The rain comes just once every thousand years, in torrents of liquid methane. The noxious air dims sunshine to an eternal orange twilight. The cold—290 degrees below zero Fahrenheit—is a lethal assault. And beyond the hazy sky looms the ringed planet Saturn.
Yet here on Saturn's outsize moon Titan is a world eerily like our own."Titan is a Peter Pan world," says Tobias Owen of the University of Hawaii's Institute for Astronomy. "It's got all the materials and elements to develop into a planet like Earth," he says, "but it never had the chance to grow up." The dense atmosphere is ﬁlled with hydrocarbon smog, "like L.A. on a bad day," Owen says. The rare methane monsoons create sudden rivers that cut deep channels in Titan's low hills and run down to a great sandy plain. Like Earth, Titan may have geologic activity and volcanism—a slow, chilly version that erupts a lavalike mix of half-melted water and ammonia. Most tantalizing of all, Titan's gentle winds carry a rich brew of organic molecules, some reminiscent of compounds that provided the raw material for life on Earth.
Owen and his fellow planetary scientists are used to picturing Titan in their imaginations. Now they've visited, if only by remote control. For the past two-and-a-half years, a space probe called Cassini has hobnobbed with the moons and rings of Saturn and gazed down on the giant planet. Soon after arriving, Cassini even launched a second, smaller probe called Huygens, which touched down on Titan's surface.
The Titan encounter was a high point in what has amounted to a voyage back in time. From the exotic metallic hydrogen in its interior to the ﬁne rubble of its rings, on moons that range from the icy oddball Phoebe to Enceladus, which spurts warm geysers, Saturn carries clues to how the solar system took shape 4.6 billion years ago and gave rise to life. The planet and its orbiting retinue, says planetary scientist Jeff Cuzzi of NASA's Ames Research Center, "connects us to solar system structure and evolution on the grandest scale."
Saturn has been slow to give up its secrets. In 1610 Galileo discovered what turned out to be its most amazing feature, the rings, but through his primitive telescope, he mistook them for two smaller bodies flanking Saturn. Only in 1656 did Dutch astronomer Christiaan Huygens (the namesake of the Titan probe) recognize what they were. Huygens also discerned a faint spark outside the rings—a moon later named Titan, after the Titans of Greek mythology, who reigned during Earth's early days.
Since then, decade by decade, astronomers have picked out smaller moons, 56 at last count. In the 1940s, as telescopes improved, they discerned a haze around Titan, the ﬁrst sign that, unlike any other moon in the solar system, it has a dense atmosphere. Finally, the ﬁrst space probes flew past Saturn—Pioneer 11 in 1979 and Voyagers 1 and 2 in 1980 and 1981. Speeding by, they snapped close-ups of the planet, rings, and moons and gleaned the ﬁrst hints that Titan is a frozen time capsule of conditions similar to those found on the very early Earth.
Now, after centuries of curiosity and anticipation, scientists are taking a long, close look at Saturn. A metal cylinder 22 feet tall, bristling with scientiﬁc instruments and topped by a white saucerlike antenna, Cassini-Huygens was built by NASA, the European Space Agency, and the Italian Space Agency. It rocketed toward Saturn in 1997 and arrived on June 30, 2004, to begin at least four years of exploration.
As it neared the end of its 2.2-billion-mile journey, Cassini had to shed speed so that Saturn's gravity could capture it. The spacecraft ﬁred its engines and dropped to within 13,000 miles of the planet's butterscotch clouds, making a daring passage between the outer rings. "White-knuckle time," Cassini project manager Robert Mitchell recalls.
The rings look crisp and manicured, but they are actually swarms of debris: billions of particles from mite- to mansion-size. A single stray pebble slamming into Cassini as it sped through the rings at over 68,000 miles an hour could have ended the 3.4-billion-dollar mission. Mitchell's team at NASA's Jet Propulsion Laboratory in Pasadena, California, anxiously monitored signals until Cassini, intact, settled into orbit and began to look around.
Exceeded in size only by Jupiter, Saturn could hold more than 700 Earths. Yet the planet, made almost entirely of hydrogen, is lighter than water. Dropped into an ocean big enough to contain its 75,000-mile diameter, it would float, bobbing like a colossal yellow sponge ball. It spins so fast that it bulges to a diameter 7,300 miles greater at the equator than at the poles, so fast that a Saturn day lasts less than 11 hours.
Because Saturn is mostly gas, it has no ﬁxed landmarks to reveal its exact rotation rate. But its dense interior generates a powerful magnetic ﬁeld that spins with the planet. Over the past two years, Cassini has clocked the ﬁeld's rotation at 10 hours, 47 minutes, and 6 seconds, although no one is sure the planet itself spins at exactly the same rate. But the ﬁeld also opens a window into the heart of Saturn.
Saturn began in the disk-shaped cloud of dust and gas that swirled around the newborn sun 4.6 billion years ago. Bit by bit, particles stuck together until gravity could take over, drawing material into ever larger lumps of iron and rock. One of them, perhaps several times the mass of Earth, was the seed that grew into Saturn.
Over time, the gravity of this rocky core attracted great clouds of hydrogen gas. The gas settled around the core, and the planet's mass rapidly grew. Pressures mounted, squeezing the innermost layer of hydrogen so hard that scientists believe it turned into a liquid metal—a superb electrical conductor. Currents surging through the metallic hydrogen generate Saturn's immense magnetic ﬁeld.
More than four billion years later the core still retains heat from its formation, which stirs massive updrafts in the planet's deep atmosphere. They whip up supersonic winds, among the fastest in the solar system at up to a thousand miles an hour, and drive vast weather systems. "We see storms, lightning, zones of clouds, and strange wavelike features in the atmosphere," says Kevin Baines of NASA's Jet Propulsion Laboratory. In images from Cassini's infrared camera, the heat rising from deep in the atmosphere sets the planet aglow. "We're seeing backlit clouds," Baines says. "We can watch the weather day and night. It's a revelation."
Only at the very top of Saturn's atmosphere, capped by a yellowish layer of haze, does the turmoil subside. Here, on calmer clouds, the distant sun inscribes shifting patterns of shadows cast by Saturn's vast system of rings.
From edge to edge, the main rings span some 165,000 miles, over two-thirds the distance from Earth to the moon. Yet the thickness of these bands of icy rubble averages only 150 feet. "Think of a sheet of paper spread over ten football ﬁelds," says Cuzzi, who studies the rings.
No one knows how the rings formed, although some scientists speculate that Saturn's gravity tore apart an icy moon or a comet, strewing debris that provided the raw material. Whatever their source, the rings are recent, cosmically speaking. If they had existed for the life of the solar system, Cuzzi says, their subtle pinks, yellows, and tans—the result of dust buildup—would have darkened. But they offer a model for something ancient: the disk of particles orbiting the young sun and its interplay with the newborn planets.
In Saturn's rings today, tiny moons play the role of the planets. Each moon's gravitational tug is minute, Cuzzi says, "about the same effect as a passing truck's gravity has on you." Yet the moons' gravity helps maintain the rings by keeping the particles from straying from their orbits. A moon can also carve a gap between rings, and its gravity can send waves of density rippling through a nearby ring, like trafﬁc speeding and slowing on a crowded freeway.
The Voyager probes glimpsed this dance, but Cassini is adding new detail. During its dash through the rings in June 2004, for example, it spotted evidence of miniature moons in the gauzy A ring, the outermost of the main rings. These moonlets—likely to number in the millions—are only a few hundred yards in diameter, but their feeble gravity is enough to leave wakes in the ring. In the F ring, farther from the planet, Cassini imaged a skein of narrow ringlets, accompanied by moonlets that sweep material into clumps and then break them up again.
"We're seeing ringlets interacting with moons and moonlets sculpting rings," Cuzzi says—and gaining new insights into how solar systems develop. "It helps explain how planets form in protoplanetary disks." The tiny moonlets in the A ring spiral slowly inward as they churn the ring particles, in a process that could also have shaped some of the bizarre solar systems detected around other stars. There, Jupiter-size planets are found right next to their suns, in orbits closer than Mercury's—perhaps because of a similar migration process.
One relic of our own early solar system still orbits Saturn: the moon Phoebe. Phoebe revolves in the opposite direction from most of the other moons, a hint that it has an unusual history. Cassini took a close look on its approach to Saturn in 2004 and found that the 130-mile-diameter moon is a hodgepodge of ice, rock, and carbon compounds—much like the Kuiper belt objects, small, icy bodies in the outer solar system that are thought to be leftover building blocks for the outer planets. As the solar system formed, most of the Kuiper belt worlds were flung far beyond Pluto. But Phoebe could be a Kuiper belt object that got left behind, trapped in orbit about the young Saturn.
Saturn's other major moons probably were born in the same clump of gas, dust, and rock that created the planet itself, but they are a study in diversity. Cassini revealed that some are little more than loose collections of rubble, including Hyperion, a potato-shaped mass 215 miles long. Larger moons are denser and have distinctive surface features, sculpted by accidents of history or by internal heat and the geologic activity it drives.
Images from Voyager, for example, showed that the 905-mile-diameter moon Iapetus is divided into black and white hemispheres, like a cosmic yin-yang symbol. Researchers suspect that the moon is made of nearly pure ice, which is exposed in the bright hemisphere and cloaked in rock and organic material on the dark side.
Cassini discovered new mysteries. Iapetus bulges in the middle, like Saturn, and has a ridge twice as high as the Himalaya running a thousand miles along the equator, mostly in the moon's dark hemisphere. "Nobody can ﬁgure this out," says Peter Thomas of Cornell University. "The bright and dark sides of Iapetus were one puzzle. That's moved back to third place with these new questions."
The most tantalizing of Saturn's moons is the largest: Titan. On December 25, 2004, six months after arriving at Saturn, Cassini launched the crown jewel of the mission, a saucer-shaped probe named Huygens that had ridden piggyback from Earth. Three weeks later, Huygens plunged into Titan's smoggy atmosphere.
At a gleaming white ofﬁce complex of the European Space Operations Center in Darmstadt, Germany, hundreds of scientists, students, and journalists jammed an auditorium and waited for the ﬁrst signals from Titan. A huge yellow model of Saturn dominated the front of the room, bright in the blaze of TV lights. Conversations in English, French, German, Spanish, and Italian flew around like stray meteoroids, reflecting the international origins of Huygens, which was also on display in a full-size mock-up.
The real thing, moving ten times faster than a rifle bullet, had slammed into Titan's outer atmosphere just an hour earlier. Pummeled by air friction, the probe's heat shield reached a temperature of thousands of degrees. Within minutes Huygens slowed and cooled. Parachutes opened, the heat shield dropped away, and Huygens drifted like a leaf on the winds of Titan, cameras and microphones recording the weather on a distant world.
"We'll be looking for lightning, but we might as well listen for thunder," said David Southwood, head of space science for the European Space Agency, who explained Huygens's progress to the crowded auditorium in Germany. "It's going to be very romantic," said Southwood, elegant in silver hair and a dark tweed suit.
As Huygens descended, people crowded into the big auditorium. Mission controllers were already receiving signals from Huygens, evidence that it had survived the descent. Relayed through Cassini, they took 67 minutes to travel from Saturn to Earth. Finally, at 5 p.m., Southwood took the podium and formally announced the probe's safe arrival. "We are the ﬁrst visitors to Titan."
Now the wait began for the signals to be computer-processed into images. Hours dragged by. Suddenly a grainy black-and-white image appeared on televisions ringing the auditorium. Taken on the way down, it showed lumpy hills and a dark plain.
Crowds surged toward the televisions, and the moon named for the gods commanded a moment of ritualistic, almost worshipful media attention. TV crews ﬁlmed the image of Titan. Photographers snapped pictures of the TV crews ﬁlming. Radio reporters held microphones toward the commentary coming from the sets.
More images followed, including a hastily constructed mosaic showing a broad aerial pano-rama. Finally the ﬁrst picture from the ground appeared. This was in color—a garish orange landscape, strewn with rocks. Low hills appeared in the distance. Long into the night, crowds clustered around the screens as they flashed the images, which had been transmitted hours before from Huygens to Cassini, stored on the mother ship, and then relayed to Earth.
By that time Huygens's short mission was already over. Cassini's orbit had carried it out of contact with the lander. Huygens continued to broadcast into the void for another two hours—far longer than expected—before its batteries went dead.
A glitch had marred the landing. Half the pictures—350 of them—were missing because of a communications failure. Even if everything had gone perfectly, Huygens could see only a small section of Titan, much like viewing an elephant at close range through a drinking straw. But it saw enough to answer some key questions.
Beforehand, no one knew whether Huygens would touch down on solid rock or squishy goo, or in an oily methane ocean. In fact the probe found no pools of liquid, but there were plenty of signs that the surface—crusty on top, soft below, like crème brûlée—is sometimes awash.
"We see signs of liquid methane scouring out river valleys," says Larry Soderblom of the USGS. "Titan may be like dry African deserts, but where rain only falls every century, or even every millennium. But when it comes, there may be a lot, like flash floods." The poles may be rainier. On a July flyby of the northern polar region, Cassini saw a landscape dappled with methane lakes—an otherworldly Minnesota.
The methane originates beneath Titan's crust, brewed in deep, warm reservoirs of water and organic material or trapped in icy deposits. Escaping to the atmosphere, some of the methane falls back to the surface as rain while ultraviolet light transforms other methane molecules into more complex organic compounds, which fall as a toxic sleet. "Titan is the best organic factory in the solar system," says Hunter Waite of the Southwest Research Institute. "There's a layer of frozen hydrocarbons, similar to gasoline, covering much of the moon. If you could mine Titan, you'd never have to worry about oil shortages."
Over millions of years, Titan's winds sculpted this vast sea of hydrocarbon sand, sweeping it into dunes over 300 feet high that run in parallel rows for hundreds of miles. "Dead ringers for dunes in the Arabian desert," says Ralph Lorenz, a Titan expert at Johns Hopkins University's Applied Physics Laboratory.
Like our atmosphere, Titan's is largely made of nitrogen, which is a key component of life. So are complex carbon compounds like those in its smoggy air. Titan preserves some of the conditions needed to start life, though it is far too cold for the spark of life to ignite. But in Cassini's most surprising discovery so far, scientists stumbled on hints that another moon might actually be hospitable to simple life-forms.
Bright as a beacon, ice-covered Enceladus reflects more light than any other body in the solar system. A quarter of a century ago, Voyager images showed only a few large craters marring the moon's surface, leading scientists to suspect that geologic processes were somehow erasing the scars. Yet at only 310 miles across, Enceladus seemed too small to generate the heat needed to drive internal activity. In another puzzle, Enceladus seemed to be feeding material into the tenuous E ring, which is densest close to the moon.
Cassini swooped in to investigate. On two encounters early in 2005, it detected an odd disturbance of Saturn's magnetic ﬁeld. Before the next encounter, Michele Dougherty of Imperial College London, chief of the magnetics team, pleaded with spacecraft controllers to set a course that would take Cassini close to the moon's south pole, where her team had measured the strongest disturbances.
On July 14, 2005, the spacecraft descended to a hundred miles above Enceladus's south polar region. Working in concert, its many instruments probed the enigmatic moon, monitoring surface heat, chemical traces, and magnetic ﬁelds. The data indicated that plumes of material were erupting near the south pole. Four months later, as the distant sun silhouetted Enceladus, Cassini made images that showed geyserlike eruptions of water vapor and ice particles shooting far into space.
The temperature near the south pole was at least 100 degrees F higher than expected—warm enough to melt ice just below the surface and feed the plumes, which erupt from long ﬁssures that cut across the ice, dubbed "tiger stripes." In freshly fallen snow around the ﬁssures, Cassini detected simple carbon compounds.
One mystery was solved. The E ring bulges near the moon because the plumes are pumping ice particles into it. Now a new puzzle arose: the source of the heat. It could be generated by radioactive elements trapped inside Enceladus or by Saturn's powerful gravity as it squeezes and flexes the moon.
A greater question: Could this modest moon harbor life? Life as we know it requires liquid water, energy, and organic molecules, says Bob Brown of the University of Arizona. "Evidence of all three are here," he says. "We have the cocktail." That same cocktail, Brown says, may exist on Jupiter's moon Europa, in a briny ocean shielded under miles of ice. It may have existed long ago on Mars, when that planet was warm enough to harbor open water. It was present on Earth as early as 3.8 billion years ago. "But we know it exists right now on Enceladus."
Life might be hiding just a few dozen feet below the ice in pockets of warm water, living off dissolved organic compounds and reproducing using some alien version of DNA—or an entirely different kind of genetic material. "We're looking for places where we might ﬁnd bugs," says Brown. "We don't expect anything intelligent, or highly developed, but here you have a place where life is possible."
Cassini is scheduled to revisit Enceladus once more and, if space budgets allow, may extend its mission beyond 2008 to make more flybys of Enceladus, Titan, and other key targets. But scientists are already thinking ahead to future space probes that could actually look for life on Enceladus and study the precursors of life on Titan—exploration that would take us closer to under-standing our own origins.
Some dream of a robot that would land at the south pole of Enceladus and drop a probe through the vents to look for life. Others picture a satellite that would orbit Titan and launch blimp-like rovers into its atmosphere for a leisurely survey of its hills and plains. Jonathan Lunine of the University of Arizona, a Cassini-Huygens scientist who also studies planets around other stars, sees the quest in the biggest terms. "These places," he says, "will write new chapters in the book on how life began in the universe"