P. Hedelt1, H. Rauer1,2, L. Grenfell1, B. Stracke1, R. Titz1, P. von Paris1

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Spectral appearance of terrestrial exoplanets

• P. Hedelt1, H. Rauer1,2, L. Grenfell1, B.
  Stracke1, R. Titz1, P. von Paris1
• 1 Institut für PlanetenforschungDeutsches
  Zentrum für Luft- und Raumfahrt e.V. (DLR)
• 2 Zentrum für Astronomie und AstrophysikTechnisch
  e Universität Berlin

20.09.2007
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Aims Scope

• Future satellite missions (DARWIN, TPF) will
  detect terrestrial exoplanets
• Emission/transmission spectra will be measured
• An Earth twin is highly unlikely
• need for a parameter study to
• classify terrestrial exoplanets
• help in examining atmospheres
• explicitly search for biomarker signals
• define suitable wavelength intervals for future
  missions
• ? Construction of a spectral catalogue

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Parameter-Variation

• Surface pressure ? Mass of atmosphere
• Gravitation ? Mass Radius of planet
• Atmospheric composition
• Age
• Stellar luminosity
• Distance to star
• Stellar type
• 7 dimensional parameter space - other planetary
  and geological parameters not yet considered
• Vegetation, Land/ocean distribution, Rotation,
  Ice coverage,

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Parameters considered so far
Literature Spectral evolution of an earthlike
planet Kaltenegger et al. 2007 Spectra for
earthlike planets around different types of
stars Segura et al. 2003 Spectra for varied
abundances, using fixed T profile Des Marais et
al. 2002 This work
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Atmospheric Models

• 1D coupled photochemical, radiative-convective
  model (Segura et al. 2003, Grenfell et al. 2007)
• IR radiative transfer scheme RRTM (Rapid
  Radiative Transfer Model, Mlawer et al. 1997)
• Validated for present earth conditions
• 1D radiative-convective model
• IR radiative transfer scheme MRAC (Modified
  RRTM for Application in CO2-dominated
  atmospheres, von Paris et al., 2007, submitted)
• Validated for dense atmospheres, high CO2

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Radiative Transfer Model

• SQuIRRL (Schreier and Böttger, 2001)Schwarzschi
  ld Quadrature InfraRed Line-by-line
• Line-by-line radiative transfer model
• Cloud and haze free
• No scattering
• Assumes LTE
• HITRAN/HITEMP-Database (Rothman et al., 2005)

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Pressure variationN2/O2 dominated atmospheres
T inversion 10 bar
T inversion 1 bar
Ozone layer becomes thicker T inversion layer
moves upwards
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Pressure variationN2/O2 dominated atmospheres
Coupled photochemical, radiative-convective
model 78 N2, 21 O2, 355 ppm CO2
CO2 4.3 µmH2O 6.3 µm CH4 7.7 µm
O3 8.8 µm O3 9.6 µm CO2 10 µm
O3 13.9 µm CO2 15 µm H2O rotation
1 bar 5 bar 10 bar
O3 not well-mixed
CH4 masked

• Temperature inversion
• O3 not well-mixed

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Pressure variation N2/O2 dominated atmospheres
Coupled photochemical, radiative-convective
model 78 N2, 21 O2, 355 ppm CO2
O3
O3 CO2
1 bar 5 bar10 bar

• Ozone distribution determineable using 9.6µm and
  14µm band
• Temperature structure determinable using 9.6µm
  and 15µm band
• 15µm CO2 band not sensitive to pressure
• Ozone visible in 8.8µm band at high surface
  pressure

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Stellar Type Variation N2/O2 dominated
atmospheres
F2V G2V K2V
F2V More O3 in stratospheredue to increased O2
dissociation
F2V Higher T in stratosphere K2V Weak
inversion layer
For more details see Grenfell et al. (2007)
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Stellar Type Variation N2/O2 dominated
atmospheres
O3 9.6µm
Coupled photochemical, radiative- convective
model 78 N2, 21 O2, 355 ppm CO2
Radiance W/m2/sr/µm
F2V G2V K2V
Wavelength µm
? Model validated with Segura et al. (2003)
Segura et al. (2003)
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Stellar Type Variation N2/O2 dominated
atmospheres
CO2 15µm
Coupled photochemical, radiative- convective
model 78 N2, 21 O2, 355 ppm CO2
Segura et al. (2003)
F2V G2V K2V
? Effect of temperature inversion
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Gravity variation N2/O2 dominated atmospheres
Ansatz Fixing surface pressure, increasing g
psurf 1 bar, no O3
0.2 g 1 g 3.9 g
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Gravity variation N2/O2 dominated atmospheres
Radiative-convective model 1 bar 78 N2, 21
O2, 355 ppm CO2
0.2 g 1 g 3.9 g
H2O 6.3 µm band
CO2 4.3 µm
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Gravity variation N2/O2 dominated atmospheres
Radiative-convective model 1 bar 78 N2, 21
O2, 355 ppm CO2
0.2 g 1 g 3.9 g
CO2 15 µm band

• 15µm CO2 absorption not sensitive to gravity /
  atmospheric mass

Weak temperature inversion
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Pressure and CO2 variation CO2 dominated
atmospheres Early Mars
1 bar, 75 CO2 1 bar, 90 CO2 1 bar, 95 CO2
3 bar, 75 CO2 2 bar, 75 CO2 1 bar, 75 CO2
? Weak effect on temperature structure
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Mixing ratio variation CO2 dominated
atmospheres Early Mars
Radiative-convective model
CO2 15 µm band
75 CO2, 1 bar 95 CO2, 1 bar
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Pressure variation CO2 dominated atmospheres
Early Mars
Radiative-convective model
CO2 15 µm band
75 CO2, 1 bar 75 CO2, 2 bar

• 15µm CO2 absorption not sensitive to mixing
  ratio
• 15µm CO2 absorption sensitive to surface
  pressure
• Temperature structure visible in 15µm band

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Summary

• N2/O2 dominated atmospheres
• Temperature information from 9.6µm and 15µm band
• Ozone distribution visible in 14µm band
• Ozone visible in 8.8µm band at high surface
  pressures
• Ozone distribution clearly visible in 9.6µm band
• CO2 dominated atmospheres
• Saturated lines prevent CO2 mixing ratio
  determination
• Surface pressure visible in 15µm band

Need for high resolution measurements in more
than one absorption band to characterize the
atmosphere of a terrestrial planet
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Future work

• Search for sensitive lines/absorption bands
• Construct catalogue
• Apply photochemistry models to all calculations
• Vary more parameters

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N2/O2 dominated atmospheresPressure variation
spectrum
Coupled photochemical, radiative-convective
model 78 N2, 21 O2, 355 ppm CO2
CO2 4.3 µmH2O 6.3 µm CH4 7.7 µm
O3 8.8 µm O3 9.6 µm CO2 10 µm
O3 14 µm CO2 15 µm H2O rotation
1 bar 5 bar 10 bar
T inversion 10 bar
T inversion 1 bar
O3 not well-mixed
CH4 masked

• Temperature inversion
• CO2 well-mixed
• O3 not well-mixed

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N2/O2 dominated atmospheresStellar Type
Variation - spectrum
O3 9.6µm
Coupled photochemical, radiative- convective
model 78 N2, 21 O2, 355 ppm CO2
Radiance W/m2/sr/µm
F2V G2V K2V
Segura et al. (2003)
Wavelength µm
For more details see Grenfell et al. (2007)