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Gautier, Owen, and Ip are among the latest in a long line of investigators that stretches back to the era just after the invention of the telescope. One of the first observers of Titan was the Dutch physicist and astronomer Christiaan Huygens, who deduced in 1655 that Titan was in orbit around Saturn. At the time, little information could be gleaned, aside from the fact that Titan was fairly large. The state of knowledge about the moon remained much as Huygens had left it until 1944, when Gerard Kuiper, a Dutch astronomer working in the U.S., detected the unmistakable signature of methane gas in the spectrum of Titan, revealing the existence of an atmosphere.
The Voyager flybys were a great leap forward in the human knowledge of Titan. Precise measurements pegged the moon’s diameter at 3,200 miles, larger than the diameter of Mercury. But it is Titan’s atmosphere, not its size, which makes the moon of such enormous interest. The Voyager spacecraft revealed that atmosphere to be thicker than Earth’s—its column mass, the total mass of gas in a column extending from the surface to the top of the atmosphere, is some ten times that of our own planet’s atmosphere. (Titan’s relatively small surface gravity, compared with the Earth’s, makes its pressure at “sea” level, as measured by Voyager 2, about 1.5 times that of the Earth’s surface atmospheric pressure.) Furthermore, the composition of Titan’s atmosphere is complex: it is made up primarily of nitrogen, but a variety of hydrocarbons, such as methane, ethane, ethyne, and propane, are also present.
Titan is the only satellite in the solar system to possess an atmosphere, though trace amounts of gases are present on a few other satellites. But Titan’s atmosphere is unique in the solar system, primarily because of its mixture of hydrocarbons. Theoretical models predict that methane gas exposed to ultraviolet light (even the small amount that reaches Titan from the distant Sun) undergoes a series of chemical reactions. Those reactions could give rise to the approximate concentrations of the various gases other than methane that the Voyager flybys observed. Furthermore, the theoretical models predict that, with time, the ultraviolet radiation would supply the energy needed to form increasing quantities of complex, long-chain hydrocarbon molecules. The most intriguing conclusion of this line of reasoning is that among the long-chain molecules could be precursors of molecules needed for life.
Another intriguing feature of Titan’s atmosphere is its apparent similarity to the atmosphere of the early Earth. The air we breathe today is actually a “secondary” atmosphere; its composition has changed substantially from what it originally was. New gases such as diatomic oxygen (O2), not present in the original terrestrial atmosphere, were added to the mix as photosynthetic organisms evolved.
It thus seems likely that many of the chemical reactions taking place in Titan’s atmosphere today are analogous to what took place here on Earth more than 3 billion years ago. So in some respects, by studying Titan’s atmosphere, planetologists are probing the early history of the Earth, particularly its early atmospheric chemistry. Such studies could have great significance for the understanding of the development of life on Earth.
But lest anyone get carried away with the idea that life could develop in Titan’s atmosphere or on its surface, it is worth noting some important differences between Titan’s present atmosphere and our planet’s primitive one. Perhaps most critical is the temperature. Titan is some ten times farther from the Sun than Earth is, and so it is extremely cold. Voyager 2 measured a surface temperature of about –285 degrees Fahrenheit. Furthermore, only a minuscule amount of oxygen exists in Titan’s atmosphere, bound together with other atoms to form carbon monoxide, carbon dioxide, and water. Those two factors alone make it quite unlikely for even the simplest life to develop there.
Atmosphere of Titan, a moon of Saturn, was imaged by Cassini on the spacecraft’s first flyby of the moon. Titan’s inner atmosphere, which is a mixture of nitrogen and hydrocarbon molecules, is seen in its actual yellowish color; the outer atmosphere, where ultraviolet light causes methane to undergo chemical reactions, has been falsely colored purple to enhance clarity.
© The Cassini Imaging Team/NASA/JPL/Space Station Institute |
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The exploration of Titan, however, amounts to more than just an investigation of its atmosphere. The data from the Voyager flybys suggested that chemical reactions would consume the atmospheric methane in only about 10 million years, far less than the roughly 4 billion years that Titan (and the rest of the solar system) has existed.
That presented a scientific conundrum. Astronomers tend to resist explanations that depend on human good fortune. Why should we happen to live during some unique period in the history of the universe that has no antecedents and that will soon—in a few million years, that is—come to an end? In the case of Titan’s atmosphere, why should we happen to live during the short period of time, less than 1 percent of Titan’s total history, when methane is present? Astronomers tend to discount such possibilities. They looked instead for a different explanation.
The easiest way out of the conundrum seemed to be to propose the existence of a reservoir of Titanian methane, which must be replenishing what is continuously being consumed in chemical reactions. And the likeliest place for such a reservoir is on, or very close to, the surface.
The reservoir hypothesis, combined with two aspects of Titan’s physical environment, led to a remarkable prediction. At the temperature and pressure measured for the surface of Titan, methane is likely to be a liquid. So in 1983 Jonathan I. Lunine of the University of Arizona in Tucson and David J. Stevenson and Yuk Yung of the California Institute of Technology in Pasadena hypothesized that Titan’s surface is dotted with lakes or even covered with seas of liquid hydrocarbon. More detailed theoretical modeling showed that such a surface liquid would probably be made up of ethane as well as methane, with some nitrogen gas dissolved into the mixture.
To check for visual clues that might confirm the surface-reservoir hypothesis, investigators examined 2,000 images taken by the Voyagers. Unfortunately, what the images showed was a body that appeared permanently enveloped by an orange smog or haze. In spite of extensive processing to remove some of the obscuring cloud cover, not one image gave any direct clues about the nature of the surface. Remarkably, the most detailed images of Titan before this past year were made by astronomers using the Hubble Space Telescope and ground-based instruments. They photographed Titan at wavelengths thought not to be blocked by the haze of Titan’s atmosphere. In those images, some parts of the surface are darker than others, perhaps indicating the presence of hydrocarbon seas.
With its fascinating environment and an atmosphere that could offer a window into Earth’s past, Titan stands out as being especially worthy of study. In 1988, my colleagues and I began to prepare in earnest for a space probe’s visit to the body. The Saturn orbiter, which would carry the Titan probe, was named Cassini, in honor of Jean-Dominique Cassini, director of the Paris Observatory in the late seventeenth and early eighteenth centuries and one of the first observers of Saturn’s rings. The probe itself was christened Huygens, after the seventeenth-century Dutch astronomer.
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Anatomy of the Huygens probe includes the front shield (red) to slow the probe and absorb the heat of atmospheric friction during the initial descent, before being ejected; the fore dome, with openings for gas and dust samples to reach instrument packages on the experiment platform; the aft cone, which encases the platforms; the top platform, which has mechanisms for ejecting parachutes, antennas, and other equipment, and exhaust pipes for some of the experiment systems; and the back cover, which will be ejected as the parachutes open.
Illustration by Tom Miller |
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But far more difficult than naming the probe was designing it. Because even the basic nature of Titan’s surface was (and remains) unknown—was it liquid? was it solid?—the project’s designers decided early on that the main scientific measurements should be made during the probe’s descent. Although we took great pains to ensure its survival after landing and for some time thereafter, we had to treat such a favorable outcome as a bonus, not an expected result.
Six instruments were developed to measure and analyze the composition of the atmosphere, detect the chemical and physical processes taking place there, and determine the nature of the surface. Of the instruments, one is a camera that works at several wavelengths of light and will gather panoramic images of both the atmosphere and the surface. The photographs will also convey information about the interaction of atmospheric particles with light, which should help investigators determine the particles’ size, shape, and composition. A second instrument will collect the particles that make up Titan’s haze, and heat them, sending vaporized traces of the particulates into a third instrument, a combination gas chromatograph and mass spectrometer. The latter instrument will measure the composition of the particles and gases in the atmosphere and determine which isotopes are present there.
The fourth onboard instrument is a miniaturized meteorological station, which will measure the temperature, pressure, and density of the atmosphere as the Huygens probe descends through it. The weather station also carries a simple microphone to record whatever sounds are made on Titan. The fifth system will monitor the changing position of Huygens relative to Cassini; in so doing, it will measure wind speed and direction by measuring the Doppler shift in the frequency of the radio signals sent from the probe to the orbiter. As the wind blows, the net motion of the two spacecraft toward or away from each other will cause slight compression or rarefaction of the radio waves, from which the properties of the wind can be inferred.
The sixth instrument—though it will operate during descent—is optimized for making measurements after landing. It includes nine sensors that together should make it possible to determine many of the physical properties of Titan’s surface at the landing site—in particular, whether the landing site is solid or liquid.
As difficult as it was to plan precisely what Huygens could accomplish during its descent through Titan’s atmosphere, it was just as hard to ensure that the probe would survive its trip. When Huygens arrives at the top of Titan’s atmosphere, its saucerlike front shield will slow it down, much as a heat shield slows down the space shuttle as the shuttle returns to Earth. Once the probe has slowed from its initial 13,500 miles an hour to about 1,000, three parachutes will deploy in a carefully choreographed sequence. The parachutes should ultimately slow the probe to about eleven miles an hour, slow enough for Huygens to examine the moon’s atmosphere and surface, and then survive impact and continue to communicate with Cassini once it lands.
Titan’s atmosphere, which extends more than 300 miles upward from the moon’s surface, is predominantly a mix of hydrocarbons and nitrogen gas. Three cloudy or hazy regions exist at roughly 30, 140, and 180 miles above "sea level." Interestingly, Titan’s atmosphere—unlike Earth’s atmosphere—is coldest at the bottom, where it dips to –290 degrees Fahrenheit from a high of –170 degrees F. at an altitude of 300 miles. Initially Huygens will be slowed by friction between its front shield and the atmosphere. Then, some 100 to 140 miles above the surface, three parachutes will deploy in carefully choreographed sequence, and the probe’s outer casing will eject.
Illustration by Ron Miller |
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That might sound easy, but the technical challenges were daunting. The front shield and the delicate equipment behind it must withstand temperatures of several thousand degrees as the probe plummets through the upper atmosphere. The shield must then be ejected, and the three parachutes must open in sequence so that the duration of the descent is roughly two and a half hours—neither so short that the scientific objectives cannot be met nor so long that Huygens’s batteries run out before the probe reaches the surface.
Adding to those difficulties, no one had ever designed parachutes to work in an environment quite like Titan’s: an unusual atmosphere, gravity just 15 percent as strong as Earth’s, and an ambient temperature as low as –290 degrees F. Moreover, after the instruments pass through the scorching entry phase, they, too, must survive the unimaginable cold of the descent.
The final difficulty for the instrument designers was that Huygens, after detaching from Cassini, must be totally autonomous, capable of making good responses to whatever it encounters. Part of the need for that autonomy comes from the fact that the travel time for light between Earth and Saturn—and thus the travel time for a command radioed from Earth to the probe—will be about ninety minutes. It will take a further ninety minutes for any feedback about a command to be relayed back to Earth. Thus any “conversation” between Earth and the spacecraft has a built-in lag of around three hours, roughly the duration of Huygens’s entire scientific mission. Commanding the probe in real time would be close to useless.
Cassini and Huygens have been locked together since the mission was launched in October 1997. They went into orbit around Saturn in July 2004, reaching the vicinity of the planet in the “short” time they did by getting four gravitational boosts from planets less distant than Saturn: two boosts from Venus, one from the Earth, and one from Jupiter [see illustration below]. Their coupling should end on December 24 (December 25 in Europe), when Huygens is launched on a collision course with Titan. A few days later, Cassini will adjust its own orbit to fly past Titan just as Huygens is plunging into the moon’s atmosphere, thereby putting the orbiter in position to receive, on January 14, the precious data beamed from the probe.
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Gravity-assisted acceleration by three planets was necessary to propel Cassini, along with its payload, Huygens, from Earth to Saturn. After launch from Earth in 1997 (blue ball at roughly half-past five o’clock with respect to the Sun), it flew by Venus in April 1998 (orange ball at nine o’clock) and again in June 1999 (orange ball at ten o’clock) before approaching Earth in August 1999 (blue ball at seven o’clock). Cassini passed Jupiter in December 2000 (white ball) before making the long trip to Saturn, which it reached on July 1, 2004.
Illustration by Ron Miller |
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For many of the scientists and engineers who have taken part in the Huygens project, the three hours’ worth of data that should be received that day will represent the culmination of fifteen years of work. But if all goes according to plan, the secrets of Titan that will emerge from that data will certainly be worth the toil and wait. Is the atmosphere what we suspect it to be? Does the surface have lakes or seas of methane and ethane? Does the moon harbor any of the molecular building blocks of life? Those questions, among others, should be answered.
Of course, the Huygens mission will not be the end of the exploration of Titan. Even though the first trip is not yet finished, and the first questions not yet answered, planetary scientists are already busy planning the next generation of missions to Titan. Because of the thick atmosphere, some investigators have proposed deploying balloons or even a helicopter in Titan’s atmosphere.
A balloon would probably take about two weeks to make a complete circuit around Titan’s equator. Along the way, instruments could make a detailed survey of the land or sea below. Mission scientists could then choose sites for further inspection. On subsequent circuits the balloon could descend and hover just above the surface while instruments make appropriate measurements of the surroundings. Above a liquid surface, the balloon could siphon some of the liquid into instruments for analysis. After several such circuits, the risky business of landing and taking off again could be contemplated.
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round 4:30 a.m. eastern standard time on the morning of January 14, 2005, a flying-saucer-shaped object named Huygens will encounter an atmosphere for the first time since it left Earth, in 1997. In that atmosphere’s thin, cold gas, the object, roughly nine feet in diameter and hurtling through space at 13,500 miles an hour, will make its first palpable contact with Titan, the largest moon of the planet Saturn. Ever so slightly, the friction with Titan’s atmosphere will slow down the spacecraft, triggering a complex sequence of events that, in the ensuing few hours, should unravel some of the secrets of what could be the most exotic environment in our solar system. By all signs to date, that environment could be dominated by complex organic molecules and seas made of liquefied hydrocarbons. A day at a Titanian beach would be spent freezing to death under hazy skies made of methane and nitrogen, a scenario similar to what would have taken place on the early Earth (though our young planet probably wasn’t freezing).
