Atmospheric Photochemistry and Potential Biosignatures on Terrestrial Exoplanets

Renyu Hu (MIT)

I will present a comprehensive photochemistry model developed from the ground up to explore the atmospheric compositions of terrestrial exoplanets. With updated numerical algorithms, the photochemistry model has desirable features for exoplanet exploration, notably the capacity to treat both reducing and oxidizing atmospheres and the elimination of the need for fine-tuned initial conditions. I have used my photochemistry model to simulate both reducing and oxidizing atmospheres on terrestrial exoplanets. Highlights of my findings are: (1) sulfur gases on virtually all types of terrestrial exoplanet atmospheres are short-lived; (2) oxygen and ozone may build up in 1-bar CO2 atmospheres to levels that have conventionally been accepted as unique signatures of life, if there is no surface emission of reducing gases; and (3) ammonia is a plausible biosignature gas in hydrogen-rich atmospheres on habitable planets.

In addition to thin atmospheres, I have extended the photochemistry model to the pressures at which thermochemical equilibrium can be effectively achieved, and developed a new photochemistry-thermochemistry model for non-hydrogen-dominated atmospheres on super-Earths. Using the model, I have summarized a 'zoo of super-Earths' - including water planets, hydrocarbon planets, and even oxygen planets - which depend on the C-H-O elemental abundances of their atmospheres. In particular, I found that 50% water vapor is not compatible with 50% CO or 50% CH_4 on super-Earth GJ 1214b, which implies that chemical stability has to be taken into account when seeking a fit to the spectrum of a super-Earth.