what makes a planet habitable? atmospheric science...

GEOS22060/GEOS32060/ASTR45900

Lecture3Thursday11April2019

Whatmakesaplanethabitable?Atmosphericsciencebasics

Forlast3meLecture2keypoints:FromEarthhistorytotheCircumstellarHabitableZoneconcept

•  SmallimbalancesbetweenthegeologicreleaserateofpCO2andthegeologicconsump3onrateofpCO2wouldhaveseriouslythreatenedEarth’shabitability.

•  Thisstronglysuggestsanega3vefeedbackhasregulatedpCO2overgeologic3me.–  Inthelast~10years,theevidenceforanegaJvefeedbackhasgoLenstronger-somepeoplewouldgofurtherandsayaweatheringfeedbackisrequiredtoexplainthegeologicdata.

•  Acandidatemechanismfortheweatheringfeedbackiscarbonate-silicateweathering.

•  TheHabitableZoneisdefinedastherangeofdistancesfromastarwithinwhichaweatheringfeedbackmightoperate.

Reminder:ClasswebsiteishLp://geosci.uchicago.edu/~kite/geos32060_2019/

(Walkeretal.,JGR,1981;KasJngetal.,Icarus,1993)

OnEarth,long-termclimatestabilityinvolvesthenonlineartemperaturedependenceofgreenhousegasdrawdownbyweathering.

*increase

stabilizeGHGconcentraJon

increasetemperature

increaseGHGconcentraJon

increase

Stabilizingfeedback:

Defini3onsofahabitableplanet(OperaJonal)definiJonofhabitableplanetusedinthiscourse:Habitableplanet≈maintainssurfaceliquidwateroverJmescalesrelevanttobiologicalmacroevoluJon

DependentonwhichatmosphericvolaJlesareavailable

Feedbacksinthecarbonate-silicatecycle

Langmuir&

BroeckerC

h.9

•  VolunteersneededforpresentaJonofsecJon3.1-3.6ofCatling&KasJngonTue16April

•  Homework1willbeonwebsiteSaturday(youwillgetanoJficaJonemail),anddueinclassonThu18April

•  Officehours:Thursdays,11a-noon(aeerclass).Hinds467,orbyappointmentkite@uchicago.edu.Pleaseemailatleast48hoursinadvancewitharangeofsuggestedJmes.

Lecture3:Atmosphericsciencebasics:Keypoints

•  Describeandqualita3velyexplainver3caltemperaturestructureofrocky-planetatmospheres.

•  Applyelementarymodelsofradia3onbalance.•  Explainthegreenhouseeffectintermsofver3caltemperaturestructureandopacityatvisibleandthermalwavelengths.

•  (Explainthetheore3calbasisforexpec3nganatmosphere-surfacetemperatureoffset).

HorizontalstructureJunoCamonJUNOspacecrae(Jupiterpolarorbiter)

VerJcalstructure LORRI(LongRangeReconnaissanceImager)/NewHorizons

Catling&KasJngAtmosphericevolu0ononinhabitedandlifelessworlds,Chapter1

mostlyconvec3veheattransport.opJcallythick(opaque)inthethermal,opJcallythin(translucent)inthevisibleundercloud-freecondiJons.

mostlyradia3veheattransportstra0fied

stratosphericabsorpJonof

sunlightSTRATOSPHERE

TROPOSPHERE

tropopause

ForEarth,sunlight(mostenergyin0.1-1μmwavelengths)ismostlyabsorbedatthegroundThermalemission(mostenergyat4-20μmforhabitablesurfaces)ismostlyre-radiatedtospaceintheuppertroposphere

HydrostaJcbalanceandatmosphericpressurevs.height

idealgaslaw

AtmosphericverJcalthermalstructureDRYADIABATICLAPSERATE

dependsonatm.composiJon

O(101)m/s^2(forrockyplanets,neveroralmostnever102m/s^2,perLeslieRogers’work)

Describesthetroposphereunderdry,convecJvecondiJons

SpectrumofblackbodyradiaJon

Pierrehu

mbe

rt201

0

Rayleigh-Je

anstail

Wien’sdisplacementlaw:

b=2900microns/K

ConsidertheradiaJonfieldwithinaclosedcavitythatperfectlyabsorbsandemitsradiaJonatallwavelengths.Atequilibrium,producJonandlossofradiaJonbalanceandtheintensityoftheradiaJonfieldisuniform.MaxPlanckfoundthespectrumtobeuniquelyrelatedtothecavitywalltemperature,T,bythePlanckfuncJon.

DefiniJonofBond/planetaryalbedo(usedforclimatecalculaJons)

Keplerspacetelescopemeasuresthedipinvisiblelightfromanexoplanet+starwhentheexoplanetpassesbehindtheexoplanet’shoststar.(a)Whatisthephaseangleforthismeasurement?(b)DoesthismeasurementconstraintheBond/planetaryalbedoorthegeometricalbedo?

Energybalance:

Langmuir&Broecker

Energybalance:

~2/3fromH2O;~1/3fromCO2However,“H2OisaslavetoCO2.”

CO2

CO2

Titan:complicatedduetointerplayofCH4,H2,andanJgreenhousehaze

Theglobalmeansurfacetemperaturecanbealotlessthantheeffec3vetemperature

Top-of-atmosphereenergyin=top-of-atmosphereenergyout

•  HowquicklywouldEarth’ssurfacetemperaturedoublewitha1W/m^2(<1%)top-of-atmosphereenergyimbalance?

•  HowquicklydidEarth’ssurfacetemperaturereturntonormalfollowingtheChicxulubimpact(~15kmdiameterasteroid,~30km/s)?

isaverygoodapproximaJonforthepurposesofthiscourse

PierrehumbertPrinciplesofPlanetaryClimate

Pierrehumbert,“Principlesofplanetaryclimate”

Howtoreadaplanet’sthermalemissionspectrumEarth,Nigervalley,warmseasonà

dashedlines:theoreJcalblackbodycurvessolidline:spacecraeobservaJonschemicalspecies:absorpJonbandsWhatcouldweconcludefromthisspectrumifwehadobtaineditforahabitable-zone1-Earth-radiusexoplanet?

CasJng&Katling,chapter2

Absorp3onofoutgoingEarthlight

Pierrehumbert2010

ERBE=EarthRadiaJonBudgetExperiment

RelaJonshipbetweensurfacetemperatureandatmospherictemperature

Bonusslides

what makes a planet habitable? atmospheric science...

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