AtmosphericChemistryandRadia3onintheSolarSystemasGuidestoExoplanetAtmospheres

DavidCatling&TylerRobinson

Univ.ofWashington(+KevinZahnle@NASAAmes)

E‐mail:dcatling@uw.edu

Myhomeworkassignment

“…overviewtalkon“Composi?on/chemistry/aerosols/radia?on”…toiden?fycri?calprocessesthatarecommontotheEarth,otherplanetsinoursolarsystem,andexoplanets,andtodiscusstheirinterac?ons…”

From: david.crisp@jpl.nasa.govDate: February5,20132:03:37PMPST

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

Part 1: Atmospheric composition

Theexistenceofatmospheres

Nature(vola?le‐richorigin)vs.nurture(evolu?on)?Nurturemustul?matelyrulebecause

atmospheresmustbestableagainstoblitera?onby:

1)  Impacterosion–lossfromlargebodyimpacts

2)  Irradia?on‐drivenhydrodynamicescape(thermalescapeend‐member)

Good versus bad neighborhoods

no atmosphere

Ref: Catling & Zahnle (2013), 44th Lunar Planet. Sci. Conf., 2665

Transiting exoplanets

prediction: no atmosphere

Kepler 70c Kepler 70c

Kepler 70b

Some Super-Earths

Ref: Catling & Zahnle (2013), 44th Lunar Planet. Sci. Conf., 2665

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

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

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

Fateofhydrogeninreducingatmospheresakeydifferenceinsmallvs.largebodies

Titan

giants

Reducing

photochemicalhaze

photochemicalhaze

nitriles,

Note:SformspolysulfurFrom:Catling&Kas?ng(2014)AtmosphericEvolu1ononInhabitedandLifelessWorlds,CambridgeUniv.Press.

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

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)

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

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)

Part2:Radia?on,structure

•  Chemistry‐radia?onconnec?on•  1stordercommonali?esanddifferencesinstratospheres,tropospheres,tropopauses

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

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

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

Commonalities in atmospheric structure

convec?veprofile

tropopause

BacktobasicsRadia?ve‐convec?veboundary

AnoteonterminologyI’musing

gray

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

McClatchey+(1972);Lindal+(1983);Moroz&Zasova(1997);Moses+(2005)

~0.1barwithinfactorof2

Globalmeantropopauseminima@~0.1bar

Yes,dynamicsmodulatesthe

tropopausepressureonEarthby±x2andisimportantforla?tudinal

differencesonVenus

HereI’mconcernedwitha1storderglobalmeanstatesetbyradia?ve‐

convec?veequilibrium

I know, I know…

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)

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

Howmolecularabsorp?onofIRcomesin

! = ! 0pp0

!"#

$%&

n 1barreferencepressure

IRop?caldepthatp0

n=2inTROPOSPHERES&LOWERSTRATOSPHERES:pressurebroadening(moleculesgainorloseenergyduringcollisionssoabsorp?onoverawiderrangeofphotonenergies)orcollision‐inducedbroadening(collision‐induceddipolesallownon‐polarmoleculestobegreenhousegases;alsodimersorsymmetry‐breakingforforbiddentransi?ons)

n=1inUPPERSTRATOSPHERES(Dopplerbroadening)

SimplemodelisdecentmatchtotropospheresandlowerstratospheresofTitan,Venus,Earth+4giants

Titan

Unselfishcoopera?oninresearch:IDLsourceonline

Ref:Robinson&Catling(2012)Ananaly?cradia?ve‐convec?vemodelforplanetaryatmospheres,Ap.J.757,104

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.

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.

Grayop?caldepthsat1bararealways~2‐10forhypothe?calTitan‐like,Earth‐like,andJupiter‐likeworlds

From:Robinson&Catling(2013),NatureGeosci.,inrevision

Aside:HotJupitersarefarfromthenorm

0.5‐1%ofallplanets(Howard,2013,Science)

So,Ichoosetofocusontheotherones.

OCCUPYTHESOLARSYSTEM:Favorthe99%notthe1%!

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

THEUSERMANUAL:

this“0.1bar”tropopauseminimumruledoesnotapplywhenthere’snominimum,

i.e.,whenthecondi?onforastratosphericinversionfails

e.g.,pumpupCO2onEarth;makemoiststratosphere

Stratosphericinversionandtropopauseminimumvanish,consistentwiththeory:At64xCO2,~10xH2Oinstratospherekstrato=goesfrom~90to<2,thevalueneededforatropopauseminimum.Henceno0.1bartropopauseminimumathighCO2becauseTHEREISNOMINIMUM

Hansenetal.(2013,submi~ed)

shortwave optical depth

infrared optical depth

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.

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.

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.