Raw Data: A Moon Full of Smust

Smog-and-ethane powder rains on Saturn's giant moon Titan—and covers it in a mile-thick layer.

By Josie Glausiusz|Friday, February 02, 2007

The Study: "The Sequestration of Ethane on Titan in Smog Particles" by Donald M. Hunten, published in the journal Nature.

The Motive: Since the early 1980s, astronomers have postulated that a vast, sloshing sea of liquid ethane half a mile deep may cover the surface of Saturn's giant moon, Titan. That would be the only other known ocean in the cosmos besides Earth's. In theory, sunlight should glint off Titan's ethane waves, yet measurements using NASA's Infrared Telescope Facility in Hawaii have failed to detect such reflections. Moreover, when the Huygens probe landed on Titan, it hit fairly solid ground. So where's the juice? Donald Hunten of the Lunar and Planetary Laboratory at the University of Arizona at Tucson claims to know the answer: His observations of falling particles in Titan's atmosphere indicate that the putative ocean consists of a solid pile of "smust." 

Although smust may sound like a festering, foul-smelling secretion left beneath your bed by an unwelcome rodent, it is in fact a dusty combination of smog and ethane that coats Titan in a layer more than a mile thick. Hunten, an 81-year-old physicist and planetary scientist, coined the term "smust" from the words dust and smog. He argues that shifting dunes of the fluffy stuff—not liquid ethane—must have accumulated on Saturn's moon over the 4.5-billion-year life span of the solar system.

The Methods: Photochemical modeling by a Caltech team indicates that ethane, a hydrocarbon detected on Titan by both of the Voyager spacecraft and the European Infrared Space Observatory, is created when solar ultraviolet light breaks down methane, a minor component of Titan's nitrogen-rich atmosphere, into a stew of organic molecules that form a dense orange-brown smog. Originally it was thought that the ethane condenses into droplets, and rains down in the frigid cold of Titan's –290 degree Fahrenheit atmosphere.

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.

The Meaning: Many astronomers were disappointed to find no ocean on Titan, but smust gives the moon its own exotic settings. Drifting downward over eons, smust eventually settles into windswept dunes, which seem to have been seen in radar images from the Cassini spacecraft. This could mean treacherous conditions for future astronauts: "I would expect you would go ankle- to hip-deep in this crumby fine dust," Hunten says. "It would probably not feel comfortable at all." Still, Titan's organic smust may hold clues about the origin of life. "We think that there are probably all of these interesting molecules lying around which resemble those that existed in a primitive Earth, and which most people think led to the evolution of life on Earth," Hunten says. "So it's kind of a model of what we think Earth might once have been like."
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