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Figure
12.4
shows that O
3
can potentially build up in the 1-bar CO
2
-dominated
atmosphere to a false positive level even on a planet with active hydrological cycle.
Segura et al. (
2007
) have based their conclusion on simulations of 20 % CO
2
1-bar
atmospheres with and without emission of H
2
and CH
4
and simulations of 2-bar
CO
2
atmospheres with emission of H
2
and CH
4
. Where Hu et al. (
2012
) models
differ from Segura et al. (
2007
)isthatHuetal.(
2012
) model successfully simulated
90 % CO
2
1-bar atmospheres with minimal volcanic reducing gas emission. This is
a parameter space that Segura et al. (
2007
) did not cover, but this is the parameter
space for high abiotic O
2
. Recently, it has further been found that around M dwarf
stars that have low near-UV radiation and strong far-UV radiation, O
2
produced
from CO
2
photodissociation is even easier to build up in the atmosphere (Tian et al.
2014
).
Therefore one should exercise caution to use spectral features of O
2
as a probe of
oxygenic photosynthesis on a terrestrial exoplanet. The risk of such false positives
would affect the inference of photosynthesis via O
2
features detected in the visible
wavelengths, potentially by either the Terrestrial Planet Finder - Coronagraph (e.g.,
Beichman et al.
2006
) or the cross-correlation method applied to high-resolution
spectroscopy on the 40-m class telescopes (Snellen et al.
2013
). The risk of false
positive is however not relevant to the detection of O
3
(a photochemical derivative of
O
2
) features in the mid-infrared, because the O
3
feature would be masked by strong
CO
2
features and therefore not detectable for CO
2
-dominated atmospheres (Selsis
et al.
2002
). Eventually, detecting both O
2
features and CH
4
features may mitigate
the risk of false positive. A methane mixing ratio of
10 ppm would imply a surface
source of reducing gases that could prevent the abiotic buildup of O
2
(Fig.
12.4
).
12.6
Prospect of Terrestrial Exoplanet Characterization
Determination of the atmospheric compositions on terrestrial exoplanets is one
of the most significant challenges facing astronomers. To achieve this goal, both
advanced observational techniques and suitable targets are required (e.g., Deming
et al.
2009
). The ratio between the radiation from a terrestrial exoplanet and that
from its parent star is in the orders of 10
10
in the visible wavelengths and 10
7
in
the mid-infrared wavelengths. This means in the transit scenario, many observations
of transits need to be stacked to lower the noise level in order to reveal the planet's
signal (e.g., Seager and Deming
2010
; Kreidberg et al.
2014
), and in the direct
imaging scenario, the stellar radiation has to be almost perfectly annihilated in
order to reveal the planet (e.g., Kuchner and Traub
2002
; Lawson and Dooley
2005
; Trauger and Traub
2007
). Even with advanced observational techniques,
atmospheric characterization of terrestrial exoplanets will be confined to nearby
systems (e.g., within tens of parsecs; Guyon et al.
2006
; Oppenheimer and Hinkley
2009
;Beluetal.
2011
).
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