ClimateClimate on on terrestrialterrestrial planetsplanets

H. RauerZentrum für Astronomie und Astrophysik,

TU Berlin

und

Institut für Planetenforschung,

DLR, Berlin-Adlershof

Venus Earth Mars

Atmosphere:

77% N2

21 % O2

1 % H2O

T = 288 K p = 1 bar

Atmosphere:

96% CO2

3,5 % N2

T = 735 K p = 90 bar

Atmosphere:

95% CO2

2,7 % N2

T = 216 K p = 0.007 bar

TerrestrialTerrestrial PlanetsPlanets withwith AtmospheresAtmospheres in in ourour Solar SystemSolar System

What are the relevant processesfor a stable climate?

What are the relevant processesfor a stable climate?

A stable climateneeds a stableatmosphere!

A stable climateneeds a stableatmosphere!

Three ways to gain a (secondary) atmosphere

Ways to loose an atmosphere

Could also be a gain

ve=√(2GM/R)1/2mve2-GMm/R=0

Die Fluchtgeschwindigkeit

Ek+Ep=0

Ep = -GmM/R

Ek l= 1/2mv2

Für Ek<Ep wird das Molekül zurückkehren

Für Ek≥Ep wird das Molekül die Atmosphäre verlassen

Die kleinst möglichste Geschwindigkeit, die für das Verlassennotwendig ist hat das Molekül für den Fall:

Thermischer Verlust (Jeans Escape)

Einzelne Moleküle können von der obersten Schicht der Atmosphäreentweichen, wenn sie genügend Energie besitzen

Die Moleküle folgen einer Maxwell-Boltzmann Verteilung:

Mittlere quadratische Geschwindigkeit:

v=√(2kT/m)

- gas giants are massive enough to keep H-He-atmospheres

- terrestrial planets atmospheres can have CO2, N2, O2, CH4, H2O, …,

but little H and He

Large escapevelocities for the giantsand ice planets

Mars escape velocityis ~½ ve(Earth)

Additional loss processes are important:

Planets with magnetosphereare generally better protectedfrom non-thermal lossprocesses.

Three terrestrial planets with atmospheres

We first focus on this one

Continue on: Other relevant processes for stable climate

heating cooling

Solar visible and near-IR radiationabsorbed in the atmosphere IR radiation in thermal IR range

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Eccentricity(orbit shape)100,000 yrs400,000 yrs

Obliquity(tilt--21.5 to 24.5o)41,000 yrs

Precession(wobble)19,000 yrs23,000 yrs

Variation of Variation of incomingincoming solar solar energyenergy duesdues to orbital to orbital variationsvariations

Radiation budget of Earth atmosphere

Relevant factors:

Energy input from Sun

clouds (also: dust, aerosols…)greenhouse gases

surface albedo, heatabsorption

Treibhauseffekt

Effektive Treibhausgase sind:

• H2O, verantwortlich für 2/3 des Treibhauseffektes auf der Erde

• CO2, verantwortlich für ca. 1/3

• CH4, N2O, O3 und anthropogene CFCs, verantwortlich für einige Prozent

solare Strahlung

Boden wird aufgeheizt

thermische Strahlung

wird absorbiert und re-emittiert

zum Boden gerichteter Anteil führt zu weiterer Heizung

CO2-Gehalt und Temperatur korrelieren

• CO2, verantwortlich für ca. 1/3 des Treibhauseffekts

Global and local dynamics affect temperature distribution

lecture by U. Langematz

Die fünf Faktoren, die unser Klima beeinflussen:

• Die Atmosphäre: Treibhauseffekt, globale Zirkulation, …

• Die Biosphäre: jahreszeitliche Wechsel der Vegetation, Entwicklung von Leben, …

• Die Hydrosphäre: Verdunstung, Kondensation, Temperaturspeicher, globaler Transport, …

• Die Kryosphäre: Eisbedeckung, Variation der Albedo,…

• Die Pedosphäre: (Lithosphäre): Tektonik, Vulkanismus, …

Earth is 4.5 billion years old.

What stabilizes climate over long time scales?

Tsurf and IR flux are negative feedback loops

* Tsurf increases outgoing thermal IR flux increases Tsurf decreasesdue to cooling effect

This negative feedback causes the stable Earth climate on shorttimescales.

Increase in Tsurf

Outgoing thermal IR flux increases

Tsurf decreases

For example: H2O Feedback loops are positive feedback loops

Climate cools

saturation vaporpressure drops

atmospheric watervapor concentrationdecreasesless greenhouse effect

Climate heats up

saturation vaporpressure rises

atmospheric watervapor concentrationrisesmore greenhouse effect

An initial cooling or heating of the atmosphere will be enhanced!

The long-term climate stability is controlled by several feed-back loops

Runawaygreenhouseeffect

snow/ice Feedback loops are positive feed-back loops

Increase in Tsurf

Less snow and icecover on surface

Decrease of planetaryalbedo

Eine Aufheizung verstärkt sich selbst.

Entsprechend: Eine Eiszeit verstärkt sich selbst.

Cloud/albedo Feedback loops are positive or negativefeedback loops and required detailed modelling

Climate heats up

atmospheric watervapor concentrationrises

Clouds form

Enhanced Greenhouseeffect

increased albedo

Tsurf decreases

The long-term climate stability is controlled by several feed-back loops

Depending on cloud propertiesand height

clouds have a net cooling effect on the surfaceclouds are necessary to achieve a surface temperature of 288 K

THE EFFECT OF CLOUDS ON SURFACE TEMPERATURE

1D mean Earth column modelKitzmann et al. 2009

The anorganic carbon cycle:

* CO2 dissolves in rain water and form carbonic acid which dissolves rocks

the products of silicate weathering are transported to the oceansorganisms in the ocean use them for shells of calcium carbonate and siliciaeventually these shells fall into the deep ocean and built sedimentssubduction of plates leads to heating of sediments and to form silicates againand releases CO2 the CO2 enters the atmosphere through vulcanism

This cycle replenishes all CO2 in about 0.5 million years!

But: timescales of 0.5- 1 106 years!

The carbon-silicate cycle is a negative feed-back loop

Climate heats up

Weathering increases(chemical reactionsfaster and more rain)

Decrease of CO2 in atmosphere

less greenhouse effect by CO2Tsurfdecreases

The long-term climate stability is controlled by several feed-back loops

The carbonate-silicate cycle controls the long-term climate on Earth on timescales of 0.5- 1x106 years!

An important negative feed-back loop:

Climate on Earth changed over time

Die des Planeten ist verbunden mit der Entwicklung des Zentralsterns

SOHO

11 jähriger Aktivitätszyklus:

Variabilität im UV

Variation on short time scales

The Sun in time

The early Sun had higher UV-fluxes than today.

Ribas et al. 2005

t [Gyr]4.5 4 3 2 1 present

[Newkirk, Jr.: Geochi. Cosmochi. Acta Suppl., 13, 293–301;Kulikov et al.: PSS, 54, 1325, 2006]

The total solar luminosity was 25-30% lower!The solar wind velocity washigher than today.

Affects lossprocesses and climate.

LookingLooking back in timeback in time……..

The total solar luminosity was 25-30% lower!

early Earth and early Mars should have been frozen!

(„faint young Sun paradox“)

early Earth and early Mars should have been frozen!

(„faint young Sun paradox“)

- The young Sun was dimmer than today, but geological evidence still indicates liquid water on the surface of Earth.

- The young Sun was dimmer than today, but geological evidence still indicates liquid water on the surface of Earth.

TheThe Earth Earth timelinetimeline……

To warm the early Earth, a strong greenhouse has in the atmosphere is required!

e.g.:

warming via H2O

warming via CO2

warming via CH4?

Proposed solutions to the„Faint Young Sun 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

Hydrocarbons(i.e., ethane)

Haqq-Misra et al. 2008 Photochemistry

P.v. Paris

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, muchless 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

-Hydrocarbons: Formation dependent on ratio between methane and carbondioxide

P.v. Paris

Temperature profile• stratosphere: radiative equilibrium

• IR flux von Paris et al. (2008) or Mlawer et al. (1997)

• solar flux Kasting et al. (1988)• troposphere: convection (moist/wet adiabat)

H2O profile• troposphere: fixed relative humidity• Stratosphere: concentration constant at cold

trap value

1D radiative-convective cloud-free model from the surface to themid-mesosphere (e.g., Kasting et al.,1984; Segura et al., 2003)

Atmospheric Model

P.v. Paris

Results: Evolution of CO2 partial pressureMinimal CO2 partial pressures required for 273 K (lower line) and 288 K (upper line)

Adapted from v. Paris et al. (2008):PSS, vol 56, p. 1244-1259

P.v. Paris

Compatible withpaleosol data(~2.5 Ga ago)

additional greenhouse gasesneeded

Earth atmosphere composition in time

Lammer et al. 2009, Kasting 1994

Why some people like CH4 instead of (or in addition to) CO2

• Atmospheric O2 levels were low prior to about 2 billion years ago

• Methanogens are evolutionarily ancient

Methanogenicbacteria

Courtesy ofNorm Pace

“Universal”(rRNA) tree

of life

Earth in timeWhat has kept early Earth sufficiently warm for liquid surface water despite the„faint young Sun“?

But more future work needed: Include additional effects, e.g. clouds, other greenhousegases,…

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von Paris et al. 2008

Model: pure CO2, H2O, N2 atmosphere

Around ~2.5 Ga liquid water possible withoutadditional methane!

But: at present no generallyaccepted solution to the „faint

young Sun problem“

Recent model calculationsindicate that much less

additional greenhouse gasesare needed.

Earth’s prebiotic atmosphere• Dominantly N2 and CO2

• Possibly enhanced methane

Then, around 2.2-2.4 Ga, atmospheric O2concentrations suddenly rose ⇒

What caused the rise of O2?Cyanobacteria!

cyanobacteria

Rise of O2 in Earth atmosphere evolution

• Organisms with photosynthesize developed:

CO2 + H2O CH2O + O2

• this O2 produced is the main O2 source in today‘sEarth atmosphere

• first bacteria believed to start photosynthesis arecyanobacteria, found e.g. in 2.7 Ga old rocks

• there seems to be a 400 million year differencebetween the origin of cyanobacteria and the later risein O2 in the atmosphere. This is presently unclear.

Rise of O3 in Earth atmosphere evolution

• The increase in O2 in Earth evolution should go along with an increase in stratospheric O3

• O3 absorbs UV radiation at 200-300 nm that would harm organismson Earth

• O3 is also a strong potential biomarker by absorptions at 9.6 micron

• an O3 shielding was probably built very quickly after the firstbacteria producing O2.

Venus Earth Mars

Atmosphere:

77% N2

21 % O2

1 % H2O

T = 288 K p = 1 bar

Atmosphere:

96% CO2

3,5 % N2

T = 735 K p = 90 bar

Atmosphere:

95% CO2

2,7 % N2

T = 216 K p = 0.007 bar

TerrestrialTerrestrial PlanetsPlanets withwith AtmospheresAtmospheres in in ourour Solar SystemSolar System

Gesamtinhalt an volatilen Molekülen

Anmerkungen: es handelt sich um untere Abschätzungen, die durch die neueren Missionen aktualisiert werden müssen. Das Diagram gibt aber einen Anhaltspunkt hier…

• Auf der Erde H2O hauptsächlich in Ozeanen und in Polkappen

• H2O auf Venus und Mars nicht in flüssiger Form möglich

• Mars: Polkappen sind Reservoir für H2O und CO2

• CO2 auf der Erde hauptsächlich in Gesteinen gebunden

McBride

Gesamtlänge: Untere Grenze für Gesamt-massenanteil im Planeten

Gefüllte Säule: Anteil in Atmosphäre

Gesamtinhalt an volatilen Molekülen

Anmerkungen: es handelt sich um untere Abschätzungen, die durch die neueren Missionen aktualisiert werden müssen. Das Diagram gibt aber einen Anhaltspunkt hier…

• Der CO2-Gehalt ist ausgeglichener, wenn man den Gesamtinhalt betrachtet!

• Der hohe relative N2-Gehalt in der Atmosphäre der Erde liegt nicht an mehr N2, sondern daran, dass mehr CO2 gebunden ist.

• Es scheint weniger H2O auf der Venus zu geben und wenig N2 auf Mars.

McBride

Gesamtlänge: Untere Grenze für Gesamt-massenanteil im Planeten

Gefüllte Säule: Anteil in Atmosphäre

Why did the other terrestrial planets develop so differently?

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Venus

• 93-bar, CO2-rich atmosphere• Practically no water (10-5

times Earth)• D/H ratio = 150 times that on

Earth

What went wrong with Venus?

Too hot!Just right!

Classical “runaway greenhouse”

Goody and Walker, Atmospheres (1972)After Rasool and deBergh, Nature (1970)

Assumptions:• Start from an airless

planet• Outgas pure H2Oor a mixture of H2Oand CO2

• Calculate greenhouseeffect with a grayatmosphere model

Classical “runaway greenhouse”

Goody and Walker, Atmospheres (1972)After Rasool and deBergh, Nature (1970)

• water vapor pressure increases due to evaporating ocean • Ts rises due to Greenhouse effect

Positive feed-back

Venus: T well above saturation pressure curve

runaway Greenhouse effect, all water evaporated if effective mixing is postulated: vapor in upper atmosphere, dissociation,H escapes to space

* solar luminosity increases

Mars fromHST

From: NASA PlanetaryPhotojournal

Mars Orbit and Climate

Mars is ~half the size of Earthand only ~11% of the mass!Weaker gravity has resulted instronger escape hence a thinatmosphere. Mars’ orbit is moreeccentric compared with theEarth so strong climate effectsare expected.

1.38AU 1.65AU

45% changein solar input

From: J. K. Beatty et al.,The New Solar System, 4th ed

WarregoVallis

(Viking)

~200 km

Old theory for warming early Mars:

• Dense CO2 atmosphere• Volcanism and impacts generated lots of CO2• But “faint young Sun”: Solar luminosity was 25-30%

lower prior to 3.8 Ga, when most of the valleys are thought to have formed…

• It is difficult to warm early Mars with a CO2/H2O atmosphere:– Condensation of CO2 reduces the tropospheric

lapse rate, thereby lowering the greenhouse effect– CO2 is a good Rayleigh scatterer (2.5 times better

than air) ⇒ increase in albedo through clouds may outweigh the increase in the greenhouse effect

• There is also a problem with carbonates: where are they?

• ‘Scattering’ greenhouse effect of CO2 clouds (F. Forget and R. Pierrehumbert, Science, 1997)

Alternatives for keeping early Mars warm

• CO2 ice crystals are expected to be 10-50 μm in size, comparable to thermal-IR wavelengths

• Outgoing thermal-IR radiation is therefore backscattered more effectively than incoming (visible/near-IR) solar radiation

⇒ surface warms…

Ref.: Forget and Pierrehumbert, Science (1997)

CO2 opticalproperties in

the thermal IR

Reflectivity

Transmissivity

Emissivity

15 μm

R

T

ECO2 ice cloud(τext = 10)

• ‘Scattering’ greenhouse effect of CO2 clouds (F. Forget and R. Pierrehumbert, Science, 1997)

Alternatives for keeping early Mars warm

• CO2 ice crystals are expected to be 10-50 μm in size, comparable to thermal-IR wavelengths

• Outgoing thermal-IR radiation is therefore backscattered more effectively than incoming (visible/near-IR) solar radiation

⇒ surface warms…

• Problems with the scattering greenhouse hypothesis:– Need near 100% cloud cover– Low (or thick) clouds can cool, as they do on Earth– CO2 clouds create localized heating which, in turn,

makes them disappear (T. Colaprete et al., JGR, 2003)

Methane on Mars?(From Mars Express Planetary Fourier Spectrometer)

Formisano et al., Science Express (28 Oct., 2004)

CH4(3018 cm-1)

H2O

H2O

H2O

Solar

0 ppbv CH4

10-50 ppbv CH4

• Mars’ early atmosphere, like Earth’s was probably weakly reducing

• Photochemical lifetime of CH4 is relatively short (~10 years) in Earth’s atmosphere today, but its lifetime in a low-O2atmosphere would be ~1000 times longer

• A biological flux comparable to Earth’s would therefore sustain ~1000 ppmv CH4, as compared to 1.7 ppmv on present Earth

CH4 Greenhouse?

further modelling required

J. Kasting

NASA

Finally, Mars lost most of its atmosphere…

Development of Earth-Venus-Mars

Proto Venus

Moon(4.5GYr)

Sun gets hotterWater vaporises

Runaway greenhouse

MagnetosphereCarbon-silicate cycle

Oxygen riseOzone layer

No magnetic fieldAtmosphere lostWater freezes

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Proto Earth Proto Mars

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