Geoscience Reference
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Fig. 12.1
Architecture of the photochemistry-thermochemistry model for terrestrial exoplanet
atmospheres developed by Hu et al. (
2012
) and Hu and Seager (
2014
). The central equation to
solve is a continuity equation that has terms for the chemical and photochemical production and
loss, and terms for transport
The main components of a thin atmosphere of a terrestrial exoplanet result from
its long-term geological evolution. For example, the N
2
-O
2
atmosphere on Earth,
the CO
2
atmosphere on Mars, and the N
2
atmosphere on Titan are results of long-
term evolution (e.g., Kasting and Catling
2003
; Coustenis
2005
). For terrestrial
exoplanets, the main components of their thin atmospheres can only be determined
by observations, and the oxidation states of the thin atmospheres can range from
reducing (e.g., H
2
atmospheres), to oxidized (e.g., N
2
and CO
2
atmospheres), to
even oxic (O
2
atmospheres).
The photon-driven chemical reactions are especially important for thin atmo-
spheres. UV and some visible-wavelength photons can dissociate gases, produce
reactive radicals, and facilitate the conversion from emitted gases to photochemical
products. These processes are key for thin atmospheres because (1) ultraviolet
photons that cause photodissociation penetrate to the pressure levels of
0.1 bar,
relevant to the bulk part of a thin atmospheres (Yung and Demore
1999
;Hu
2013
),
and (2) in many cases, the photochemical processes in a thin atmosphere are irre-
versible. For example, the photochemical production of unsaturated hydrocarbons
and haze from CH
4
occurs in the upper atmosphere of Titan, and the photochemical
formation of C
2
H
6
is irreversible and is therefore the dominant sink for CH
4
on
Titan (Yung et al.
1984
). This is in contrast to Jupiter's atmosphere, in which the
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