Chapter 7: The Giant Planets and Their Moons

As we might expect, the temperatures of the giant planets reflect their increasing distance from the Sun. Received radiation decreases as the inverse square of the distance from an energy source. Put simply, distant planets receive less of the Sun's warming rays than inner planets. If we compare temperatures at the depth in the atmosphere where the pressure is equal to sea-level pressure on Earth, the typical temperatures of Earth, Jupiter, Saturn, Uranus, and Neptune are: 290 K, 170 K, 150 K, 78 K, and 69 K. (In degrees Fahrenheit, the sequence from Earth out to Neptune is: 63 °F, -153 °F, -189 °F, -319 °F, and -335 °F.) The trend toward colder conditions as we move away from the Sun is clear.

Our studies of other planets' atmospheres are still in their infancy. Results from the Voyager and Galileo probes and flybys gave us a better understanding of the atmospheric conditions of all four giants than we could glean from Earth. Data show that the upper atmospheres are cold, but below the cloud layers, their lower atmospheres are hot and have high air pressure. The vertical structure of any planet's atmosphere follows physical principles that relate temperature, density, and pressure. The deeper you go into an atmosphere, the higher the temperature, density, and pressure. Major differences between the vertical structures of the atmospheres of different planets are caused by gravity. A more massive planet has stronger gravity. Enormous gravitational force creates conditions of crushing pressure and blistering heat at the center of a giant planet.

The image shows the vertical structure of the atmosphere of Jupiter. The temperature is shown as a function of pressure (black line). The approximate altitudes of atmospheric transitions troposhere–stratopshere (tropopause) and stratosphere–thermosphere are shown, as well as tropospheric cloud layers.

As the most massive planet, Jupiter has the hottest atmosphere at its lower levels. This heat provides the energy to drive storm systems such as the Red Spot, and other upwellings of colorful clouds from the warmer regions below. On all the gas giants, the bright zones usually represent higher, brighter clouds, and the dark belts represent gaps in the high clouds where we see lower and more colorful clouds.

These gas planets lack many of the characteristics that our experience Earth has taught us to expect. On the terrestrial planets, we would travel down through a gaseous atmosphere to a hard, rocky surface. If we descended below the visible clouds of any of the Jovian planets, we would find the atmosphere simply grows thicker and hotter with no clear delineation from one gas level to the next. The physical state of the gas we'd pass through, primarily hydrogen, is unfamiliar to us in ordinary life. No place on Earth reaches the very high pressures deep in these planets, caused by the extreme weight of the overlying layers. If the temperature were cool enough, an ocean of liquid hydrogen might exist 100 kilometers below the clouds. But the temperature is too high, so the gas simply gets denser at lower depths, resembling a thick, hot liquid, with no well-defined surface. Sunlight can't penetrate very far into these atmospheres. If we could survive the crushing pressure deep within the giant planets, we would enter a murky "twilight zone" between vapor and liquid. Deeper still, we would find liquid gradually turning into solid. There would be nothing firm to stand on, but a slushy mixture of ice and rock.

Carefully applying simple physical principles, scientists have hypothesized the internal structures of the giant planets. They know the planets' chemical composition and overall density. They apply the idea of differentiation — the tendency for heavier elements and compounds to sink to the center of a planet. The law of gravity and the gas laws govern how temperature varies with depth in the atmosphere. To determine the state of hydrogen, helium, and other materials inside these planets, scientists use laboratory data on their properties at high pressure. The results give an idea of the interior structure of the four Jovian planets. We can thus use simple physics and chemistry to deduce the conditions in places we've never visited!

Interior layers of the gas giants

Jupiter has a bulk internal composition of roughly ⅔ hydrogen, with the rest being helium mixed with small amounts of rocky silicates, metals, and other impurities. Jupiter's cloudy atmosphere gradually grades into a liquid form of hydrogen, called liquid molecular hydrogen. At these levels, it's cold enough that hydrogen atoms still stick together in H₂ molecules, but dense enough that the hydrogen is a liquid. At 20,000 kilometers below the clouds of Jupiter, the pressure is about 4 million times Earth's atmospheric pressure. The high pressure and temperature cause hydrogen atoms to collide frequently at high speed and their electrons are stripped away. Loose electrons surround loose protons. This exotic state of hydrogen is called liquid metallic hydrogen. It's metallic because the electrons flow freely and could carry an electric current, as in a metal. This mass of high-pressure liquid metallic hydrogen covers the deeper solid core.

Saturn has a similar composition and interior structure to that of Jupiter. In Uranus and Neptune, however, the pressures are much lower, not great enough to produce liquid metallic hydrogen. On these two planets, vast oceans of liquid molecular ammonia, methane, and hydrogen extend from the base of the atmosphere down to the ice/rock core. These oceans are at very high pressure, and they reach temperatures of several thousand Kelvin. Uranus and Neptune also differ compositionally in having a higher proportion of silicates, metals, and impurities than Jupiter and Saturn.

All four of the giant planets are believed to have central cores that are rocky and metallic, about 1.5 to 3 times the diameter of the Earth's core. In Saturn, Uranus, and Neptune these Earth-like cores are surrounded by icy outer cores made of frozen water, methane, and ammonia. In all four giant planets, deep mantles of hydrogen in various forms surround these solid cores.

You can visualize the Jovian planets as cold super-Earths, roughly two or three times as big as our planet, buried in vast ocean-like mantles of high-pressure hydrogen. A deep hydrogen atmosphere full of clouds surrounds each planet. These massive envelopes of hydrogen-rich materials extend the total diameter of the giant planets to four to ten times the Earth's diameter.