Water ice forms, not in massive blocks as it does on Earth, but as fine particles, perhaps acting on Titan's shores the way sand does on terrestrial beaches.

At first glance, a planet such as this - Titan is a moon, but it is larger than Mercury and Pluto and its diameter is three-quarters the size of Mars - would not be considered a prime candidate for life. Yet certain hints are beginning to appear that counter such an assumption.

Could the discovery of living organisms elsewhere in the solar system occur not on the red planet next door, but the orange-hazed world nearly a billion miles away?

During its closest flyby of Titan on April 16, NASA's Cassini spacecraft detected some surprisingly complex organic molecules floating in its upper atmosphere.

Specifically, the spacecraft's mass spectrometer picked up the presence of a variety of hydrocarbons, including ethane and even octane - the same substance that boosts performance in automotive engines.

The discovery goes against pre-existing concepts about Titan's atmosphere, particularly that nothing complex could remain there because of the low temperatures.

The nitrogen and methane that compose the bulk of the atmosphere were expected to form larger hydrocarbon molecules in reactions with sunlight, or with electromagnetic energy emanating from Saturn, but those molecules were supposed to be rare and quickly rain down to the surface.

Cassini's latest data therefore pose a mystery: what kind of chemistry is driving hydrocarbon production in Titan's atmosphere?

Is it the same process that governed the early atmosphere of Earth, which helped life to spring forth? Or, could it be those molecules are the signs of living systems at work on the surface?

"We are beginning to appreciate the role of the upper atmosphere in the complex carbon cycle that occurs on Titan," said Dr. Hunter Waite, principal investigator of the Cassini ion and neutral mass spectrometer - the instrument that detected the molecules - and professor at the University of Michigan in Ann Arbor.

"Ultimately, this information from the Saturn system will help us determine the origins of organic matter within the entire solar system."

In other words, Waite asked, "What is the ultimate source of the organics in the solar system?"

The presence of organic molecules on Titan is not the issue. Astronomers have inferred the presence of organics even in the vast clouds of dust found between the stars.

One such cloud - the remnant of an ancient giant star that exploded in a supernova - formed the sun, Earth and the rest of the solar system.

The key ingredient is oxygen. The atmospheres of the four giant outer planets - Jupiter, Saturn, Uranus and Neptune - plus Titan are rich methane and nitrogen, but are largely devoid of oxygen, which has acted as the catalyst for the proliferation of life on Earth.

In those cases, organics are produced by the interaction of ultraviolet light from the sun, or energetic particle radiation - from Saturn's magnetic field in the case of Titan - with atmospheric nitrogen and methane.

The question is, what else might be going on?

Cassini's April 16 flyby of Titan was its sixth in a series of 45 planned encounters over the next several years.

So far, along with the latest discovery, the spacecraft's instruments have observed a surface that seems very young by geological standards, meaning it is active, like Earth's.

An active surface means there are sources of heat, possibly from volcanism or the gravitational tugs of the more than 30 other worlds orbiting Saturn.

Where there is heat and nourishment - hydrocarbons falling from the sky could be food in some biosystems - there could be life. It is not an impossibility.

Cassini's repeated observations should help scientists answer some of the emerging questions about Titan, but the best way to explore this fascinating place is to send another probe to the surface.

Perhaps then, the answer to another mystery might be found.

Last Jan. 14, during its descent to Titan and brief lifetime on the surface, the European Huygens probe snapped a rather striking photo.

For one thing, it revealed the moon's landscape looking amazingly like a shoreline bordered by rolling, snow-covered hills. Of course, the ocean and snow, if it existed, would have been composed of methane.

Also appearing in the photo, however, is a structure consisting of two straight sides joined at a point, like a very large, v-shaped wall.

The image is rather fuzzy and, no doubt, there is some logical explanation for the feature that does not involve the Titanian equivalent of China's Great Wall.

Still, the intriguing thought remains: Did Huygens casually capture and transmit the first photograph of an alien artifact?

Unlikely, but not impossible.

In the Stars is a series examining new discoveries about the cosmos, by Phil Berardelli, UPI's Science & Technology editor. E-mail: [email protected]

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