Saturn: Sixth planet in order of distance from the Sun, and the second largest in the solar system.  The Romans named the planet after their god of agriculture.  Saturn's most distinctive feature is its ring system, which was first seen in 1610 by Italian scientist Galileo, using one of the first telescopes.  He did not understand that the rings were separate from the body of the planet, so he described them as handles (ansae).  The Dutch astronomer Christiaan Huygens was the first to describe the rings correctly.  In 1655, desiring further time to verify his explanation without losing his claim to priority, Huygens wrote a series of letters in code, which when properly arranged formed a Latin sentence that read in translation, "It is girdled by a thin flat ring, nowhere touching, inclined to the ecliptic."  The rings are named in order of their discovery, and from the planet outward they are known as the D, C, B, A, F, G, and E rings.  These rings are now known to comprise more than 100,000 individual ringlets, each of which circles the planet.


Saturn is the least dense of the solar system’s planets.  The mean density of Saturn is eight times less than that of Earth because the planet consists mainly of the lightweight gas hydrogen.  The enormous weight of Saturn's atmosphere causes the atmospheric pressure to increase rapidly toward the interior, where the hydrogen gas condenses into a liquid.  Closer to the center of the planet, the liquid hydrogen is compressed into metallic hydrogen, which is an electrical conductor.  Electrical currents in this metallic hydrogen are responsible for the planet's magnetic field.  At the center of Saturn, heavy elements have probably settled into a small rocky core with a temperature close to 15,000°C (27,000°F).  Both Jupiter and Saturn are still settling gravitationally, following their original accretion from the gas and dust nebula from which the solar system was formed more than 4.7 billion years ago.  This contraction generates heat, causing Saturn to radiate into space three times as much heat as it receives from the Sun.


Saturn's atmospheric constituents are, in order by mass, hydrogen (88 percent) and helium (11 percent); and traces of methane, ammonia, ammonia crystals, and such other gases as ethane, acetylene, and phosphine comprise the remainder.  Voyager images showed whirls and eddies of clouds occurring deep in a haze that is much thicker than that of Jupiter because of Saturn's lower temperature.  The temperatures of Saturn's cloud tops are close to -176°C (-285°F), about 27 degrees Celsius (49 degrees Fahrenheit) lower than such locations on Jupiter.


The visible rings stretch out to a distance of 136,200 km (84,650 mi) from Saturn's center, but in many regions they may be only 5 m (16.4 ft) thick.  They are thought to consist of aggregates of rock, frozen gases, and water ice ranging in size from less than 0.0005 cm (0.0002 in) in diameter to about 10 m (33 ft) in diameter -- from dust to boulder size.  An instrument aboard Voyager 2 counted more than 100,000 ringlets in the Saturnian system.


Saturn has more than 30 moons.  They range up to 2,575 km (1,600 mi) in radius.  They consist mostly of the lighter, icy substances that prevailed in the outer parts of the gas and dust nebula from which the solar system was formed and where radiation from the distant Sun could not evaporate the frozen gases.


. . . The five larger inner satellites -- Mimas, Enceladus, Tethys, Dione, and Rhea -- are roughly spherical in shape and composed mostly of water ice.  Rocky material may constitute up to 40 percent of Dione's mass.  The surfaces of the five moons are heavily cratered by meteorite impacts.  Enceladus has a smoother surface than the others, the least cratered area on its surface being less than a few hundred million years old.  Astronomers suspect that Enceladus supplies particles to the E ring, which neighbors Enceladus's orbit.  Mimas, far from being smooth, displays an impact crater the diameter of which is one-third of the diameter of the satellite itself.  Tethys also bears a large crater and a valley 100 km (62 mi) in width that stretches more than 2,000 km (1,200 mi) across the surface.  Both Dione and Rhea have bright, wispy streaks on their already highly reflective surfaces.  Some scientists conjecture these were caused either by ice ejected from craters by meteorites, or by fresh ice that has migrated from the interior.


Several small moons have been discovered immediately outside the A ring and close to the F and G rings.  Possibly four so-called Trojan satellites of Tethys and one of Dione have also been discovered.  Trojan satellites occur in regions of stability that lead or follow a body in its orbit around a massive central body, in this case, Saturn.


The outer satellites Hyperion and Iapetus also consist mainly of water ice.  Iapetus has a very dark region in contrast to most of its surface, which is bright.  This dark region and the rotation of the satellite are the cause of the variations of brightness that were noticed by Cassini in 1671.  Phoebe, the farthest large satellite, moves in a retrograde orbit (in the opposite direction of the orbits of most of the other satellites) that is at a sharp angle to Saturn's equator.  Phoebe is probably a cometary body captured by Saturn's gravitational field.


Titan, Saturn's largest moon, orbits the planet between the inner and outer satellites.  Titan’s radius is 2,575 km (1,600 mi), larger even than the planet Mercury.  The moon appears nearly featureless to optical telescopes.  A dense orange haze hides the surface, but astronomers have glimpsed distinct methane clouds.  Titan’s atmosphere is largely composed of nitrogen with traces of methane, ethane, hydrogen cyanide, carbon dioxide, water vapor, and several other organic compounds.  The Cassini spacecraft imaged the moon close-up using various wavelengths of light and mapped it with radar.  Titan has a geologically young surface, with no sharply defined impact craters.  The Huygens probe descended to the surface in January 2005 and sent back pictures of what appear to be drainage channels leading to a flood plain.  The surface temperature is about -180°C (-290°F), so the liquid eroding the surface cannot be water.  Scientists believe methane, which is liquid at that temperature, to be responsible.  The interior of Titan probably consists of equal amounts of rock and water ice.

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