what makes a planet (unin)habitable? runaway...

GEOS22060/GEOS32060/ASTR45900

Lecture8Tuesday30April2019

Whatmakesaplanet(unin)habitable?Runawaygreenhouse

LogisEcs•  Homework1andHomework2aregraded•  Homework3willbeissuedonWedorThuanddueonFri10•  Totalnumberofhomeworkswillbe6(hopefully7)•  Midtermfeedbackformresults:

CourseoutlineFounda6ons(1-2weeks)•  Earthhistory•  HZconcept,atmosphericscienceessenEals•  Post-HadeanEarthsystem

Principles–howarehabitableplanetsini6atedandsustained?(4-5weeks)•  VolaElesupply,volaEleescape•  Runawaygreenhouse,moistgreenhouse•  Long-termclimateevoluEon

•  Specifics(2.5weeks)•  HyperthermalsonEarth•  EarlyMars•  Oceanswithinice-coveredmoons•  Exoplanetarysystemse.g.TRAPPIST-1system

TODAY

Earthscience

planetaryscience

Lecture7wrap-up•  Energy-limit:XUVdrivenescapemore-likely-than-notsculptstheexoplanetradius-perioddistribuEon(‘photo-evaporaEonvalley’)

•  Diffusionlimit:whatregulatesHlossfromVenus,EarthandMarstoday

•  Impacterosion–giantimpactsandplanetesimalimpacts

Wrap-up:impacterosion

Two-stagegasgun

NucleartestsHydrocode

Terrestrialimpactcraters

FormaEonofEarth-sizedplanetsinvolvesgiant(oligarchic)impacts.

TheoutputunderlyingthisplotwasgeneratedbyC.Cossou.

*=giantimpacts

MassesofresulEngplanets(Earths)

Simula8onintendedtoreproduce“typical”Keplersystemofshort-period,8ghtly-packedinnerplanets

TheMoon-formingimpactwasnotthelastbigimpactonEarth,butitwasthelastEmethatEarthhitanotherplanet.

SchlichEng&Mukhopadhay2018

Theatmosphere-lossescapeefficiencyofgiantimpactsissetbytheground-moEonspeed

TherehavealsobeenmajorrecentdevelopmentsinourunderstandingofMoonforma8on,theMoon’sorbitalevolu8on,andMoon-induced8dalhea8ng,butorbital/8daleffectsarenotpartofthiscourse.

Oceanremovalbygiantimpacts?(OceanvaporizaEon!=oceanremoval)

SimulaEonssuggestthattheMoon-formingimpactwasmarginallyabletoremoveanypre-exisEngEarthocean

Stewartetal.LPSC2014

Qs~ve2foroligarchicimpact

escapevelocity

SchlichEng&Mukhopadhay2018

TotalimpactormassneededtoejecttheatmosphereasafuncEonofimpactorradius

Impactsbysmallasteroids/cometsefficientlyeject~1baratmospheres

ThepeakwouldmovetolargerriftheiniEalatmosphericpressureweregreater

SchlichEng&Mukhopadhay2018

EARTH-SPECIFIC

AnupEckinbombardment~3.9Ga?

Catling&KasEngch.6(Fig.6.18)

Effectofbasin-formingimpactsonhabitability:impactfrustraEonoflifeestablishingitselfonEarth?

e.g.Maher&Stevenson1987

Lecture8Runawaygreenhouse–keypoints

•  The(H2O-)runawaygreenhouseisageologicallyrapidincreaseinplanetsurfacetemperaturefrom<500Kto>1000KcausedbyaposiEvefeedbackbetweenthesaturaEonvaporpressureofwatervaporandtheplanetsurfacetemperature

•  Beabletoexplainthemechanismoftherunawaygreenhouse

•  ItisalmostcertainthatreleaseofCO2byhumanscannotcausearunawaygreenhouse

•  Theexactthresholdfortherunawaygreenhousedependsoncloudcover,landfracEon,andplanetrotaEonrate

ChalkboardEnergybalanceßàPlanckfeedbackEffectoffixed-T-offsethigh-alEtudeopEcallythickIRabsorberonPlanckfeedbackRunawayGHrepresentsfailureofthePlanckfeedback

Venusisdrytoday

Theinneredgeofthehabitablezoneisdefinedbytherunawaygreenhouselimit

Earthin<2Gyr(uninhabitable)

Earthnow(habitable)

Clasius-ClapeyronrelaEon:exponen6alincreaseinwatervaporparEalpressurewithincreasingT(7%perdegreeK,linearizingaroundmodernEarthT)

secEon1.1.3.5ofCatling&KasEng,ch.1

DefiniEons:adiabatandmoistadiabat(1/2)

Earthtoday

DefiniEons:adiabatandmoistadiabat(2/2)Condensible“c”(e.g.water)andnoncondensible“a”(e.g.O2/N2)AssumeinstantprecipitaEonofcondensate

MixingraEoatsaturaEon

AssumingsaturaEon(relaEvehumidity=1),

zerobydefiniEonofadiabat

Inthelimitrsat=0(dryatmosphere),thisequaEongivesthedryadiabatInthelimitwherewatervaporisthedominantconsEtuentoftheatmosphere,thistendstothesaturaEonvaporpressurecurve.

Astemperatureincreases,themoistadiabatincreasinglydivergesfromthedryadiabat

RELATIVEHUMIDITY=1EARTHAIR,1BAR(DRY)Pierrehumbert2010

dryadiabat:

Stratosphericcold-trap

potenEaltemperature:

Therunawaygreenhouseleadstotheendofhabitability

Goldblax

&W

atson2012,followingNakajim

a1992

Condensablegreenhousegasesleadtoclimateinstability(Kombayashi-Ingersolllimit)

T

z

Consequences:(allareimportant)H2OdissociaEonVNIRwindowSurfacemagmaAllwaterlosttospaceCarbonatesdecompose

AssumpEons:

ConstantRH(RH=relaEvehumidity)

Pierrehu

mbe

rt,N

ature,2002

TheswitchfromadrytoamoiststratospherehappensoveranarrowrangeofTsurf–becauseoftheexponenEaldependenceofs.v.p.onT,combinedwiththelapseratefeedback

Goldblax&Watson2012

Goldblax&Watson2012

Grayatmosphere:

OpEcaldepthattropopause:

(1/g)inbrackets

Goldblax&Watson2012

slightlylessthantheblack-bodyfluxcorrespondingtothetemperatureoneopEcaldepthdownfromthetopoftheatmosphere

Pierrehumbert2010figure4.4

Therunawaygreenhouseleadstothelossofsurfaceliquidwater(However,asupercriEcalH2O-richC-poorphasemaypersist)

Whatstopsthetemperaturefromgoingupindefinitely?(Whyisitlimitedto~1500K?)(It’snotthelatentheatofmagmamel8ng–it’scoincidencethattherunawayGHstopsaroundthedry-rocksolidus).TwokeywindowsinthewatervaporabsorpEonspectrum:8-12micronsand<~4microns

Anthropogenicgreenhousegases(probably)cannottriggertherunawaygreenhouse.

Goldblaxetal.NatureGeoscience2014

BlackGreenBlueRedpurple

Dashedlines:RH<1Solidlines:RH=1

Modernalbedo

Earthtodayisclosetotherunawaygreenhouselimit.Therunawaygreenhousecanbetriggeredbyanincreaseinsolarluminosity

Leconteetal.2013Nature

waterlost(bars)

Hlossflux(atomscm-2s-1)

Timeforloss(109yr)

Highgravitymovestherunawaygreenhouselimitclosertothestare.g.metal-richplanet,larger-radiussuper-Earth

TemperatureàvaporpressureVaporpressurexhumidity/gravityàcolumnmassofgreenhousegasColumnmassàgreenhouseeffect.

MaxOLR~=slightlylessthantheblackbodyfluxcorrespondingtothetemperatureoneopEcaldepthdownfromthetopoftheatmosphere

High-albedocloudscanshiztherunawayGHboundaryclosertothestar.High-albedocloudsareatthesubstellarpointwhentheplanetisslowlyrotaEng(e.g.,Edallylockedandinthehabitablezone).

Yangetal.2013

RunawaysduetotheGH-effectofacondensableapplytonon-H2Ocondensatesaswell–e.g.,CO2onMars

Kiteetal.arXiv:1709.08302

Also:N2runawayonEarlyTitan?

Therunaway&moistgreenhouses:underthehood

RaisingtemperatureraisestheH2OmixingraEoatthecoldtrap(assumedisothermal)

KasEng1988

KasEng1988

Howwasthelast10thofVenus’oceanremoved?

KasEng1988

Runawaygreenhouse–keypoints•  The(H2O-)runawaygreenhouseisageologicallyrapidincreaseinplanetsurfacetemperaturefrom<400Kto>1000KcausedbyaposiEvefeedbackbetweenthesaturaEonvaporpressureofwatervaporandthe

•  Beabletoexplainthemechanismoftherunawaygreenhouse

•  ItisalmostcertainthatreleaseofCO2byhumanscannotcausearunawaygreenhouse

•  Theexactthresholdfortherunawaygreenhousedependsoncloudcover,landfracEon,andplanetrotaEonrate

Backup/addi6onalslides