Venus,EarthandTitan

�  Venus’lackofwater�  TheRunawayGreenhouse

�  Earth’sgainofO2�  Evolutionaccordingtothegeologicalrecord

�  Titan’sdwindlingCH4�  What’shappeninghere.

�  VenusformedclosertotheSun,whereH2OwasdepletedcomparedtothebuildingblocksofEarth.

�  Venuswaswarmenoughtocatastrophicallylooseallit’swaterinaRunawayGreenhouseevent.*

*Thisfurthersuggeststhatthereisatippingpointforwhichatmospherescanirreversiblyreachanentirelydifferentstate.

Convectioninvolvestheriseofhotair,whichismorebuoyantthantheoverlyingair.Thisoccursfastenoughthatlittleheatisexchangedandtheprocessisadiabatic.

Assumeidealgas:(1)(2)Differentiate(1):Include(2):(3)Firstlawofthermodynamics:Foradiabaticmotion(dq=0):Considering(3) Andhydrostaticequilibrium:

Where

Liquid

Saturated vapor

Trc

Stratosphere (radiative equilibrium) Optical depth due to vapor alone

Convective region Dry lapse rate Flux not conserved

Radiative Equilibrium

Assume hydrostatic equilibrium:

Write condensable mass density:

Assume grey atmosphere:

Then:

Let: Then:

H is the humidity, Ps the saturation pressure, Pv is the partial pressure of the vapor , k the absorption coefficient, mv and m the mean molecular weights of the vapor and the atmosphere, and g the gravity

Optical depth at Trc

RT equation: Hydrostatic Equilibrium: Optical Depth: Outgoing Flux:

For T small: F ~ T4 solutions increase with T For T large: F ~ T4/Ps solutions decrease with T -> Solutions exist only for values below a maximum F -> This critical value depends on Ps and K

Cold Trap

Cold Trap

VENUS

Only H2O

Photons

Consider: K= 0.1 cm2/g (appropriate for 8-20 um window H = 100% m = 29 g (N2) and mv = 18 g (H2O) g = 10 m/s Po = 8 mm Hg Fmax = 0.63 cal cm-2 min-1

T = 260 K Average incident sunlight on Earth is 0.5 cal cm-2 min-1 Average incident sunlight on Earth is 0.9 cal cm-2 min-1 But the albedos of Earth is 0.3 and for Venus is 0.78 So… both Venus and Earth absorb 0.3 They are subcritical.

But what if Venus was not as cloudy as in the past

Note: runaway greenhouses grow from below

�  Atmosphereistransparentatshortwavelengths�  ThisprohibitsvastcloudcoverabovetheTrclevel

�  Surfacehasalargecondensablereservoir� Whenvaporizedthecondensableisopticallythickatlongwavelengths.

However,thisstudyindicatesthatanatmospherecanreachatippingpoint,beyondwhichaplanetirreversiblyendsupinaradicallydifferentstate.

BIF

Namibia

TitanGanymedeCallistoR=2575kmR=2631kmR=2410kmd=1.88gm/cm3 d=1.93gm/cm3d=1.83gm/cm3

Whydifferent?Formedfromdifferentices,perhapswithmorecarbon?Acquiredanatmospherebutlostitthroughimpacts.(Griffith&Zahnle1995)

ImagefromUCLPlanetsgroup

N2CH4H2O

Amess.

MainPlayers:C2H6(l)C2H2(s)HCN(s)Haze(s)

FromCassini

Horstetal.2012

Griffith2009

Particlesscattermostefficientlyatwavelengthsequaltoorsmallerthantheparticlesize.

FromLorenz2000

Howcanamoleculeoratomtransitionbetween2states?

LTE:theoccupationissetbycollisionsandfollowsaBoltzmanndistributionNon-LTE:spontaneousandstimulatedemissioncandepopulatestates

Spontaneousemission

Stimulatedemission

Absorption

Collisionexcitation

Collisiondeexcitation

Lunine,StevensonandYung1982,Sagan&Dermott1982

Equivalentto1meterofCH4

Theatmospherehastheequivalentof5mofmethane.Itisclosetosaturation.

�  Justrunningoutofmethaneafterbillionsofyears�  Idea:CH4outgassedearlyon,heatede.g.byaccretion�  Consequence:thesurfacehas0.5kmoforganics*

�  Hasarecentboutofgeologicalactivity&outgassing�  Idea:Titan’sinteriorisfreezingnow,whichcircularizestheorbit�  Advantage:ExplainsTitan’snon-circularorbit�  Consequence:therearenotalotoforganicscoatingthesurface

�  IsinsomesortofequilibriumwithsubsurfaceCH4�  Idea:SubsurfaceaquifersormethanogensbalanceCH4loss�  Consequence:There’smorethanmeetstheeye.

*ThebyproductsofCH4&N2photolysis

Tobieetal.2006

Griffithetal.Nature,486,238,2012

PCAanalysisofCassiniData

Griffith,inpreparation

�  HotJupiters�  Compositionofabundantelementslikelyestablishedby

thermochemicalequilibriumbecausehotand“surfaceless”�  Secondarymoleculesaffectedbyphotochemistry�  Complications:magnetosphericeffectsonatmosphericstructure

andchemistry,disentanglinghoststar&planetsignals,andeffectsofcloudsonmeasurementsofgasabundancesandtemporaturestructure.

�  RockyPlanets�  Ifhot,potentiallysomewhatinthermochemicalequilibrium,but

surfaceinteraction/effectsareinevitable.�  Complicatednon-equilibriumeffectsinevitableandperhapsmost

interesting

�  Planets�  InsitumeasurementsofV,E,M,J,A,T

�  Surface,Atmosphere,Magnetosphere,Ionosphere,�  Spatiallyresolvedobservations(e.g.1meterbyHiRISE)�  DetailedinformationoftheSun’sattributestoo

�  Exoplanets�  Moreexamples�  Greaterbreadthofconditionstotestprocesses

�  E.g.TheC/Oratioingiantplanets�  FutureProspects

�  Morephotons(TMT)�  Morespectralcoverage(JWST)�  Butnotspatiallyresolved,norinsitu…ornot…

A=Asteroid(Bennu,acarbonaceousasteroid)Mission:OSIRIS-Rex(UniversityofArizona)

TheStarChipcanbemass-producedatthecostofaniPhoneandbesentonmissionsinlargenumberstoprovideredundancyandcoverage

Lightbeamtopropelgram-scale‘nanocrafts’to20%speedoflightcouldreachAlphaCentauri(4.37lightyears)withinabout20yearsofitslaunch.

Components:StarChip:cameras,photonthrusters,powersupply,navigationandcommunicationequipment,andconstitutingafullyfunctionalspaceprobe.Lightsail:Advancesinnanotechnologyareproducingincreasinglythinandlight-weightmetamaterials,promisingtoenablethefabricationofmeter-scalesailsnomorethanafewhundredatomsthickandatgram-scalemass.2.LightBeamerTherisingpowerandfallingcostoflasers,consistentwithMoore’slaw,leadtosignificantadvancesinlightbeamingtechnology.Meanwhile,phasedarraysoflasers(the‘lightbeamer’)couldpotentiallybescaleduptothe100gigawattlevel.

TobeleadbyPeteWorden,theformerdirectorofNASAAMESResearchCenter