ClimateClimate onon terrestrialterrestrial planetsplanets H. Rauer Zentrum für Astronomie und Astrophysik, TU Berlin und Institut für Planetenforschung, DLR, Berlin-Adlershof Terrestrial Planets with Atmospheres in our Solar System Venus Earth Mars T = 735 K T = 288 K T = 216 K p = 90 bar p = 1 bar p = 0.007 bar Atmosphere: Atmosphere: Atmosphere: 96% CO2 77% N2 95% CO2 3,5 % N2 21 % O2 2,7 % N2 1 % H2O WhatWhatare aret thehe relevant relevant processesprocesses forfora a stablestablec climate?limate? AA stablestable climate climate needsneeds a a stablestable atmosphere!atmosphere! Three ways to gain a (secondary) atmosphere Ways to loose an atmosphere Could also be a gain Die Fluchtgeschwindigkeit Ep = -GmM/R 2 Ekl= 1/2mv Für Ek<Ep wird das Molekül zurückkehren Für Ek≥Ep wird das Molekül die Atmosphäre verlassen Die kleinst möglichste Geschwindigkeit, die für das Verlassen notwendig ist hat das Molekül für den Fall: Ek+Ep=0 2 1/2mve -GMm/R=0 ve=√(2GM/R) Thermischer Verlust (Jeans Escape) Einzelne Moleküle können von der obersten Schicht der Atmosphäre entweichen, wenn sie genügend Energie besitzen Die Moleküle folgen einer Maxwell-Boltzmann Verteilung: Mittlere quadratische Geschwindigkeit: v=√(2kT/m) Large escape velocities for the giants and ice planets Mars escape velocity is ~½ ve(Earth) - 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 Additional loss processes are important: Planets with magnetosphere are generally better protected from non-thermal loss processes. Continue on: Other relevant processes for stable climate Three terrestrial planets with atmospheres We first focus on this one heating cooling Solar visible and near-IR radiation absorbed in the atmosphere IR radiation in thermal IR range Variation of incoming solar energy dues to orbital variations Eccentricity (orbit shape) c20/lec20.html 100,000 yrs 400,000 yrs Obliquity rlb/COURSES/205/Le (tilt--21.5 to 24.5o) 41,000 yrs Precession (wobble) 19,000 yrs http://www.geo.lsa.umich.edu/~c 23,000 yrs Radiation budget of Earth atmosphere Relevant factors: Å Energy input from Sun Å clouds (also: dust, aerosols…) Å greenhouse gases Å surface albedo, heat absorption Treibhauseffekt solare Strahlung wird absorbiert und re-emittiert zum Boden gerichteter Anteil thermische führt zu weiterer Strahlung Heizung Boden wird aufgeheizt 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 • CO2, verantwortlich für ca. 1/3 des Treibhauseffekts CO2-Gehalt und Temperatur korrelieren Global and local dynamics affect temperature distribution Æ 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 Æ outgoing thermal IR flux increases Æ Tsurf decreases due to cooling effect Outgoing thermal IR flux increases Increase in Tsurf Tsurf decreases This negative feedback causes the stable Earth climate on short timescales. The long-term climate stability is controlled by several feed-back loops For example: H2O Feedback loops are positive feedback loops saturation vapor pressure drops Climate cools atmospheric water vapor concentration less greenhouse effect decreases saturation vapor pressure rises Climate heats up Runaway atmospheric water greenhouse vapor concentration more greenhouse effect rises effect An initial cooling or heating of the atmosphere will be enhanced! snow/ice Feedback loops are positive feed-back loops Less snow and ice cover on surface Increase in Tsurf Decrease of planetary albedo Eine Aufheizung verstärkt sich selbst. Entsprechend: Eine Eiszeit verstärkt sich selbst. The long-term climate stability is controlled by several feed-back loops Cloud/albedo Feedback loops are positive or negative feedback loops and required detailed modelling atmospheric water vapor concentration Climate heats up rises Depending on Clouds form cloud properties and height Enhanced Greenhouse increased albedo effect Tsurf decreases THE EFFECT OF CLOUDS ON SURFACE TEMPERATURE Kitzmann et al. 2009 1D mean Earth column model Æ clouds have a net cooling effect on the surface Æ clouds are necessary to achieve a surface temperature of 288 K The carbon-silicate cycle is a negative feed-back loop The anorganic carbon cycle: * CO2 dissolves in rain water and form carbonic acid which dissolves rocks Æ the products of silicate weathering are transported to the oceans Æ organisms in the ocean use them for shells of calcium carbonate and silicia Æ eventually these shells fall into the deep ocean and built sediments Æ subduction of plates leads to heating of sediments and to form silicates again and releases CO2 Æ the CO2 enters the atmosphere through vulcanism This cycle replenishes all CO2 in about 0.5 million years! Weathering increases (chemical reactions faster and more rain) Climate heats up Decrease of CO2 in atmosphere T surf less greenhouse effect by CO2 decreases But: timescales of 0.5- 1 106 years! The long-term climate stability is controlled by several feed-back loops An important negative feed-back loop: The carbonate-silicate cycle controls the long-term climate on Earth on timescales of 0.5- 1x106 years! Climate on Earth changed over time Die des Planeten ist verbunden mit der Entwicklung des Zentralsterns SOHO Variation on short time scales 11 jähriger Aktivitätszyklus: Variabilität im UV The Sun in time Affects loss processes and climate. 4.5 4 3 t [Gyr] 2 1 present The early Sun had higher UV- The solar wind velocity was The total solar luminosity was 25-30% lower! fluxes than today. higher than today. [Newkirk, Jr.: Geochi. Cosmochi. Acta Suppl., 13, 293–301; Kulikov et al.: PSS, 54, 1325, 2006] Ribas et al. 2005 Looking back in time…. The total solar luminosity was 25-30% lower! ÆÆearlyearlyE Eartharth andand earlyearlyM Marsars shouldshouldh haveave b beeneen f frozen!rozen! („faint(„fainty youngoung Sun Sun paradox“)paradox“) The Earth timeline… --The Theyoung youngSun Sun was was dimmer dimmert thanhan t today,oday, but butgeological geologicalevidence evidences stilltill indicatesindicatesliquid liquid water wateron on the thes surfaceurface of of Earth. Earth. To warm the early Earth, a strong greenhouse has in the atmosphere is required! e.g.: Æ warming via H2O Æ warming via CO2 Æ 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, Biology, Sagan & Chyba 1997 photochemistry Carbon Dioxide Kasting et al. 1984, Outgassing Kasting 1987 Methane Pavlov et al. 2000 Outgassing, methanogenes Hydrocarbons Haqq-Misra et al. 2008 Photochemistry (i.e., ethane) 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, 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 -Hydrocarbons: Formation dependent on ratio between methane and carbon dioxide P.v. Paris Atmospheric Model 1D radiative-convective cloud-free model from the surface to the mid-mesosphere (e.g., Kasting et al.,1984; Segura et al., 2003) 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 P.v. Paris Results: Evolution of CO2 partial pressure Minimal CO2 partial pressures required for 273 K (lower line) and 288 K (upper line) Compatible with paleosol data (~2.5 Ga ago) Adapted from v. Paris et al. (2008): additional PSS, vol 56, p. 1244-1259 greenhouse gases P.v. Paris needed 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 Methanogenic bacteria “Universal” (rRNA) tree of life Courtesy of Norm Pace Earth in time What has kept early Earth sufficientlyBut: warm at for present liquid no generally surface water despite the „faint youngaccepted Sun“? solution to? the „faint young Sun problem“ Model: pure CO2, H2O, N2 atmosphere Recent model calculations Around ~2.5 Ga liquid indicate that muchwaterlesspossible without additional greenhouseadditionalgases methane! But more future work needed: are needed.Include additional effects, e.g. clouds, other greenhouse gases,… von Paris et al. 2008 Earth’s prebiotic atmosphere