| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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Skywatcher Circling Saturn
Many astronomers can recount the thrill they felt when they focused their first telescope on Saturn. From a billion miles away, the solar system's second-largest planet resembles a bright orb girdled by a glowing hula hoop. Lately, scientists have been getting a far better view of the real Lord of the Rings. On July 1, 2004, the spacecraft Cassini arrived at Saturn to take a four-year-long close-up look at the planet, its atmosphere, its rings, and its moons. Cassini is the fourth spacecraft to reach Saturn and the first to go into orbit around it. It's been a long, strange journey for the bus-sized spacecraft. Beginning in 1997, Cassini made two loops through the inner solar system, passing Venus twice before swinging past Earth, and building up speed with each maneuver. Using Earth's gravity as a "slingshot" gave Cassini the velocity it needed-about 50,000 miles per hour-to cross the asteroid belt between Mars and Jupiter. In December 2000, the spacecraft entered the realm of the giant planets, passing Jupiter and using that planet's gravity for the final gravitational boost that would take it to Saturn. Cassini's seven-year journey is the culmination of centuries of Saturn studies. Galileo Galilei, the first astronomer to use a telescope, was the first to see Saturn up close. In the early 1600s, he described what he called "handles" or "ears" flanking the ball of Saturn, but his crude, 30-power "optik tube" wasn't quite up to the task of resolving the rings clearly. It fell to later astronomers with better telescopes to uncover the true nature of Saturn's system. Christiaan Huygens was the first to identify the enigmatic rings. Composed of ice particles ranging in size from specks of dust to perhaps house-size boulders, the rings are sculpted by the gravity of Saturn's 47 known moons (as of May 2005). For example, 12-mile wide Pan keeps its orbit clean of stray dust. As Cassini approached its final destination, its view of the planet and the magnificent rings surrounding it gradually improved. What had been thought from Earth-based observations to consist of three simple, broad rings-labeled, from outside in, as "A," "B," and "C"-resolved into a much more complex system. Individual, tightly-packed ringlets comprise the broader bands, much like the grooves of an old-fashioned phonograph record. This had been seen by the Voyager missions, but never with such clarity. In some areas, ringlets were squished close together, while in adjacent regions they were spread apart. Astronomers believe a compression wave, likely caused by gravitational influences from Saturn's moons, may have swept across them much like the ripples in a pond. Though Jupiter, Uranus, and Neptune are also encircled by planetary rings, Saturn's are still the brightest and most easily observed. The rings of the other giant planets are composed of dark dust, making them fairly nonreflective and hard to observe from Earth. A curious feature photographed by the earlier Voyager probes was a pattern of radial spokes seen against the B ring (the brightest of the three main segments), which seemed to rotate mysteriously around the planet. These were thought to be clouds of dust levitated above the plane of the rings by static electricity. However, no photos taken by Cassini, which has much finer instruments aboard, appear to show anything resembling the spokes. The mystery of what happened to them remains unresolved. There's more to the rings, too: inside the innermost C ring is a very diffuse ring, labeled "D," and out beyond the main rings, there are now known to be E, F, and G rings, the latter two being very narrow and E being very broad. These extend out millions of miles-far enough to encompass several of the planet's large moons. To enter orbit around Saturn, Cassini had to pass between two of these rings. The planet itself, a colossal ball of mostly hydrogen and helium, resembles a yellowish, blander version of Jupiter. Saturn has a high-altitude haze of ammonia that obscures our view of much of the activity in its atmosphere, so to earthbound eyes it appears more sedate than its riotous big brother. But from Cassini's closer perspective, Saturn's clouds are shown to be peppered with hundreds of circular storm systems, while wind speeds around its equator reach 1,000 miles per hour-ten times the speed of Earth's jet stream. While still 8 million miles from the ringed planet, Cassini passed Saturn's outermost known moon, Phoebe. This 133-mile wide rock has long been known to possess a retrograde orbit-that is, it orbits Saturn in the opposite direction from the other moons and the way the planet itself rotates. This suggests that Phoebe may be an asteroid or "Kuiper body" that passed close to Saturn and was captured by the planet's gravity. (A Kuiper body is an object from the Kuiper Belt, a zone of comet nuclei orbiting the Sun just beyond Neptune. Pluto is the largest object in the Kuiper Belt.) Moving in past Phoebe, Cassini swung into orbit around Saturn, passing twice through the plane of the rings by threading the nearly 20,000-mile gap between rings F and G. On its way, the probe took numerous images of the F ring and its two shepherds, the tiny moons Pandora and Prometheus. Orbiting on either side of the rings, the gravity of these two tiny objects, less than 100 miles wide, keep the F ring intact. Over the next four years, Cassini will make 74 oblong loops around the ringed planet in a path resembling the petals of a flower. It will weave in and out past many of Saturn's other moons, largely avoiding the dust rings. Most of these encounters will be with Titan, one of the largest moons in the solar system. In January 14, 2005, the European Space Agency's Huygens probe, piggybacked aboard Cassini, parachuted through the atmosphere into Titan's opaque skies. Titan may be more like primordial Earth than any other body orbiting our Sun. Floating in its skies are hydrocarbon molecules called tholins. When mixed with water in laboratory experiments, tholins form amino acids, the chemical precursors of life. One question scientists have been asking is whether or not tholins are involved in the origin of life. If so, Titan may be the most likely place to find life beyond Earth. During its descent, Huygens found that the murkiness cleared below about 20 kilometers, giving a sharp view of the landing zone. Although Huygens took 700 images during the descent, about half were lost to a command error from ground control. What was successfully transmitted, however, were views of a completely unknown world. Its features may include riverbeds and even a possible coastline. The fluid involved most certainly isn't water, which would freeze in the -170¼C weather, but probably liquid methane. Like water in Earth's atmosphere, methane may form clouds in Titan's atmosphere and maybe even rain. An onboard microphone recorded the sound of Titan's winds on the way down. After a two-and-a-half-hour descent, Huygens made a softer landing than anticipated, plopping onto what is thought to be a sandy or even muddy surface. The bump was so gentle that it didn't even cause a hiccup in the probe's radio messages. Huygen's onboard batteries continued operating for 72 minutes after landing. Photos taken from the surface show small, fist-sized rocks that turned out to be water ice, hardened to the consistency of rock by Titan's intensely cold temperatures. Instruments also suggested that volcanic activity on Titan produced not lava but water ice and ammonia. Technically, the mission was known as Cassini-Huygens, since it was a joint project between NASA, ESA, and the Italian Space Agency. A joke circulating around NASA was that Cassini was a shipping company, and Huygens was the cargo, carried 2.2 billion miles and delivered on time, with delivery confirmed. Though Huygens' mission is now over, Cassini still has a lot more trucking to do through Saturn's system. Cassini has also made early surveys of other large moons. Mimas, resembling a 250-mile-wide eyeball, is a major architect of many of Saturn's rings. It governs the Cassini Division between the A and B rings and is the sculptor of several bright ringlets within the A ring. A near-catastrophic impact on Mimas created the 80-mile-wide crater dubbed Herschel, and opened up a huge system of fractures on the other side of the moon. Unfortunately, because Mimas is so close to the main ring system, Cassini will not have any especially close flybys. Its only scheduled encounter, at a distance of 30,000 miles, is set for August 2005. A little farther out, the frozen moons Enceladus and Tethys are safer targets for study. Located farther away from the rings, they are therefore less subject to possible collisions with the ice and dust that make up the ring particles. Like Mimas, Tethys suffered a devastating impact that resulted in a 250-mile wide crater nearly half the diameter of the moon itself. Enceladus is thought to have been partly resurfaced in the not too distant past by fresh ice. The ice may even have been released from underground as water, which then quickly froze solid. Large areas of its 300-mile-wide surface appear less blemished by impact craters than those of other moons. The first flyby of Iapetus, nearly 900 miles across, reveals a rough-hewn surface with a curious feature-a towering, eight-mile-high ridge that stretches for at least 800 miles. The ridge closely parallels the moon's equator, seemingly separating it into two halves. One other close flyby of Iapetus is scheduled, and it may shed more light on this mysterious "Great Wall." As much a puzzle is the dark, tarry material covering Iapetus' leading surface (that is, the side facing the direction in which the moon moves in its orbit around Saturn). Scientists don't know if it was spewed out from beneath the crust by the two-toned moon itself, or if it was ejected by other moons as a cloud of dust and then splattered Iapetus as it passed through. Though Cassini's mission nominally ends in 2008, it could possibly go on making discoveries for as long as its machinery continues to function. In hopes of solving many of the mysteries about Saturn's system that remain, Cassini is slated for further flybys of the battered moons Rhea, Dione, and Hyperion during the next few years. Each encounter will be a visit to another new world, for like Jupiter, Saturn almost constitutes a miniature solar system of its own, with a myriad of curiosities yet to be revealed. Bing F. Quock is acting chairman of the Morrison Planetarium at the California Academy of Sciences. |
