10. Saturn: lord of the rings

Beneath the clouds of Saturn

The internal constitution of Saturn

Saturnís low mass density indicates that the lightest element, hydrogen, is the main ingredient inside the planet, just as it is for Jupiter and the Sun. The lightweight material, just 68.8 percent as dense as water, is hurled outward in its equatorial regions by the planetís rapid 10.6562-hour rotation, making Saturn the most oblate planet in the solar system. Its equatorial bulge amounts to about 10 percent of the radius, and is about as big in extent as the Earth. Or, as some view it, the polar regions of Saturn are squashed and flattened by this amount.

The oblong shape of Saturn can be seen with a small telescope, and measured precisely from its satellite orbits and ring positions as well as by the trajectories of the passing Voyager spacecraft. When these measurements are combined with Saturnís known mass, volume and rotation rate, scientists can obtain information about its internal distribution of mass.

Fig. .. 

The model the experts come up with is just a scaled down version of Jupiter, with a small, dense core of melted ice and molten rock surrounded by a vast globe of liquid hydrogen and topped by a thin gaseous atmosphere. Like Jupiter, giant Saturn is not a solid world, and is essentially a great big drop of liquid.

Deep down inside, the liquid hydrogen is compressed to such high pressures that it conducts electricity like a metal. But since Saturn is less than three times as massive as Jupiter, and only slightly smaller, the internal pressure at a given depth is less, and the liquid hydrogen turns into a metal further down in the ringed planet. Saturn therefore has a smaller shell of liquid metallic hydrogen.

Interior heat and helium rain

Precise measurements from the Voyager 1 and 2 spacecraft indicate that Saturn is radiating 1.78 times more energy in visible and infrared light than it absorbs from incoming sunlight. This excess energy must be coming from within the planet. It implies that Saturn, like Jupiter, is an incandescent globe with an internal source of heat.

Both Jupiter and Saturn radiate almost twice as much energy as they receive from the Sun, but the dominant source of internal heat is different for the two giant planets. Jupiterís internal heat is primarily primordial heat liberated during the gravitational collapse when it was formed, and Saturn must have also started out hot inside as the result of its similar formation. But being somewhat smaller and less massive than Jupiter, the planet Saturn was not as hot in its beginning and has had time to cool. As a result, Saturn lost most of its primordial heat and there must be another source for most of its internal heat.

Saturnís excess heat is generated by the precipitation of helium into its metallic hydrogen core. The heavier helium separates from the lighter hydrogen and drops toward the center, somewhat like the heavier ingredients of a salad dressing that hasnít been shaken for awhile. Small helium droplets form where it is cool enough, precipitate or rain down, and then dissolve at hotter deeper levels. As the helium at a higher level drizzles down through the surrounding hydrogen, the helium converts some of its energy to heat. In much the same way, raindrops on Earth become slightly warmer when they fall and strike the ground; their energy of motion Ė acquired from gravity Ė is converted to heat.

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Copyright 2010, Professor Kenneth R. Lang, Tufts University