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 20.09.2007

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

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, …

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 0.5 - 1.6 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

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

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)

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

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

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

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)

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)

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

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

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

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

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

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

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

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

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

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

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)