Saturn

Saturn orbits the Sun at a mean distance of 9.539 astronomical units (AU, where 1 AU is the average distance between the Earth and the Sun). Its slightly eccentric (noncircular) orbit, however, allows the planet to be far from the Sun as 10.069 AU and as close as 9.008 AU. Saturn takes 29.46 Earth years to complete one orbit around the Sun. Saturn has an equatorial diameter of 74,855 mi (120,540 km), making it the second-largest planet in the solar system (Jupiter is about the same size as Saturn but is 3.35 times more massive). Saturn spins on its axis 2.25 times more rapidly than the Earth, that is, every 10 hours 14 minutes. This rapid rotation causes it to be 8,073 mi (13,000 km) wider at the equator than it is from pole to pole.

Saturn has an average density of 0.69 g/cm³, less than that of water (1.0 g/cm³) and lowest of all the planets in the solar system. This low density indicates that the planet must be composed mainly of hydrogen and helium (the most abundant elements in the Universe). Theoretical models suggest that Saturn has a rocky inner core that accounts for only about 26% of its mass. This central core is surrounded by a thick layer of liquid metallic hydrogen, a form of hydrogen that occurs only under extreme pressure. This mantle is surrounded by an atmosphere composed mostly of molecular hydrogen and helium that is liquid at its base and gradually becomes less dense at higher altitudes, finally becoming a gaseous atmosphere at the highest levels.

When the two Voyager spacecraft flew past Saturn in 1980 and 1981, they confirmed that Saturn has a magnetic

Saturn, the second largest planet in the solar system, and its system of rings. U.S. National Aeronautics and Space Administration (NASA).

field. Like Jupiter's magnetic field, Saturn's is probably produced in the planet's metallic-hydrogen mantle. The magnetic field at Saturn's cloud tops, however, is about one tenth that observed on Jupiter. Indeed, Saturn's equatorial magnetic field is only about twothirds as strong as Earth's.

Careful measurements of Saturn's energy budget (balance of energy absorbed versus energy radiated) show that the planet radiates 1.5–2.5 times more energy into space than it receives from the Sun. This radiated energy indicates that the planet must have an internal heat source. Scientists accept that Saturn draws its extra energy from two sources: (1) heat left over from the planet's formation approximately 4.5 billion years ago, still radiating out into space, and (2) the "raining out" of atmospheric helium. Just as water condenses in terrestrial clouds to produce rain, droplets of liquid helium form in Saturn's atmosphere. As these droplets fall through Saturn's atmosphere they acquire kinetic energy. This energy is absorbed into deeper layers where the droplets meet resistance and slow their fall, and the temperature in those regions increases. This thermal energy is eventually circulated by convection back up through the higher layers of the atmosphere and radiated into space. The helium raining out of Saturn's upper layers is left over from the planet's formation; in about two billion more years all of Saturn's helium will have sunk deep into the planet, at which time heating by helium condensation will cease.

Support for the helium-condensation model was obtained during the Voyager encounters, when it was found that the abundance of helium in Saturn's atmosphere was much lower than that observed in Jupiter's. Upper-atmospheric depletion of helium has not yet occurred on Jupiter because its atmosphere has only recently become cool enough to permit helium condensation; on Saturn, in contrast, helium has been raining out for about two billion years, settling half the available helium toward the core.