Evolution of primary planetary atmospheres
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Transcript of Evolution of primary planetary atmospheres
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Evolution of primary planetary atmospheres
Philip von Paris1), John Lee Grenfell1), Pascal Hedelt1), Heike Rauer1,2), Barbara Stracke1)
1)Institut für Planetenforschung, DLR Berlin2)Zentrum für Astronomie und Astrophysik, TU Berlin
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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
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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
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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:
Gas Reference Source
Ammonia Sagan & Mullen 1972, Sagan & Chyba 1997
Biology, photochemistry
Carbon dioxide Kasting et al. 1984, Kasting 1987
Outgassing
Methane Pavlov et al. 2000 Outgassing, methanogenes
Ethane Haqq-Misra et al. 2007 Photochemistry
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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. 2003 <-> Kharecha et al. 2005), dominating photochemical sink not well established
- Ethane: Formation of needed hydrocarbon haze dependent on ratio between methane and carbon dioxide
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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)
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IR-Fluss
Stratosphäre Strahlungsgleichgewicht,
F aus Strahlungstransportgl.
Troposphäre trocken- oder feuchtadiabatisches T-Profil
stellarer Fluss
Klima Chemie
biogene-Flüsse
Chemisches Reaktions-netzwerk
berücksichtigt 55 Spezies, 220 Reaktionen
T, p Profil Profile chemischer Konzentrationen
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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
- CO2 continuum absorption as opacity source in IR
- k-distributions recalculated for CO2-enhanced background atmosphere with 5% CO2 and 95% N2 to include line broadening by carbon dioxide
This work: Model description
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Density function
Distribution function
Absorption coefficient k(v) Net Flux
Net Flux
2
1
)(
dkFFnet
1
0
)( dgkFF gnet
0
)( kndkkf
k
dkkfkg0
')'()(
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Validation of modified radiation scheme
Validation of k-distributions for normal air background, i.e. modern Earth
Example: CO2 fundamental band at 15µm
RRTM
(Mlawer et al. 1997)
MRAC
(This work)
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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 CO2, other gases (ozone, methane and nitrous oxide) removed
RRTM
(Mlawer et al. 1997)
MRAC
(This work)
Convective regime
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Importance of T-grid for absorption coefficients
For shown validation run:
Calculated temperatures outside RRTMtemperature grid
extrapolation is performed
This yields negative absorption cross sections, in contrast to interpolation in MRAC
RRTMMRACTabulated values
Calculated values
„True“ values
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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 Tsurf isreached
This work: Model runs
Runs Tsurf/K
1-6 273
7-12 288
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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 samealtitudes
Results: Atmospheric structure
Temperature inversion
Convective regime
Cold trap
S=0.8
S=0.85
S=0.8
S=0.85
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Results: Keeping the surface warm
Minimal CO2 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 CO2 /
(for 2 approximations of solar luminosity) Late Archaean:
values of 3-4 mb
compatible
Tsurf= 273 K,
from Kasting (1987)Tsurf= 273 K
Tsurf= 288 K
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- Much less CO2 needed to keep surface of early Earth above 273K
- Calculated amount of CO2 (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