Atmospheric Biosignatures

Life has likely coevolved with the Earth system in time in various ways. Our oxygen-rich atmosphere and the protective ozone layer are mainly the result of photosynthetic activity. Additionally, bacteria emit greenhouse gases such as methane and nitrous oxide into the atmosphere, and vegetation can emit a variety of organic molecules. In an exoplanetary context, it is important to consider whether such gas-phase species – so-called atmospheric biosignatures – could be detected spectroscopically and attributed to extraterrestrial life. Another signature of life on Earth is the so-called redox disequilibrium of its atmosphere. This refers to the presence of simultaneously oxidizing and reducing species (e.g., molecular oxygen and methane). Without life, such species would react and be removed on relatively fast timescales. Since Earth’s atmosphere has changed considerably during its history, we will also consider atmospheric biosignatures in the context of the early Earth. This chapter will present a brief literature review of atmospheric biosignatures. We will discuss the main photochemical responses of such species in the modern and early Earth’s atmosphere and their potential to act as atmospheric biosignatures in an exoplanetary context