Hunten, who has pondered Titan's atmosphere for more than three decades, investigated the puzzle of Titan's missing seas by focusing on a nearby planet that he dubs his laboratory: Jupiter, whose atmosphere also contains ethane. At an average of 887 million miles from the sun, Titan is about twice as far from the sun as Jupiter, but according to Hunten, "the photochemistry is similar enough on the two objects that you can apply the same principles to both of them." Observing the Jovian atmosphere in 1995 with the mass spectrometer aboard the Galileo probe, he found that ethane molecules were too sparse at the cloud tops to condense into a liquid—just 2 trillion molecules per cubic centimeter. To condense into droplets at Jupiter's temperature of –189°F, the concentration of ethane would have to be far higher, 30 quadrillion molecules per cubic centimeter. On Titan, the concentration of ethane is about 4 quadrillion molecules per cubic centimeter—too low, at the moon's far colder temperatures, to condense into anything but the most tenuous clouds.
Hunten contends that molecules of ethane can't form droplets, so they instead cling to smog particles in Titan's atmosphere. "Smog particles are known to be very, very fluffy structures, full of holes, so they are excellent sites for ethane to grab onto," he says. Another clue came from the now-frozen Huygens probe, which parachuted to the surface of Titan on January 14, 2005. As it drifted down, the probe pointed infrared imaging instruments at the sun. By observing the sun's fuzzy halo, and the way the light was scattered by smog particles, planetary scientists could discern that these particles were indeed fluffy—with a density of only 0.1 gram per cubic centimeter and a radius of 0.9 micrometer—and not globular, like droplets.