helium. The Sun is a bigger ball of hydrogen and helium. If Jupiter were about 100 times bigger, it would have sufficient mass to begin fusion, the process that generates energy in the Sun. Jupiter's composition is basically identical to that of a star, but too small to ignite, so studying Jupiter can lead to insights into how stars and solar systems form.

The giant planets are cool enough to also contain molecules, the primary ones being water (H20), ammonia (NH3), and methane (CH4). These molecules contribute to much of the color seen in the atmospheres of the giant planets.

Exploration of the Outer Solar System

Most of what we've learned about the giants comes from the spacecraft that have been sent there. Six spacecraft have been to the outer planets and a seventh is on the way.

The spacecraft that make this journey must be rugged, reliable, and capable to react to emergencies independent of Earth controllers. The flight to the giant planets takes years before the craft begins the work it was designed for. And the time to receive a signal and send a reply is about an hour for a craft near Jupiter, and several hours for a craft near Neptune.

The craft must carry its own power source because the sunlight is too weak for solar cells to supply all the power. I believe that all the spacecraft mentioned carried a power source that relied on radioactive decay.

The craft must survive a trip through the asteroid belt.

Finally, the craft must survive the radiation present in the magnetospheres of the giant planets.

Galileo and Cassini

The Jovian Planets

Basic Characteristics

The basic properties of the Jovian planets are summarized in Table 10.3. Their distance from the sun ranges from 5.2 AU for Jupiter to 30 AU for Neptune. Their periods of rotation are correspondingly large, 12 (Earth) years for Jupiter and 165 for Neptune.

While their masses are large, their densities are about the same as the density of water on Earth, 1g/cm3. Saturn's density of only 0.7 g/cm3 means it would float if we could find a body of water big enough to put it in.

Appearance and Rotation

All that we are able to see of the giant planets is their atmosphere. Mostly we see the uppermost clouds. This makes it difficult to accurately determine the planet's rotation -- even on Earth, clouds move at a different rate than the surface. But the rotation can be determined by measuring the rotation of the magnetic field.

What is found is that their rotation rates range from 10 hours for Jupiter to 17 hours for Uranus. This is rather fast for such a large planet, and may account for much of the interesting weather patterns seen in their atmospheres.

Recall that seasons on Earth result from the tilt of the axis of rotation to the plane of orbit. The giant planets can also have a tilt. The tilt for Jupiter is small, about 3°. For Saturn and Neptune, the tilt is slightly more than for the Earth, 27° and 29° respectively. Uranus has an unusually large tilt of 98°. This means that Uranus basically rotates "on its side" (see Figure 10.6). The seasons on Uranus unusual, at least compared to our experience. It is believed that this unusual tilt resulted from a collision with a large body.

(Note the trend: "collision with a large body" is the catch-all explanation for all the unusual properties seen, the formation of the Moon, the rotations of Venus and Uranus, the composition of Mercury, and the existence of the asteroid belt.)

Composition and Structure

Our understanding of the composition and internal structure of the giant planets is not based on direct measurement, but rather inferred from calculations of mass, density, and gravity. Improved measurements or calculations may lead us to alter our understanding.

The outer part of the giants consists mostly of molecular hydrogen (H2), gaseous in the outermost layer, then turning to liquid at the larger pressure present deeper in. In Jupiter and Saturn, the pressure becomes so great that the hydrogen goes into a metallic state! A metal is capable of conducting electricity, and this plays a role in the formation of the magnetic fields around Jupiter and Saturn.

In the innermost core of the giants is believed to be material composed of carbon, nitrogen, oxygen, and hydrogen (called ice), and then rock (any heavier elements).

Internal Heat Sources

Magnetic Fields

All of the giant planets have magnetic fields. The magnetic fields trap charged particles from the Sun, just as happens around the Earth. These charged particles pose a radiation hazard to spacecraft sent to study the planets.

10.3 Atmospheres of the Jovian Planets

Atmospheric Composition

The atmosphere is all that we see of the giant planets. They show a striking array of colors resulting from the compounds in the upper atmosphere: ammonia, methane, sulfides, and phosphates.

Clouds and Atmospheric Structure

Jupiter and Saturn have bands of clouds moving east-west at different speeds (and in different directions, see Figure 10.14). The bands seem to be stable of rather long periods of time. Uranus and Neptune have similar "winds".

Winds and Weather


Along with east-west winds we find circulating storms, similar to high and low pressure systems on Earth. The most famous of these storms is the Giant Red Spot on Jupiter, a storm system larger in diameter in the Earth and believed to be stable for centuries. Neptune had a large storm known as the Great Dark Spot, seen in 1989, but not visible now. Many smaller storms exist with lifetimes of years or decades.

© Robert Harr 2001