disparate evolutions - caitlin griffith
TRANSCRIPT
� 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
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
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.
TitanGanymedeCallistoR=2575kmR=2631kmR=2410kmd=1.88gm/cm3 d=1.93gm/cm3d=1.83gm/cm3
Whydifferent?Formedfromdifferentices,perhapswithmorecarbon?Acquiredanatmospherebutlostitthroughimpacts.(Griffith&Zahnle1995)
Howcanamoleculeoratomtransitionbetween2states?
LTE:theoccupationissetbycollisionsandfollowsaBoltzmanndistributionNon-LTE:spontaneousandstimulatedemissioncandepopulatestates
Spontaneousemission
Stimulatedemission
Absorption
Collisionexcitation
Collisiondeexcitation
� 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
� 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