CHAPTER 3: Gas-phase Prebiotic Chemistry Driven by Ultraviolet Photolysis of Simple Molecules

The photolysis of stable simple molecules by means of solar ultraviolet photons is known to be the first step in the chemistry of planetary atmospheres. The transient species produced in this way can react and form more complex molecules that are normally present in trace amounts. Trying to tackle the problem associated with the massive organic system that allegedly preceded the emergence of life on Earth according to the abiotic origin of life theory, we consider here the possibility that prebiotic chemistry on Earth started in its upper atmosphere. A proof-of-concept of this scenario can be provided by investigating the atmospheric chemistry of other planets and moons. In particular, if one searches for good test cases in our Solar System, the atmosphere of Titan stands out for its rich organic chemistry. In this chapter, our comprehension of the chemistry leading to N-rich organic molecules and macromolecules as made possible by resorting to a multidisciplinary approach based on photochemical models and dedicated laboratory experiments will be illustrated. The focus will be on neutral bimolecular reactions leading to nitriles, imines and other organic N-bearing molecules which can react further in the upper atmosphere of Titan up to the formation of macromolecules. Nitriles, imines and N-containing macromolecules are known as possible precursors of amino acids and nucleobases once they come into contact with water.