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Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Named after the Roman god of agriculture, Saturn, its astronomical symbol (") represents the god's sickle. Saturn is a gas giant with an average radius about nine times that of Earth.[10][11] While only one-eighth the average density of Earth, with its larger volume Saturn is just over 95 times more massive than Earth.[12][13][14]
Saturn's interior is probably composed of a core of iron, nickel and rock (silicon and oxygen compounds), surrounded by a deep layer of metallic hydrogen, an intermediate layer of liquid hydrogen and liquid helium and an outer gaseous layer.[15] The planet exhibits a pale yellow hue due to ammonia crystals in its upper atmosphere. Electrical current within the metallic hydrogen layer is thought to give rise to Saturn's planetary magnetic field, which is slightly weaker than Earth's and around one-twentieth the strength of Jupiter's.[16] The outer atmosphere is generally bland and lacking in contrast, although long-lived features can appear. Wind speeds on Saturn can reach 1,800 km/h (1,100 mph), faster than on Jupiter, but not as fast as those on Neptune.[17]
Saturn has a prominent ring system that consists of nine continuous main rings and three discontinuous arcs, composed mostly of ice particles with a smaller amount of rocky debris and dust. Sixty-two[18] known moons orbit the planet; fifty-three are officially named. This does not include the hundreds of "moonlets" within the rings. Titan, Saturn's largest and the Solar System's second largest moon, is larger than the planet Mercury and is the only moon in the Solar System to retain a substantial atmosphere.[19]
Saturn is classified as a gas giant planet because the exterior is predominantly composed of gas and it lacks a definite surface, although it may have a solid core.[20] The rotation of the planet causes it to take the shape of an oblate spheroid; that is, it is flattened at the poles and bulges at the equator. Its equatorial and polar radii differ by almost 10% " 60,268 km versus 54,364 km, respectively.[3] Jupiter, Uranus, and Neptune, the other gas giants in the Solar System, are also oblate but to a lesser extent. Saturn is the only planet of the Solar System that is less dense than water " about 30% less.[21] Although Saturn's core is considerably denser than water, the average specific density of the planet is 0.69 g/cm3 due to the gaseous atmosphere. Jupiter has 318 times the Earth's mass,[22] while Saturn is 95 times the mass of the Earth,[3] Together, Jupiter and Saturn hold 92% of the total planetary mass in the Solar System.[23]
Saturn is termed a gas giant, but it is not entirely gaseous. The planet primarily consists of hydrogen, which becomes a non-ideal liquid when the density is above 0.01 g/cm3. This density is reached at a radius containing 99.9% of Saturn's mass. The temperature, pressure and density inside the planet all rise steadily toward the core, which, in the deeper layers of the planet, cause hydrogen to transition into a metal.[23]
Standard planetary models suggest that the interior of Saturn is similar to that of Jupiter, having a small rocky core surrounded by hydrogen and helium with trace amounts of various volatiles.[24] This core is similar in composition to the Earth, but more dense. Examination of the gravitational moment of the planet, in combination with physical models of the interior, allowed French astronomers Didier Saumon and Tristan Guillot to place constraints on the mass of the planet's core. In 2004, they estimated that the core must be 9"22 times the mass of the Earth,[25][26] which corresponds to a diameter of about 25,000 km.[27] This is surrounded by a thicker liquid metallic hydrogen layer, followed by a liquid layer of helium-saturated molecular hydrogen that gradually transitions into gas with increasing altitude. The outermost layer spans 1,000 km and consists of a gaseous atmosphere.[28][29][30]
Saturn has a very hot interior, reaching 11,700 °C at the core, and the planet radiates 2.5 times more energy into space than it receives from the Sun. Most of this extra energy is generated by the Kelvin"Helmholtz mechanism of slow gravitational compression, but this alone may not be sufficient to explain Saturn's heat production. An additional mechanism may be at play whereby Saturn generates some of its heat through the "raining out" of droplets of helium deep in its interior. As the droplets descend through the lower-density hydrogen, the process releases heat by friction and leaves the outer layers of the planet depleted of helium.[31][32] These descending droplets may have accumulated into a helium shell surrounding the core.[24]
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