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Introduction
The mystery of the origin of life on Earth will never be solved if our studies are confined to our own planet. Life originated sometime during the first billion years of Earth's history, perhaps more than once, from a subtle pre-biotic chemistry involving two key ingredients: carbon-based molecules and liquid water. The life we know today probably began some time near the end ofEarth's accretional bombardment by icy and rocky planetesimals about 4000 million years ago. The record of the conditions and processes from that time has been obliterated by erosion and plate tectonics. We do not even know what the composition of the atmosphere was while the early steps in chemical evolution were occurring. To discover how chemistry became pre-biotic chemistry and then biology, wemust go to another world where a primitive environment has been preserved and a complex organic - potentially pre-biotic - chemistry is still at work. That is one of the reasons why we are so eager to explore Titan.

Figure 1. To discover how chemistry on Earth became pre-biotic chemistry and then biology, we must go to another world where a primitive environment has been preserved and a complexorganic chemistry is still at work
Why Titan?
Thanks to Voyager 1, we already know that Saturn's largest satellite has a predominantly nitrogen atmosphere containing a few percent of methane. Both of these compounds are being continuously broken apart by solar UV photons, precipitating electrons from Saturn's magnetosphere, and cosmic rays. The fragments of the parent molecules recombine to makenew compounds, while the liberated hydrogen escapes into space (to become a species in Saturn's magnetosphere). Six simple hydrocarbons in addition to methane and five nitriles have been identified, as well as CO and a tiny trace of CO2. Titan's visible atmosphere is filled with smog, which must be a mixture of simple condensates of the identified gases and polymers that have built up frommolecules such as HCN and C2H2.
While this ubiquitous smog prevented Voyager from seeing Titan's surface, we do know that the average surface temperature is very low, at 94 K (-179°C). Water ice is almost certainly the main constituent of Titan's crust and upper mantle, but the vapour pressure of H20 is so low at this temperature that this abundant compound cannot supply the oxygen that is necessary tochange the chemistry of Titan's atmosphere to an oxidising condition. A small amount of OH is supplied by ice grains from Saturn's rings and icy satellites and by impacting comets. This is adequate to convert some CH4 to CO and some CO to CO2, but it is not sufficient to produce a CO2/N2 atmosphere, such as we find on Mars and Venus. CH4 is still the most abundant form of carbon, just as it is inthe atmospheres of the giant planets.
In other words, Titan provides us with an opportunity to travel back in time. Conditions on Titan today resemble the anoxic environment on Earth in which the chemical reactions necessary for the origin of life must have taken place. The fundamental difference from the early Earth is Titan's low temperature. As we discussed earlier, there is no chance ofthere being liquid water on Titan's surface, except from possible transient heating events such as vulcanism (if there is any) or from impacts by comets or meteorites - possibilities to be examined by Cassini/Huygens. The absence of liquid water prevents the origin of life as we know it on Earth. Instead, we must focus our investigations on the nature of the chemical reactions taking placespontaneously in Titan's atmosphere and on the surface, where the environment will again be different from that on Earth. It is very doubtful that much bedrock is exposed. However, if there are rocks, any liquid water or ammonia would act on them to produce clays or other active silicate surfaces that could serve as templates for complex organic polymers. While such activity is viewed as being very limited...