A Titan of a Mission

On Jan. 14, a flying saucer will parachute through the thick orange haze of a distant moon’s atmosphere. Descending through the hydrocarbon smog, the probe could crash into an icy mountain, plop in a pool of organic goo, or dive into a methane ocean. Welcome to Saturn’s largest moon, Titan, a place where organic chemistry appears to be a carbon copy of the infant Earth’s just before life got a foothold. The saucer-shaped Huygens probe, named for the 17th-century Dutch astronomer who discovered Titan, has been riding piggyback on the Cassini spacecraft since it left Earth in October 1997. The craft arrived at Saturn on June 30 and has now embarked on a 4-year tour of the planet and its moons.

Radar data from Cassini, taken during its first close flyby of Titan on Oct. 26, reveal dark patches that might be lakes of methane. Streaks imaged by visible-light cameras during that flyby could be caused by the flow of a hydrocarbon fluid or by wind eroding solid material (SN: 11/6/04, p. 291: Available to subscribers at Titanic Close-up: Cassini eyes Saturn’s big moon).

Titan has fascinated researchers for 6 decades, ever since astronomer Gerard Kuiper analyzed sunlight reflecting off the moon and discovered methane in its atmosphere. But interest escalated in 1980, when the Voyager 1 spacecraft revealed that methane is a small but key component of a nitrogen-rich atmosphere too thick to see through. The craft also confirmed the presence of ethane, acetylene, propane, and other hydrocarbons. Bombarded by energetic charged particles from Saturn as well as by ultraviolet light from the sun, methane breaks down in Titan’s upper atmosphere to form this complex array of organic compounds.

The chemicals may even rain out of the atmosphere to form hydrocarbon ponds or vast lakes on the moon’s surface. Molecules that evaporate from these liquid reservoirs would end up back in the atmosphere, replenishing the supply, just as water in Earth’s oceans resupplies our planet’s atmosphere.

That would make Titan, the second-biggest moon in the solar system (after Jupiter’s moon Ganymede), the only one with liquid at its surface. Radar beamed from Earth suggests that radio waves are indeed reflecting off a Titan lake or ocean, but results from visible-light studies are less clear-cut.

Yet even if Huygens doesn’t plunge into a methane bath, its findings are likely to make quite a splash.

It isn’t just Titan’s mix of organic compounds that intrigues planetary scientists. The moon also has reserves of frozen water that occasionally melt when struck by comets. The overall chemical cocktail appears to offer researchers the only available glimpse of conditions like those on Earth just before life got started. On our planet, traces of this long-ago era have been erased by the actions of life itself. But Titan, residing in the chilly outer solar system and protected by a thick atmosphere, may have preserved for billions of years the conditions that were necessary for life to begin.

Life is unlikely to have sprung up on Titan, which has an average temperature of –180°C. In exploring the moon, “we’re trying to understand about the origin of life in the solar system, which is very different from searching for life,” says Larry Soderblom of the U.S. Geological Survey in Flagstaff, Ariz. Titan could reveal how the raw materials for life—organic compounds—collected into pockets of varying concentrations, where biological action could begin, he adds.

“Titan is more like the prebiotic Earth than any other site in the solar system,” asserts planetary scientist Jonathan I. Lunine of the University of Arizona in Tucson.

Prepare to dive

Before Huygens can take the big plunge, it will have to execute the big escape—separating from its mother craft, Cassini. On Christmas Day, engineers will radio a final set of commands for the parting. Explosive bolts will fire, springs will give a gentle push to the probe, and Huygens will coast into space. A device on Cassini will twirl Huygens as it detaches, giving the probe a spin of seven revolutions per minute that will prevent it from tumbling end over end.

During Huygens’ 22-day coast to Titan, all the detectors on the probe will be asleep. But three quartz clocks will continue to operate, set to power up the detectors 45 minutes before Huygens reaches the top of Titan’s extended atmosphere. The moon’s atmosphere reaches to a height of 600 kilometers, or 10 times the height of Earth’s.

While Huygens sleeps, Cassini will fire its engines, reorienting itself so it will pass over Titan at a relatively slow speed during the probe’s descent. That’s crucial because Cassini is the only relay for the precious data to be collected by the parachuting probe.

Huygens’ suite of instruments will have only a few hours to record data. Once it hits the atmosphere, it will take about 2.5 hours to descend to the surface. Whether the probe survives the landing depends on the surface it encounters. Huygens wasn’t designed as a lander, but if it falls into liquid, it may float for a while.

Even if it survives impact, the battery-operated craft will have no more than about 2 hours to study the surface after it’s landed. By that time, Cassini will have disappeared over the horizon of the landing site, continuing on its tour of the Saturn system.

Down to work

The 319-kilogram probe packs six sophisticated instruments that will attempt to measure in different ways the basic properties of Titan’s atmosphere: its temperature, pressure, wind speed and direction, and composition.

The Huygens Atmosphere Structure Instrument (HASI) will kick in early, beginning at an altitude of 2,000 km. Accelerometers will measure how rapidly the probe slows down from its initial speed of 6 km per second. That deceleration will indicate gas density and wind gusts in these upper reaches of the atmosphere.

During the initial, rapid descent, the searing heat beneath Huygens will make temperature measurements of the atmosphere impossible. But once the parachute opens, at a height of 170 km above Titan, a thermometer on HASI will be put to work.

HASI will also explore the electrical conductivity of Titan’s atmosphere. Titan lacks the magnetic shield that protects Earth from galactic cosmic rays, which are energetic enough to ionize gas molecules. Titan’s atmosphere is much more highly charged and conductive than that of Earth.

HASI also carries a microphone that can listen to the sounds of Titan—such as thunder—as the probe falls. If this device succeeds, Titan will be one of the few places beyond Earth where sound has been recorded.

Relying on an ultrasteady radio signal from Huygens to Cassini, the Doppler Wind Experiment (DWE) will measure the strength of Titan’s wind. As winds buffet the parachuting probe to and fro, the radio signal detected by Cassini will shift between slightly higher and lower frequencies. This Doppler shift will indicate wind speed to an accuracy of a meter per second.

The gas chromatograph/mass spectrometer (GCMS) will measure the composition of the atmosphere in two ways. One instrument sorts molecules by weight and the other, by chemical reactivity. If Huygens manages to land intact on Titan, the GCMS will also measure the composition of the solids or liquids it encounters on the moon’s surface. To accomplish that feat, the device will be heated just before impact so that it vaporizes the first surface material with which it comes into contact. Hasso Niemann of NASA’s Goddard Space Flight Center in Greenbelt, Md., who built the device, also designed a simpler, spectrometer-only instrument that successfully parachuted into Jupiter in 1995 (SN: 12/23&30/95, p. 420).

A related instrument on Huygens, the aerosol collector and pyrolyser, will suck Titan’s atmosphere through a filter and heat the trapped particles in miniature ovens. Twice during the descent, the vaporized samples will be piped to the GCMS.

Say cheese

Only one group of instruments on Huygens will graphically document the exploration. The descent imager/spectral radiometer (DISR) will take both visible-light and near-infrared pictures, beginning at 150-km altitude and continuing down to the surface, where its cameras will resolve features just a few centimeters across. As the spinning probe descends, the series of images will form overlapping panoramic views that scientists can covert into stereo pictures, providing a three-dimensional perspective of Titan.

DISR’s visible-light and infrared spectrometers will analyze the feeble sunlight reflected from the surface of Titan back through its atmosphere, revealing the composition of clouds and the size of aerosol particles. As Huygens descends to an altitude of 700 meters, DISR will switch on a 20-watt lamp that will illuminate the surface, enabling the spectrometer to analyze the reflected light.

Mounted on the underside of the probe, the surface-science package (SSP) is the primary tool for studying the nature of the landing site. Because the craft’s survival after impact is highly uncertain, researchers have designed the SSP so that it can begin its exploration of Titan near the top of the atmosphere, 2.5 hours before touchdown.

Using a high-frequency signal generator and receiver, the SSP will attempt to measure the speed of sound at different altitudes in Titan’s atmosphere. At each altitude, the sound speed indicates the composition of the atmosphere, notes John Zarnecki of the Open University in Milton Keynes, England, who helped design the European-built instrument.

Using two devices akin to carpenter’s levels, the SSP will record the tilt of the probe as it plunges through the atmosphere as well as Huygens’ final orientation on the ground. The tilt during descent indicates the strength of the wind buffeting the probe, complementing more-precise measurements by the DWE.

On the surface, the amount of tilt may indicate whether Huygens has landed on solid ground or is bobbing on an ocean, rocked by waves that could be as high as 15 m. Measuring the frequency of such waves “would be the first time we’ve conducted an experiment in oceanography” on a place other than Earth, says Zarnecki.

The SSP also comes equipped with an acoustic sounder, similar to sonar, that can send signals down through the atmosphere and listen for an echo. Should Huygens pass through an extremely dense methane cloud, the reflected signal could reveal the condensation of methane droplets, or rain. As Huygens comes within a few hundred meters of the surface, the echo may indicate the bumpiness of the terrain. If the probe lands in a lake or an ocean, an echo may reveal the depth of the reservoir.

By aiming a light beam into any substantial body of liquid ethane or methane, the SSP will attempt to record the index of refraction of the fluid, another indication of its density.

A short carbon fiber protruding from the probe’s underbelly will be the first part of Huygens to strike the surface. Four sensitive transducers connected to the tip of the stick will detect the force of impact, indicating whether the landing site is solid, gooey, or liquid.

The SSP also includes a miniature float, like a fishing bobber. In the event of a liquid landing, the portion of the float sticking out of the lake or ocean will indicate the fluid’s density.

Several times during the years when he and colleagues were designing and testing the SSP, they questioned why they were putting so much effort into a device that might last only 3 minutes on Titan’s surface, he recalls.

“Still, today, if you offered me 3 minutes to explore the surface of Titan, I would grab it,” Zarnecki says. “We all want to know what the composition of this stuff is.”

Says Huygens researcher William Borucki of NASA’s Jet Propulsion Laboratory in Pasadena, Calif.: “The idea in back of all our minds, with each of these instruments, is to try to understand conditions that might make it possible for life to emerge.”

“We’re mad to undertake this venture at all, but at least we’re not completely mad,” says Zarnecki.

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