Earth: Atmospheric Evolution of a Habitable Planet

Our present-day atmosphere is often used as an analog for potentially\nhabitable exoplanets, but Earth's atmosphere has changed dramatically\nthroughout its 4.5 billion year history. For example, molecular oxygen is\nabundant in the atmosphere today but was absent on the early Earth. Meanwhile,\nthe physical and chemical evolution of Earth's atmosphere has also resulted in\nmajor swings in surface temperature, at times resulting in extreme glaciation\nor warm greenhouse climates. Despite this dynamic and occasionally dramatic\nhistory, the Earth has been persistently habitable--and, in fact,\ninhabited--for roughly 4 billion years. Understanding Earth's momentous changes\nand its enduring habitability is essential as a guide to the diversity of\nhabitable planetary environments that may exist beyond our solar system and for\nultimately recognizing spectroscopic fingerprints of life elsewhere in the\nUniverse. Here, we review long-term trends in the composition of Earth's\natmosphere as it relates to both planetary habitability and inhabitation. We\nfocus on gases that may serve as habitability markers (CO2, N2) or\nbiosignatures (CH4, O2), especially as related to the redox evolution of the\natmosphere and the coupled evolution of Earth's climate system. We emphasize\nthat in the search for Earth-like planets we must be mindful that the example\nprovided by the modern atmosphere merely represents a single snapshot of\nEarth's long-term evolution. In exploring the many former states of our own\nplanet, we emphasize Earth's atmospheric evolution during the Archean,\nProterozoic, and Phanerozoic eons, but we conclude with a brief discussion of\npotential atmospheric trajectories into the distant future, many millions to\nbillions of years from now. All of these 'Alternative Earth' scenarios provide\ninsight to the potential diversity of Earth-like, habitable, and inhabited\nworlds.