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.
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.
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 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
Worse news emerged when the imaging team began digging into data from the camera’s supporting instruments. Huygens contained a sensor to record each time the spinning probe swept past the sun’s location in the sky. That information was supposed to record the probe’s precise orientation when it took each image. In principle, creating a map of Titan would simply be a matter of snapping each part of the mosaic into place. But the atmosphere turned out to be a lot soupier than expected, so the sun faded once Huygens sank below an altitude of 30 miles. Tomasko grimaced stoically at the news. “Maybe we came down through a methane shower,” he said.
Huygens also was buffeted by those high winds that Gurvits measured. “We had a hell of a ride,” said Laura Ellen Dafoe, lead systems engineer of the Huygens optical package. “We prepared for a 10-degree tip, but we got a 60-degree tip.” Every picture therefore arrived with almost no information about where the camera was pointed.
“It’s like trying to assemble a jigsaw puzzle without half the pieces and with no picture on the box,” Tomasko said. With the reporters and TV crews waiting, the imaging team huddled around their computers to reconstruct meaningful views of Titan. The first night after Titan’s Saturday landing, Tomasko released three individual frames—“raw images, hot off the computer”—that gave intriguing glimpses of riverbeds, a dark coastline, and Titan’s pebble-strewn surface. Sunday he and his teammates hunkered down in their temporary headquarters, a pair of connected trailers plopped in a weedy courtyard next door to the control room, and the second frenzy began.
While the telephone kept ringing—When will we see more pictures? Can you do an interview?—the researchers ricocheted between glee and disappointment, divided up the tasks, and tore into the work. One group cleaned up the images; another tried to deduce the color of the surface; yet another sifted for directional clues buried in the probe’s readings of scattered sunlight. Tomasko puzzled over the nature of Titan’s haze while Laurence Soderblom of the U.S. Geological Survey, a geologist on the team, dissected the geometry of the images. Soderblom groaned good-naturedly at his task: “It’s the arithmetic that kills you.”
By Tuesday giddy grins appeared in the makeshift imaging lab. “Yesterday was the worst day,” said Lisa McFarlane, a postdoc at the University of Arizona. Using improvised computer programs, she had started to figure out Huygens’s orientation at the time of each snapshot, which made it much easier to construct large panoramas of Titan’s surface. Soderblom had pinpointed the probe’s landing site. Tomasko could finally relax and put his worst fears in the past tense: “We came so close to losing everything.”
Few people outside of Tomasko’s group knew about the restless problem solving, number crunching, and seat-of-the-pants programming that followed the Huygens landing. Everyone could see the triumphant result. NASA and the European Space Agency released an imposing mosaic view of Titan’s surface, a version of the Titanian road map that came together, piece by piece, on the trailer’s wall. Three days earlier, Titan was a celestial question mark. Now it was succumbing to geologists, chemists, and cartographers, much as the Antarctic had a century earlier.
With each refinement, the picture looked more and more like a world we already knew. “Titan was supposed to be bizarre,” said Soderblom. “Instead it’s the most Earth-like planet in the solar system.” (Calling Titan a planet is a telling slip. Technically it is a satellite, but everyone on the Huygens team regards Titan as the equal of Mars, which is only slightly larger and has much less atmosphere.)
The bright highlands are most likely outcroppings of water ice. On his laptop computer in the trailer, Soderblom compared stereo images of the hills and estimated that they are a few hundred feet high. Organic soot steadily settles out of Titan’s atmosphere, so something—possibly methane rain—must scour these light areas clean. Dirty runoff evidently flows into the branching channels seen in the first-released Titan image. The channels lead to dark, flat lowlands—lake beds, perhaps.
Much to Tomasko’s frustration, the Huygens images did not clearly show whether any of the lakes contain liquid right now. On the other hand, everyone on the imaging team was overwhelmed by the evidence for rainfall and flooding in the relatively recent past. Soderblom pointed to formations that look like sandbars and barrier islands. The view from the surface shows rounded blocks of water ice, like eroded stones in a creek bed. One set of images shows patchy fog, which could be subsurface methane evaporating into the atmosphere; another reveals what looks like channels flowing outward from a methane spring. “This is an active, young surface,” Soderblom said.
An onboard accelerometer and ground penetrometer kicked in additional hints about the nature of the ground where Huygens landed. John Zarnecki of the Open University in the United Kingdom, who led the experiment, reported that the 705-pound Huygens, falling 10 miles per hour, first broke through a relatively stiff layer, about half an inch thick, then sank about six inches. An icy crust atop a muddy mix of water-ice “sand” and liquid methane would match the readings nicely. As Zarnecki watched workers dismantle the conference table where he had presented these results, he was already formulating plans to drop a test target into various combinations of sand, mud, and gravel to simulate Titan’s surface texture.
Results from one of Huygens’s chemical sniffers bolstered the idea that the probe landed on soil that had recently been soaked with methane dew or rain. About three minutes after the probe landed, it picked up a tendril of methane wafting from the surface, as if the probe’s heat were boiling some of the liquid trapped just below. Traces of argon gas found in the atmosphere, along with radar readings from Cassini, even hint at volcanic activity. At Titan’s frigid temperatures, however, the lava would be a syrupy mix of water ice and ammonia.
“Titan is a strange, through-the-looking-glass kind of world,” Tomasko said. The description fits not just its appearance but also its chemistry. Four billion years ago, Earth’s atmosphere might have broadly resembled the one Titan has today. Cold, sluggish Titan has evolved much more slowly, so the dirty orange organic chemicals that tint its surface—most likely dominated by tarry compounds called tholins—could be a deep-freeze model of early Earth.
Guy Israel of the University of Paris has been decoding the chemical brew Huygens encountered during its descent. He hardly expects to find signs of life. Biochemistry as we know it relies on reactions that can happen only in solution, “and that is not a possibility—unless we bring along the liquid water,” he said.
By the end of the week, officials at the European Space Agency were already looking forward to their next missions, to the moon and Venus, but the researchers in Darmstadt couldn’t escape the spell of Titan, a world that looks and acts so much like our own. Even as they began scattering to their homes in Arizona, Paris, London, and Bonn, the Huygens scientists were dreaming about how to return. Tomasko was tantalized by Titan’s dense atmosphere and luxuriously low gravity: “It would be a wonderful place to explore with a balloon.”
