The Story Of O_2

摘要

Two gases overwhelmingly dominate Earth's atmosphere: N_2 and O_2. The former is primordial, and its presence and abundance are not driven by biological processes. Indeed, N_2 is virtually inert and has an atmospheric lifetime on the order of 1 billion years (1). In contrast, O_2 is continuously produced biologically via the oxidation of water driven by energy from the Sun. The gas was almost certainly virtually nonexistent in Earth's early atmosphere, is highly reactive, and has an atmospheric lifetime of ～4 million years (2). Yet despite this comparatively short atmospheric lifetime, O_2 came to constitute ～ 10 to 30% of the atmospheric volume for the past ～500 million years (3,4).