Saturn

Saturn

**Saturn** is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius about nine times that of Earth. Although only one-eighth the average density of Earth, with its larger volume Saturn is just over 95 times more massive.

Saturn has a prominent ring system
 that consists of nine continuous main rings and three discontinuous
arcs and that is composed mostly of ice particles with a smaller amount
of rocky debris and dust. Sixty-two moons are known to orbit Saturn, of which fifty-three are officially named. This does not include the hundreds of moonlets comprising the rings. Titan, Saturn's largest moon, and the second-largest in the Solar System, is larger than the planet Mercury, although less massive, and is the only moon in the Solar System to have a substantial atmosphere.

## Physical Characteristics

Saturn is a gas giant
 because it is predominantly composed of hydrogen and helium ('gas'). It
 lacks a definite surface, though it may have a solid core. Saturn's rotation causes it to have the shape of an oblate spheroid; that is, it is flattened at the poles and bulges at its equator.

Saturn's atmosphere exhibits a banded pattern similar to Jupiter's, but
Saturn's bands are much fainter and are much wider near the equator. Saturn's finer cloud patterns were not observed until the flybys of the Voyager spacecraft during the 1980s. The composition of the clouds varies with depth and increasing pressure.
 In the upper cloud layers, with the temperature in the range 100–160 K
and pressures extending between 0.5–2 bar, the clouds consist of ammonia ice. Water ice clouds
 begin at a level where the pressure is about 2.5 bar and extend down to
 9.5 bar, where temperatures range from 185–270 K. Intermixed in this
layer is a band of ammonium hydrosulfide ice, lying in the pressure
range 3–6 bar with temperatures of 290–235 K. Finally, the lower layers,
 where pressures are between 10–20 bar and temperatures are 270–330 K,
contains a region of water droplets with ammonia in aqueous solution.

Saturn's usually bland atmosphere occasionally exhibits long-lived ovals and other features common on Jupiter. In 1990, the Hubble Space Telescope
 imaged an enormous white cloud near Saturn's equator that was not
present during the Voyager encounters and in 1994, another, smaller
storm was observed. The 1990 storm was an example of a Great White Spot,
 a unique but short-lived phenomenon that occurs once every Saturnian
year, roughly every 30 Earth years, around the time of the northern
hemisphere's summer solstice.

## Satellites

Saturn has 62 known moons, 53 of which have formal names. In addition, there is evidence of dozens to hundreds of moonlets with diameters of 40–500 meters in Saturn's rings. Titan, the largest moon, comprises more than 90% of the mass in orbit around Saturn, including the rings. Saturn's second-largest moon, Rhea, may have a tenuous ring system of its own.

Many of the other moons are small: 34 are less than 10 km in diameter and another 14 between 10 and 50 km in diameter. Traditionally, most of Saturn's moons have been named after Titans of Greek mythology. Titan is the only satellite in the Solar System with a major atmosphere, in which a complex organic chemistry occurs. It is the only satellite with hydrocarbon lakes.

Saturn's moon Enceladus, which seems similar in chemical makeup to comets, has often been regarded as a potential habitat for microbial life. Evidence of this possibility includes the satellite's salt-rich
particles having an "ocean-like" composition that indicates most of
Enceladus's expelled ice comes from the evaporation of liquid salt water.

### Rings of Saturn

Saturn is probably best known for the system of planetary rings that makes it visually unique. The rings extend from 6,630 km to 120,700 km outward from Saturn's
equator, average approximately 20 meters in thickness and are composed
of 93% water ice with traces of tholin impurities and 7% amorphous carbon. While the other gas giants also have ring systems, Saturn's is the largest and most visible.

There are two main hypotheses regarding the origin of the rings. One
hypothesis is that the rings are remnants of a destroyed moon of Saturn.
 The second hypothesis is that the rings are left over from the original
 nebular material from which Saturn formed. Some ice in the E ring comes from the moon Enceladus's geysers.

Beyond the main rings at a distance of 12 million km from the planet is
the sparse Phoebe ring, which is tilted at an angle of 27° to the other
rings and, like Phoebe, orbits in retrograde fashion.

Some of the moons of Saturn, including Pandora and Prometheus, act as shepherd moons to confine the rings and prevent them from spreading out.

## Colonization

### Population

117,520 (Titan, Enceladus and orbiting stations)