Tracking organic chemistry into space

To help answer the question of whether life has emerged elsewhere in the solar system than on Earth, astrobiologists look to Mars and Jupiter's ice-sheathed moon, Europa, as potential incubators.

But for a look at how key chemical ingredients - carbon, seasoned with hydrogen, nitrogen, and other elements - may have mixed to form the pre-biotic building blocks necessary for organic life to emerge, researchers are looking to Saturn's moon, Titan.

Taken together, researchers say, these three - Mars, Europa, and Titan - may unlock the secrets of the evolution of organic material in the universe, from simple atoms forged in stars to the rich complexity of organic life on Earth. Of the three, Titan remains the most enigmatic, says Jonathan Lunine, a planetary scientist at the University of Arizona in Tucson and a member of the Cassini-Huygens mission to study Saturn and its systems of rings and moons.

Researchers will get their first close-up look at the moon and its chemical stew at the end of 2004, when the Cassini spacecraft begins the first in a series of 45 close fly-bys of Titan during its four-year mission. In January 2005, Cassini is slated to drop the European-built Huygens probe into Titan's atmosphere. If all goes well, the probe should continue to return data from the moon's surface for up to half an hour before falling silent.

"We think there are enormous quantities of organics in the atmosphere and on the surface, and for that reason alone, it's an interesting astrobiological target," Dr. Lunine says. "It may be an even more interesting target because there's the possibility that there have been energy sources and maybe liquid water in times and places on the surface that would have allowed these organics to evolve the kind of chemistry that might lead towards life."

Titan's atmosphere is thought to resemble that of Earth a few hundred million years after it coalesced from the disk of dust and gas that surrounded the young sun. Studies have shown that Titan's atmosphere presents a chemical face only an organic chemist could love, noted Lunine, during a series of briefings here this week conducted by the Council for the Advancement of Science Writing.

Once thought to be largely methane, Titan's atmosphere is now known to consist mainly of molecular nitrogen. Through chemical reactions triggered by sunlight, the nitrogen and methane recombine to form a range of organic compounds. Titan's temperatures range between 70 and 90 Kelvin (-334 to -298 degrees F.). At these temperatures, many of the compounds in Titan's dense atmosphere condense and fall to the surface.

Researchers have calculated that over the life span of the solar system, this chemical fallout would have encased Titan's rocky surface in what Lunine calls "the solar system's largest oil spill": a layer of solid and liquid hydrocarbons a kilometer (0.62 miles) thick.

One of the big mysteries scientists hope to solve with Cassini-Huygens might be dubbed the case of the reappearing methane. Methane condenses and rains out of the lower atmosphere. And in Titan's upper atmosphere, where the photochemistry takes place, hydrogen is dislodged from methane molecules, from which it escapes into space. This should leave ever-shrinking amounts of hydrogen to recombine with carbon and reform atmospheric methane. "It's a one-way reaction," Lunine says.

Indeed, researchers calculate that the amount of methane on Titan detected by the Voyager craft would vanish over some 48 million years, about 1 percent of the age of the solar system. Yet it is still found in abundance on the frigid moon. "We don't know where the methane comes from, or where it goes, and we don't know much about the chemistry that is going on at the surface," Lunine says. That surface is thought to consist of a layer of solid and liquid hydrocarbons slightly more than half a mile thick.

One of the surface-chemistry mysteries nagging at astrobiologists involves water. Titan is too cold to host liquid water for geologically long periods of time. But events such as volcanic eruptions or comet collisions could have generated liquid water stable enough to generate some fascinating chemistry.

For example, Lunine describes simulations that suggest a comet, striking the surface at about 10 kilometers a second, would punch a crater in the surface, leaving a relatively small amount of water. The decaying heat from the impact would keep the water liquid for 1,000 to 10,000 years.

During its four-year mission to the Saturn system, Cassini will use radar and infrared mapping techniques to help answer some of these questions, while the Huygens probe takes a more up-close and personal look at the atmosphere and surface.

Data can then be used to form target maps for future missions.