1. Spectral appearance of terrestrial exoplanets
P. Hedelt1, H. Rauer1,2, L. Grenfell1, B. Stracke1, R. Titz1, P. von Paris1
1 Institut für Planetenforschung Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR)
2 Zentrum für Astronomie und Astrophysik Technische Universität Berlin

2. 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

3. 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, …

4. 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:
Background
atmosphere
Surface
Pressure
Gravity [g]
Composition
Stellar parameters
a [AU]
Age [Gyr] 1
N2/O2
1 – 10 bar
Earthlike
S0, G2
Present 2
1 bar
F2/G2/K2 3
0.2 – 3.9 4
CO2
1 – 2 bar
0.38
75% CO2
0.75 S0, G2
1.52
3.8 5
75% -95% CO2

5. 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

6. Radiative Transfer Model
SQuIRRL (Schreier and Böttger, 2001): „Schwarzschild 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)

7. Pressure variation: N2/O2 dominated atmospheres
1 bar
2 bar
5 bar
10 bar
T inversion: 10 bar
T inversion: 1 bar
Ozone layer becomes thicker
T inversion layer moves upwards

8. Pressure variation N2/O2 dominated atmospheres:
Coupled photochemical, radiative-convective model; 78% N2, 21% O2, 355 ppm CO2
CO2: 4.3 µm H2O: 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
CH4 masked
O3 not well-mixed
Temperature inversion
O3 not well-mixed

9. Pressure variation N2/O2 dominated atmospheres:
Coupled photochemical, radiative-convective model; 78% N2, 21% O2, 355 ppm CO2 O3
O3 + CO2
1 bar 5 bar 10 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

10. Stellar Type Variation N2/O2 dominated atmospheres:
F2V
G2V
K2V
F2V: More O3 in stratosphere due to increased O2 dissociation
F2V: Higher T in stratosphere
K2V: Weak inversion layer
For more details see Grenfell et al. (2007)

11. Stellar Type Variation N2/O2 dominated atmospheres:
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)

12. Stellar Type Variation N2/O2 dominated atmospheres:
CO2: 15µm
Coupled photochemical, radiative-
convective model;
78% N2, 21% O2, 355 ppm CO2
F2V
G2V
K2V
Segura et al. (2003)
 Effect of temperature inversion

13. 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

14. Gravity variation N2/O2 dominated atmospheres:
Radiative-convective model; 1 bar; 78% N2, 21% O2, 355 ppm CO2
CO2: 4.3 µm
H2O: 6.3 µm band
: 0.2 g
: 1 g
: 3.9 g

15. Gravity variation N2/O2 dominated atmospheres:
Radiative-convective model; 1 bar; 78% N2, 21% O2, 355 ppm CO2
CO2 15 µm band
0.2 g
1 g
3.9 g
15µm CO2 absorption not sensitive to gravity / atmospheric mass
Weak temperature inversion

16. 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

17. Mixing ratio variation CO2 dominated atmospheres: Early Mars
Radiative-convective model
CO2 15 µm band
75% CO2, 1 bar
95% CO2, 1 bar

18. 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

19. 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

20. Future work Search for sensitive lines/absorption bands
Construct catalogue
Apply photochemistry models to all calculations
Vary more parameters

21. N2/O2 dominated atmospheres: Pressure variation – spectrum
Coupled photochemical, radiative-convective model; 78% N2, 21% O2, 355 ppm CO2
1 bar
2 bar
5 bar
10 bar
CO2: 4.3 µm H2O: 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
CH4 masked
O3 not well-mixed
Temperature inversion
CO2 well-mixed
O3 not well-mixed

22. N2/O2 dominated atmospheres: Stellar Type Variation - spectrum
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)