Photochemistry of Terrestrial Exoplanet Atmospheres - Planetary Exploration and Science: Recent Results and Advances - page 288

et al. 2014 ), and the super-Earth/mini Neptune GJ 1214 b is being observed in as
much detail as possible (e.g., Bean et al. 2010 ; Croll et al. 2011 ; Desert et al. 2011 ;
Berta et al. 2012 ; de Mooij et al. 2012 ; Kreidberg et al. 2014 ).
Besides spectral characterization, exoplanet atmospheres have also been studied
via the phase curves (e.g., Seager et al. 2000 ; Knutson et al. 2007 ), and Kepler has
made the first observation of a phase curve from a terrestrial exoplanet (Batalha
et al. 2011 ; Fogtmann-Schulz et al. 2014 ).

12.3 Physical and Chemical Processes in Terrestrial

Exoplanet Atmospheres

Central in the studies of terrestrial exoplanets is to characterize their atmospheres
and to search for potential biosignature gases, i.e., the atmospheric components that
indicate biogenic surface emissions. For this goal, a deep understanding of the key
physical and chemical processes that control the atmospheric composition is crucial.
One important process for terrestrial exoplanet atmospheres is chemical and
photochemical reactions. The network of chemical reactions in the atmosphere
may serve as sources for certain gases and sinks for the others. Chemical reactions
occur when two molecules collide, and the reaction rates are therefore proportional
to the number density of both molecules. Certain reactions would require a third
body in the collision to remove excess energy or angular momentum. The rates
of such termolecular reactions are therefore also dependent on the total number
density of the atmosphere. Near the top of the atmosphere, photon-driven reactions
contribute dominantly to the source and sink, as ultraviolet (UV) photons from the
parent star that could dissociate molecules usually penetrate to the pressure levels of
0.1 bar (e.g., Yung and Demore 1999 ;Huetal. 2012 ). The UV photodissociation
produces reactive radicals that facilitate some reactions that are otherwise kinetically
prohibited. A generic reaction network should include bimolecular reactions, ter-
molecular reactions, photodissociation reactions, and thermodissociation reactions
for the study of terrestrial exoplanet atmospheres (Hu 2013 ).
The other process that controls the compositions is transport. Both large-scale
mean flows and small-scale turbulence and instability can transport molecules in
the atmosphere and affect the composition (e.g., Brasseur and Solomon 2005 ;
Seinfeld and Pandis 2006 ). One could focus on the transport in the vertical direction
and explore the compositions of terrestrial exoplanet atmospheres as a function of
altitude. Altitude is the most important dimension because the temperature and
the pressure are strong functions of altitude. For example, the composition of
Earth's atmosphere is primarily a function of altitude instead of longitude or latitude
(Seinfeld and Pandis 2006 ). Also, the vertically resolved compositions are critical
for prediction and interpretation of spectra of a terrestrial exoplanet, because the
spectra probe different altitudes of the atmosphere depending on the wavelength
(Seager 2010 ).