Cassini, by contrast, will become a resident, orbiting at least 76 times around Saturn. Some NASA scientists think the probe will fling an average of one gigabit of data back every day for four years, including up to 750,000 pictures. That’s “more than we’ve gotten back from any other planetary probe,” Mitchell says. Anticipating what the results will bring is a bit like anticipating a first date—you may have impressions from a glimpse of the opposite sex, but time and proximity make all the difference. “We don’t know what Cassini is going to be like,” Cuzzi says. “We don’t really have an analogue.”

When Cassini fired its rocket and eased into orbit around Saturn on June 30, one of the first orders of business became trying to make sense of those dazzling rings. The spacecraft’s namesake, Italian astronomer Giovanni Cassini, deduced in the 17th century that the rings are composed of a swarm of particles. Later studies added only a few details: The rings are composed largely of water ice and measure about 175,000 miles in diameter yet are just a few tens of yards thick. Voyagers 1 and 2 provided some amazing views of the rings but did not do much to explain two key questions: What controls the structure of the rings, and why are they there?

The why question is especially confounding. Computer simulations suggest that planetary rings are not durable. Within a half-billion years or so—scarcely more than one-tenth the age of the solar system—Saturn’s rings should have dispersed or should at least have degenerated into something much less spectacular, like the dark, wispy rings around Uranus. It wouldn’t be so surprising that we are witnessing Saturn’s spectacular display right now if planetary rings were popping up all the time, but they are not. According to what little we know, Saturn’s rings must have formed as the result of rare, highly improbable events.

There are two theories of how the rings originated—both plausible, but only barely. In one scenario, a comet or something similar struck one of Saturn’s moons, blowing it to bits. The rubble then went into orbit so close to Saturn that the planet’s gravity prevented the particles from pulling back together, and they became rings. In the second, a gigantic icy object from the extreme outer solar system swooped in, got too close to Saturn, and was ripped apart by gravitational forces.

Figuring out what explanation is correct requires knowing the precise composition of the 10 percent of the rings that isn’t water ice. To find out, Cassini will analyze radiation bouncing off the ring particles to look for silicates, carbon, ammonia, organics, and other substances. When the spacecraft passes through the rings, it will also use its instruments to sniff out their composition. In the end, neither origin theory looks terribly promising. It is extremely unlikely that a comet would strike a moon in just such a way as to destroy it, but the odds of a huge ice ball wandering close enough to Saturn to be torn apart by gravity are also long.

Which brings us to another possibility: The rings are not as short-lived as scientists think. Their stability depends on an elaborate dance between them, the surrounding moons, passing meteoroids, and Saturn. The rings contain objects ranging in scale from grains of dust to flying mountains. Regardless of size, each bit of ring follows its own orbit about the planet, suspended not only in Saturn’s gravity but also in its own minute gravitational field. Needless to say, a trillion objects tugging at each other are going to interact in complicated ways. Broadly, though, there is one overarching trend—toward destruction.

Saturn’s inner moons, orbiting just beyond, constantly steal momentum from the ring particles. As a result, the moons spiral outward, and the rings eventually fall into Saturn. This is the chief reason why scientists think they cannot be more than about 500 million years old. Another observation supports that timescale. Interplanetary dust constantly falls onto the rings, as it does onto all objects in the solar system. Whereas interplanetary dust is dark, Saturn’s rings are bright. If the rings were old, they would have darkened.

The Early Days: What Cassini Has Already Delivered

Photographs, clockwise from top left: Courtesy of NASA/JPL/Cassini Imaging Team/University of Arizona; Courtesy of NASA/JPL; Courtesy of NASA/JPL/University of Arizona

Although it has taken Cassini seven years to reach Saturn, the spacecraft has made the most of its agonizing, 2-billion-mile windup. Along the way it has set its sights on a wide range of passing solar-system sights—including our moon, Jupiter’s color-striped clouds, the Jovian moon Io, and a small asteroid called Masursky—so that engineers could test the craft’s instruments and work out any glitches. As a bonus, Cassini acquired a trove of exciting data and images long before its formal mission began.

After launch, Cassini swooped through the inner solar system to pick up gravitational assists from Earth and Venus. In the summer of 1999, it made a final swing past Earth and turned its cameras on the moon, yielding some of the sharpest images (A) ever taken of our natural satellite. Six months later, Cassini trained its cameras on the roughly 10-mile-wide asteroid Masursky as it passed. To scientists’ surprise, the object seems to have a different composition from other asteroids in that part of the solar system.

By late 2000 Cassini was closing in on another intermediate destination, Jupiter. The spacecraft’s radar instrument listened in on radio waves from the planet, producing a unique map of Jupiter’s harsh radiation environment (B). The emissions come from electrons racing around at nearly the speed of light in Jupiter’s enormously powerful magnetic field. Yellow denotes the most intense emissions in this image; blue signifies the least intense. Farther in toward Jupiter, Cassini’s cameras captured detailed panoramas of atmospheric storms and novel views of tiny moons orbiting in Jupiter’s tenuous ring. One of the most dramatic Cassini images was a portrait of the volcanic satellite Io seeming to float just above its parent planet (C). Despite appearances, Jupiter and its moon were nearly a quarter of a million miles apart.

Last year, Cassini began focusing on its final destination, taking a series of snapshots that documented Saturn growing ever larger through the narrow-angle camera. The spacecraft hasn’t looked back since.         

—Maia Weinstock

Ion and neutral mass spectrometer

Identifies atoms and subatomic particles around Saturn and Titan to study their atmospheres.

Radio science subsystem

Watches how radio signals from Earth change as they pass through atmospheres and through the rings.

Some caveats are in order. Life on Titan would not be a walk in the park. From the vantage point of the Saturn system, the sun is a rather dim bulb. Titan is therefore one cold place: Surface temperatures average 92 degrees Kelvin, or about –300 degrees Fahrenheit. At those temperatures, water is a rock, and it would flow only from volcanoes. Although Titan is half water, there’s no place to get a drink. Any aspiring living organism will also immediately find that there’s no oxygen in Titan’s atmosphere—it’s all locked up inside water ice. The only hope for life as we know it, and it’s an exceedingly slim one, is that water mixed with ammonia may get warm enough deep below the surface to liquefy. If so, life could possibly eke out an underground living much like the hardy microbes that surround Earth’s hydrothermal vents.

The bitterly cold temperatures that make Titan so forbidding for life in some ways make it more intriguing to people like Toby Owen, a planetary scientist at the University of Hawaii and a Cassini coinvestigator. Titan’s chilly climate keeps things in a state of preservation, like a freezer. If a cosmic tugboat were to drag Titan into the inner solar system, the sun would quickly boil off the water and just about everything else, leaving Titan as nothing more than a modest rock. Out near Saturn, however, Titan can hang on to most of the substances that it acquired during its 4.5 billion years in the solar system. The organic reactions that may have established the starting conditions for life on the early Earth are long gone, erased by our planet’s high-speed chemical and geologic evolution. On Titan, similar reactions may still be sitting in deep storage.

Voyager gave only a tantalizing hints of these remarkable possibilities. Huygens, a disk-shaped probe built by the European Space Agency (and named after Christiaan Huygens, the Dutch astronomer who discovered Titan), should rectify the situation. After parting ways with Cassini, the probe will drop into Titan’s atmosphere, open a parachute, and float down to the surface, taking measurements every step of the way.

While Huygens sniffs around for organic chemicals, it will also search for clues that could explain why Titan has its unique atmosphere. Nitrogen doesn’t come from rocks; it had to be acquired somehow. One possibility is that Titan’s nitrogen was deposited by comets or by the icy planetoids that came together during its formation. Huygens will sample methane in the atmosphere and measure the relative abundance of light hydrogen and heavy hydrogen to produce a chemical fingerprint that will enable scientists to compare Titan’s composition with that of comets. If the probe happens to land on ice, it will evaporate a sample with a little projecting tube and measure the hydrogen there as well.

The most dramatic phase of Huygens’s explorations, though, won’t be about chemicals. It will be about the pictures the probe will snap during its two-and-a-half-hour parachute ride to the surface. Assuming tholins really account for Titan’s haze, the atmosphere should grow fairly clear less than 20 miles above the surface, and Huygens’s cameras will be able to grab some great panoramas. Titan promises to be a strange sight indeed. There may be oceans, rain, rivers, and waterfalls—but instead of water, the predominant liquid on Titan is probably a substance akin to liquefied natural gas. “We anticipate seeing some remarkable stuff,” Owen says. “We haven’t really seen the surface yet. We can get a dim picture of it from Earth, but at a very low resolution. We know that it’s not all covered by one thing, but we don’t know what’s there.”

Planetary scientists have created a small publishing industry of speculations about what those things might be. When ultraviolet rays from the sun fall on methane, they break the molecule into components that form ethane, a constituent of natural gas on Earth. Ethane freezes at –295ºF and boils at –128ºF. Titan’s average surface temperature lies somewhere in between, so researchers expect to find lakes and seas of liquid ethane on the surface. Ethane there may act the way water does on Earth. It may evaporate from the surface and condense in the atmosphere to form clouds, which in turn release rain. Over 4.5 billion years, Titan could have accumulated enough ethane to cover a good portion of the surface. Last year astronomers used the telescope at Arecibo, Puerto Rico, to send a beam of radio waves to Titan. They got back what look like surface reflections from lakes or oceans of liquid, like the glint of sun on a lake. “I like to think of Titan as the dream world of the Exxon people. It’s a flammable surface, but it doesn’t explode, because the oxygen it would need to burn is trapped in water ice,” Owen says.

Compared with its showy rings and haze-obscured moon, Saturn itself seems downright ordinary. Yet the 75,000-mile-wide planet—the second largest in the solar system, 95 times as massive as Earth—holds some serious interest of its own.

Like Jupiter, Saturn is a gas giant, a relatively small ball of rock surrounded by a vast envelope of helium, hydrogen, and various hydrogen compounds. In many ways, it is like Jupiter’s strange little brother. Saturn is half as dense—less dense than even water, cubic inch for cubic inch. It releases less internal heat than Jupiter, but given its smaller size scientists aren’t sure why it radiates any heat at all. Saturn’s storms are, surprisingly, more powerful than Jupiter’s, and its supersonic jet streams are much faster. The planet looks blander, however, because a thick haze of ammonia crystals obscures the colorful banding seen so readily on Jupiter. Cassini scientists hope that studying these differences will tell us about how giant planets form, how weather systems work under different conditions, and what planets around other stars might be like.

Four of Cassini’s instruments will directly explore the magnetic field that surrounds Saturn. If our eyes could see radio waves, this field, not the rings, would be Saturn’s largest and most distinctive feature—a teardrop-shaped envelope of radio-emitting particles that may extend a million miles across. Here, too, Saturn seems like a junior version of Jupiter, whose field is 10 times more powerful. The now-defunct Galileo probe revealed spectacular activity in Jupiter’s magnetosphere (the region of magnetically disrupted space around the planet) that confounded the scientific models. “Jupiter is much more complex,” says Tamas Gombosi, head of Cassini’s magnetosphere team. “By understanding Saturn, perhaps we’ll be able to understand Jupiter.” The answers could lead to a better appreciation of Earth’s magnetic shield as well.

Cassini scientists keep repeating such sentiments. Right now they know so little about Saturn that they aren’t even sure what questions to ask. By the end of the mission, they will have so much information about Saturn that they’ll have a hard time deciding where to start figuring out the answers. “It’s going to take a long time for all this to sink in,” says Cuzzi of NASA-Ames. “It’s not going to be one of these three-day wonders with Voyager. It’s going to take four years to get all the data. Understanding the data may take 40 years.”

The most immediate result of the Cassini mission may be one that Galileo would have dearly appreciated back in 1610: new photos that finally show Saturn with crystalline clarity. Thirty-six years ago Sir Arthur C. Clarke put the action of his novel 2001: A Space Odyssey in the vicinity of Saturn. For the movie version, director Stanley Kubrick decided on Jupiter instead. “Saturn was too big a challenge for the special-effects boys,” Clarke recalls. They couldn’t make even convincing guesses about what it would look like up close. “The popcorn set just wouldn’t believe it,” says Clarke.

Soon, they will.

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