Photochemistry of Terrestrial Exoplanet Atmospheres - Planetary Exploration and Science: Recent Results and Advances - page 295

Third, volcanic carbon compounds (i.e., CH 4 and CO 2 ) are chemically long-lived
and tend to be well mixed in terrestrial exoplanet atmospheres, whereas volcanic
sulfur compounds (i.e., H 2 SandSO 2 ) are short-lived (Fig. 12.3 ). CH 4 and CO 2
have chemical lifetime longer than 10,000 years in all three benchmark atmospheres
ranging from reducing to oxidizing, implying that a relatively small volcanic input
can result in a high steady-state mixing ratio. The chemical lifetime CO, another
possible volcanic carbon compound, ranges from 0.1 to 700 years depending on the
OH abundance in the atmosphere. Unlike carbon compounds, both H 2 SandSO 2 are
chemically short-lived in virtually all types of atmospheres on terrestrial exoplanets
(Hu et al. 2013 ). This implies that the carbon compounds are more likely to be
spectroscopically detected than the sulfur compounds. In particular, direct detection
of surface sulfur emission is unlikely, as their surface emission rates need to be
extremely high (>1,000 times Earth's volcanic sulfur emission) for these gases to
build up to a detectable level. Sulfur compounds emitted from the surface will lead
to photochemical formation of elemental sulfur and sulfuric acid in the atmosphere,
which would condense to form aerosols if saturated.

12.5 Is O 2 a Biosignature Gas?

Oxygen and ozone are the most studied biosignature gases for terrestrial exoplanet
characterization, due to their biogenic origin on Earth and their strong spectral
features at visible and infrared wavelengths (e.g., Angel et al. 1986 ; Leger et al.
1993 , 1996 ; Beichman et al. 1999 ; Snellen et al. 2013 ). When we consider using
O 2 /O 3 as biosignature gases, a natural question is whether O 2 may be produced
without involving life. Indeed, photodissociation of H 2 OandCO 2 produce free
oxygen in the atmosphere (Fig. 12.3 ).
The abiotic production of oxygen in terrestrial atmospheres has been studied
either for understanding prebiotic Earth's atmosphere (e.g., Walker 1977 ; Kasting
et al. 1979 ; Kasting and Catling 2003 ) or for assessing whether abiotic oxygen
can be a false positive for detecting photosynthesis on habitable exoplanets (Selsis
et al. 2002 ; Segura et al. 2007 ;Huetal. 2012 ; Tian et al. 2014 ; Wordsworth and
Pierrehumbert 2014 ). Selsis et al. ( 2002 ) found that photochemically produced
oxygen may build up in CO 2 -dominated atmospheres, if there is no surface emission
or deposition. The results of Selsis et al. ( 2002 ) was later challenged by Segura et al.
( 2007 ), who had additionally considered the surface emission of reducing gases
including H 2 and CH 4 , and found that abiotic oxygen would not build up in the
atmosphere on a planet having active hydrological cycle.
Hu et al. ( 2012 ) first pointed out that the steady-state number density of O 2
and O 3 in the CO 2 -dominated atmosphere is mainly controlled by the surface
emission of reducing gases such as H 2 and CH 4 , and without surface emission of
reducing gas, photochemically produced O 2 can build up in a 1-bar CO 2 -dominated
atmosphere. Figure 12.4 shows the simulated CO 2 -dominated atmospheres with
relatively low and zero emission rates of H 2 and CH 4 .TheO 2 mixing ratio near