The Breath of Life – Oxygen in Earth’s Atmosphere and the Search for Life Beyond

Earth’s atmosphere is remarkable among the planets in our Solar System for its high concentration of molecular oxygen (O2), which makes this planet habitable for humans. This abundance of O2 is indisputably a signature of O2-producing photosynthesis – and hence of life. Therefore, the search for life on planets outside our Solar System understandably focuses on looking for evidence of O2. However, for half its history – before the Great Oxidation Event that began ~ 2.4 billion years ago – Earth’s atmosphere was nearly devoid of O2. Accumulating evidence suggests that this was the case even though O2 was already being produced by photosynthesis. Why? What controls the abundance of O2 in the atmosphere? And how does our understanding of Earth’s atmospheric O2 inform our use of this gas as a signature of life on other worlds? Frontier research on these topics will be presented.