Bistability of Atmospheric Oxygen and the Great Oxidation: Implications for Life Detection

摘要

Earth's atmospheric evolution was punctuated by a rapid and nonlinear transition in oxygen inventory 2.4 - 2.3 billion years ago, from < 2 x 10~(-6) atm to > 10~(-3) atm, known as the Great Oxidation. The cause of the Great Oxidation has been a major problem in understanding the evolution of the Earth system. In particular, oxygenic photosynthesis is thought to have evolved by 2.7 billion years ago, at least 300 million years before the Great Ox-idation. We have shown that the origin of oxygenic photosynthesis gave rise to two simultaneously stable steady states for atmospheric oxygen. The exis-tence of a low oxygen steady state explains how a reducing atmosphere persisted long after the onset of oxygenic photosynthesis. The Great Oxidation can be understood as a switch to the high oxygen steady state. The bistability arises because ultraviolet shielding of the troposphere by ozone becomes effective once oxygen exceeds 2 x 10~(-6) atm, causing a nonlinear increase in the lifetime of at-mospheric oxygen. Identification of oxygen or ozone in an extrasolar planetary atmosphere has been proposed as air indicator of life, but the existence of a low oxygen stable steady state introduces an important false negative case of planets with stable low (undetectable) oxygen but thriving oxygenic photosynthesis. An oxygen rich atmosphere, detectable remotely and capable of supporting complex life, is likely to occur only on planets of similar size to Earth; too small and bulk atmospheric loss occurs, too large and energetic limitation of hydrogen escape prevents atmospheric oxidation.