J/ApJ/761/166 Terrestrial exoplanet atmospheres. I. (Hu+, 2012)
Photochemistry in terrestrial exoplanet atmospheres.
I. Photochemistry model and benchmark cases.
Hu R., Seager S., Bains W.
<Astrophys. J., 761, 166 (2012)>
=2012ApJ...761..166H 2012ApJ...761..166H
ADC_Keywords: Atomic physics ; Models, atmosphere ; Planets
Keywords: astrobiology; atmospheric effects; radiative transfer;
planetary systems; techniques: spectroscopic
Abstract:
We present a comprehensive photochemistry model for exploration of the
chemical composition of terrestrial exoplanet atmospheres. The
photochemistry model is designed from the ground up to have the
capacity to treat all types of terrestrial planet atmospheres, ranging
from oxidizing through reducing, which makes the code suitable for
applications for the wide range of anticipated terrestrial exoplanet
compositions. The one-dimensional chemical transport model treats up
to 800 chemical reactions, photochemical processes, dry and wet
deposition, surface emission, and thermal escape of O, H, C, N, and S
bearing species, as well as formation and deposition of elemental
sulfur and sulfuric acid aerosols. We validate the model by computing
the atmospheric composition of current Earth and Mars and find
agreement with observations of major trace gases in Earth's and Mars'
atmospheres. We simulate several plausible atmospheric scenarios of
terrestrial exoplanets and choose three benchmark cases for
atmospheres from reducing to oxidizing. The most interesting finding
is that atomic hydrogen is always a more abundant reactive radical
than the hydroxyl radical in anoxic atmospheres. Whether atomic
hydrogen is the most important removal path for a molecule of interest
also depends on the relevant reaction rates. We also find that
volcanic carbon compounds (i.e., CH4 and CO2) are chemically
long-lived and tend to be well mixed in both reducing and oxidizing
atmospheres, and their dry deposition velocities to the surface
control the atmospheric oxidation states. Furthermore, we revisit
whether photochemically produced oxygen can cause false positives for
detecting oxygenic photosynthesis, and find that in 1 bar CO2-rich
atmospheres oxygen and ozone may build up to levels that have
conventionally been accepted as signatures of life, if there is no
surface emission of reducing gases. The atmospheric scenarios
presented in this paper can serve as the benchmark atmospheres for
quickly assessing the lifetime of trace gases in reducing, weakly
oxidizing, and highly oxidizing atmospheres on terrestrial exoplanets
for the exploration of possible biosignature gases.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 139 848 Reaction rates of bi-molecular reactions (R),
ter-molecular reactions (M), and
thermo-dissociation reactions (T) in the
photochemistry model
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See also:
J/A+A/539/A28 : New transit phot. for super-Earth 55 Cnc e (Gillon+, 2012)
J/ApJ/747/35 : HST/WFC3 transit observation of GJ1214b (Berta+, 2012)
J/ApJ/745/77 : Photochemical model for planet WASP-12b (Kopparapu+, 2012)
J/A+A/533/A114 : Transit of super-Earth 55 Cnc e (Demory+, 2011)
J/A+A/528/A111 : GJ3634 radial velocity and 4.5um flux (Bonfils+, 2011)
J/ApJ/736/19 : Kepler planetary candidates. II. (Borucki+, 2011)
J/ApJ/708/1366 : Radial velocities for 61 Vir (Vogt+, 2010)
J/A+A/493/645 : Gl 176 radial velocities (Forveille+, 2009)
J/A+A/493/639 : Velocity curves of HD 40307 (Mayor+, 2009)
J/A+A/469/L43 : Radial velocities of Gl 581 (Udry+, 2007)
http://kinetics.nist.gov/kinetics : NIST Chemical Kinetics database
http://jpldataeval.jpl.nasa.gov/ : JPL data evaluation
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 4 A4 --- Model Model number (1)
6- 19 A14 --- Reactant Reactants
21- 36 A16 --- Product Products
38- 89 A52 --- RRate Reaction rate (2)
91-112 A22 --- Ref Reference (4)
114-117 I4 K Tmin ? Minimum of temperature range RRate is valid
118 A1 --- --- [-]
119-122 I4 K Tmax ? Maximum of temperature range RRate is valid
124-139 A16 --- Note Additional note (3)
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Note (1): Model identification as follows:
R = bi-molecular reaction;
M = ter-molecular reaction;
T = thermo-dissociation reaction.
Note (2): The unit of the reaction rate is cm3/s for bi-molecular reactions,
cm6/s for ter-molecular reactions, and s-1 for thermo-dissociation
reactions. The unit of the temperature T is K and the unit of the
total number density N is cm-3.
Note (3): We annotate "Low Temperature" and "High Temperature" to the reaction
rates only valid at temperatures lower than 300K and higher than
1000K, respectively. For low-temperature applications, including
explorations of Solar-System terrestrial planets and habitable
exoplanets, we exclude those reactions marked as "High Temperature".
Note (4): References are:
JPL = Jet Propulsion Laboratory (NASA)
NIST = National Institute of Standards and Technology
Kasting (1990) = "Bolide impacts and the oxidation state of carbon in
the earth's early atmosphere" (1990OLEB...20..199K 1990OLEB...20..199K )
Moses et al. (2002) = "Photochemistry of a Volcanically Driven Atmosphere
on Io" (2002Icar..156...76M 2002Icar..156...76M)
Turco et al. (1982) = "Stratospheric Aerosols: Observation and Theory"
(1982RvGeo..20..233T 1982RvGeo..20..233T)
Yung and Demore (1999) = "Photochemistry of planetary atmospheres"
Oxford University Press (1999ppa..conf.....Y)
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History:
From electronic version of the journal
References:
Hu et al. Paper II. 2013ApJ...769....6H 2013ApJ...769....6H
Hu & Seager Paper III. 2014ApJ...784...63H 2014ApJ...784...63H
(End) Greg Schwarz [AAS], Emmanuelle Perret [CDS] 22-Aug-2014