CHEMICAL EVOLUTION ON TITAN - COMPARISONS TO THE PREBIOTIC EARTH

Models for the origin of Titan's atmosphere, the processing of the atm osphere and surface and its exobiological role are reviewed. Titan has gained widespread acceptance in the origin of life field as a model f or the types of evolutionary processes that could have occurred on pre biotic Earth. Both Titan and Earth possess significant atmospheres (gr eater than or equal to 1 atm) composed mainly of molecular nitrogen wi th smaller amounts of more reactive species. Both of these atmospheres are processed primarily by solar ultraviolet light with high energy p articles interactions contributing to a lesser extent. The products of these reactions condense or are dissolved in other atmospheric specie s (aerosols/clouds) and fall to the surface. There these products may have been further processed on Titan and the primitive Earth by impact ing comets and meteorites. While the low temperatures on Titan (simila r to 72-180 K) preclude the presence of permanent liquid water on the surface, it has been suggested that tectonic activity or impacts by me teors and comets could produce liquid water pools on the surface for t housands of years. Hydrolysis and oligomerization reactions in these p ools might form chemicals of prebiological significance. Other direct comparisons between the conditions on present day Titan and those prop osed for prebiotic Earth are also presented.