Saturn

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.

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.

V

THE MAGNETOSPHERE

Saturn's magnetic field is substantially weaker than that of Jupiter, and Saturn's magnetosphere is about one-third the size of Jupiter's. Saturn's magnetosphere consists of a set of doughnut-shaped radiation belts in which electrons and atomic nuclei are trapped. The belts extend to more than 2 million km (1.3 million mi) from the center of Saturn and even farther in the direction away from the Sun, although the size of the magnetosphere fluctuates, depending on the intensity of the solar wind (the flow of charged particles from the Sun). The solar wind and Saturn's rings and satellites supply the particles that are trapped in the radiation belts. The magnetosphere interacts with the ionosphere, the topmost layer of Saturn's atmosphere, causing auroral emissions of ultraviolet radiation.

Measuring the rotation rate of Saturn’s magnetosphere apparently does not indicate the true rotation rate for the body of the planet according to findings made by the Cassini space probe. Scientists have used radio signals generated by magnetic fields to estimate the rotation periods of the other giant planets—Jupiter, Uranus, and Neptune—on the assumption that the magnetic field and the planet rotate together. The giant planets have constantly changing atmospheres and do not have solid surfaces with features that could be used to determine their true rotation rates directly. Researchers reported in May 2006 that an instrument on the Cassini orbiter detected a radio period in the planet’s magnetic field of about 10 hours and 47 minutes—about six minutes longer than the rotation period estimated from measurements of the magnetic field made by the Voyager space probes in the 1980s.

Additional Cassini findings reported in March 2007 suggested that particles originating from geysers on the moon Enceladus may provide an explanation for the change. The neutral gas particles become electrically charged and are captured by Saturn’s magnetic field, forming a disk of hot, ionized gas around the planet’s equator. The charged particles interact with the magnetic field and slow down the rotation of the ionized gas, causing the radio period associated with the magnetic field to be longer than Saturn’s true rotation period. The period of radio signals from the magnetosphere apparently varies over time, possibly reflecting the activity levels of the geysers on Enceladus that create the ionized gas disk. The exact rotation period for the body of Saturn is not known for sure, but it is probably about 10.5 hours.

Surrounding the Saturnian satellite Titan and Titan's orbit, and extending to the orbit of Saturn's moon Rhea, is an enormous doughnut-shaped cloud of neutral hydrogen atoms. A disk of plasma, composed of hydrogen and possibly oxygen ions, extends from outside the orbit of the moon Tethys almost to the orbit of Titan. The plasma rotates in nearly perfect synchrony with Saturn's magnetic field.

VI

THE RING SYSTEM

When the Italian scientist Galileo saw Saturn’s ring system through a small telescope in 1610, he did not understand that the rings were separate from the body of the planet. He described the rings as handles (ansae). The Dutch astronomer Christiaan Huygens was the first person 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. The Cassini probe discovered an additional faint ring between the G ring and the F ring in 2006. The main rings are now known to comprise more than 100,000 individual ringlets, each of which circles the planet. 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. The mass of the entire ring system is about one-sixth the mass of Earth’s Moon.

The apparent separation between the A and B rings is called Cassini's division, after its discoverer, the French astronomer Giovanni Cassini. Voyager's television showed five new faint rings within Cassini's division. The wide B and C rings appear to consist of hundreds of ringlets, some slightly elliptical, that have ripples of varying density. The gravitational interaction between rings and satellites, which causes these density waves, is still not completely understood. The B ring appears bright when viewed from the side illuminated by the Sun, but dark on the other side because it is dense enough to block most of the sunlight. Voyager images have also revealed radial, rotating spokelike patterns in the B ring. These spokelike patterns appear to be seasonal and were not visible when Cassini began orbiting Saturn in 2004. The patterns may be caused by electrostatic effects that elevate tiny particles above the ring plane. The spokes may reappear when the angle of the rings to the Sun changes.

Although Saturn itself formed about 4.6 billion years ago, most of its rings may have been created as recently as 100 million years ago, a time when dinosaurs roamed the Earth. One theory is that a comet or an asteroid smashed a small moon that orbited the planet. The debris from the collision spread out to form the main body of the rings (rings D through F). Images from Cassini confirm that the rings contain chunks and particles of rock and ice in a full range of sizes as expected from a collision. The origin of the tenuous G ring is not known. The faint E ring that stretches from the orbits of the moons Mimas to past Rhea is mainly made of tiny particles released by active geysers on the moon Enceladus and is constantly being renewed.

The gravitational pull from some of Saturn’s moons helps shape parts of the rings. The Cassini division is thought to be caused by the moon Mimas, which orbits Saturn once for every two orbits made by ring particles near the gap. In some cases small moons that orbit within the ring system affect the shape of some of the rings, keeping them narrow or causing the rings to have braided or scalloped shapes.

VII

MOONS

Saturn has at least 56 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 discovery of 12 of Saturn’s moons was reported as recently as May 2005 and 9 more were announced in June 2006. These moons are irregular in shape and small, ranging in diameter from 3 km (2 mi) to 7 km (4 mi). Most orbit Saturn in a direction opposite that of Saturn’s larger moons, suggesting that these recently discovered satellites were originally asteroids that were captured by Saturn’s gravitational field.

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 Saturn’s other moons, the least cratered area on its surface being less than a few hundred million years old. The detection of geysers on Enceladus suggests that liquid water below the surface is being heated by some source. Among the possibilities are tidal forces—the gravitational pull of Saturn and other moons. These tidal forces could cause friction that heats rocks within the interior of the moon. Heat released by the radioactive decay of rocks deep in the moon could also melt some of the ice. The Cassini spacecraft detected oxygen atoms in the geyser plumes that jet out from the moon’s southern polar region. The plumes reach about 418 km (about 260 mi) into space. Scientists concluded that the geysers were spewing out water molecules that then broke down into oxygen and hydrogen atoms. Astronomers think that Enceladus supplies most of the particles in 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 or coorbital satellites of Tethys and one of Dione have also been discovered. Trojan satellites occur in regions of gravitational 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. The Huygens probe descended to the surface in January 2005 and sent back pictures of large rocklike objects probably made of ice.

Radar observations reported in 2006 revealed sandlike dunes remarkably similar to those found in the Sahara and the Namib Desert on Earth. In some cases the dunes are as high as 100 m (330 ft) and stretch for as long as 1,500 km (930 mi), running parallel to each other like those in the Sahara. The fine sandlike grains are probably made of ice or organic solids or a mixture of both, rather than the silicates that make up sand on Earth. Scientists theorized that the dunes are formed by winds created by the tidal forces that Saturn exerts on Titan. Scientists recently learned that those forces are 400 times more powerful than the tidal forces that the Moon exerts on Earth. Previously, scientists believed that there was little wind on Titan because the amount of sunlight it receives is insufficient to supply the energy for atmospheric circulation. Tidal forces exerted by Saturn, however, could be enough to create winds averaging nearly a kilometer per hour.

Titan’s north pole has areas that appear to be lakes based on radar images returned in July 2006 and afterward. The shapes and smooth surfaces of the bodies suggest they contain liquid, probably methane mixed with ethane. Such lakes may form and evaporate depending on Titan’s seasons. Other recent research indicates large methane storms can occur in the dense, cold atmosphere and may precipitate out as hydrocarbon rain. The topography on Titan shows evidence of channels and other drainage features similar to those created by water erosion on Earth. Radar also indicates a continent-like highland area named Xanadu has features resembling river courses, hills, and mountains over 1 km (0.6 mi) high. The elevated region is thought to be formed from rock-hard water ice and has a rugged terrain that suggests methane rains may have riddled the ground with caverns.

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