sara seager - lecture3 - mit
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“AllRoadsLeadtoTPF/Darwin”
SaraSeagerMIT
BiosignaturesandtheSearchforHabitableWorlds
WhyAstrobiology
Theexoplanetatmosphereistheonlywaytoinferwhetherornotaplanetishabitableorlikelyinhabited
Theplanetaryatmosphereisourwindowintotemperatures,habitabilityindicators,andbiosignaturegases
BiosignaturesandHabitablePlanets
Overview Review Introduction: Earth’s Biosignatures The Thermodynamics of Biosignatures The Search for Habitable Worlds
KnownPlanets1995
Based on data compiled by J. Schneider
KnownPlanets1996
Based on data compiled by J. Schneider
KnownPlanets2000
Based on data compiled by J. Schneider
KnownPlanets2005
Based on data compiled by J. Schneider
KnownPlanets2010
Based on data compiled by J. Schneider
Planets 2000
Planets 2005
Planets June 14 2010
Planet Candidates June 15 2010
KeplerPlanetCandidatesannouncedJune15,2010
AtmosphereInterpretaRon
H2OandCH4intransmissionfromHSTSwainetal.(2008)SeealsoGrillmairetal.2008.
IdenficaRonofatomsandmolecules
Day‐nighttemperaturegradients
Possiblevariability
Advancesinretrievaltechniques
SeeSeagerandDemingARAA2010
OverviewPoints
MonumentalexoplanetachievementsweremadedespitewidespreadskepRcism
Thehugepromiseforthefutureisbasedontheachievementsofthepastdecade+
Keepthisinmindfortherestofthetalk
BiosignaturesandHabitablePlanets
Introduction: Earth’s Biosignatures The Thermodynamics of Biosignatures The Search for Habitable Worlds
Earth,Venus,Mars
1DmodelsfromVikkiMeadowsVirtualPlanetaryLaboratory
Earth,Venus,Mars
1DmodelsfromVikkiMeadowsVirtualPlanetaryLaboratory
Earth’sSpectrum
Turnbulletal.2007
PearlandChristensen1997
S. Seager
InsRtuteforAdvancedStudy,Princeton,July2002
VegetaRonasaSurfaceBiomarker
S. Seager
S. Seager
VegetaRonasaSurfaceBiomarker
TheVegetaRonBiosignature
• ChlorophyllcausesstrongabsorpRonatwavelengthsshorterthan0.7µm
• Lightscaceringinairgapsbetweenwater‐filledplantcellscausesstrongredreflectance
• Plantsabsorbenergyatshortwavelengthsforphotosynthesis;reflectandtransmitradiaRonatlongwavelengthsforthermalbalance
Seageretal.2005DatafromClark1993
EarthBiosignatureSummary• O2/O3:thesmokinggun• N2O:biologicalbutweaksignature
• H2O:evidenceforliquidwater
• CH4:biological/abiological
• (VegetaRonrededge)
But,wearestuckinaworldofterracentricbiosignatures.
BiosignaturesandHabitablePlanets
Introduction: Earth’s Biosignatures The Thermodynamics of Biosignatures The Search for Habitable Worlds
NothingwouldbemoretragicintheAmericanexploraRonofspacethantoencounteralienlifeandfailtorecognizeitNRCreport2007
ConstraintsforLifeintheUniverse
AllLife
Liquidwater
Carbon‐based
ChemicalPotenRalEnergy
biologist
chemist
physicist
CH2O→CO2
Fuels
Redo
xPo
tenR
al(V
)
Oxidants
H2→H+ H+→H2
CO2→CH2O
NH4+→N2
N2→NH4+
CH4→CO2 CO2→CH4
H2S→SO42‐
SO42‐→H2S
O2→H2OH2O→O2
N2→NO3‐
NO3‐→N2
Nottoscale
ElectronTower
AkerLane,NatureMay2006
CH2O→CO2
Fuels
Red
oxPoten
Ral(V)
Oxidants
H2→H+ H+→H2
CO2→CH2O
NH4+→N2
N2→NH4+
CH4→CO2 CO2→CH4
H2S→SO42‐
SO42‐→H2S
O2→H2OH2O→O2
N2→NO3‐
NO3‐→N2
Nottoscale
CH2O→CO2
Fuels
Redo
xPo
tenR
al(V
)
Oxidants
H2→H+ H+→H2
CO2→CH2O
NH4+→N2
N2→NH4+
CH4→CO2 CO2→CH4
H2S→SO42‐
SO42‐→H2S
O2→H2OH2O→O2
N2→NO3‐
NO3‐→N2
Nottoscale
ElectronTower
AkerLane,NatureMay2006
BiosignatureThermodynamics
• MetabolicreacRonsareredoxreacRons• TheGibbsfreeenergyisusedtodeterminetheenergyyieldofareacRon
• Recall:theGibbsfreeenergyisanenergypotenRal
• TheenergypotenRalcanbeconvertedintovoltsviatheNernstequaRon.
See“BiologicalThermodynamics”byD.T.Haynie.
O2,H2,CO2,N2,N2O,NO,NO2,H2S,CH4,SO2,H2O,NH3
AllEarth‐basedmetabolicbyproducts
Gaseousmetabolicbyproducts
Unique
Generatedbygeologyor
photochemistry
Nothighlysolubleinocean
Notrapidlyassimilated
SeagerandSchrenk,“AnAstrophysicalViewofEarth‐BasedMetabolismsubmicedtoAstrobiology,.
SupportedbyFQXI
H2,CO2,H2S,CH4,SO2
O2,O3,N2O
Differentatmospheres/stellarUVcouldhavedifferentbiosignatures.
RadiaRvetransfer
AtmosphericComposiRon
Chemicalequilibrium/disequilibrium
Photochemistry
Atmosphericescape
AtmosphericcirculaRon
ConnecRonwithobservaRons
Clouds
Biosignatures
BiosignatureFramework • Foraspectralfeaturetobesignificant,whatistherequiredbiosignatureflux?
• ThisisnotonlyaradiaRvetransferproblem,butalsorelatedtophotochemistrysourcesandsinks,andputaRvechemistryoftheplanetcrustandatmosphere
• Realitycheck:converttobiomassbyequaRontheGibbsfreeenergy(yield)tomaintenanceenergyoftheorganism
Seageretal.inprep.FigurefromDeWit
BiosignatureSummary
• Lifeusesandexploitschemicalenergygradients
• ReacRonsthatareenergeRcallyfavorablebutkineRcallyinhibited
• ThereisaquanRtaRvepathforwardforredoxbiosignaturefluxesinthecontextofatmosphericradiaRvetransferandchemistrymodels–givesachancetofindnewbiosignatures.
BiosignaturesandHabitablePlanets
Introduction: Earth’s Biosignatures The Thermodynamics of Biosignatures The Search for Habitable Worlds
RequirestechnologyinvestmentsAndnewspace‐basedfaciliRes
Prong1:TranrsiRngplanetswithafocusonMdwarfs
Prong2:directimagingofF,G,Kdwarfs
Two‐ProngedStrategy
Fast‐trackground‐based,andexisRngspaceassets
TransitSurveyTelescopes
TESSconcept6to9lensesonthesameplaqorm
CNES/Corot30cmmirrorPolarorbit
NASA/Kepler1mapertureEarth‐trailingorbit
ExoplanetSatconceptAtripleCubeSatconstellaRonofnanosats
PlatosciencepayloadUnderstudybyESO
Allroadsleadto…TerrestrialPlanetFinder/Darwin
DirectImaging
Marois,MacIntosh,Doyonetal.2008
A5V40pcPlanetsat:24,38,and60AUPlanet‐starcontrast104or105
Direct Imaging Telescope
Diffractedlightproblem
Direct Imaging Telescope
DiffractedlightsoluRons
…requireanearperfecttelescope
Cash2006
Cash2006
Ledby RemiSoummerandWebCashRecentSPIEpaper R.A.Brown,I.Jordan,A.Roberge,T.Glassman,A.Lo,S.Seager,L.Pueyoandothers
Technicalissues 70mocculterat70,000km StaRonkeeping ‐notopRmizedatvisible‐interesRngfeaturesnearamicron undersamplingofPSFatvisible‐limitstohighS/N limitedRme‐9%ofJWSTRme,limitssearchtoabout30stars
Principleofastarshadeonaseparatespacecraktoblockthelightfromthestar,whileallowingthelightfromanexoplanettopasstheedgeoftheocculterunimpeded.NorthrupGrumman
SeetheNASA‐sponsoredstudiesTHEIA(ledbyDavidSpergel),NWOledbyWebsterCash.SeeCash2006
SimulaRonsfora12‐dayobservingRmeforasuperEarthorbiRngasun‐likestarat10pc.
R=1000binneddowntoR=200
R.Soummeretal.SPIEinprep.ModelsbyS.Seager
SearchforHabitableWorldsSummary
• TransitsordirectimagingwillprovideavaluablehandfulofpotenRallyhabitableplanets
• VastresourcesintermsoftelescopeRmefortransitsandtechnologydevelopmentforspace‐baseddirectimagingareneeded
• Anear‐termTerrestrialPlanetFinderpossibilityistheocculter+theJWST
A`
SummaryBiosignaturegoal:understandpossible
metabolicbyproductsandtheirlifeRmesonnon‐Earth‐likeexoplanets NWO
TransiRngexoplanets:findthemviaground‐basedorspacebasedtransitsearches.
MeasurespectraoftransiRngsuperEarthsorbiRngMstarsusingtheJamesWebbSpaceTelescope
DirectimagingfromspacetofindandcharacterizetrueEarthanalogs.Theexternalocculterispromisingtechnologytoblockoutstarlightofasun‐likestartoalevelof10billionatvisiblewavelengths
seagerexoplanets.mit.edu
KeplerDataBriceDemory (EAPSpostdoc)JoshCarter (Physicspostdoc)K.Berry (PhysicsUROP)K.Singh (PhysicsUROP)
ExoplanetAtmospheresandInteriorsB.Benneke (Aero/Astrogradstudent)M.Braunstein(EAPSgradstudent)R.Hu (EAPSgradstudent)M.Nikku (EAPSpostdoc)A.‐M.Piso (PhysicsUROP)L.Rogers (Physicsgradstudent)J.DeWit SupAeromastersintern
ExoplanetSatJ.Villasenor (Kavli)G.Farmer (EAPSgradstudent)C.Pong (Aero/Astrogradstudent)M.Smith (Aero/Astrogradstudent)M.Knapp (Aero/AstroUROP)B.Stavely (Aero/AstroUROP)
ThankyoutomystudentsandpostdocsSupportedbyMIT,NASA,andFQXI