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photochemistry, is the main reactive species. One could expect that the atmospheric
composition of exoplanets will be highly varied, based on the nearly continuous
range of masses and orbits of exoplanets. In this large parameter space, the primary
dimension of chemical characterization for terrestrial exoplanet atmospheres is their
oxidation states.
The main components of the atmosphere in large part, and the surface emission
ad deposition of trace gases to a lesser extent, determine its oxidation power. In the
extreme cases of the atmospheric redox state, i.e., the H
2
-dominated atmospheres
and the O
2
-rich atmospheres, the atmospheric redox power is surely reducing or
oxidizing for a wide range of surface emission or deposition fluxes. However, for
an intermediate redox state, the atmosphere would be composed of redox-neutral
species like N
2
and CO
2
, and the redox power of the atmosphere can be mainly
controlled by the emission and the deposition fluxes of trace gases (i.e., H
2
,CH
4
,
and H
2
S) from the surface. For example, the higher the emission of reducing gases
is, the more reducing the atmosphere becomes.
Here we describe the benchmark scenarios proposed by Hu et al. (
2012
)for
reducing, weakly oxidizing, and strongly oxidizing atmospheres on an Earth-size
and Earth-mass habitable terrestrial exoplanet around a Sun-like star. The three
scenarios are a reducing (90 % H
2
-10 % N
2
) atmosphere, a weakly oxidizing
N2 atmosphere (>99 % N
2
), and a highly oxidizing (90 % CO
2
-10 % N
2
)
atmosphere. Hu et al. (
2012
) consider Earth-like volcanic gas emission rate and
composition that consists of CO
2
,H
2
,SO
2
,CH
4
,andH
2
S and assume that the
planet surface has a substantial fraction of its surface covered by a liquid water
ocean so that water is transported from the surface and buffered by the balance
of evaporation/condensation. Key nonequilibrium processes in these scenarios are
schematically shown in Fig.
12.2
, and the molecular composition of the three
benchmark scenarios is shown in Fig.
12.3
.
Summarizing the benchmark scenarios, several general chemistry properties of
thin atmospheres on habitable terrestrial exoplanets stand out. These properties are
results of physical structures of molecules, and how they interact, and therefore are
independent of detailed planetary scenarios (Hu et al.
2012
;Hu
2013
).
First, atomic hydrogen (H) is a more abundant reactive radical than hydroxyl
radical (OH) in anoxic atmospheres. Atomic hydrogen is mainly produced by water
vapor photodissociation (Hu et al.
2012
; Seager et al.
2013
). The production of
atomic hydrogen is catalyzed by water vapor:
H
2
O
C
hv
!
H
C
OH;
OH
C
H
2
!
H
2
O
C
H;
Net
W
H
2
C
hv
!
2H:
It is difficult to remove hydrogen once produced in anoxic atmospheres, which is
in contrast to oxygen-rich atmospheres (e.g., current Earth's atmosphere) in which
H can be quickly consumed by O
2
. As a result, removal of a gas by H is likely to be
an important removal path for trace gases in an anoxic atmosphere. Atomic oxygen
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