Enshrouded in an atmosphere impenetrable to the visible light, Saturn's largest moon has never revealed its surface. No one has been able to see through the orange-brown atmospheric haze and admire the unknown world below.

Still, researchers know that Titan is a planet-size organic reactor where "building blocks" of life are being generated as they might have been created 4 billion years ago on Earth.

In some ways, Titan resembles early Earth. Its dense atmosphere is mostly composed of nitrogen and some methane. Scientists once believed that early Earth's atmosphere was reducing like Titan's and that it allowed fast assembly of long organic molecules. Today many argue that Earth's primordial atmosphere contained nitrogen and a lot of carbon dioxide.

"This type of atmosphere is neutral for oxidation and reduction reactions and does not allow an easy and direct formation of long chains of organic molecules," says University of Arizona planetary sciences Professor Jonathan I. Lunine. "Some particular circumstances may be required to create them. Although there isn't much carbon dioxide on Titan, if we see that complex organic molecules are created on Titan, it would be a very important lesson about the early Earth and the environment in which life originated."

"Titan has organics, but in what form and how much is not clear. These molecules are generated in the atmosphere and over time are deposited on the moon's surface. Until recently, researchers have been very careful in their speculations about what might be happening after these molecules get to the surface of Titan," Lunine says.

The atmospheric pressure at Titan's surface is 50 percent higher than on Earth, which is pressure comparable with pressure at the bottom of a 10-foot-deep swimming pool. Titan's thick atmosphere protects the surface and organics from harmful cosmic rays and ultraviolet radiation.

The NASA Cassini spacecraft launched in 1997 with the mission to study Saturn and its moons will reach its target in 2004. It carries the European Space Agency's Huygens probe, which will descend through Titan's atmosphere and land on the surface. The Cassini-Huygens mission will conduct a 4-year survey of Titan's surface and atmosphere through remote sensing and in-situ techniques.

"The Cassini mission has the potential to teach us as much about Titan as we know about Mars today. We will learn about the surface composition, find out more about the atmosphere, and see what the surface looks like. The Cassini orbiter will measure the shape of Titan's gravitational field, which will help determine the nature of Titan's interior," Lunine says.

"Titan will be full of surprises. One of them will be organic chemistry processes on the surface. It would be interesting to see what their products might be," he adds.

"I also hope that Cassini-Huygens will tell us if there are places on Titan where the organic molecules look different, and therefore, might be modified over time. Particularly exciting would be finding out if there are any variations in the apparent organic composition that are correlated with impact carters or sites of volcanism. If that turns out to be true, these should be the places to visit in the future," he says.

Could Titan host primitive life? "It is not the right place, it is too cold," Lunine says. "Others have argued that life could exist in the deep interior of Titan where liquid water may be available all the time. It is possible, but finding it would be extremely difficult. I do not see Titan as the place to search for life. But it certainly is the place to explore the chemistry that may have led to its origin."

For life to be possible, Titan would need liquid water, which is not stable for long because Titan is too cold. However, many of the large icy moons in the outer solar system host active water volcanism. Most of them contain a lot of liquid water, which flows across their surfaces in the same way lava does on Earth. Their internal heat initiates a melt that rises to the surface. These moons also contain various substances that are antifreezes (e.g. ammonia or formaldehyde). They are mixed into the water which lowers the density of liquid water and helps the water come up to the surface through the more dense icy crust. Titan is the second largest moon in the solar system, and if it hosts such volcanic processes, then water exists temporarily on the surface.

Titan can also be heated with large impacts. In the early 1990s, Carl Sagan and W. Reid Thompson of Cornell University suggested that impacts on the surface of Titan would melt the icy crust and produce liquid water. Lunine and a colleague from Moscow have been modeling impacts on Titan to see what fraction of the crater would become liquid due to an impact. They calculate that an impact of a one-kilometer-diameter comet can turn about 5 percent of a craterıs interior into liquid. Their simulations also show that the areas potentially containing organic matter would not be heavily shocked in an impact. Organic material survives such events and would be tossed in the crater where the liquid water would exist. When life on Earth originated about 4 billion years ago, large impacts were frequent.

"An organic soup on Earth did not have much uninterrupted time to form products relevant to life. Undoubtedly, the environment was changing dramatically, as young Earth was struck by other impacts or altered by volcanism," Lunine says.

Although today the solar system is relatively a quiet place, a one-kilometer-diameter object could hit Titan once every 10 million - 50 million years.

"There should be areas that haven't been changed in geologically recent time and where the products of organic processes that happened after that impact should be preserved. These may not be possible to investigate with the Cassini-Huygens probe, but could be done with the following missions. We are very optimistic that there are places on Titan where organic matter might be dropped into the liquid water at the bottom of the crater after an impact. This water can be available for hundreds or even up to a thousand years," Lunine says. A thousand years is very short on a geologic time scale, but it's a long time for organic chemistry.

"No scientist has a thousand years, so we can't proceed at this time scale in the laboratory. Though we don't have a chance to see organic reaction on Titan in action, we may find the products of organic chemistry if we go to the right place," he adds. "If Cassini finds that organic matter looks the same everywhere on the surface, then this probably did not happen. But we need to go and see."

The chance that the Huygens probe will land in the right place is infinitesimal, but the Cassini orbiter can map the surface and tell if amino acids or peptides might be present. "We have been designing miniature laboratory equipment that may be eventually sent to Titan to analyze the properties of organic molecules on the surface," Lunine says. The search would be for fossil organics, not fossil life, that have been modified at the bottoms of craters.