Then, Jan. 14, in a make-or-break plunge into the atmosphere, the European Space Agency's Huygens probe, which has spent the last seven years piggybacking aboard Cassini, will attempt to collect more data about Saturn's giant moon than has been compiled in all previous missions and ground-based observations -- including Cassini's previous two flybys.

The whole mission will be tricky. Huygens is unpowered, which means its separation from Cassini must occur at the precise moment to place it on a correct heading to 1) rendezvous with Titan and 2) enter its atmosphere at the planned location and proper angle. As with all entries, too steep an angle could burn up the probe, while too slight and it will deflect back into space.

Even more critical will be the descent to the surface, where Huygens will transmit data and images for only a few minutes. The reason is the little probe is not powerful enough to send signals the 800 million miles back to Earth, so it must rely on Cassini to relay them. Cassini, however, will be racing past Titan and very quickly will be on the far side of the moon and blocked from receiving the Huygens transmissions.

If the probe's parachute array works too well, the descent will take longer than the planned time of about two-and-a-half hours, so by the time Huygens comes to rest Cassini will be out of reach. So mission engineers packed an extra parachute that will allow a faster descent if necessary.

In either case, the probe's onboard computer must make the decision, because a radio signal traveling to Earth and back will take about 150 minutes. To hear the mission scientists tell it, how ever long it takes to derive new information about Titan, it will be worth the wait.

Titan is still deeply mystifying to us, said Carolyn Porco, Cassini's imaging team leader, at a recent briefing on the mission.

Chief among those mysteries is whether the moon's surface is completely dry or contains slow-rolling seas of liquid methane or other hydrocarbons. Cassini's latest flyby, which took place Dec. 13, revealed surface patterns of light and dark that look very much like an ocean rimmed by a shoreline, Porco said, though quickly added Titan might not be wet anymore because the oceanic material might have solidified because of the extreme cold surface temperatures.

Whatever Huygens finds, Titan's surface is relatively young and dynamic. Craters provide the common benchmark for age. Earth's moon, for example, is still pocked by craters it received billions of years ago, while Earth's surface contains only a handful of visible craters retained from impacts within the past few hundred thousand years or so.

Some craters still remain on Titan, but most have been disturbed by subsequent features, such as cracks in the frozen surface.

The moon's atmosphere also is dynamic. Cassini's growing collection of images is showing rapidly changing cloud patterns and other data culled from images suggests there are ground features that have been shaped by wind action.

Still, geology and meteorology do not constitute the big mystery of the moment. That is a question of chemistry -- specifically, why methane has been detected in Titan's atmosphere.

During this latest Cassini flyby, the spacecraft aimed its mass spectrometer at that atmosphere, which was backlit by the sun, and found the methane.

Scientists are fascinated by this discovery for the same reason they are pursuing it in the Martian atmosphere: Methane is a short-lived gas in any location it is exposed to sunlight -- even the relatively dim sunlight shining on Titan. Eventually, atmospheric methane gets broken down into other hydrocarbons. For methane to persist in the atmosphere, it must be replenished, and among the possible sources of that replenishment -- along with simple evaporation from methane seas -- is microbial action.

It is an admitted long shot, because the extreme cold would seem to preclude the chances of life gaining a foothold so far from the sun. Still, life on Titan is not an impossibility. The basic building blocks are there, so any heat source sufficient to melt water ice, even in small pockets, could have gotten things started there.

Gaining even a hint of organic action in the Saturnian system would be a major discovery, and a further strengthening of the evidence that the processes capable of giving rise to life are active all over the universe. Cassini and Huygens are on the verge of providing that evidence if it is there to be found.

In the Stars is a weekly series by UPI examining new discoveries about the cosmos.

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