Formation of organic molecules on Titan

Abstract

The unique atmospheric environment on Titan has stimulated great interest in its organic chemistry. Recently¹ we proposed that simple organic-nitrogen compounds such as HCN could be efficiently formed by cosmic ray bombardment of a nitrogen-containing atmosphere on Titan. Voyager I has now verified that molecular nitrogen is indeed the major constituent on Titan^2–4 and that HCN is also present². Based on these new data, we now propose that even more complex organic-nitrogen molecules such as ethyl cyanide (CH₃CH₂CN), vinyl cyanide (CH₂CHCN), and cyanoacetylene (HCCCN) may be formed efficiently in the lower atmosphere of Titan, where lower temperatures and higher densities will ensure the efficacy of three-body ion–molecule association reactions. Interestingly, these compounds have been found in several dark interstellar clouds^5,6, thus the chemistry suggested here is analogous to that proposed as an explanation of interstellar cyanopolyynes⁷. The only difference is the role played by three-body association reactions in the dense lower atmosphere of Titan. The mechanism proposed here rests upon a firmer experimental foundation than the analogous radiative association reactions in interstellar clouds.