Photochemistry of Terrestrial Exoplanet Atmospheres - Planetary Exploration and Science: Recent Results and Advances - page 294

10 −1
10 0
10 1
10 2
10 3
10 4
10 5

H 2 H 2

H 2 O
CO 2

CO
CH 4

H 2

10 −1
10 0
10 1
10 2
10 3
10 4
10 5

N 2 N 2

H
OH
O 2
O
O 3

H 2 S

SO 2
H 2 SO 4 (A)

S
S 2
S 8 (A)
N 2

10 −1
10 0
10 1
10 2
10 3
10 4

CO 2

CO 2

10 5

10 −14 10 −12

10 −10 10 −8 10 −6 10 −4 10 −2 10 0 10 −14 10 −12 10 −10 10 −8 10 −6 10 −4 10 −2 10 0

Mixing Ratio Mixing Ratio

Fig. 12.3 Compositions of the benchmark scenarios of rocky exoplanet atmospheres. The left
column shows mixing ratios of H and O species, and the right column shows mixing ratios of N,
C, and S species. From top to bottom , the three panels correspond to the reducing (H 2 -dominated),
oxidized (N 2 -dominated), and highly oxidized (CO 2 -dominated) atmospheres. The vertical scales
are expressed in pressure, which allows comparison between different scenarios that have very
different mean molecular masses. Thick lines highlight the profiles of three reactive species, H,
OH, and O (Reproduced from Hu et al. 2012 with the permission of the AAS)

is likely to be the most abundant reactive radical in CO 2 -dominated atmospheres.
Due to the photochemical origin of the reactive species, their abundances in the
atmosphere around a quiet M dwarf star are 2 orders of magnitude lower than their
abundances around a Sun-like star, because a quiet M dwarf emits much less UV
radiation than a Sun-like star.
Second, dry deposition velocities of long-lived compounds, notably major vol-
canic carbon compounds including methane, carbon monoxide, and carbon dioxide,
have significant effects on the atmospheric oxidation states. The specific choice
of dry deposition velocities for emitted gases and their major photochemical by-
products in the atmosphere is critical to determine the atmospheric composition
and the redox power on terrestrial exoplanets (Hu et al. 2012 ). For example, if
the dry deposition velocity of CO were greater for a CO 2 -dominated atmosphere,
the steady-state mixing ratio of CO in the atmosphere would be lower, and
consequently, the atmosphere would have less H 2 and more O 2 .