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The Lure of Titan
On July 1, 2004, the Cassini spacecraft arrived at Saturn after a journey from Earth lasting almost seven years. At 6.8 m in length, this monstrous robotic explorer was the largest western spacecraft ever to be dispatched on an interplanetary mission. Its battery of scientific instruments was designed to return images and data not only from the giant planet itself and its spectacular ring system, but also from members of Saturn’s family of over fifty moons. Foremost in interest among the diverse collection of icy worlds in orbit around Saturn was Titan, a body so special, so intriguing in its own right that Cassini carried with it a detachable package of instruments—named the Huygens probe—that would parachute through Titan’s atmosphere to observe its surface.
By any reckoning, Titan is an unusual moon. It is 5,150 km across— nearly 50 percent bigger than our own Moon and 6 percent larger than Mercury. If it happened to orbit around the Sun, its size and character would easily make it as much a planet as Mercury, Venus, Earth, and Mars. But the landscape of this extraordinary world remained hidden to us throughout the first decades of the space age, partially because of Titan’s remote location and partially because it is swathed in a thick and visually impenetrable blanket of haze. Thanks to Cassini–Huygens and the technological advances that have vastly extended the reach of ground-based telescopes, the situation has now changed dramatically. Titan is undergoing an all-out scientific assault both by the most powerful telescopes on Earth and from the cameras and radar aboard Cassini, the flagship international space mission. This observational barrage, topped off by the Huygens probe’s daring drop down to the surface of Titan, is serving to unveil this enigmatic moon, revealing more of its intriguing features than we have ever seen before.
THE IMPERATIVE TO EXPLORE
When the two Voyager craft sped past Jupiter and Saturn between 1979 and 1981, they returned a wealth of new information about the two giant planets and their moons. But the images and data received from these missions were essentially snapshots—fleeting opportunistic glances at worlds demanding more serious and systematic attention. And as far as Titan was concerned, the results of these flybys were especially disappointing.
Observing Titan was a high priority for the planners of the Voyager missions, and in November 1980, Voyager 1 passed Titan at a distance of 4,394 km. The encounter sent the spacecraft hurtling out of the plane of the solar system and prevented it from exploring any more moons or planets. However, curiosity about Titan was so great that the sacrifice was considered worth making.
A principal reason for the great interest in Titan was the fact that it possesses a significant atmosphere. Astronomers had been aware of Titan’s atmosphere since 1944, when Gerard Kuiper announced that spectra he had taken of Titan revealed the presence of methane gas. Therefore, planetary scientists were not going to be surprised to find haze or clouds in the atmosphere, but at the very least, they hoped that parts of Titan’s surface would be visible when Voyager arrived.
Unfortunately, those hopes were completely dashed. The whole of Titan proved to be shrouded from pole to pole in opaque orange haze. Voyager’s camera was sensitive only to visible light, and the spacecraft carried no instruments (such as an infrared camera or imaging radar) capable of probing below the haze. Voyager was able to return some important new data about the atmosphere but virtually nothing about the surface.
The exploration of the Jupiter and Saturn systems continued to beckon, however, and the next logical step was to send orbiters to make close and detailed observations over a sustained period of time.
Between the two of them, Jupiter and Saturn possess five of the seven largest moons of the solar system, and they both have far more known moons than any of the other major planets. With such a variety of planetary bodies to observe from close quarters, not to mention Saturn’s iconic ring system, the urge to send orbiters was very compelling. As the nearer of the two, Jupiter was the first to be targeted. The Galileo spacecraft was launched on its six-year journey to Jupiter from the space shuttle in 1989. It operated successfully between 1995 and 2003 and was deliberately crashed into Jupiter at the end of its useful life.
An orbiter for Saturn was scheduled to follow, and Titan was firmly in the sights of the Saturn mission planners. The Voyager experience generated an overwhelming incentive to design a mission to the Saturnian system capable of discovering what lay below Titan’s haze. Both the National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA) were involved from early on with the conception and development of the mission; the idea from the beginning was to send an orbiter carrying a Titan probe. In what turned out to be a highly successful international collaboration, NASA provided the orbiter and ESA built the probe. The orbiter was named in honor of Giovanni Domenico Cassini, the French-Italian astronomer who discovered four of Saturn’s moons and the gap separating the two main rings. The probe was named after Christiaan Huygens, the Dutch astronomer who discovered Titan. Cassini would be equipped with radar and infrared imaging capabilities for penetrating the haze; the independent probe was to parachute through the haze and radio back via Cassini the data collected by its instruments and camera.
No mission as complex as Cassini–Huygens had ever before been undertaken at such an immense distance from Earth. Even when Earth and Saturn are at their closest, the gulf between them is around 1.3 billion km. By the time Cassini was launched in 1997, Galileo had been performing well at Jupiter for nearly two years, even though its main communications dish had failed to unfurl correctly. But Saturn is roughly twice as far away as Jupiter. Light and radio signals take over an hour to make the one-way trip between Saturn and Earth, and even getting to Saturn at all would be less than straightforward.
Cassini’s route was necessarily a convoluted one. The 5.5-ton spacecraft was launched by a powerful rocket but could not make the journey in a reasonable time without extra impetus. To get some additional kicks, the mission design relied on “gravity assist”—a maneuvering technique whereby spacecraft pick up speed from close encounters with planets. Before it could set out properly on the main leg of its journey to Saturn, Cassini made two loops around the inner solar system. To gather enough speed, it skimmed close to Venus on two separate passes and then swung by Earth. Some two years after it had been launched on October 15, 1997, Cassini was finally catapulted away from the vicinity of Earth and toward the outer solar system. It received a final boost at Jupiter, about the halfway point. After being maneuvered into orbit around Saturn in July 2004, it embarked on a long series of loops, carefully planned to allow scrutiny of the planet, rings, and moons by its eleven instruments. If all went well, it would keep going for at least four more years.
On December 25, 2004, Huygens parted company with Cassini and for twenty days followed an independent orbit that would bring it close to Titan. Then, on January 14, 2005, Huygens plunged into Titan’s atmosphere. As it descended to the surface, it transmitted data for two hours and twenty-eight minutes and conducted operations for over three hours after landing, until its batteries were dead. Unfortunately, after one hour and twelve minutes, Cassini was below the horizon and could no longer relay the probe's data back to Earth; also, a technical glitch caused the loss of some information for one experiment (though it was largely recovered by radio telescopes observing from Earth). Otherwise, to the delight of the triumphant science teams who anxiously monitored its progress, the probe worked almost entirely according to plan.
Even before Huygens reached Titan, the Cassini orbiter had begun its own program of mapping and remote sensing that would take it on dozens of close encounters with Titan. All eleven of its instruments were to be used to collect data on Titan; the expected deluge of information began to arrive on cue in the second half of 2004. The time had come to test the many hypotheses and speculations surrounding what would be found on Titan.
In the following chapters, we tell the story of how Cassini and Huygens have finally begun to lift the veil of mystery surrounding Titan, beginning with advancements in our understanding of Titan that took place in the decade preceding Cassini’s arrival. Some predictions have proved gratifyingly accurate; others have turned out to be misconceived, however plausible they may have seemed initially. Though many questions can now be answered—even some that no one thought to ask—they have quickly been replaced by a torrent of new and deeper puzzles.
But before we get to that story, we should set the scene. In broad-brush terms, when was Titan discovered, what kind of world is it, and where in our solar system does it fit in the scheme of things?
Born in The Hague in the Netherlands, Christiaan Huygens (1629–95) discovered Titan on March 25, 1655. He announced the existence of Saturn’s moon a year later, and then went on to famously develop the wave theory of light and to become one of the greatest scientists of the seventeenth century. Besides possessing outstanding abilities as a theorist and mathematician, his many talents also included a practical bent. With his brother Constantyn, he designed and constructed a machine that could produce telescope lenses of better optical quality than any other at the time. Using a telescope made with one of these home-produced high-quality lenses, Huygens identified Titan as a moon of Saturn. Titan was the first planetary satellite to be discovered since 1610, when Galileo had found the four large “Galilean” moons of Jupiter, later named Io, Europa, Ganymede, and Callisto.
Though he discovered Titan, Huygens did not call it anything other than “Luna Saturni.” For nearly two hundred years, the world we now call Titan was anonymous. The relatively small number of then-known planetary satellites were referred to by numbers. By the middle of the nineteenth century, however, new discoveries of more moons had rendered ambiguous the existing numbering system (wherein satellites were numbered in order of distance from their primary), so Sir John Herschel proposed the idea of giving moons individual names. From about 1848 on, astronomers happily adopted the names from classical mythology, including Titan, that Herschel had suggested.
ONE OF A FAMILY
Living up to its name, Titan truly dwarfs the rest of Saturn’s natural satellites. In sheer size, Titan shares more in common with its four substantial cousins in orbit around Jupiter. What its siblings lack in size, however, they make up for in number. As we write, the total count of Saturn’s moons is at least fifty-six. The number of known satellites began to rise dramatically in 2000 because the Saturnian system was under close scrutiny from Earth in advance of Cassini’s arrival. In 2004, Cassini itself took up the search and found yet more. Saturn, perhaps more than any other planetary body, prompts the question, What is a moon? After all, each of the countless millions of ring particles is a distinct, rigid body, following its own orbit around Saturn, but it would be silly to call them all satellites.
Titan’s size was not determined conclusively until the flyby of Voyager 1 in 1980. Early estimates were all based on the tricky business of measuring Titan’s apparent diameter when it is seen as a “flat” disk in the sky. (This measurement is tricky because Titan is dark toward its edges, unlike the disk of the Moon, which is nearly uniformly bright right to its edges.) The best measurements indicated a size of about two-thirds of an arcsecond (about the size of a golf ball eight miles [13 km] away). The opaque atmosphere further complicated the issue by making the visible disk look larger than Titan’s solid body really is. As a result, its diameter was overestimated. Experiments conducted during an occultation in the 1970s, when the Moon crossed in front of Titan, produced a figure of 5,800 km. For a time, Titan was thought to be the largest moon in the solar system, but then it was demoted to second place in the rankings when the results came back from Voyager 1’s radio science experiment.
As Voyager 1 passed behind Titan’s atmosphere, the spacecraft’s radio signals were first deflected (though not blocked) by the moon’s atmospheric gas. Analysis of the degree of deflection provided information on the temperature and pressure of Titan’s atmosphere at various altitudes. Then, the spacecraft’s radio signals were cut off completely when the spacecraft went behind Titan’s solid globe. With these data, Titan’s true diameter could be assessed: 5,150 km (to within 1 km), or 60 km less than that of Jupiter’s moon Ganymede.
Titan’s mass was first estimated in the nineteenth century by its effect on the orbit of Hyperion, the next Saturnian satellite out from Titan. The effect of Titan’s gravity on the trajectory of Voyager 1 allowed an even more precise measurement. Combining its size and mass (1.346 × 1023 kg) tells us that the average density of Titan is 1.88 times that of water, which is slightly higher than that of any of Saturn’s other larger satellites. By comparison, the value for our rocky Moon is 3.34, and Earth, with its iron core, has a value of 5.52. Considering average density alone, Titan must be some mixture of ice and rock. Most likely, it consists of a rocky core overlain by a mantle chiefly made of ice.
It is no surprise to find that Titan, like all other satellites in the cold outer solar system (apart from Jupiter’s exceptional moon, Io), has a substantial proportion of ice. The temperature at Titan’s surface is around 94 K (or −179°C). Solar heating is so feeble and temperatures are generally so low that water ice is as hard as rock is on Earth—although like rock on Earth, the ice may be soft or even molten in the deep interior of Titan.
Jupiter’s volcanically active moon Io (diameter 3,642 km), closest to Jupiter of its four large satellites, is the odd one out among the satellites of the outer planets, particularly with regard to composition. Io is made of rock and sulfur, and has virtually no water. Its interior temperature is raised to melting point by tidal energy resulting from its orbital motion within Jupiter’s powerful gravity field. (The mechanism of tidal heating is similar to the way the gravity of the Sun and Moon raises tides in Earth’s oceans.)
Europa (3,130 km), the second of Jupiter’s Galilean moons, must be principally rock according to its average density. However, its surface layers are mainly water. Although its outer crust is frozen, a great deal of evidence strongly suggests that the crust floats on a global ocean of liquid water. Like Io, Europa is heated below its surface by tidal energy.
The other two large moons orbiting Jupiter, Ganymede (5,268 km) and Callisto (4,806 km), both have a higher proportion of ice and are more like Titan in this regard, though Ganymede’s density is a bit higher than Titan’s and Callisto’s is a little lower. Unexpectedly, magnetic measurements made by the Galileo spacecraft hinted that Callisto, like Europa, may have an internal ocean, even though the tidal heating it experiences is not nearly as great as Europa’s. These measurements also raise the intriguing possibility that Titan might have a subsurface ocean too.
Titan’s more immediate neighbors in the Saturnian system each have individual characteristics and mysteries of their own, but looking at them alongside Titan only emphasizes the unique qualities of exceptional size and atmosphere that make Titan particularly fascinating. And if these lesser worlds have such varied and unexpected features, what greater surprises might be waiting on Titan?
The second largest Saturnian moon, Rhea, is only 30 percent the size of Titan (and only one-sixtieth the mass). It is one of six satellites in the 400–1,500 km class, which are massive enough to have shapes close to spherical. In order of size they are Rhea, Iapetus, Dione, Tethys, Enceladus, and Mimas. Their predominantly icy nature is confirmed by their densities: all of them are only a little denser than water. Before Cassini, virtually everything known about their surfaces came from the encounters of Voyagers 1 and 2, but one of the most thrilling aspects of Cassini’s bounty from the early phase of its mission was the spectacularly detailed and comprehensive images of these distinctive worlds.
Moving inward from Titan’s orbit, we first encounter Rhea and then Dione. Between them is a certain resemblance. Both are heavily cratered, and on each of them the leading hemisphere (the side facing the direction in which the satellite orbits) is markedly different from the other side (the trailing hemisphere). Rhea’s leading side is brighter and more heavily cratered. A network of bright streaks crosses darker terrain on the other half. Although the impression from Voyager’s distant views was that the streaks might have been bright material sprayed out from the interior, the close-up views of Dione from Cassini revealed that they are the bright edges of cliffs where the crust has fractured.
Tethys is next. Its most striking feature is a huge impact crater called Odysseus. With a diameter of 400 km, this basin is nearly half the size of Tethys, though its once deep relief has sagged over time. The other distinctive feature on Tethys is a vast canyon called Ithaca Chasma. About 100 km wide, 3–5 km deep, and 2,000 km long, it stretches three-quarters of the way around the moon’s circumference. A darker, less heavily cratered belt of terrain crossed by cracks is evidence that activity of some kind has altered part of the surface in the past.
We can only speculate about the activity that altered Tethys long ago, but to the delight of Cassini scientists, Enceladus proved to be active now, right in front of our eyes. Multiple jets of water vapor and ice particles are spewing from surface cracks, dubbed “tiger stripes,” in the south polar region. Cassini even flew through the plume, which extends upward as much as 500 km. Some source of energy—tides perhaps, or radioactivity—is warming the material escaping through the cracks and is driving the geyserlike eruptions. Enceladus, along with Dione, Tethys, and Mimas, orbits within Saturn’s tenuous outer ring, the E ring. It seems that Enceladus itself is the main source of the particles that make up that ring.
Mimas is the innermost of the larger moons. The dramatic 140-km crater Herschel, with its central peak, makes Mimas’s crater-saturated face unmistakable and has earned it comparisons with the Death Star of the Star Wars movies. The gravitational action of Mimas was also responsible for clearing material from the Cassini division, which separates Saturn’s A and B rings.
Moving out from Titan, the next moon we encounter, between Titan and Iapetus, is Hyperion. Though not one of the larger moons of Saturn, it is certainly one of the most curious. It is the largest known moon anywhere to have an irregular shape. When imaged by Cassini, it looked for all the world like a cosmic sponge. The many craters on Hyperion seem to have been deepened by a process called “thermal erosion.” Solar radiation warms up dark colored dust deposited in the bottom of the craters; the resulting heat tends to melt the ice and deepen the craters. As well as looking like a sponge, Hyperion has another spongelike property—a great deal of empty space inside. A density of only 0.6 times that of water means it must essentially be an icy pile of rubble.
Iapetus, too, is intriguing and different. Though roughly spherical, parts of it are squashed in, and a strange ridge 20 km wide and 13 km high runs for 1,300 km around its midriff. But the most bizarre thing about Iapetus is the contrast between its leading and trailing sides. A huge dark reddish-brown area called Cassini Regio, which covers much of the leading side, reflects no more than 5 percent of the light falling on it, while the other side and the poles are ten times brighter. The explanation for Iapetus’s duplicitous character remains disputed. One favored theory is that the dark region is coated with a layer of dust that came from one or more of Saturn’s numerous outer moons.
Beyond the large inner satellites orbiting in stately order in the ring plane, at distances ten to twenty times farther from Saturn than Titan, we find a rabble of smaller moons (typically 5–40 km across). Their orbits are tilted to Saturn’s equatorial plane by large angles, and a group of them are in retrograde (backward) orbits compared with the rest. This is seriously abnormal behavior for planetary satellites, and it suggests they were not born alongside Saturn in the same way as the inner satellites were. Saturn enlarged its family by adoption sometime after it had condensed out of the solar nebula and had already developed its own primordial satellite system. The evidence points to the errant moons as once having been wanderers through the outer solar system. Straying too close to Saturn’s gravitational influence, they found themselves captured. And it seems they did not arrive randomly, since there are several distinct groups made up of members whose orbits share common features. These subsets of related moons are picturesquely known as the “Inuit group,” the “Norse group,” and the “Gallic group” and have been named individually after characters in the mythology of the respective culture. With so many moons to name, the committee tasked with the responsibility clearly decided it was time to tap into new resources or face the danger of exhausting the supply of names from Greek and Roman mythology!
Phoebe deserves special mention. It was discovered back in 1899, orbiting far out from Saturn in an exotic retrograde orbit. For a century it appeared to be a lone oddball, though we now know it belongs to the Norse group of Saturn’s outer swarm of diminutive moons. However, Phoebe is still distinguished by size; it measures about 220 km across, which makes it an order of magnitude bigger than the other outer moons and explains why it was discovered so much earlier. In some ways, Phoebe’s situation was much like Pluto’s. Pluto was a puzzling misfit among the major planets for more than six decades after its discovery in 1930. Sixty-two years later, it was found to be just one of the larger members of the Kuiper Belt of icy bodies beyond Neptune. Interestingly, the parallel between Phoebe and Pluto does not stop there.
Smart mission planners arranged for Cassini to take a close-up look at Phoebe from a mere 2,068 km away as the spacecraft approached Saturn in June 2004. What they saw was a heavily cratered moon with a varied surface composed for the most part of water ice, but also laced with minerals and organic compounds. Phoebe had been regarded as a prime candidate for the mysterious source of dark material coating Iapetus, so it was a puzzle that Cassini’s data showed Phoebe’s composition not to be a match for the dark part of Iapetus. Phoebe turned out not to be like the rocky asteroids in the belt between Mars and Jupiter, but is instead more akin to Kuiper Belt objects. Perhaps Saturn captured a miniature Kuiper Belt all of its own.
Not all of Saturn’s small moons lie on the remote fringe. A collection of them are much closer to Saturn and are actually within the ring system. Some share orbits with each other or with larger siblings. Pan circulates in the Encke division and tiny Daphnis in the Keeler gap, both within the bright A ring. Prometheus and Pandora are “shepherds,” herding the F-ring particles into a narrow ribbon. Complex interactions between the particles that make up the rings and the inner moons govern their orbits as they jostle by each other, responding to countless gravitational tugs.
These, then, are Titan’s immediate family. Titan, of course, does not necessarily share any of their individual characteristics beyond being chiefly composed of ice and located in the same corner of space. However, as a group they set the context for the environment in which Titan formed and evolved.
MOON IN MOTION
Titan revolves around Saturn some 1.22 million km from the center of its parent planet, a distance equivalent to about twenty Saturnian radii. It is considerably farther out than the ring system. For comparison, the easily visible A and B rings extend to about 2.3 radii from Saturn’s center. Though more distant from Saturn than the rings, Titan’s orbit is, like the rings, around Saturn’s equator—or very nearly so; it is tilted by only one-third of a degree. Rather than being precisely circular, its orbit is slightly elliptical so that Titan’s distance from Saturn varies by 71,000 km, or 6 percent.
Tied to its parent planet, Titan makes a circuit of the Sun each 29.458 years. And because Saturn’s equator is tilted to its orbit by nearly twenty-seven degrees, Titan too is tilted to the same extent, relative to its path around the Sun. This significant tilt (or “obliquity”) means that both Saturn and Titan experience marked seasons, much as Earth does with a tilt about three degrees less. When Cassini arrived in 2004, it was late summer in the southern hemispheres of Saturn and Titan; the solstice had been back in 2002. By the time the mission reaches its planned finish date in 2008, the equinox will be approaching—spring in the northern hemisphere and autumn in the south.
If a planet is tilted, it goes through a cycle of seasons each time it circles the Sun regardless of the shape of its orbit. Earth travels on anorbit that is not quite circular. Its distance from the Sun ranges between 147 and 152 million km, and our closest approach to the Sun occurs around the third of January each year. However, this change in distance causes only a few percent change in the amount of sunlight reaching points on the Earth’s surface, an effect that is much smaller than that of the tilt.
The same is not true of Saturn and Titan. Saturn’s distance from the Sun varies from 1,347 to 1,507 million km. Between the two extremes, the strength of the sunlight at Saturn changes by around 20 percent. When Huygens arrived at Titan in 2005, about two and one-half years had elapsed since Saturn was last at perihelion—its closest approach to the Sun. However, the effects that this variation of solar radiation have throughout Titan’s “year” are likely to be relatively subtle compared with the changes in Titan’s atmosphere induced by the march of the seasons.
One of the few conclusions that could be drawn from Voyager 1’s almost featureless images of Titan was that the northern haze cover looked darker than the southern hemisphere. Perhaps it was a seasonal effect? A decade later, Hubble Space Telescope images revealed that there had been a switch and the southern hemisphere had become the darker. Subsequent monitoring captured further changes to the haze, as if with the seasons it is blown back and forth between north and south.
The very first suggestion that Titan might have an atmosphere dates back to the beginning of the twentieth century. José Comas Sola` studied Titan visually using a 38-cm telescope at the Fabra Observatory in Barcelona, Spain. On August 13, 1907, he made a sketch of Titan, which was published the following year. He wrote, “[W]ith a clear image and using a magnification of 750, I observed Titan with very darkened edges” and concluded, “We may reasonably suppose that the darkening of the edges demonstrates the existence of a strongly absorbing atmosphere around Titan.”
What Comas Sola` claimed to have seen was certainly at the absolute limit of the capability of the human eye, if not beyond it. No one else ventured to claim they could see as much, but in 1925 the British astrophysicist Sir James Jeans used the kinetic theory of gases he had developed to demonstrate that it was theoretically possible for Titan to have an atmosphere.
Jeans pictured the molecules of a gas whizzing about, colliding with each other and anything in their way. The hotter and the more lightweight they are, the faster they move and vice versa. With nothing to hold them back—a container for example—the molecules soon fly apart and the gas disperses. A layer of gas around a planet or moon has gravity pulling it down, but with no lid on the top, any molecules traveling fast enough can escape from the gravity and shoot off into space. If a planet is going to hold on to an atmosphere for a length of time comparable with the age of the solar system, the gas molecules have to be cold enough and heavy enough to be confined in the gravity trap. Jeans calculated that, in the frigid conditions surrounding Saturn, Titan’s gravity was strong enough to have kept hold of an atmosphere for as long as the solar system had been in existence. But there was a proviso. Lightweight molecules such as hydrogen and helium would have escaped long ago. The atmosphere, if it existed, would have to consist of heavier substances such as argon, neon, nitrogen, and methane.
The first proof that Titan indeed has an atmosphere came when Gerard Kuiper’s spectra taken in 1943–44 showed the distinctive signature of methane. Methane was not the whole story, though. Even a small amount of methane shows up strongly in a spectrum. By contrast, the gas that forms the bulk of Titan’s atmosphere does not announce its presence so readily in spectra. Data collected by Voyager 1 in 1980 showed that Titan’s atmosphere is overwhelmingly made of nitrogen, the gas that accounts for 80 percent of Earth’s atmosphere, with methane amounting to no more than 5 percent. Further, atmospheric pressure at Titan’s surface is 1.5 bar—that is, 50 percent greater than the pressure at Earth’s surface. All at once, with regard to composition and pressure, Titan was understood to be the world in the solar system with the most Earth-like atmosphere.
Methane may be a minor constituent in Titan’s atmosphere, but chemically, its importance is immense. A methane molecule consists of one carbon atom and four hydrogen atoms. In ultraviolet light, it breaks up into one or two hydrogen atoms and a fragment containing the carbon atom and the remaining hydrogens. The freed-up hydrogen ultimately tends to escape from Titan in the form of H2 molecules, though not so easily from the much stronger gravity of Saturn. The hydrocarbon fragments combine with each other and with nitrogen in countless different ways to produce a vast range of organic compounds. Many were identified and their concentrations determined from infrared spectra recorded by Voyager 1. The most abundant are ethane (C2H6) and acetylene (C2H2). This process of photolysis is the origin of Titan’s haze.
Haze particles form high in Titan’s stratosphere where sunlight can penetrate to break up the methane. The concentration of organic molecules builds up to a level where clumps condense outward and stick to each other, then begin to drift downward very slowly. In fact, the haze is not really very dense. It is opaque chiefly because there is such a thick layer of it. Huygens found haze particles all the way down to the surface, although the atmosphere was clear enough to get good images of the ground from heights of 40 km and below. The logical consequence of the steady drizzle of haze is that Titan’s surface becomes contaminated all over by a layer of organic sludge.
Here on Earth, the interplay between the atmosphere, the oceans, and radiation from the Sun makes for dynamic and often dramatic weather. Weather is so important to human well-being that monitoring it, predicting it, and just talking about it are major preoccupations. Since Titan has an atmosphere as impressive as ours, could it too have interesting weather?
Our weather is all about water and a chance combination of natural circumstances. It is a matter of everyday experience that water can be a liquid, a vapor, or a frozen solid in the range of temperatures encountered on Earth. What is less commonly remembered is that atmospheric pressure affects water’s boiling point, though anyone who has used a household pressure cooker or tried to brew tea at a high altitude will be familiar with the idea. So on Mars, for example, where the pressure is only 0.007 of that on Earth, water’s boiling point is barely above 0°C. Liquid water put there would literally boil and freeze at the same time. Under Earth’s atmospheric pressure the situation is entirely different, since all of 100°C separate water’s freezing and boiling points. On Earth, of course, we have vast reservoirs of liquid water in the oceans, lakes, and rivers. Water evaporates from them, then condenses into clouds of droplets from which rain falls—or hail or snow if it is cold enough. This hydrological cycle combined with winds effectively moves water around our planet, including from oceans to land.
So how does Titan’s weather compare? Unlike on Earth, liquid water and water vapor are not generally present; it is far too cold. On Titan, water in its rock-hard state plays the role that silicate rocks do on Earth. But in such frigid conditions, one substance can mimic the behavior of water on balmy Earth: methane. In the Titan regime, methane is able to evaporate, condense into clouds, and fall as droplets of rain. Going up through the atmosphere on Earth or on Titan, the temperature falls until it reaches a minimum. This altitude, called the “tropopause,” is about 10 km high on Earth; on Titan it is some 40 km above its surface. Above that level, the temperature rises again through the stratosphere. It is below the tropopause, in the troposphere, that clouds form.
Despite the theoretical reasons for thinking that Titan should have clouds, first proof of their existence under much of the haze was not easy. The elusive cloud finally appeared in 1995 when Caitlin Griffith and Toby Owen were making infrared observations of Titan in the hope of discovering something about its surface composition. On September 4 they recorded wildly anomalous data amounting to twice as much radiation as they would have expected at a wavelength of three microns— more than could possibly have come from Titan’s surface even if it were perfectly white. By careful analysis of subtleties in the spectrum, they recorded that they could account for the anomaly if clouds at a height of 10–15 km covered about 10 percent of Titan. It seems, however, that such large clouds may be rare.
More recently, clouds on Titan have been observed regularly both by Cassini and by one of the 10 m ground-based W. M. Keck telescopes. Curiously, these clouds have been restricted to a stormy patch over the south pole and a band around Titan at a latitude of 40° south. Within this band, individual elongated clouds stretch for over 1,000 km, and clouds seem to bunch up at two particular longitudes. The clouds rise up to a height of about 40 km and then dissipate, all in several hours.
The realization that methane, and one of the by-products of the atmospheric photochemistry—ethane—could both be liquid on Titan, and were present in large enough quantities, led to the idea that substantial lakes or oceans might exist. It did not seem unreasonable to take the parallel with water on Earth that far. So seriously was the possibility considered, the Huygens probe was designed so that it could float and test the depth to the solid floor below it, should it come down on a body of liquid. As was widely reported in the media at the time, Huygens landed not in liquid but on soft ground with the consistency of loose, wet sand. The chemical evidence suggested that liquid methane was making this surface material damp. Nothing from Huygens or from Cassini’s images and soundings in the very early part of its mission confirmed directly the presence of oceans, lakes, or actively flowing rivers. But even those earliest results contained strong hints from the landscape that liquid had been at work and had left its mark. We return to the unraveling of this Titan enigma in a later chapter.
Huygens and Cassini have transformed years of speculation about the nature of Titan’s surface into a gradual discovery of the truth. Cassini is filling in detail on the tentative broad-brush maps begun in the mid1990s with observations made by the Hubble Space Telescope, as well as ground-based infrared images. On a large scale, even the pre-Cassini observations revealed a highly varied surface with contrasting dark and bright areas. The largest and most conspicuous bright region is Xanadu, a plateau about the size of Australia just south of the equator. An extensive dark area, Shangri-La, lies to the west of Xanadu. (Huygens landed on the western periphery of Shangri-La.)
Features on Titan like Xanadu and Shangri-La, which are distinguished by their albedo (noticeably dark or bright areas), are named after sacred or enchanted places in world mythologies and literature. As happens with all planetary bodies, a system had to be devised for categorizing the types of features and giving them individual names. This is often a tortuous process because the informal names researchers adopt as soon as they spot interesting things on images have to be translated into “approved” names within the system. So, for example, the crater on Titan informally dubbed “Circus Maximus” became the Etruscan goddess Menrva, since the rules say that craters are to be named after “deities of wisdom.”
The most detailed views of Titan’s landscape came from the images recorded by Huygens as it descended, and from Cassini’s imaging radar, which at each pass cuts a swathe across the surface a few hundred kilometers wide and several thousand kilometers long. Even the earliest sweeps revealed a variety of features as diverse as we have on Earth, including windblown dunes, craters, chains of mountains, and what looks like a volcanic dome. Most intriguing of all are the channels, apparent shorelines, and lakelike features that say liquid has flowed on Titan in the past, or does flow from time to time even if it does not stand in oceans and large lakes at the present. Huygens landed close to what resembles a shoreline between bright and dark areas, and returned pictures of dark-colored branching channels cutting across the bright area.
AN EARTH-LIKE WORLD?
The thrill of uncovering any extraterrestrial world and the challenge of demystifying the solar system piece by piece is enough to stimulate interplanetary exploration. But as humans, we cannot help being extra-fascinated by places that remind us of our home planet in some way. Mars is one such place. Titan, though so far away in the frigid depths of the outer solar system, is another. The argument that Titan can tell us something special about Earth and how life emerged here stirs the enthusiasm of many who find the solar system’s inactive and unchanging balls of rock and ice less stimulating. Titan has been much vaunted as an Earth analogue in a variety of ways. But how far does the comparison go?
One thing planetary scientists agree on: Titan is not like an Earth in the deep freeze. The bulk compositions of the two bodies are totally different and reflect their origins at vastly different distances from the Sun. Nearly all would agree that Titan is not a likely home for extraterrestrial life despite the complex organic chemistry, though some have pointed out that niches for life might be created if internal or external heating causes water to melt.
Arguably, the most significant similarity between Earth and Titan is the likeness of their atmospheres. The presence of an atmosphere dramatically changes the nature of a planetary surface. There are physical processes, such as erosion and deposition of surface materials by wind and weather, and the temperature regime that would exist in the absence of any atmosphere is altered. And then there is the chemical interplay of the atmosphere with the substance of the solid surface beneath it and electromagnetic energy and high-speed particles bombarding it from above. Interactions of these kinds have shaped Titan as surely as they have characterized Earth. Of course, there is not an exact comparison between Titan’s atmosphere and Earth’s, since Titan’s contains no oxygen and Earth’s oxygen has come about through biological processes. However, an intriguing question is the extent to which Titan’s present atmosphere resembles Earth’s atmosphere in remote geological history. Certainly, the atmospheres both worlds possess now are not the same as ones they had in the remote past.
Today there is considerable concern about the buildup of so-called greenhouse gases, chiefly carbon dioxide, in Earth’s atmosphere and the global warming that follows as a consequence. Titan has its own very potent greenhouse gas in the form of naturally occurring methane. There is also a sense in which Titan’s haze plays a role similar to the ozone layer high in Earth’s atmosphere, under threat from human disturbance of atmospheric chemistry. Though the chemical pathways involved are fundamentally different, the formation of both ozone and haze prevent ultraviolet radiation from penetrating nearer the surface.
But perhaps the most satisfying reward for all the endeavors to unveil Titan so far is the revelation of a world with landscapes bearing an uncanny resemblance to our home planet, despite the significant differences of temperature and composition. As the details are revealed one by one, it is sometimes hard to remember we are seeing ice in the role of rock and methane substituting for water. Titan really does provoke more comparisons with Earth than with any other planet or moon. In many ways we expected as much.
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