Origin of planetary atmospheres and their role in the evolution of life

The successful synthesis of amino acids in a reducing gas mixture presumably resembling the primary Earth atmosphere, by the Miller-Urey experiment, had a long lasting influence on the general perception of such an early atmosphere. Based on atmospheric escape, such a reducing atmosphere, however, has a lifetime of only millions of years. According to present view the early atmosphere of Earth may have resulted from mantle degassing of volatiles (H2O, CO2, N-2) followed by later accumulation of O-2 as product of life (photosynthesis). For Mars, with similar degassing products, evolution of life could have been possible in its early history. Non-reducing atmospheres (with liquid water) although not conducive for local synthesis of prebiotic molecules, may provide a benign environment for the evolution of life via ubiquitous supply of "building blocks" from external sources.