Temperature effects on the ozone abundance in terrestrial exoplanets' atmospheres
Résumé
The search for extrasolar terrestrial planets raises currently a considerable scientific interest. The first one to be discovered are on close-in orbits around their parent stars. Known terrestrial exoplanets can therefore be as hot as a few thousand K, such as Corot-7b and Kepler-10b. The detectability of any spectral features in exoplanetary atmospheres depends mainly on two main parameters: their chemical composition and their temperature profile. However, the competi- tion between photochemical kinetics and thermochemistry susceptible to exist in hot terrestrial atmospheres prevents us from generalizing the processes occurring in Earth's atmosphere and initiating potential departures from equilibrium. It requires therefore detailed modelling in order to estimate the risk of false-positive and/or false-negative occurrences when seeking spectroscopic evidence of habitable conditions and life. We apply here a one-dimensional model coupling photochemical and thermochemical kinetics and vertical diffusion to study the effects of disequilibrium chemistry on the atmospheric composition of hot terrestrial exoplanets. We will show that the ozone abundance is very sensitive to the temperature profile and is consequently a source of notable uncertainties when modelling hot terrestrial atmospheres. Indeed, high atmospheric temperatures seem to inhibit very efficiently the production of an ozone O3 layer when considering thermochemical kinetics and when fully reversing kinetic reaction rates. In the Earth's atmosphere, the destruction of ozone O3 occurs through a large number of reactions, among which are some catalytic cycles involving mainly hydrogenous compounds (H, OH, HO2) and competiting efficiently with the regular Chapman cycle. Globally, the higher the atmospheric temperatures, the larger the increase in some of these active compounds abundances over equilibrium predictions.