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
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 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 http://www.geo.lsa.umich.edu/~crlb/COURSES/205/Lec20/lec20.html energy energy Variation of Variation of Precession Obliquity Eccentricity (tilt 400,000 yrs 100,000 yrs (orbit shape) 23,000 yrs 19,000 yrs (wobble) 41,000 yrs dues dues -- 21.5 to24.5 to orbital to orbital incoming incoming variations variations o solar solar ) 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 • Dominantly N2 and CO2 • Possibly enhanced methane Then, around 2.2-2.4 Ga, atmospheric O2 concentrations suddenly rose ⇒ What caused the rise of O2? Cyanobacteria! cyanobacteria Rise of O2 in Earth atmosphere evolution • Organisms with photosynthesize developed: CO2 + H2O Æ CH2O + O2 • this O2 produced is the main O2 source in today‘s Earth atmosphere • first bacteria believed to start photosynthesis are cyanobacteria, found e.g. in 2.7 Ga old rocks • there seems to be a 400 million year difference between the origin of cyanobacteria and the later rise in 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 organisms on Earth • O3 is also a strong potential biomarker by absorptions at 9.6 micron • an O3 shielding was probably built very quickly after the first bacteria producing O2. 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 Gesamtinhalt an volatilen Molekülen Gesamtlänge: Untere Grenze für Gesamt- massenanteil im Planeten • 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 Gefüllte Säule: Anteil in Atmosphäre Anmerkungen: es handelt sich um untere Abschätzungen, die durch die neueren Missionen aktualisiert werden müssen. Das Diagram gibt aber einen Anhaltspunkt hier… McBride Gesamtinhalt an volatilen Molekülen Gesamtlänge: Untere Grenze für Gesamt- massenanteil im Planeten •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. Gefüllte Säule: Anteil in Atmosphäre Anmerkungen: es handelt sich um untere Abschätzungen, die durch die neueren Missionen aktualisiert werden müssen. Das Diagram gibt aber einen Anhaltspunkt hier… McBride Why did the other terrestrial planets develop so differently? ? ? Venus • 93-bar, CO2-rich atmosphere • Practically no water (10-5 times Earth) • D/H ratio = 150 times that on Earth Just right! Too hot! What went wrong with Venus? Classical “runaway greenhouse” Assumptions: • Start from an airless planet • Outgas pure H2O or a mixture of H2O and CO2 • Calculate greenhouse effect with a gray atmosphere model Goody and Walker, Atmospheres (1972) After Rasool and deBergh, Nature (1970) Classical “runaway greenhouse” * solar luminosity increases • water vapor pressure increases due to evaporating ocean •Ts rises due to Greenhouse effect ÆPositive feed-back Venus: T well above saturation pressure curve Æ runaway Greenhouse effect, all water evaporated if effective mixing is postulated: vapor in upper atmosphere, dissociation, Goody and Walker, Atmospheres (1972) H escapes to space After Rasool and deBergh, Nature (1970) Mars from HST From: NASA Planetary Photojournal Mars Orbit and Climate Mars is ~half the size of Earth and only ~11% of the mass! 1.38AU 1.65AU Weaker gravity has resulted in stronger escape hence a thin atmosphere. Mars’ orbit is more 45% change eccentric compared with the in solar input Earth so strong climate effects are expected. ~200 km Warrego Vallis (Viking) From: J. K. Beatty et al.,The New Solar System, 4th ed 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? Alternatives for keeping early Mars warm • ‘Scattering’ greenhouse effect of CO2 clouds (F. Forget and R. Pierrehumbert, Science, 1997) • 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… 15 μm CO2 optical properties in CO ice cloud the thermal IR 2 E (τext = 10) R Reflectivity T Transmissivity Emissivity Ref.: Forget and Pierrehumbert, Science (1997) Alternatives for keeping early Mars warm • ‘Scattering’ greenhouse effect of CO2 clouds (F. Forget and R. Pierrehumbert, Science, 1997) • 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) 0 ppbv CH4 10-50 ppbv CH4 H2O H2O H2O CH4 (3018 cm-1) Solar Formisano et al., Science Express (28 Oct., 2004) CH4 Greenhouse? • 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-O2 atmosphere 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 Æ further modelling required J. Kasting Finally, Mars lost most of its atmosphere… NASA Development of Earth-Venus-Mars Proto Venus Proto Earth Proto Mars Moon ? Magnetosphere(4.5GYr) ? Carbon-silicate cycle No magnetic field Sun gets hotter Oxygen rise Atmosphere lost Water vaporises Ozone layer Water freezes Runaway greenhouse You are here