The Sciences

Saturn Spectacular

Cassini's first photos from the ringed planet promise enough awe and wonder to last a lifetime.

By Fred GuterlAug 2, 2004 12:00 AM
Saturn fills the entire frame of Cassini’s narrow-angle camera in this natural-color image taken while the spacecraft was still 29.7 million miles away. The blue sliver above the rings results from sunlight scattered off Saturn’s clear upper atmosphere; unknown compounds cause the color variations in the subtly banded clouds. | Courtesy of NASA/JPL/Space Science Institute


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In 1610 Galileo pointed his crude spyglass at Saturn and was dumbfounded by what he saw: “The planet Saturn is not alone, but is composed of three, which almost touch one another and never move nor change with respect to one another.” Worse, the two bulging planets on each side of the main planet had disappeared when he looked again a few months later. “What is to be said concerning such a strange metamorphosis?” he cried. Eventually, the frustrated Galileo decided never to look at Saturn again.

Now, of course, we have much better telescopes, and we know that Galileo was looking at the planet’s unique set of wide, thin rings. Seen broadside, they resembled companion planets through Galileo’s weak lenses; later, seen edge on, they shrank to nothingness—an invisible sliver. But nearly 400 years after Galileo’s observations, Saturn still teases astronomers, and the closer we look, the more oddities we see. Saturn’s magnificent rings, for example, consist of trillions of bits of ice, some no bigger than a speck of dust, making up a weird and complex system of satellites. The ring particles are so puny that you would expect them to quickly scatter and fall into the planet, yet they are still there. And the planet is so loosely built that it would float on water.

At another extreme, Saturn’s giant moon Titan seems more like a planet in its own right, larger than Mercury and cloaked in a dense atmosphere. Titan’s surface seems to be covered with ethane oceans and an organic goo that may resemble the Earth’s early surface chemistry, but nobody knows for sure, because astronomers can’t see through the moon’s maddeningly opaque orange fog. In between, Saturn has at least 30 other smaller moons, some smooth, some battered, some strangely mottled.

When NASA sized up Saturn’s expansive mysteries and decided to get a probe there, it chose the scientific equivalent of the big cannons on a battleship: a 22-foot-tall, 12,600-pound spacecraft called Cassini-Huygens. This behemoth is the largest object ever launched into deep space by the United States. And at a total cost of $3.3 billion, it is also among the most expensive planetary missions. Cassini is so hefty that NASA did not have a rocket powerful enough to send it on a direct course, so engineers devised a 2.2-billion-mile loop-the-loop trajectory that exploited the gravity of Venus, Earth, and Jupiter to hurl the spacecraft to its ringed destination.

At the end of June, Cassini reached Saturn and finally began erasing the question marks that have surrounded the planet ever since Galileo studied it. Aboard are a dozen instruments, including sensors for visible light, ultraviolet, and infrared as well as magnetometers, radar, and plasma detectors. All the equipment will be powered by three plutonium-driven generators cranking out 750 watts of electricity. The smaller Huygens probe will separate from Cassini on December 24 and set off for a rendezvous with Titan. Huygens has a suite of cameras and sensors all its own. “You want a complete set of instruments to make measurements simultaneously so you can correlate all the results,” says Robert Mitchell, the Cassini program manager at the Jet Propulsion Laboratory in Pasadena, California.

Many of the scientists on the Cassini team have waited a good fraction of their professional careers for this moment. Pioneer 11, the first craft to visit Saturn, returned a handful of provocative pictures in 1979. In the early 1980s, Voyagers 1 and 2 flew by, sending back vastly sharper images of Saturn’s rings and intriguing data about Titan’s atmosphere. “We knew very little before Voyager,” says planetary scientist Jeff Cuzzi of NASA’s Ames Research Center, a member of the Cassini mission’s scientific executive committee and a veteran of the Voyager imaging team. “It was a stunning and an exhilarating and a humbling experience.” But the Voyagers used 1960s-era technology, and they did not hang around long. Like typical American tourists, they swept in, took a few snapshots, and left. Planetary scientists then spent two decades sifting and resifting through a limited cache of data.

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.

This orange blur is the best image of Titan from Earth. On January 14, the Huygens probe will parachute to the surface and show what lies beneath the haze: Light areas may be ice continents; darker regions could be chilly ethane seas. | Photograph from European Southern Observatory (ESO)

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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