Photochemistry of Terrestrial Exoplanet Atmospheres - Planetary Exploration and Science: Recent Results and Advances - page 289

The major mechanisms for vertical transport in an irradiated atmosphere on a
terrestrial exoplanet include convection (the same mechanism required to transport
heat), small-scale instability driven by shear of horizontal flows, and molecular
diffusion. The first two processes can be approximated by the so-called eddy
diffusion coefficients (e.g., Seinfeld and Pandis 2006 ), and the last process can be
approximated by the molecular diffusion coefficients. Therefore, the atmospheres
on terrestrial exoplanets can be treated as gravitationally stratified, plan-parallel
irradiated atmospheres, in which vertical mixing can be parameterized (Hu 2013 ).
Models for terrestrial exoplanet atmospheres can be developed to compute
the chemical reaction kinetics and transport processes. Such models are often
called “photochemistry-thermochemistry kinetic-transport model” or simply
“photochemistry-thermochemistry model.” The purpose of such models is to
provide a tool to predict the amounts of component gases in the atmospheres
of terrestrial exoplanets and in the meantime quantify the links between the
observables (e.g., abundances of trace gases and their spectral signatures) and the
fundamental unknowns (e.g., geological and biological processes on the planetary
surface, mixing and escape of atmosphere gases, heat sources from planetary
interior and exterior).
Photochemistry models have been successful in simulating the compositions of
the atmospheres of Earth (Seinfeld and Pandis 2006 ) and the atmospheres of planets
in our Solar System (Yung and Demore 1999 ). The photochemistry models are
also critical for the study of molecular compositions of any exoplanet atmosphere,
including the atmospheres of terrestrial exoplanets, because the composition of the
observable part of an exoplanet atmosphere (0.1 mbar to 1 bar, depending on the
wavelength) is controlled by the competition between chemical reaction kinetics
and transport. Figure 12.1 schematically shows the architecture of such a model
developed by Hu et al. ( 2012 ) and Hu and Seager ( 2014 ). A handful of other
photochemistry models are available (e.g., Kasting et al. 1985 ; Yung and Demore
1999 ; Liang et al. 2003 ; Atreya et al. 2006 ; Zahnle et al. 2009 ; Line et al. 2010 ;
Moses et al. 2011 , 2013 ), and these models solve the same continuity equation in
Fig. 12.1 and likely have similar architectures.
Eventually, the steady-state composition of a terrestrial exoplanet atmosphere
is controlled by the boundary conditions. The upper boundary conditions are
the fluxes of atmospheric escape or the material exchange fluxes between the
neutral atmospheres and the ionospheres (not modeled) above. The lower boundary
conditions depend on whether or not thermochemical equilibrium holds near the
lower boundary. One could therefore consider the following two categories of
atmospheres on terrestrial exoplanets: thin atmospheres and thick atmospheres. The
thick atmospheres are defined as the atmospheres that are thick enough to maintain
thermochemical equilibrium at high pressures, and the thin atmospheres are defined
as the atmospheres at the surface of which achieving thermochemical equilibrium
is kinetically prohibited. In the following we will focus on thin atmospheres
because the thin atmospheres are more akin to creating a potentially habitable
environment.