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.”