Elementary reactions of N atoms with hydrocarbons: first steps towards the formation of prebiotic N-containing molecules in planetary atmospheres.

Gas-phase reactions involving atomic nitrogen in the ground (4)S and first excited (2)D electronic states with simple hydrocarbons or hydrocarbon radicals lead to the formation of prebiotic N-containing organic molecules. These reactions are now active in the upper atmosphere of Titan (a massive moon of Saturn) and might have played an important role in nitrogen fixation in the primitive upper terrestrial atmosphere, assuming that it is similar to the present atmosphere of Titan. The products of these reactions (nitriles, imines and radicals) are the precursors of larger N-containing molecules, which form the dense haze aerosols that completely cover the moon of Saturn. If anything similar to Titan's haze has ever existed on our planet, it is reasonable to imagine that, once deposited on the surface of the oceans, further chemical evolution might have transformed these molecules into aminoacids and nucleobases, the molecular building blocks of living entities. The experimental techniques necessary to investigate these reactions in detail are presented and the main results reviewed.