Winds Blow Organic Molecules Across Planet-sized Moon

Researchers from NASA and other institutions have developed an atmospheric model lending insights to decades-old mysteries surrounding Saturn’s moon Titan that could shed light on the chemical processes that may have jump-started life on Earth.

These mysteries have especially intrigued astrobiologists, who view Titan as a model for the young Earth before life began. Other than Earth, Titan is the only other moon or planet in our solar system with a thick, nitrogen-dominated atmosphere. Its thick organic haze also appears very similar to smog on Earth.

“Titan is an interesting world. Its organic haze may be an example of the prebiotic organic chemistry that led to life on Earth,” said Dr. Christopher McKay, a scientist at NASA Ames Research Center in California’s Silicon Valley, and co-author of a research paper published yesterday in the journal Nature titled “A Wind Origin For Titan’s Haze.”

On Titan, methane and nitrogen molecules are thought to be converted into complex organic materials such as hydrocarbons and possibly amino acids, which are the building blocks of life on Earth. “We think similar processes once happened here, and life may have started that way,” said McKay.

Titan has long puzzled scientists because of several unexplained features in its thick, hazy atmosphere, composed largely of solid organic materials. Voyager images taken in 1980, for example, show that the haze is much brighter at Titan’s summer hemisphere than at its winter hemisphere. Earth-based observations also show that this difference in brightness changes with Titan’s seasons. Each season on Titan lasts for four Earth years. Titan’s haze also is much thicker near the polar caps than anywhere else. But perhaps most puzzling, a layer of the haze is detached from the rest of Titan’s atmosphere, appearing like a ghostly shell floating in space.

The research outlined in the paper provides the first ‘coupled’ model, linking Titan’s organic haze with atmospheric winds and with the sunlight that heats the haze. According to the group’s model, sunlight heats the haze that drives the wind, which, in turn, carries the haze. The smallest haze particles also can be carried from one pole to the other within one Titan season. And according to the model, the detached haze arises because very small particles of haze formed high in Titan’s atmosphere are blown to the pole before they can fall, becoming detached.

“We found that the main features of Titan’s organic haze arise from a strong feedback loop between the haze, the sunlight and the wind,” said McKay. “This is a critical new factor in understanding Titan.”

The model is precursor research for a NASA/European Space Agency probe expected to enter Titan’s atmosphere in January 2005. The Huygens probe, part of NASA’s Cassini mission, will take measurements and samples of Titan’s haze. NASA’s Jet Propulsion Laboratory, Pasadena, Calif., manages the Cassini-Huygens mission.

The lead author of the paper is Dr. Pascal Rannou of the University of Paris and the University of Versailles-St. Quentin. The other co-author is Dr. Frederic Hourdin of the University of Paris.