Internal Heat Production

Saturn, along with Jupiter, radiates more energy than it receives from the Sun. Saturn's heat budget is not as extreme as Jupiter's, though; some measurements from Pioneer and Voyager indicated that Saturn receives almost as much energy from the Sun as it produces internally. More recent and exact measurements show that Saturn produces 82 percent more heat internally than it receives from the Sun. The degree of internal energy required to match the solar input is not as excessive as it would be for the Earth: Saturn receives only 1 percent of the solar flux that the Earth receives.

Excess internal energy may come from compositional differentiation or from internal gravitational collapse. In the case of internal compositional differentiation, scientists hypothesize that a fractiona-tion of helium from hydrogen may be the driving force. At pressures of 3 to 5 million bars inside Saturn, hydrogen changes to a metallic state, where atoms share electrons as if they were a gas among the atomic nuclei.When hydrogen reaches this metallic state, helium can no longer mix homogeneously with the hydrogen, and the helium condenses into a liquid and drops further into Saturn's interior as helium rain.The dropping helium loses potential energy as it falls into the gravity field of the planet and releases heat. This kind of falling may continue until material reaches the core of the planet, which may be as hot as 21,600°F (12,000°C). Saturn may also be collapsing gradually under its own gravitational field. Contraction similarly releases heat from both the loss of potential energy and from friction, and so collapse may add to the internal heat budget of the planet.

Saturn and Jupiter have similar radii (37,450 miles, or 60,268 km, for Saturn versus 44,424 miles, or 71,492 km, for Jupiter), though Saturn is considerably less massive than Jupiter (1.25 x 1026 pounds, or 5.69 x 1026 kg, for Saturn versus 4.2 x 1027 pounds, or 1.9 x 1027 kg, for Jupiter). The similarity in radii can be explained if helium differentiation took place much earlier on Jupiter than it did on Saturn. Jupiter may thus hold the majority of its mass in a dense state near its core, while on Saturn, helium is still differentiating and sinking deeper into the planet. Unfortunately, this theory is not supported by helium measurements in the outer portions of these planets: Saturn has a lower helium to hydrogen ratio than Jupiter, implying that less of its helium remains in the outer portions of the planet.

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