Introduction to Close encounters with Saturn

Descriptions of the international Cassini-Huygens mission to Saturn tend toward superlatives. The Cassini orbiter and the Huygens probe, which slid into orbit around this famously ringed planet in July 2004, represent the most ambitious effort in the history of interplanetary space exploration. Three probes launched by the United States National Aeronautics and Space Administration (NASA) had already flown by Saturn in the 1970's and 1980's. But the Cassini-Huygens spacecraft, the first to orbit Saturn, dwarfs all three in size and weight and in the complexity and power of its instruments. Scientists from NASA, the European Space Agency (ESA), and the Italian space agency, Agenzia Spaziale Italiana (ASI), spent more than 20 years and billions of dollars to complete the mammoth spacecraft, which stands 7 meters (22 feet) tall and weighs 5,700 kilograms (12,600 pounds). In fact, the spacecraft is so huge that it needed to swing by three planets—Earth, Venus, and Jupiter—to get the energy boost it needed for its seven-year journey to Saturn.

Instruments on the Cassini spacecraft have given us the most spectacular images as well as the most detailed information ever collected about Saturn. For example, Cassini's instruments recorded hurricanelike storms a million times stronger than those on Earth as well as the sudden appearance of a strange oxygen cloud that surrounded the planet. Cassini's cameras caught Saturn's moon Prometheus stealing particles from one of the planet's rings. A camera also spotted a previously unknown moon hidden within the rings. Cassini scientists were delighted to confirm that Titan, Saturn's planet-sized moon, has methane rain and an upper atmosphere with complex compounds similar to the chemical building blocks of life on Earth.

In January 2005, the ESA's Huygens probe—carried piggyback on Cassini during the journey from Earth—became the first human-made object to land on Titan. The probe, which collected and transmitted data for twice as long as anyone had expected, revealed its share of amazing discoveries, including detailed images of Titan's surface. As the probe floated through Titan's haze-choked atmosphere, its cameras revealed a hauntingly Earthlike landscape, complete with streamlike drainage channels, evidence of erosion, features that look like shorelines, and rocks (though the rocks are made of ice).

The Cassini mission is scheduled to last until at least 2008, by which time it will have orbited Saturn 76 times. The data the orbiter collects will keep scientists busy for years. Perhaps these data will answer some of the questions intriguing scientists. Where, for example, did the material in Saturn's rings originate? Why are the rings subtly colored? How many more moons does Saturn have? (By mid-2005, the count had reached 47.) How does the moon Enceladus produce particles for one of Saturn's rings? Why does the moon Iapetus have a huge mountain range around its equator? And where are the lakes and oceans of liquid methane and hydrocarbons that scientists expected to see on Titan?

A Surprising Turn

Saturn, which can be seen in the night sky with the unaided eye, was the farthest planet from Earth known to ancient astronomers. But studying this intriguing planet is not easy. When we look at Saturn through telescopes or even from spacecraft, we see only its dense cloud-covered atmosphere, partially obscured by a brightly lit high-altitude haze. The Italian astronomer and physicist Galileo (1564-1642) discovered Saturn's rings in the early 1600's. However, he thought he was seeing three satellites—and was annoyed to discover, when he looked at Saturn again a few months later, that two of the “satellites” had disappeared. (In fact, the angle of the rings relative to Earth had changed.) In the 1650's, Dutch astronomer Christiaan Huygens (1629-1695) discovered Titan and also concluded that what astronomers called “Saturn's arms” was actually a ring. Italian-born French astronomer Giovanni Domenico Cassini in 1675 discovered that Saturn had more than one ring.

Saturn is one of the oddest planets in the solar system. For example, although Saturn is the second largest planet (after Jupiter), it has the lowest density (amount of matter per unit volume) of all the planets. Saturn is the only planet that is, on average, less dense than water. In fact, if you could find a bathtub big enough to hold Saturn, the planet would float. This low density results from Saturn's composition, which is mostly hydrogen and helium, the two lightest elements. The planet gets its white and golden hues from the small amounts of ammonia and other, less common chemicals in its atmosphere.

Some of Cassini's findings have strengthened Saturn's reputation for strangeness. For example, Saturn is the only planet whose exact rotation rate continues to be a mystery to scientists. As the two Voyager spacecraft flew by Saturn in 1980 and 1981, they measured intense radio waves coming from an area of the planet near the equator. Because the most energetic of these radio waves had wavelengths about 1 kilometer (0.6 mile) in length, they were called Saturn Kilometric Radiation (SKR). (Wavelengths measure the distance between one peak or crest of a wave and the next.) Scientists believe the SKR are caused by ions (electrically charged particles) interacting with a magnetic field that originates deep inside Saturn. In the 1980's, NASA scientists used the Voyager measurements of the SKR to calculate that Saturn took 10 hours 39 minutes and 24 seconds to spin on its axis (compared with 23 hours 56 minutes and 4 seconds for Earth).

Cassini's measurements of the planet's SKR, taken by the probe's Radio and Plasma Wave Science (RPWS) instrument, however, suggested the amazing possibility that Saturn was spinning more slowly than it had in the 1980's. According to Cassini's data, Saturn was spinning on its axis once every 10 hours 45 minutes 45 seconds, about six minutes slower than in 1981. Scientists were astounded. If all Saturn's rings and all its moons had crashed into the planet, the collisions would not have decreased Saturn's spin by that much. And at any rate, astronomers on Earth certainly would have noticed any event monumental enough to affect this huge planet to such an extent.

Rather than conclude that Saturn has slowed down, Cassini scientists settled on another possibility. They speculated that Saturn's magnetic field might not rotate at the same rate, the way all of Earth's or Jupiter's does. Instead, the field's—and, thus, the planet's—rotation rate might vary with latitude, like the sun's. That is, regions nearer the poles spin at a slower rate than regions nearer the equator. Support for that idea came from RPWS data indicating that the source area for the SKR had moved to a region above Saturn's southern pole. As the mission progresses, scientists will examine additional data to try to solve Saturn's rotation-rate puzzle.

Cassini also found that Saturn's storms seem to have changed dramatically since the Voyager visits. Storms on Saturn develop along the edges of high-altitude bands of wind that encircle the planet at various latitudes. Some of these bands blow eastward; some blow in the opposite direction. The Voyager probes clocked the winds in the bands at about 1,600 kilometers (1,000 miles per hour), almost 10 times as fast as Earth's high-altitude winds. Along the boundaries between bands, the conflicting winds can churn into hurricanelike storms. The storms detected by the Voyager probes appeared almost exclusively near Saturn's equator and lasted for many months.

Cassini searched for storms using its RPWS instrument, which can detect a type of radio wave generated by lightning. On Earth, these radio waves cause the crackle and pop that you hear when listening to AM radio during a thunderstorm. The RPWS instrument found that the storms were widely distributed over mid- to high-latitude areas of Saturn's Southern Hemisphere.

Scientists think that the differences in storm patterns may be linked to the location of the shadow cast by Saturn's rings. As Saturn orbits the sun, the degree to which its axis tilts toward or away from the sun varies from 0 degrees to 26 degrees. The rings circle Saturn around its equator, and so they tilt with respect to the sun at the same angle as the planet.

In the 1980's, Saturn had almost no tilt with respect to the sun, and, thus, the rings cast a narrow, deep shadow near the equator. The result was turbulence between the cold, shadowed region near the equator and the warmer sunlit regions elsewhere. (In general, winds are fueled by differences in regional atmospheric temperatures based on the amount of energy received from the sun. The greater the temperature difference, the more violent the winds.) In 2004 and 2005, however, Saturn was tilting at its maximum angle with respect to the sun, and so the ring shadow covered most of its Northern Hemisphere. As a result, storms occurred over a broader region. In addition, because the temperature differences between regions were less, the storms lasted for shorter periods.

Storms on Saturn can blow for weeks, months, or even years. Storms on Earth rarely last more than one week, fading as they lose energy. On Saturn, storms tend to merge and continue their sweep across the planet. These combined storms also fade. But because Saturn does not have a solid surface to create friction, the storms fade more slowly than they do on Earth. On March 20, 2004, Cassini photographed the merger of two hurricanelike storms, each 370 kilometers (600 miles) in diameter. Both lasted for about a month before they approached each other, spun counterclockwise around each other, and then combined into an even larger storm.

New Belt

Another of Cassini's unexpected discoveries was a radiation belt existing much closer to the planet than scientists had thought possible. Saturn has one other radiation belt, outside the rings. Cassini's magnetospheric imaging instrument (MIMI) discovered the new belt encircling the planet between the cloud tops and the inner edge of the D ring, the innermost ring. Before the Cassini mission, scientists had thought that a radiation belt could not exist between the D ring and Saturn's atmosphere because Saturn's magnetic field would prevent the ions from entering the rings. Apparently, however, as particles trapped in the main radiation belt collide with Saturn's upper atmosphere, some of those particles “leak” into the region inside the innermost ring, creating a second radiation belt.

In early 2004, while Cassini was still en route to Saturn, the spacecraft's ultraviolet imaging spectrometer (UVIS) detected the sudden and surprising appearance of an immense cloud of oxygen surrounding Saturn. Scientists theorized that the oxygen atoms emerged from ice particles created as small moonlets in Saturn's E ring collided and broke apart. Radiation given off by Saturn freed the oxygen atoms from the ice particles. The oxygen cloud, equivalent to the total material in Saturn's E ring, dissipated within about two months. Cassini scientists will be watching to see if this startling process occurs again.

Remarkable Rings

Saturn's magnificent rings are one of the greatest mysteries of the solar system. Other planets—Uranus, Neptune, and Jupiter—have rings. But none are as massive, complex, or diverse as Saturn's rings, which make them the solar system's best laboratory for studying planetary rings in action. Cassini's mission includes orbits ideally suited to studying the rings, especially during the last year of its four-year tour. At that time, the spacecraft will orbit almost directly over Saturn's poles, providing scientists with excellent views of the rings.

Scientists using Earth-based telescopes discovered five of Saturn's seven rings, including the four main rings (A, B, C, and D), which extend from Saturn's upper atmosphere outward about 64,000 kilometers (40,000 miles). The fifth ring is the scattered E ring, which extends from just inside the orbit of Enceladus to as far away as Titan's orbit. NASA's Pioneer-Saturn spacecraft, which flew by Saturn in 1979, found the narrow F and G rings.

The Voyager probes discovered that the rings look like thousands of narrow ringlets. However, the main rings (except for a few gaps) are actually continuous rings that change in optical depth (the number of ring particles per unit area), giving the appearance of individual ringlets. In other words, the main rings' particles orbit closer together in some places and farther apart in other places, creating areas that look like alternating dense and less dense ringlets. The apparent ringlets range in size from about 2 to 100 kilometers (1 to 60 miles) in diameter. Cameras aboard the Voyager spacecraft also revealed that Saturn's rings consist of icy particles that may be as tiny as dust grains or as large as chunks the size of large houses or even mountains. The rings themselves, however, are paper-thin, no more than 90 meters (300 feet) thick in most places.

Cassini's cameras have photographed Saturn's rings in greater detail than ever seen before. At their best, the cameras can obtain a resolution of about 90 meters (300 feet), about the length of a football field. (Resolution is the ability of a lens to produce separate images of objects that are close together.) The images have revealed, for example, the existence of seven new ringlets, five orbiting within gaps between main rings.

Cassini scientists have marveled at the images of such unexpected and intriguing ring features as straws and ropy areas. These structures appear to be long clumps of particles that, for some unknown reason, have become jammed together. The straws extend from 1 to 2 kilometers (0.62 to 1.2 miles) in length, while the ropy areas may be 9.6 to 19 kilometers (6 to 12 miles) long.

Cassini's cameras also revealed the “wake” created by the tiny moonlet Pan, Saturn's innermost moon, as it speeds like a motorboat in water through a passage in the A ring known as the Encke Gap. In the images, the wake appears as scalloping along the inner edge of the gap. Scientists believe that Pan maintains the Encke Gap by sweeping away any particles that fall into it. After Cassini entered Saturn's orbit, the cameras discovered three faint new ringlets in the Encke Gap. These ringlets suggest that Pan is not the only moonlet maintaining openings through the Encke Gap. Cassini's cameras also discovered a new moonlet in the Keeler Gap, a gap near the outer edge of the A ring.

Cassini has shed some light on the development of the rings. The rings may have formed from fragments of one of Saturn's moons after a collision with another moon or with a comet, meteor, or other object traveling through space. They also may be the remains of comets or meteors that ventured too close to Saturn and were captured and then torn apart by Saturn's gravity.

Before Cassini, scientists knew that the rings are only a few hundred million years old, much younger than Saturn and the rest of the solar system, which formed about 4.6 billion years ago. However, Cassini found that some regions of the rings contain fresh material that was released in the last 10 million to 100 million years. Where did these additions come from? The fresh material might have been created when a larger ring particle was broken apart, releasing cleaner, purer material.

Cassini also discovered that the rings might lose material to Saturn's moons. On Oct. 29, 2004, Cassini cameras caught the moon Prometheus stealing particles from the slender F ring. Prometheus—and its partner, Pandora—are among Saturn's shepherd moons, pairs of moons that orbit a short distance apart, keeping ring material confined between them. As Prometheus reached its closest approach to the F ring, a thin streamer of ring particles linked the two. These images showed, for the first time, the complex interaction between the shepherding moons and the F ring.

Saturn's Icy Satellites

Except for Titan, Saturn's moons are usually referred to as icy satellites because of their rock-hard surfaces consisting mainly of water ice. Scientists have long believed that the icy satellites were too small to have enough gravity to hold on to a substantial atmosphere. For many years, scientists also thought Saturn's icy satellites were geologically dead. They assumed that any internal source of heat the satellites might have was too limited to affect the surface in any way. (On Earth, radioactive decay in the core produces heat that fuels volcanic activity and powers the movement of the tectonic plates that make up Earth's outer surface.)

Images from the Voyager probes picked up clues that the moon Enceladus was ejecting material that became part of the scattered E ring, Saturn's outermost ring. In March 2005, Cassini discovered that Enceladus, which has a diameter of only about 500 kilometers (300 miles), actually has an atmosphere. The spacecraft's cosmic dust analyzer revealed that this atmosphere consists of tiny pieces of water ice only about 1 micron (0.0000394 inch) in diameter. Because of Enceladus's light gravitational force, however, these particles float away over time, becoming part of the E ring. Thus, scientists have speculated, Enceladus must have geysers or volcanoes or another means of replenishing its atmosphere.

Cassini also gave scientists their first close look at Phoebe, one of Saturn's darkest moons. Voyager 2 saw Phoebe in 1981 from a distance of 2.2 million kilometers (1.4 million miles). The Cassini spacecraft, however, flew within 2,060 kilometers (1,285 miles) of Phoebe on June 11, 2004. It was Cassini's only opportunity to fly that close to Phoebe. But that one flyby provided more information about Phoebe than scientists had learned since the moon's discovery in 1898.

Cassini's images of Phoebe revealed a battered world scarred with impact craters and littered with landslides and small boulders. In the bottom of some of the craters lay boulders hundreds of meters (thousands of feet) wide. Cassini's visual and infrared mapping spectrometer confirmed Earth-based conclusions that Phoebe's dark surface consists of water ice darkened by a covering of carbon compounds and other material. (A spectrometer is an instrument that spreads out light and other types of electromagnetic waves into a spectrum and displays it for study.)

Scientists were elated to see images showing bright streaks on the walls of Phoebe's largest craters and bright rays originating from its small craters. They believe these features were created as meteors or other objects broke through Phoebe's dark crust during bombardments, exposing the moon's deeper layers. Cassini's infrared and ultraviolet instruments also discovered that the bright streaks are water ice with some carbon dioxide ice.

We now know that Phoebe is a mixture of water ice, rock, carbon dioxide, and primitive organic (carbon-containing) compounds, much like Pluto and Neptune's moon Triton. Scientists believe that icy bodies like Phoebe were plentiful in the outer regions of the solar system about 4.6 billion years ago and were the building blocks of the outer planets—Jupiter, Saturn, Uranus, and Neptune. As these planets formed, gravitational forces ejected most of the leftover blocks to orbits outside the solar system. Phoebe became trapped in Saturn's orbit instead. As a result, we have the first detailed images of such a primitive object.

Saturn's oddest moon, Iapetus, also revealed some of its secrets to Cassini. One side of this moon is bright, while the other side is dark. Voyagers 1 and 2 flew by Iapetus, but the dark side appeared only as an inky blackness in the probes' cameras. Cassini's more sophisticated cameras returned images of both sides. The dark-side images showed startling surface features, including a long, narrow, soaring ridge that lies almost exactly on Iapetus's equator. The ridge is about 1,300 kilometers (800 miles) long, with some mountains at least 20 kilometers (12 miles) high, about three times higher than Earth's Mount Everest. The ridge narrows as it rises, from a base that is perhaps 20 kilometers (12 miles) wide to peaks only 2 kilometers (1.2 miles) across.

At the boundary between the bright and dark materials, the images show craters with bright walls on one side and dark walls on the other. The colors most likely relate to the material making up the moon. The dark material is probably organic compounds. The light-colored material is probably water ice and carbon dioxide ice, which are very bright.

Also at the boundary between the bright and dark material, the images show feathery-looking black streaks. Scientists disagree whether this dark material spews from the interior of Iapetus or rains down onto Iapetus from elsewhere, perhaps from nearby Phoebe. From looking at the meteorites that fall to Earth, we know that carbonaceous chondrites, the most primitive type of meteorite, are rich in dark materials like those from which the sun and planets formed. Perhaps at one time, Iapetus went through a cloud of this material. Scientists hope that Cassini's next flyby of Iapetus in 2007 will uncover more clues to this mystery.

Titan's Pull

Titan, Saturn's largest moon, has exerted its own pull on Earth's scientists, independent of their fascination with its mother planet. Scientists and science-fiction writers alike have long speculated that Titan's methane-rich atmosphere might be similar to Earth's 4 billion years ago, before life appeared here.

One of the most exciting days of the Cassini-Huygens mission occurred on Jan. 14, 2005, when the Huygens probe became the first spacecraft to land on Titan. Mission scientists burst into cheers when the Green Bank radio telescope in West Virginia—1 of 17 radio telescopes around the world listening for signals from Huygens—picked up a faint but unmistakable crackling sound. The signal indicated that the probe had awakened from a 20-day “sleep” following its launch from Cassini and had come within about 1,300 kilometers (800 miles) of Titan's surface.

At that point, one exciting event happened after another. Within three minutes, Titan's dense atmosphere slowed the descending craft's speed from about 20,000 to 1,400 kilometers (12,300 to 900 miles) per hour. A small parachute deployed, further slowing the probe and pulling its main chute from its storage pack. At 120 kilometers (75 miles), another small parachute replaced the main chute for the final descent.

At an altitude of about 170 kilometers (100 miles), the probe's heat shield popped off, and its instruments began collecting and transmitting data. Then, as the probe descended for the next 2 hours and 30 minutes, Titan revealed some of its secrets.

During its descent, Huygens's instruments recorded sounds, measured wind speeds, and analyzed Titan's atmosphere. Observations by the Voyager spacecraft had revealed that nitrogen makes up most of Titan's atmosphere, as it does Earth's. In 1944, however, American astronomer Gerard Kuiper had discovered, using a spectrometer, that Titan had an atmosphere that included methane. Methane is a type of hydrocarbon, a compound of carbon and hydrogen. On Earth, hydrocarbons are found in large quantities in coal, natural gas, and petroleum. In the moon's upper atmosphere, ultraviolet sunlight breaks apart the methane molecules, the way ultraviolet light destroys ozone molecules in Earth's upper atmosphere. The fragments of methane then recombine to form propane, acetylene, benzene, and the other molecules that make up Titan's smoggy atmosphere.

Methane gas in Titan's atmosphere provides the building blocks to produce complex organic molecules that might form life. However, because of the intense cold (-178 °C [-290 °F]) at the surface, all water is frozen as hard as a rock and the chemical reactions that might have occurred on Earth and resulted in the development of life couldn't happen there.

The haziness of Titan's atmosphere surprised us. We had expected the probe to drop out of the haze when it reached an altitude of from 50 to 70 kilometers (30 to 45 miles). But the haze did not thin enough to give the probe's cameras a relatively clear view of the landscape until about 30 kilometers (20 miles) above the surface. At 20 kilometers (12 miles), Huygens found methane clouds. In fact, the Huygens probe discovered that the haze continues all the way to the surface.

As the probe descended, it took hundreds of pictures. Titan's surface, Huygens discovered, has intricate geologic features that scientists believe were shaped by physical processes similar to those that have shaped—and continue to shape—Earth. These include erosion caused by wind and, particularly, flowing liquids. On Titan, liquid methane plays the role that water does on Earth, scientists believe. The methane evaporates, condenses, forms clouds, and rains down, creating streams and rivers. The atmosphere, however, would run out of methane unless it was continually being replenished. These observations indicate that Titan's surface has been modified and changed over its history.

Finally, Huygens landed with what NASA called “a splat”—into Titanian mud. The first instrument to hit the surface was a long, sticklike device called a penetrometer, which was attached to the bottom of the probe. This device measured the force of the probe's impact and the properties of the material on the surface. Data from the penetrometer indicated something rigid—solid crust or a pebble—at the surface and clay or wet sand just below the surface.

Heat given off by Huygens warmed the landing surface, and the probe's spectrometer measured a sudden increase in methane gas boiling out of the surface. This event reinforced the idea that methane forms clouds and produces rain that erodes the surface. Much to scientists' surprise, Cassini's instruments have not detected any lakes or oceans of liquid hydrocarbons. Before Cassini's arrival at Titan, models predicted that liquid methane and ethane should have been accumulating on Titan's surface for a long time, creating large oceans and lakes. Titan's rivers and lakes appeared dry at the Huygens landing site, but methane rain may have occurred not long before the landing.

Where did all of this liquid go? Did it even ever exist? What are the details about the unusual bright spot on Titan that scientists spotted in May 2005? These are some of the questions that scientists hope to answer after reviewing the Cassini-Huygens data. These unanswered questions provide even more reason to be intrigued with the mysteries that lie behind Saturn, its dazzling rings, and its numerous moons.