  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.
