As the Huygens probe prepared to plunge into the atmosphere of Titan, the scientists at the European Space Operations Center in Darmstadt, Germany, kept warning the packed auditorium full of colleagues and journalists to “expect the unexpected.” Saturn’s planet-size moon is completely enshrouded in an orange-brown haze. It is 10 times as far from the sun as Earth is, its thick atmosphere is tinged with methane (the air would burst into flame if oxygen were present), and it has about a seventh of Earth’s surface gravity. Whatever lay under Titan’s global smog would surely boggle the imagination.

 

EUROPE SCORES A BULL’S-EYE

A life-size model of the European Space Agency’s nine-foot-wide Huygens probe (above) sat on display in Germany while the real thing parachuted onto Saturn’s moon Titan, spinning and snapping pictures as it fell. Thirty of those images yielded the grand mosaic at right.


(Spacecraft image courtesy of Ferit Kuyas; mosaic courtesy of ESA/NASA/ University of Arizona)

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On the evening of January 14, when the snapshots from Huygens started to arrive, the researchers were startled anyway. The first set of images showed formations resembling riverbeds, eroded hillsides, coastlines, sandbars, and barrier islands that made Titan look improbably like Earth. One early Huygens picture looked eerily like the New Jersey shore. “Nah, it’s too rugged,” said Martin Tomasko of the University of Arizona, lead researcher on Huygens’s camera-spectrometer package, eyeballing a pile of color printouts scattered on a table in front of him. “It’s more like the south of France.”

At second look, however, nothing on Titan is quite what it seems. The thermometer hovers around –290 degrees Fahrenheit—cold enough to provoke chemistries and states of matter never seen naturally on Earth. The hills on Titan are rock-hard frozen water. The rain is condensed methane. The dark deposits in the channels and lowlands are most likely a tar that precipitated out of the hydrocarbon-rich atmosphere.

Titan’s contrast of the recognizable and the bizarre carries a profound lesson. If we ever see Earth-like worlds around other stars, there’s a good chance they will seem familiar too. All it takes is air, fluid, and a little geologic activity to create a place that looks remarkably like home. Titan expands our perspective on the whole range of landscapes out there.

And we came horribly close to never seeing it at all.

Courtesy of NASA/JPL/ Space Science Institute

Fuzzy Moon

This false-color image, taken from the Cassini space-craft, shows Titan through two kinds of filters. Ultraviolet (blue) highlights the moon’s several-hundred-miles-thick atmosphere. Infrared (red and green) penetrates the haze to show some surface details. Huygens images will greatly help scientists interpret Cassini’s long-range pictures.

Exploring new worlds is not for the impatient. Plans for the Huygens mission began so long ago that researchers in Darmstadt joked about it: “I feel like the new guy here; I’ve only been working on this project for 11 years,” said Mark Dahl, the Cassini-Huygens program executive at NASA. Titan got on researchers’ radar screens back in the 1940s, when Dutch astronomer Gerard Kuiper discovered that it is the only moon in the solar system with a substantial atmosphere. In 1980 the Voyager 1 spacecraft revealed Titan as an orange ball blanketed in nitrogen and an opaque haze of organic molecules. The discovery hinted at something marvelous: Titan’s chemical mix might mimic the conditions on Earth 4 billion years ago, when life first appeared.

Soon after, European and American space planners struck a deal. NASA would send the six-ton Cassini spacecraft to orbit Saturn; the European Space Agency’s Huygens companion would hitch a ride and parachute down to neighboring Titan. Huygens would be Europe’s most daring space effort, aiming to land on a distant surface we’d never even seen. The joint mission took off from Kennedy Space Center on October 15, 1997, on a looping 2-billion-mile journey. On Christmas Day of last year, a set of explosive bolts fired, three compressed springs separated probe from orbiter, and Huygens began its approach to Titan. Everything seemed to go exactly as planned, right up to the moment the first data signal arrived at 5:15 p.m. Central European Time on January 14.

Then a sick look of worry swept over the faces of engineers in the control room. Huygens was supposed to transmit its findings through two radio channels, channel A and channel B, to split the risk in case one malfunctioned. The two signals would be relayed to Cassini, which would amplify them and use its large antenna to broadcast the message home. But only channel B was coming through—channel A was missing. As mission planners scrambled, they reached an agonizing conclusion. Someone had neglected to program Cassini to listen for both channels.

Cassini obediently passed along channel B while channel A leaked away into space. David Southwood, the European Space Agency’s director of science, quickly launched an inquiry (quashing rumors that NASA was responsible for the error) and told the anxious crowd in Darmstadt, “We’re human, and the gods—maybe the Titans—always demand some human aspect in every godlike activity.”

The Huygens teams did not have time to fuss; they just wanted to save the science. Most of the probe’s instruments sent redundant signals through both channels, but the Doppler Wind Experiment seemed lost. The concept behind that experiment was beautifully simple: Beam a signal to Cassini, which would record subtle radio distortion caused by winds blowing around Titan. By analyzing that distortion, researchers could reconstruct Titan’s weather patterns. The experiment relied entirely on channel A.

Fortunately, Leonid Gurvits of the Joint Institute for VLBI in Europe—a Dutch radio astronomy institute—had a backup plan. Since the launch of Cassini-Huygens, the sensitivity of radio telescopes on Earth had improved so much, he realized, that it might be possible to conduct the experiment from the ground as well. He therefore collaborated with researchers at 17 radio dishes in Australia, China, Japan, and the United States to monitor Huygens’s signal. Gurvits and his colleagues spent a sleepless night gathering the preliminary results. The next day he announced, “We will recover 100 percent of the mission goals, with the same science outcome.” As proof, he showed a crisp plot of the signal as received by the Parkes and Mopra dishes in Australia and by the Green Bank Telescope in West Virginia. Early results show Titan’s high-altitude winds bluster westerly at 250 miles per hour. By summer Gurvits expects to have a map of wind patterns accurate to about two miles per hour—all extracted from a two-watt signal that originated nearly a billion miles away.

For Huygens’s crucial imaging camera and spectrometer, the channel A mishap touched off a different set of troubles. Half of the images ran through each channel, so the number of Titan images was cut from 700 to 350. The probe’s primary camera pointed at a downward angle, and the probe spun as it descended; the resulting images were supposed to form a spiral panorama that steadily zoomed in on the ever-closer surface. With half the images missing, the panoramas would be full of gaps.

A

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D

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F

A New World Explored in 217 Minutes

The Huygens descent and landing lasted less than four hours, triggering a concentrated burst of activity. On January 13, the control room in Darmstadt, Germany, was deserted (a). The next day, it erupted with celebration (b) as radio signals proved the mission had succeeded. David Southwood, the European Space Agency’s chief scientist (c), praised Europe’s newfound prowess in space. Mission researchers scrambled to explain just-in results (d); Martin Tomasko found himself mobbed by the news media (e). Later that day, he and the rest of the imaging team (f) retired to their quarters to reassemble a Humpty-Dumpty pile of Titan imagery.

Images A-E courtesy of Ferit Kuyas; F courtesy of ESA/ESOC/University of Arizona