Saturn

Saturn is the second-largest planet in the Solar System and the sixth planet from the Sun. It orbits the Sun at a distance of about 9.5 astronomical units or 1.4 billion kilometers, with an orbital period of 29.45 years. The planet is well known for its large ring system. It also has many moons, including several with planet-like features of their own.

Saturn is one of the Solar System's four gas giants, large planets composed mainly of gas rather than solid matter. As massive as 95 Earths, Saturn has as much volume as 763.59 Earths, making it overall less dense than liquid water and the least dense planet. The outer gaseous layer and intermediate layer of liquefied gases take up most of the planet’s volume, probably surrounding a core of iron, nickel, and metallic hydrogen. Saturn’s core is believed to be somewhere between nine and twenty-two times the mass of Earth, and very hot. However, many aspects of Saturn's makeup remain obscure.

Saturn’s rotation period is only ten hours and thirty-four minutes, giving it a very fast rotational velocity of 9.87 kilometers per second at the equator. This rapid rotation “squashes” the planet, causing the massive atmosphere to bulge at the equator, creating a shape called an oblate spheroid.

Atmosphere and Climate

Like the other gas giants, Saturn has an atmosphere is made up mainly of hydrogen (about 96.3 percent of the atmosphere’s molecules) and helium (about 3.25 percent), with a mixture of ammonia, acetylene, propane, phosphine, ethane, methane, and other trace elements in unknown proportions. Compared to Jupiter, the atmosphere is much less dramatic, showing up in the Cassini probe’s images as faintly banded with an overall faded peach tint. Cloud patterns, though easily seen on Jupiter, were not perceived on Saturn until the Voyager probe flybys. The winds peak at faster speeds than on Jupiter. As with the other gas giants, clouds are composed of different materials at different atmospheric pressures: ammonia ice, water ice, ammonium hydrosulfide ice, and water droplets with ammonia, from least to greatest pressure.

The Great White Spot is a storm that occurs once a Saturnian year during the Northern Hemisphere’s summer solstice. The large white clouds that result can be seen by telescope from Earth. In the Southern Hemisphere, the Dragon Storm is a long-lived complex storm that produces periodic bright white flumes. At the North Pole, a hexagonal wave pattern persists in the atmosphere, first discovered by Voyager. The sides of the hexagon are about 13,800 kilometers long, and it is believed to be caused by a standing-wave pattern in the polar vortex. The South Pole, by contrast, has no strong vortex or standing wave, but is home to a hurricane that has persisted for an indefinitely long time, perhaps billions of years. The southern storm is notable as the first true hurricane—with a clearly defined eye wall—observed on another planet.

Planetary Rings and Moons

Saturn is famous for its prominent ring system, consisting of twelve rings, nine of them continuous and three of them broken rings called arcs. The rings are primarily (93 percent) ice with carbon and traces of tholin, and consist of particles orbiting the planet. Other planets have ring systems, but Saturn’s is the largest and most dramatic. There are several theories for the rings’ origin. While some of the ring material is known to come from the ice volcanos of the moon of Enceladus, for instance, this cannot account for all of the ring material. The remainder may be the remnants of a moon destroyed in a collision, or could include material left over from Saturn’s formation. Tilted at a 27-degree angle relative to the other rings is the Phoebe ring, which has a retrograde orbit, like the moon for which it is named.

Saturn has at least 146 moons, most of them small, and may have more. Many of these moons are not yet well-explored, but several are notable for various reasons. Rhea, for instance, is Saturn’s second-largest moon, and was discovered in 2010 to have a sparse atmosphere of oxygen and carbon dioxide. The origin of the carbon dioxide is unclear, but it may come from organic material in the moon’s ice. The moon may also have a ring system, which would be the first time a moon was discovered to have rings.

Saturn’s largest moon, Titan, is the second largest in the solar system. It is the only moon with a significant atmosphere—one composed mainly of nitrogen, with a methane cycle that transpires much like Earth’s water cycle. Many researchers have focused on Titan as one of the most promising locations for potential extraterrestrial life in the solar system.

History of Observation and Exploration

Saturn is visible with the naked eye, and is one of the five planets known by most ancient cultures. Its rings were discovered in the telescopic age, in the seventeenth century. Galileo was the first to see them, though he mistook them for moons because of the limitations of telescopes of his day.

The first flyby of Saturn was conducted by the Pioneer 11 probe in 1979, resulting in the first close study of Saturn’s rings. The first high-resolution images were produced the following year by the Voyager 1 probe. Voyager 2’s 1981 exploration of Saturn was cut short by camera problems.

The most extensive exploration of Saturn was conducted by the Cassini-Huygens spacecraft. Launched in 1997 after intensive development by seventeen countries and managed by the National Aeronautics and Space Administration (NASA), Cassini conducted flybys of Venus and Jupiter before approaching Saturn, discovering seven new moons over five years. In 2004 it began orbiting Saturn, and it explored Titan as their orbits approached one another the following day. The Huygens probe was launched from Cassini and successfully landed on Titan, although half of its images were lost due to a software error. Cassini’s discoveries included liquid water geysers on the moon Enceladus, a previously undiscovered Saturnian ring, liquid hydrocarbon lakes on Titan, the southern hurricane on Saturn, spokes in Saturn’s rings, and other phenomena. The mission ended in 2017 when Cassini was sent to burn up in Saturn's atmosphere.

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