Saturn

I

Introduction

Saturn, sixth planet in order of distance from the Sun, and the second largest in our solar system. Saturn’s most distinctive feature is a giant system of rings that surrounds the planet at its equator, stretching over twice the width of the planet itself. The first person to see the rings was the Italian scientist Galileo in 1610, using one of the earliest telescopes. Space probes have greatly increased our knowledge of Saturn, its rings, and its many moons. Flybys by the Pioneer and the Voyager probes led to the Cassini orbiter that began studying Saturn in detail in 2004. As seen from Earth, Saturn appears as a yellowish object—one of the brightest in the night sky. The planet is named for Saturn, the Roman god of agriculture.

Saturn takes about 29.5 years to orbit the Sun at an average distance of 1,435 billion km (891.5 million mi), or about 9.59 astronomical unit (AU). An AU is equal to the average distance between the Earth and the Sun, or 150 million km (93 million mi). Saturn rotates on its axis in about 10.5 hours and is tilted at about 27°, giving the planet distinct seasons.

The diameter of Saturn is about 121,000 km (75,000 mi), and its mass is equal to the mass of about 95 Earths, making it the second largest planet in our solar system after Jupiter. Saturn is 10 percent wider at its equator than at its poles and has a more oblate (flattened sphere) shape than any other planet.

II

Exploration of the Saturnian System

Observed through a telescope, Saturn’s brightest rings are easily visible, whereas only under optimal conditions can the fainter outer rings be seen. Sensitive Earth-based telescopes have detected many satellites, and in the haze of Saturn's gaseous envelope, pale belts and zones parallel to the equator can be distinguished.

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Several United States spacecraft have enormously increased knowledge of the Saturnian system. The Pioneer 11 (see Pioneer) probe flew by in September 1979, followed by Voyager 1 in November 1980 and Voyager 2 (see Voyager) in August 1981. These spacecraft carried cameras and instruments for analyzing the intensities and polarizations of radiation in the visible, ultraviolet, infrared, and radio portions of the electromagnetic spectrum (see Electromagnetic Radiation). The spacecraft were also equipped with instruments for studying magnetic fields and for detecting charged particles and interplanetary grains.

The National Aeronautics and Space Administration (NASA) launched an orbiter called the Cassini spacecraft toward Saturn in October 1997. It reached Saturn in July 2004 and began studying the planet and its moons. Cassini launched a probe (the Huygens probe) that descended to the surface of Saturn's moon Titan early in 2005. In 2006 NASA scientists reported that Cassini had detected geysers on Saturn’s moon Enceladus. Previously, Cassini had detected carbon molecules on the moon’s surface. The discovery was particularly significant because the existence of liquid water, heat, and carbon molecules represent the three ingredients essential for life.

III

The Interior of Saturn

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 about 4.6 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 receives about 1.1 percent as much sunlight as Earth does.

IV

The Atmosphere of Saturn

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 wind velocities in Saturn’s atmosphere change with the planet’s seasons and are affected by the angle of the shadows cast on the atmosphere by the planet’s rings. The Cassini space probe found evidence that the velocity of winds at Saturn’s equator has decreased from about 1,700 km/h (1,060 mph) to around 1,000 km/h (621 mph) since the early 1980s, when the Voyager probes returned data about the planet.

In 1988, from studies of Voyager photos, scientists determined an odd atmospheric feature around Saturn's north pole. What may be a standing-wave pattern (see Wave Motion), repeated six times around the planet, makes cloud bands some distance from the pole appear to form a huge, permanent hexagon. This feature was viewed in much better detail by the Cassini orbiter in 2007. Using infrared imaging, Cassini found a double hexagon pattern, with a smaller hexagon inside a larger one. The feature is about 24,000 km (15,000 m) wide and apparently rotates with the planet itself in about 10.5 hours. In contrast, Saturn’s south pole has a round hurricane-like vortex that rotates at 550 kph (342 mph), with a structure like a hurricane’s eye wall in the center. The giant south pole storm is about 8,000 km (5,000 m) across, slightly wider than the diameter of Earth.