What makes a planet (unin)habitable? Runaway...
Transcript of 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