Evolution of primary planetary atmospheres
Philip von Paris1), John Lee Grenfell1), Pascal Hedelt1), Heike Rauer1,2), Barbara Stracke1)
1)Institut
2)Zentrum
für Planetenforschung, DLR Berlin
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
CNES
- 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
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)
r
Dashed line: Gough (1981)
Plain line: Caldeira and Kasting (1992)
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
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
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
stellarer Fluss
Klima
Stratosphäre
Strahlungsgleichgewicht,
F aus
Strahlungstransportgl.
Chemie
Chemisches Reaktionsnetzwerk
berücksichtigt 55 Spezies,
220 Reaktionen
Troposphäre trockenoder feuchtadiabatisches
T-Profil
biogene-Flüsse
T, p Profil
Profile chemischer
Konzentrationen
This work: Model description
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
Absorption coefficient k(v)
Net Flux
2
Fnet   F (k )d
1
Density function

 f (k )dk  n
k
0
Distribution function
k
g (k )   f (k ' )dk '
0
1
Net Flux
Fnet   F (k g )dg
0
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)
Validation of modified radiation scheme
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
Mid-stratosphere to upper mesosphere:
slight disagreement due to extrapolation errors
for RRTM (yielding negative optical depths)
RRTM
(Mlawer et al. 1997)
Surface up to mid-stratosphere: excellent
agreement
Convective regime
MRAC
(This work)
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
Tabulated values
MRAC
RRTM
Calculated values
„True“ values
This work: Model runs
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 is
reached
Runs
Tsurf/K
1-6
273
7-12
288
Results: Atmospheric structure
S=0.8
S=0.8
S=0.85
S=0.85
Temperature inversion
Cold trap
Convective regime
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: Keeping the surface warm
Tsurf= 288 K
Tsurf= 273 K,
from Kasting (1987)
Tsurf= 273 K
Upper limits on CO2
/ 
(for 2 approximations of solar luminosity)
Late Archaean:
values of 3-4 mb
compatible
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)
Conclusions
-
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