Biomarkers set in context
In a famous paper, Sagan et al.(1993) analyzed a spectrum of the Earth taken by the Galileo probe, searching for signatures of life. They concluded that the large amount of O2 and the simultaneous presence of CH4 traces are strongly suggestive of biology. The detection of a widespread red-absorbing pigment with no likely mineral origin supports the hypothesis of biophotosynthesis. The search for signs of life on possibly very different planets implies that we need to gather as much information as possible in order to understand how the observed atmosphere physically and chemically works. The Earth-Sun intensity ratio is about 10^{-7} in the thermal infrared (10 micrometer), and about 10^{-10} in the visible (0.5 micrometer). The interferometric systems suggested for Darwin and the Terrestrial Planet Finder Interferometer (TPF-I) mission operates in the mid-IR (5 - 20 micrometer), the coronagraph suggested for Terrestrial Planet Finder Coronagraph (TPF-C) in the visible (0.5 - 1 micrometer). For the former it is thus the thermal emission emanating from the planet that is detected and analyzed while for the later the reflected stellar flux is measured. The spectrum of the planet can contain signatures of atmospheric species that are important for habitability, like CO2 and H2O, or result from biological activity (O2, O3, CH4, and N2O). Both spectral regions contain atmospheric bio-indicators. The presence or absence of these spectral features will indicate similarities or differences with the atmospheres of terrestrial planets and are discussed in detail and set into context with the physical characteristics of a planet in this chapter.