Prebiotic Synthesis in Planetary Environments

Abstract

Except for major short-term perturbations in surface environments caused by a declining flux of impactors, equable conditions for prebiotic evolution could have existed as early as 4.4 billion years ago. Giant impacts undoubtedly constrained the timing of life's origin, but quantitative statements about when the clock was set awaits stronger consensus on impactor fluxes and more refmed theoretical models. Organic matter surviving impacts or synthesized in impacts would have augmented the inventory of compounds produced endogenously in surface environments. The oxidation state of the prebiotic atmosphere remains controversial, but little question exists about the reduced state of the early ocean, which may have provided a more productive medium for prebiotic synthesis than the atmosphere. If the atmosphere has been only mildly reducing since the end of the major epoch of accretion at-4.4 Ga, the apparent lack of strong atmospheric sources for hydrogen cyanide, formaldehyde and ammonia poses a serious challenge for theories of prebiotic evolution which require these key chemical intermediates. Submarine hydrothermal systems and the wind mixed layer of the ocean are specific settings which may have favored prebiotic evolution. Especially interesting is the ocean-atmosphere interface where a complex set of physical and chemical processes operated continuously: collection of gas, aerosols and dust from the atmosphere; recycling of organic and inorganic solutes between the ocean and atmosphere through bubble formation and bursting; organic synthesis by UV radiation, cavitation and other energy sources; and formation, dissipation and reformation of surface active mono layers and bilayer vesicles. The intersection of the wind mixed ocean layer with shorelines of volcanic platforms and shallow marine hydrothermal systems may have been key sites for prebiotic evolution.