Evolution of primary planetary atmospheres Philip von Paris 1), John Lee Grenfell 1), Pascal Hedelt 1), Heike Rauer 1,2), Barbara Stracke 1) 1) Institut für Planetenforschung, DLR Berlin 2) Zentrum für Astronomie und Astrophysik, TU Berlin

Characterization of Terrestrial Exoplanets -Satellite missions are on-going or planned to look for small, rocky planets and characterize their atmospheres: CoRoT, Kepler, Darwin/TPF - Spectral signatures might be indicative of a biosphere on a terrestrial planet - Atmospheric modeling helps in mission design and data interpretation - Terrestrial planets are expected to be found at different ages: Models needed to track the atmosphere in the course of planetary evolution CNES ESA

The faint young Sun „paradoxon“ The young Sun was less bright than today. - Surface temperatures below 273 K before 2 Gyr ago if greenhouse effect was at present level (i.e., ΔT ~ 30 K) But: - Geological hints for liquid water as early as 4 Gyr ago (e.g., Mojzsis et al. 1996, Rosing & Frei 2004) Dashed line: Gough (1981)Plain line: Caldeira and Kasting (1992) r

Proposed solutions to the „paradoxon“ Atmospheric composition changed since the first primitive atmosphere, hence the greenhouse effect was more pronounced. Several greenhouse gases have been proposed: GasReferenceSource AmmoniaSagan & Mullen 1972, Sagan & Chyba 1997 Biology, photochemistry Carbon dioxide Kasting et al. 1984, Kasting 1987 Outgassing MethanePavlov et al. 2000Outgassing, methanogenes EthaneHaqq-Misra et al. 2007Photochemistry

Problems with the proposed solutions - Ammonia: Rapid photolytic destruction, UV shielding via haze formation in an anoxic atmosphere: model results not clear ( Sagan & Chyba 1997 Pavlov et al. 2001) - Carbon dioxide: Sediment data sets upper limits on partial pressure, much less than needed in model studies (Rye et al. 1995, Hessler et al. 2004) - Methane: Outgassing rates and biogenic production not well determined (Pavlov et al Kharecha et al. 2005), dominating photochemical sink not well established - Ethane: Formation of needed hydrocarbon haze dependent on ratio between methane and carbon dioxide

This work: Model description Type: 1D radiative-convective model for temperature and water profiles Based on Kasting (1984,1988), Pavlov et al. (2000) and Segura et al. (2003): Temperature profile: from radiative equilibrium and convective adjustment Water profile: from relative humidity distribution (Manabe & Wetherald 1967)

IR-Fluss Stratosphäre Strahlungsgleichgewicht, F aus Strahlungstransportgl. Troposphäre trocken- oder feuchtadiabatisches T-Profil stellarer Fluss KlimaChemie biogene-Flüsse Chemisches Reaktions- netzwerk berücksichtigt 55 Spezies, 220 Reaktionen T, p Profil Profile chemischer Konzentrationen

New in our model: Adapted IR radiation transfer modeling (MRAC) to better simulate arbitrary terrestrial atmospheres ( based on RRTM model, Mlawer et al. (1997)): - New spectral (added 1-3µm), temperature (include T<150 K) and pressure (up to 10 bar) coverage included - CO 2 continuum absorption as opacity source in IR - k-distributions recalculated for CO 2 -enhanced background atmosphere with 5% CO 2 and 95% N 2 to include line broadening by carbon dioxide This work: Model description

Density function Distribution function Absorption coefficient k(v) Net Flux

Validation of modified radiation scheme Validation of k-distributions for normal air background, i.e. modern Earth Example: CO 2 fundamental band at 15µm RRTM (Mlawer et al. 1997) MRAC (This work)

Validation of modified radiation scheme Surface up to mid-stratosphere: excellent agreement Mid-stratosphere to upper mesosphere: slight disagreement due to extrapolation errors for RRTM (yielding negative optical depths) Validation of temperature profiles calculated with RRTM and MRAC Example: present Earth, 1bar atmosphere, 78% nitrogen, 21% oxygen, 1% argon, 355ppm CO 2, other gases (ozone, methane and nitrous oxide) removed RRTM (Mlawer et al. 1997) MRAC (This work) Convective regime

Importance of T-grid for absorption coefficients For shown validation run: Calculated temperatures outside RRTM temperature grid  extrapolation is performed  This yields negative absorption cross sections, in contrast to interpolation in MRAC RRTMMRAC Tabulated values Calculated values „True“ values

Vary Earth age Solar constants of 0.70,0.75, 0.80, 0.85, 0.90, 0.95 (equivalent to times 4.6 – 0.1 Gyr ago)  Constant nitrogen background pressure of 0.77 bar  Add carbon dioxide until desired surface temperature T surf is reached This work: Model runs RunsT surf /K

The structure of early Earth atmospheres differs from the present one: (i) The tropopause moves closer to the surface (ii) Cold trap no longer associated with a temperature inversion (iii) Tropopause, cold trap and temperature inversion no longer at same altitudes Results: Atmospheric structure Temperature inversion Convective regime Cold trap S=0.8 S=0.85 S=0.8 S=0.85

Results: Keeping the surface warm Minimal CO 2 concentrations for 273 K (lower plain line) and 288 K (upper plain line), for comparison: values from Kasting (1987) for 273 K (dashed line) Upper limits on CO 2  /  (for 2 approximations of solar luminosity) Late Archaean: values of 3-4 mb compatible T surf = 273 K, from Kasting (1987) T surf = 273 K T surf = 288 K

- Much less CO 2 needed to keep surface of early Earth above 273K - Calculated amount of CO 2 (3-4 mbar) for the late Archaean and early Proterozoic is compatible with palaesol records, contrary to previous studies - Atmospheric structure very different from today‘s structure - Outlook: Model different evolutionary stages in the future Conclusions