DIX Planetary Science Seminar
The atmospheric evolution of terrestrial planets is sculpted by a range of complex astrophysical, geophysical, and geochemical processes. Interpreting observations of ostensibly habitable exoplanets will require an improved understanding of how these competing influences interact on long timescales. In particular, the interpretation of potential biosignature gases, such as oxygen, is contingent upon understanding the probable redox evolution of lifeless worlds. Here, we present a generalized model of rocky planet atmospheric evolution to anticipate observations of terrestrial exoplanets. The model connects early magma ocean evolution to subsequent, temperate geochemical cycling. The thermal evolution of the interior, cycling of volatiles, surface climate, crustal production, and atmospheric escape are explicitly coupled throughout this evolution. The redox evolution of the atmosphere is controlled by net planetary oxidation via the escape of H to space, the loss of atmospheric oxygen to the magma ocean, and other crustal oxygen sinks such as outgassing. The model can successfully reproduce the atmospheric evolution of a lifeless Earth: it consistently predicts an anoxic atmosphere and temperate surface conditions after 4.5 Gyrs of evolution. However, if initial volatile inventories are permitted to vary outside these "Earth-like" ranges, then dramatically different redox evolution trajectories are permitted. We identify three scenarios whereby Earth-sized planets in the habitable zones of sunlike stars could accumulate oxygen rich atmospheres (0.01 - 1 bar) in the absence of life. The model also sheds light on the atmospheric evolution of Venus and Venus-like exoplanets. We can successfully recover the modern state of Venus' atmosphere, including a dense CO2-dominated atmosphere with negligible water vapor and molecular oxygen. Moreover, there is a clear dichotomy in the evolutionary scenarios that recover modern Venus conditions, one in which Venus was never habitable and perpetually in runaway greenhouse since formation, and another whereby Venus experienced ~1-2 Gyr of surface habitability.