Photochemistry of Terrestrial Exoplanet Atmospheres - Planetary Exploration and Science: Recent Results and Advances - page 291

photochemically formed C 2 H 6 is converted back to CH 4 in deep atmosphere via
pyrolysis (Strobel 1969 , 1973 ; Gladstone et al. 1996 ).
The fundamental parameters that define a thin atmosphere are the surface source
(i.e., emission rates), the surface sink (i.e., deposition velocities of emitted gases
and their photochemical products in the atmosphere) of trace gases (Yung and
Demore 1999 ; Seinfeld and Pandis 2006 ), and, in some cases, atmospheric loss
to space. A photochemistry model, when applied to thin atmospheres, is to seek
steady-state mixing ratios for trace gases of interest that are either emitted from
the surface or produced by the chemical network in the atmosphere. The amounts
of these trace gases are eventually controlled by the mass exchange between the
surface and the atmosphere. It is important to study the amounts of trace gases
by photochemistry models because an atmospheric spectrum may have strong
features from spectroscopically active trace gases whose lifetime is controlled by
the full chemical network in the atmosphere, and some of these trace gases may be
hallmarks for specific atmospheric scenarios (e.g., Hu 2013 ).
The fact that the surface emission and deposition control the steady-state mixing
ratios of trace gases in thin atmospheres on terrestrial exoplanets is pivotal to the
ultimate goal of characterizing terrestrial exoplanets that might harbor life. Potential
metabolic activities on a rocky planet emit a gas to the atmosphere that is otherwise
not emitted or consume a gas that is otherwise not consumed. Both processes occur
on Earth and regulate the composition of Earth's atmosphere. For example, Earth-
based photosynthesis leads to the emission of O 2 that sustains a high O 2 mixing
ratio in Earth's atmosphere, and Earth-based hydrogen-oxidizing bacteria provide
an appreciable deposition velocity for H 2 from the atmosphere to the surface (e.g.,
Kasting and Catling 2003 ; Seinfeld and Pandis 2006 ). The photochemistry model
provides the interface between the observables (atmospheric compositions) and the
fundamental unknowns (surface source and sinks that may or may not be attributed
to life), and the photochemistry model is therefore critical for determining the
habitability of a terrestrial exoplanet and investigating whether a habitable planet
is inhabited.

12.4 Exoplanet Benchmark Scenarios

Several benchmark scenarios can be set up for the atmospheres of terrestrial
exoplanets (Hu et al. 2012 ). The goal of these benchmark scenarios is to provide
baseline models to assess the stability of molecules in different kinds of atmospheres
in order to calculate the lifetime of spectrally significant gases and, in particular, the
lifetime of potential biosignature gases.
The most important parameter that determines the molecule lifetimes is the
oxidation power of the atmosphere - the ability to reduce or oxidize a gas in the
atmosphere. The main reactive species in the atmosphere provide the oxidizing or
reducing power. In an oxidizing atmosphere, OH and O are created by photochem-
istry and are the main reactive radicals. In a reducing atmosphere, H, also created by