david catling & tyler robinson univ. of washington · 2013. 7. 8. · atmospheric chemistry and...
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AtmosphericChemistryandRadia3onintheSolarSystemasGuidestoExoplanetAtmospheres
DavidCatling&TylerRobinson
Univ.ofWashington(+KevinZahnle@NASAAmes)
E‐mail:[email protected]
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Myhomeworkassignment
“…overviewtalkon“Composi?on/chemistry/aerosols/radia?on”…toiden?fycri?calprocessesthatarecommontotheEarth,otherplanetsinoursolarsystem,andexoplanets,andtodiscusstheirinterac?ons…”
From: [email protected]: February5,20132:03:37PMPST
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Outline
COMPOSITION
RADIATION
(structure)
AerosolsLIFE
Earth(s)
Chemistry
1) Atmosphericcomposi?on:
‐Whyatmospheresexist(zero‐orderques?onofescapeand
reten?on)
‐Thetaxonomyofatmospheric
composi?on
‐Earthasbio‐atmosphericchemistry
2)Radia?onandstructure:
Chemistry‐radia?onconnec?on
1stordercommonali?esinstratospheres,tropospheres
3)Conclusions
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Part 1: Atmospheric composition
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Theexistenceofatmospheres
Nature(vola?le‐richorigin)vs.nurture(evolu?on)?Nurturemustul?matelyrulebecause
atmospheresmustbestableagainstoblitera?onby:
1) Impacterosion–lossfromlargebodyimpacts
2) Irradia?on‐drivenhydrodynamicescape(thermalescapeend‐member)
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Good versus bad neighborhoods
no atmosphere
Ref: Catling & Zahnle (2013), 44th Lunar Planet. Sci. Conf., 2665
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Transiting exoplanets
prediction: no atmosphere
Kepler 70c Kepler 70c
Kepler 70b
Some Super-Earths
Ref: Catling & Zahnle (2013), 44th Lunar Planet. Sci. Conf., 2665
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10-6
10-4
10-2
100
102
104
0.1 1 10 100
Re
lative S
tella
r H
eatin
g
Escape Velocity [km/s]
Earth
Mercury
Mars
Venus
CallistoGanymede
Moon
IoEuropa
Triton
Titan
Pluto
Rhea
Iapetus
Gliese 876d
Jupiter
Saturn
Uranus
Neptune
GJ436b
Quaoar Eris
Ceres
Hektor
Vesta
Pallas
More Asteroids
transitingextrasolarplanets
Charon 2003EL61
Sedna2005FY9
COROT7
Thermal (hydrodynamic escape) stability
b
From Zahnle & Catling (2013), The cosmic shoreline, 44th Lunar Planet. Sci. Conf., 2787
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So,atmosphereswillnotexistif:1) Bad impact regime ( has received little attention ) 2) Bad thermal regime ( prone to hydrodynamic escape )
• Pre-main sequence luminosity is under-appreciated • e.g., a 1/3M M-star is ~10x more luminous during first ~4 Myr than on the main
sequence, when planets have formed
• rocky planets in M-star habitable zone are vulnerable
• e.g. Earth @ 1/9AU from 1/3M vimpact/vesc ~3 => problem for complex life? Mars in same HZ, vimpact/vesc ~6 => I predict airless
• M-star planets form 105-106 yrs => migration is an unlikely savior; disk
dissipation takes ~106-7 yr from observations, so planet will be in its final orbit while impact regime is bad (Lissauer, 2007, Ap. J.).
• Lucky ones? Frozen volatiles on night-side; late slow, volatile-rich impactors
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Taxonomyofatmosphericcomposi?ons
Fundamentally,twotypes:(aSolarSystemperspec?ve)
Atmospheres
1.Reducing
2.Oxidizing
Titangiants
Post‐2.4GaEarthVenus(atal?tude)
Pre‐2.4GaEarthPre‐4.3Ga?Mars
I’mignoring‐verytenuousN2(CH4)ice‐vaporequilibriaairofKBOs(e.g.,Triton,Pluto)
Mars
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Fateofhydrogeninreducingatmospheresakeydifferenceinsmallvs.largebodies
Titan
giants
Reducing
photochemicalhaze
photochemicalhaze
nitriles,
Note:SformspolysulfurFrom:Catling&Kas?ng(2014)AtmosphericEvolu1ononInhabitedandLifelessWorlds,CambridgeUniv.Press.
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Snapshotoftaxa:GJ1214b‐typeatmospheresElem
entalabu
ndance,H
hydrocarbonifO‐poor
C‐oxideifO‐poor
ElementalC/Ora?o
Specified:0.014AU,Mstar470Ktop800K@1barFixedC‐H‐O
CO2
FromR.Hu(2013),44thLPSC,1428
solar
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Sulfurinoxidizingatmospheresdifferencesindryvs.wetworlds,coldvs.hot
Oxidizing
Earth
Venus(>30km)
Mars
oxidizedSinoceanicSO4(aq)H2SO4haze(Jungelayer@20‐25km)
volcanoes:~1%of~30%albedo
SulfurinairgivenhotsurfaceSO2,3rdalerCO2,N2abundancesH2SO4thickhaze,importantinIR,dominates76%visiblealbedo
oxidizedSinsoil,sedimentarySO4(s)H2SO4hazeearlyMars,adds≥10%albedo(Tianetal.,2010)
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Earth:uniquelywetandO2‐rich…• 2 key consequences for chemistry: (1) a strong control on
trace gases via the hydroxyl radical OH (2) rainfall 1) CLEARS OUT THE MUCK BY OXIDATION; PLUS RAIN OUT O3absorp?on<340nmgeneratesO(1D)
H2O+O(1D)OH+OH Oxidizes H2S,COS,DMStosulfate(NH3toNH3‐sulfateintroposphere)whichrainsout.Oxidizes CO,CH4andotherhydrocarbonstoCO2andH2O
=> air transparent to visible => sunniest planet in Solar System with an atmosphere
2) IF THERE WERE A LACK of ‘OH’ => HAZE WOULD BUILD UP e.g.,ArcheanEarth: CH4andassociatedhydrocarbons+haze
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ThicksulfateaerosolhazeIndicates a dry, volcanic planet Sort of anti-biosignature. Venus: nailing H2SO4 required polarized reflected light as a function of phase angle (Young, 1973; Hansen & Hovenis, 1974). Exoplanets: ~45 micron sulfate feature, MIR OCS, SO2 proxies. (Conversely, S8 absorption edge@300-400 nm)
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Part2:Radia?on,structure
• Chemistry‐radia?onconnec?on• 1stordercommonali?esanddifferencesinstratospheres,tropospheres,tropopauses
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World StratosphericHea3ng StratosphericCooling
Venus CO2 CO2
Earth ozone(UV) CO2
Jupiter aerosols(UV/vis);methane(NIR)
acetylene(C2H2)13.7μmethane(C2H6)12.2μm
Saturn aerosols(UV/vis);methane(NIR)
acetylene(C2H2);ethane(C2H6)
Titan haze;methane(NIR) acetylene(C2H2);ethane(C2H6);haze
Uranus aerosols(UV/vis);methane(NIR)
acetylene(C2H2);ethane(C2H6)
Neptune aerosols(UV/vis);methane(NIR)
acetylene(C2H2);ethane(C2H6)
STRATOSPHERES:ACHEMISTRY‐‐‐STRUCTURECONNECTION
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World Keygreenhouse Coolingfactors Comment
Venus CO2(15μm)
hugeSO4hazealbedo
Earth H2O(con?nuum)‐CO2(15μm)
(CH4,O3N2O)
H2Ocloudalbedo,smallSO4haze
minorCO2controlofgreenhouse
Jupiter H2‐H2,H2‐HeCIA;NH3,CH4
cloud,hazealbedo Giants:H2‐H2/‐HenearPlanckpeak
Saturn H2‐H2,H2‐HeCIA;PH3,CH4
cloud,hazealbedo
Titan N2‐N2,N2‐CH4,N2‐H2,CH4‐CH4CIA
haze‘an?‐greenhouse’absorbssolarinIRop?callythinregion
minorH2affectswindow(16.5‐25μm)+greenhouse
Uranus H2‐H2,H2‐HeCIA;CH4
cloud,hazealbedo
Neptune H2‐H2,H2‐HeCIA;CH4
cloud,hazealbedo
TROPOSPHERES:ACHEMISTRY‐‐‐STRUCTURECONNECTION
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PH3
Absorp?on,oxidizedgas:CO2(+H2O+O3forEarth)+bandcenteremission(stratosphere)
Absorp?on,reducinggas:• H2ongiants• NH3,onallgiants
(1400‐1800cm‐1);PH3onSaturn
• CH4,H2opera?ngwithN2onTitan
Emission:C2H2(729=13.7μm)C2H6(820=12.2μm),CH4
Reducing
Oxidizing
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Commonalities in atmospheric structure
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convec?veprofile
tropopause
BacktobasicsRadia?ve‐convec?veboundary
AnoteonterminologyI’musing
gray
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Earth
troposphere
stratosphere
tropopauseminimumradia?ve‐convec?veboundary
1) temperatureminimum‘tropopause’2)Shockingly:let’scalltheradia?ve‐convec?veboundary…the“radia?ve‐convec?veboundary”
Commonalities in structure: Terminology
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McClatchey+(1972);Lindal+(1983);Moroz&Zasova(1997);Moses+(2005)
~0.1barwithinfactorof2
Globalmeantropopauseminima@~0.1bar
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Yes,dynamicsmodulatesthe
tropopausepressureonEarthby±x2andisimportantforla?tudinal
differencesonVenus
HereI’mconcernedwitha1storderglobalmeanstatesetbyradia?ve‐
convec?veequilibrium
I know, I know…
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Why~0.1bartropopauses?First,somewronganswers:
1) where the IR absorption by gas changes from pressure to Doppler broadening?
2) Where IR optical depth drops below ~1? (National Academy atmos. physicist) (but on the right track)
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meanmoistadiabat
Pressure
Temperature
Strato:
Infrared(IR) {Solarflux
radia?ve‐convec?veboundary
1.WithTandfluxcon?nuity,solveforIRop?caldepthτIRat(i)radia?ve‐convec?veboundary(ii)1bar
2.Relateop?caldepthtopressuretogetprofile
Fstrato
!e!kstrato" IR
Tropo: Ftropo!e!ktropo" IR
+
(internalheat)
Analy?cradia?ve‐convec?ve:Robinson&Catling(2012)Ap.J.757,104
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Howmolecularabsorp?onofIRcomesin
! = ! 0pp0
!"#
$%&
n 1barreferencepressure
IRop?caldepthatp0
n=2inTROPOSPHERES&LOWERSTRATOSPHERES:pressurebroadening(moleculesgainorloseenergyduringcollisionssoabsorp?onoverawiderrangeofphotonenergies)orcollision‐inducedbroadening(collision‐induceddipolesallownon‐polarmoleculestobegreenhousegases;alsodimersorsymmetry‐breakingforforbiddentransi?ons)
n=1inUPPERSTRATOSPHERES(Dopplerbroadening)
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SimplemodelisdecentmatchtotropospheresandlowerstratospheresofTitan,Venus,Earth+4giants
Titan
Unselfishcoopera?oninresearch:IDLsourceonline
Ref:Robinson&Catling(2012)Ananaly?cradia?ve‐convec?vemodelforplanetaryatmospheres,Ap.J.757,104
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Pres
sure
Temperature
radiative-conv. boundary @ DτIR ~1 (or more
e.g. Titan)
tropopause @ DτIR ≈ 0.08±0.03, p~0.1 bar
@ p0 = 1 bar, τIR =8±5
Schematic: Solar System average
Disdiffusivityfactor
Ref:fromRobinson&Catling(2013)Explana?onofacommon0.1bartropopauseinthickatmospheresofplanetsandlargemoons.,NatureGeosci.,inrevision.
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So,whythe~0.1bartropopause?
ptropo
(p0 = 1 bar)!
" tropo ~ 0.05
" 0 ~ a few to several
#$%
&'(
1/2
is always ~ 0.1 bar
1. minima in T occur at τIR ≈ 0.05 (enough IR transparency for shortwave heating to start dominance)
2. pressure broadening and/or collision-induced
absorption relate optical depth to pressure τ ~ p2 => p ~ τ1/2
3. atmospheres are IR-optically thick at 1 bar
Ref:fromRobinson&Catling(2013)Explana?onofacommon0.1bartropopauseinthickatmospheresofplanetsandlargemoons.,NatureGeosci.,inrevision.
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Grayop?caldepthsat1bararealways~2‐10forhypothe?calTitan‐like,Earth‐like,andJupiter‐likeworlds
From:Robinson&Catling(2013),NatureGeosci.,inrevision
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Aside:HotJupitersarefarfromthenorm
0.5‐1%ofallplanets(Howard,2013,Science)
So,Ichoosetofocusontheotherones.
OCCUPYTHESOLARSYSTEM:Favorthe99%notthe1%!
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What’sthecondi?onforastratosphericinversiontoexist?It’sanaly?c
A tropopause minimum: differentiate and set to 0, gives:
! tp "1
kstratoln
Fstrato!
Ftropo!
+ Fi
kstrato2
D2
#1$%&
'()
*
+,,
-
.//
Only has a physical solution if kstrato>D ~1.7, usu. ~102 Hence no global mean stratospheric inversion on Venus
wherekstrato=shortwave optical depth
infrared optical depthofstratosphere
~1/100 ~1/10 ~103
~5
≈0.05
fluxabovefluxbelow
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THEUSERMANUAL:
this“0.1bar”tropopauseminimumruledoesnotapplywhenthere’snominimum,
i.e.,whenthecondi?onforastratosphericinversionfails
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e.g.,pumpupCO2onEarth;makemoiststratosphere
Stratosphericinversionandtropopauseminimumvanish,consistentwiththeory:At64xCO2,~10xH2Oinstratospherekstrato=goesfrom~90to<2,thevalueneededforatropopauseminimum.Henceno0.1bartropopauseminimumathighCO2becauseTHEREISNOMINIMUM
Hansenetal.(2013,submi~ed)
shortwave optical depth
infrared optical depth
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Extendtoexoplanets?~0.1 bar tropopause minima of Earth, Titan, giants from (1) a common IR transparency requirement and (2) a common pressure dependence on the IR opacity
A ~0.1 bar tropopause minimum is an emergent “rule” arising from common physics that will apply to many exoplanets and exomoons with thick atmospheres
TESTABLE HYPOTHESIS:
From:Robinson&Catling(2013)Explana?onofacommon0.1bartropopauseinthickatmospheresofplanetsandlargemoons.,NatureGeosci.,inrevision.
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Conclusions• Atmospheresmaynotexistinbadimpacterosionorthermalescaperegimes
‐ e.g.,doublewhammyforM‐dwarfplanets
• NoSuper‐EarthexamplesinourSolarSystem(notablypresentasexoplanets),butthebodies/modelsatleastprovidepointsofreferenceforexoplanetatmospheres.
• Atmospheresweknoware:(1)reducingwithorganichazes+/‐poly‐S,poly‐P,or(2)oxidizingwithsulfatehazesofvariableradia?vesignificancethatcanbehighvis.albedo‐ Reasonabletoexpecttosuchhazesonsimilarexoplanets–canweseesulfateorS8?‐ Reasonabletoexpectcorrespondingabsorp?on/emissionfeaturesofcommon
reducingoroxidizinggassuites• Structure:
Reducingatmospheresweknowhavestratosphericinversionsfrommethane+hazes Oxidizingatmospheresweknowdo(Earth)ordon’t(Venus)havestrat.inversions
• StratosphericinversionsrequireIR‐op?callythinstratospheresandabsorbersstronginSWrela?vetoIR.Combinedwithpressure‐broadeningorCIA=>common~0.1barlevelfortropopauseminima.Testablehypothesis:Plausiblytrueinmanyexoplanetatmospheres
• Didn’ttalkaboutevolu?on.ButobservingrunawayVenuses(futureEarths)stateson
exoplanetswouldclearlybeanextremelyvaluableconfirma?onoftheories.
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ReferencesONIMPACTEROSIONANDHYDRODYNAMICESCAPEOFATMOSPHERES:• D.C.Catling&K.J.Zahnle(2013)Animpacterosionstabilitylimitcontrollingthe
existenceofplanetaryatmospheresonexoplanetsandsolarsystembodies,44thLunarPlanet.Sci.Conf.,2665
• K.J.Zahnle&D.C.Catling(2013)Thecosmicshoreline44thLunarPlanet.Sci.Conf.,2787.
OVERVIEWOFTHECHEMISTRY,RADIATIONANDEVOLUTIONOFATMOSPHERES:• D.C.Catling&J.F.Kas?ng(2014)AtmosphericEvolu1ononInhabitedandLifelessWorlds,
CambridgeUniv.Press.RADIATION,ATMOSPHERICSTRUCTURE,andCOMMONTROPOPAUSEMINIMUMLEVEL:• T.D.Robinson&D.C.Catling(2012)Ananaly?cradia?ve‐convec?vemodelforplanetary
atmospheres,Astrophys.J.757,104.
• T.D.Robinson&D.C.Catling(2013)Explana?onofacommon0.1bartropopauseinthickatmospheresofplanetsandlargemoons.,NatureGeosci.,inrevision.