Factors affecting surface air composition on earthlike extrasolar planets

Abstract

The motivation of this study is to investigate changes in surface air pollutants across the Habitable Zone (HZ) hence estimate a more refined definition of the HZ in terms of a “Breathable Air Zone” (BAZ). This implies life as on Earth. We have performed sensitivity experiments using a tropospheric photochemical model in which the flux was increased to a Venus orbit (inner HZ run) and decreased to a Mars orbit (outer HZ run) relative to a pre-industrial Earth atmosphere control run. We then repeated the inner HZ run but with a prescribed temperature increase of 50K, which we denote as the warm inner HZ run. The original control run was also repeated but with modified stellar radiation corresponding to K2V and an F2V stars at 0.53 AU and 1.69 AU respectively [Segura et al., 2003]. The model calculated among other chemical species surface ozone (O ₃) - (an important pollutant) and hydroxyl (an important pollutant remover) concentrations. An increase in solar flux stimulated O₃ production via the established photochemical "smog" mechanism [Haagen-Smit et al., 1952] and also stimulated OH production via enhanced water photolysis. O₃ increased by a factor 6 in going from the outer to the inner HZ. Absolute levels depended sensitively upon methane (and other volatile organics), NO_x and UV, required by the smog mechanism. Levels of O₃ toxic to humans (i.e. a few hundred of ppbv, [Horstman et al., 1990]) were not reached for the runs shown here but would be approached if surface methane, NO_x and UV increase by a factor of 50 to 100, maybe attainable earlier in Earth's history.