such places, it will be valuable to look for evidence of plate tectonics, because that would indicate hydrothermal activity and the abiotic production of chemical energy and carbon sources that can support life. Studies of atmospheres of extrasolar planets will benefit from access to models of atmospheric conditions on planets that never have evolved oxygenic photosynthesis and that remain anaerobic. Earth did not accumulate oxygen during the first roughly 3 billion years, and it did not form an ozone layer until about 1.5 billion years ago. There is considerable emphasis on looking for contemporary Earth atmospheres that have oxygen and an ozone layer, but there should also be models of atmospheres with different anaerobic microbial ecosystems, atmospheres that might parallel the different stages in the evolution of Earth’s atmospheres over 4 billion years, and atmospheric conditions that could indicate the presence of a tectonically active planet.

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