* Abstracted from the article 'The Relevance of Titan and Cassini/Huygens to Pre-biotic Chemistry and the Origin of Life on Earth' by T. Owen et al., in Huygens: Science, Payload and Mission, ESA SP-1177, August 1997. That volume also includes the paper 'Titan's Organic Chemistry and Exobiology' by F. Raulin.
The relevance of Titan to the study of pre-biotic chemistry and the origin of life on Earth will be addressed by an interdisciplinary group of Cassini/Huygens scientists using the different, but synergistic, data sets obtained by NASA's Cassini Orbiter and ESA's Huygens Probe. Titan's special importance lies in the primitive, chemically-reducing nature of its atmosphere. Cassini/Huygens instruments will determine what compounds form in this environment, and the results will be compared with models for pre-biological chemical evolution on Earth.
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 of Earth'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, we must 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 complex organic chemistry is still at work
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 make new 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 from molecules 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 to change 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 in the 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 of there 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 place spontaneously 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 at best, it cannot be categorically ruled out on the basis of the evidence we have today.
What compounds are likely to be produced under these conditions? Titan offers us a kind of controlled experiment to study pre-biotic chemistry on a planetary scale. Titan is a world where organic chemistry has been proceeding for 4500 million years; at low rates, obviously, because of the low temperature, but for very long times. There may even be pools of liquid hydrocarbons in which compounds produced in the atmosphere can be concentrated and further reactions can occur. We are eager to learn what compounds are produced, and what reaction pathways are taken as chemistry proceeds from the simple, abundant molecules towards more complex compounds.
We must confess that, despite the giant steps made by Voyager, our understanding of this system is still very limited. The best photo-chemical models for Titan's atmosphere predict that ethane should be the main organic product of atmospheric reactions. If that were true, we would expect to find huge seas of ethane on Titan's surface, since the equivalent of a 1-3 km-deep global ocean of this compound should have been produced during the last 4500 million years. Yet radar and near-infrared observations from Earth have failed to find any evidence of hydrocarbon oceans. This is doubly vexing, since such oceans have also been invoked as reservoirs for Titan's atmospheric methane, which is constantly being destroyed, as has already been described.
Why is there any methane left today? How is it resupplied - by internal or external sources? Where is all the expected ethane? The fact that we have no answers to these simple, basic questions shows how far we are from understanding the chemistry of a primitive reducing atmosphere. Once we have thoroughly investigated the chemistry on Titan, we can expect to be in a much better position to deal with the far more complex issue of the origin(s) of life in the Solar System.
Several Cassini Orbiter instruments and most of those on Huygens will provide critical information on Titan's complex organic chemistry. In particular, they will provide new opportunities for the detection of organic compounds, including those not yet observedbut already assumed to be present in Titan's atmosphere. Huygens will measure vertical concentration profiles of many of the constituents, in the gas and condensed phases, and vertical profiles of energy deposition in the atmosphere - data of prime importance for understanding the processes of formation and evolution of organic matter in Titan's environment. On the Orbiter, the radar and infrared spectrographic instruments will allow many of the results obtained from Huygens' descent at a single location to be extrapolated to a global scale and to be monitored for temporal variations over the span of the Orbiter's four-year orbital tour. These data will reveal the chemical and physical nature of Titan's unknown surface - information essential for understanding the full cycle of organic chemistry that has taken place on that world over 4500 million years.
As previously noted, we do not know what the composition of the Earth's early atmosphere was at the time life began. We know that free molecular oxygen was missing; the O2 we have today is a gift of green-plant photosynthesis. The absence of O2 is beneficial to the origin of life, since all our attempts to simulate the early steps in chemical evolution have taught us that this pre-biological chemistry cannot proceed in the presence of free oxygen. But what gases were present? Only CO2, CO and N2? Or was there some CH4 and NH3, constantly regenerated by impacts?
If the volatile elements we now find in Earth's atmosphere were originally delivered by comets, as some scientists believe, we might expect the elemental composition of Titan's atmosphere to be similar to the atmosphere of the early Earth. Titan is made of 'cometary' material. It must have accumulated from ice-rich planetesimals that formed in the Saturn subnebula, augmented by real comets bombarding the satellite from outside the Saturn system. Depending on the extent of the influence of Saturn subnebula chemistry on Titan's early atmosphere, the initial atmospheric compositions on both Earth and Titan may have been very similar indeed.
In any case, Titan will serve as a full-scale 'end member' environment in studies of possible atmospheres for the early Earth. It is a world in which all the volatiles were delivered by ice-rich planetesimals and comets. It provides a reference planetary environment for studying the role of liquid water in chemical evolution since organic chemistry on Titan has been occurring for 4500 million years in the absence of this universal solvent. An important task will be to reconstruct Titan's original atmosphere from the clues provided by the present abundances and isotopic ratios of atmospheric volatiles as measured by Cassini/Huygens. This reconstructed atmosphere will then constitute the initial environment from which the organic compounds analysed by the Probe were produced.