metrics to find life from remote sensing data & their limitaons · metrics to find life from...
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DavidCatlingDept.ofEarthandSpaceSciences/AstrobiologyProgram
UniversityofWashington,Sea@le
+NASAEarth&SpaceScienceFellowshipstudent,JoshKrissansen-To=on
Metricstofindlifefromremotesensingdata&theirlimitaDons
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Part1:Background:ideasoflookingforliferemotelyPart2:A)Lookingatthermodynamicdisequilibrium(inGibbsenergy/moleair)inSolarSystematmospheresasametricforlifeB)thekineDcsofdisequilibriumPart3:Wherewe’regoingfromhere
Outline
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PART1:
Ideasforfindingliferemotely:
1. Search for Extraterrestrial Intelligence (SETI) legit approach, but not the focus of this talk/meeting
2. Biogenic surfaces
microbial or multicellular pigments-discussed yesterday somewhat
3. Biogenic gases
Which gases? How many gases? What levels constitute life detection? Why chemical disequilibrium?
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Part1Chemicaldisequilibriumasasignoflife?“Kine&cinstabilityinthecontextoflocalchemicalandphysicalcondi&ons…”JoshuaLederberg(1965)NatureMorethanone:“Searchfor…compoundsintheplanet’satmospherethatareincompa&bleonalong-termbasis”JamesLovelock(1965)Nature“gaseousoxygen…andatmosphericmethaneinextremethermodynamicdisequilibrium...arestronglysugges&veoflifeonEarth”CarlSagan(1993)Science
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Part1Chemicaldisequilibriumasasignoflife?“Kine&cinstabilityinthecontextoflocalchemicalandphysicalcondi&ons…”JoshuaLederberg(1965)NatureMorethanone:“Searchfor…compoundsintheplanet’satmospherethatareincompa&bleonalong-termbasis”JamesLovelock(1965)Nature“gaseousoxygen…andatmosphericmethaneinextremethermodynamicdisequilibrium...arestronglysugges&veoflifeonEarth”CarlSagan(1993)Science
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Part1Chemicaldisequilibriumasasignoflife?“Kine&cinstabilityinthecontextoflocalchemicalandphysicalcondi&ons…”JoshuaLederberg(1965)NatureMorethanone:“Searchfor…compoundsintheplanet’satmospherethatareincompa&bleonalong-termbasis”JamesLovelock(1965)Nature“gaseousoxygen…andatmosphericmethaneinextremethermodynamicdisequilibrium...arestronglysugges&veoflifeonEarth”CarlSagan(1993)Science
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Modern incarnation: Biogenic gases in reflectance spectra
If photosynthesis ceased, O2 decreases exponentially to <0.4% in ~10 m.y. Proposed search to Earth-like exoplanets for O2 and CH4
<0.3 ppmv O2
0.1% O2
21% O2
Brightne
sste
mpe
rature(K
)
wavelength(μm)
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Disequilibrium applies to waste biogenic gases, but it’s nuanced:
1) All planetary atmospheres are in disequilibrium
- Geophysics competes with biology. How much?
2) Life feeds on disequilibrium so sometimes disequilibrium might mean “no one home” i.e.,
uneaten free food=> no grad students no life
uneaten free food=> no grad students
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Atmosphericdisequilibriumasabiosignatureonexoplanets?
9
Part1
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e.g.,back-of-envelopees[mate:CH4infixedbulkair
CH4 + 2O2 ! CO2 + 2H2OΔGreaction
0 = Gproducts0∑ − Greactants
0∑ = −817.9 kJ mol-1
Keq, 298K = activity product of productsactivity product of reactants
= aGgaH
h
aBbaC
c = exp −ΔGreaction0
RT⎛⎝⎜
⎞⎠⎟
Nega[ve:thermodynamicallyfavored,withequilibriumconstantfor
pCH4 ~ (pCO2 )(aH2O )(pO2 )2Keq
= 380 ×10−6
(0.21)2 ×10143.29 = 10−145 bar
Hence,equilibriummethanein0.21barO2and380ppmvCO2is:
(net)
ButactualCH4abundanceis1.8x10-6bar
bB + cC = gG + hH of
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Equilibriumofeachgaswithfixedbulkair:O2,N2,CO2,H2O(g)
Gas Abundance Equilibriumabundance
Source DetectableinGalileoNIRspectraofEarth?
CS2 10-11–10-10 ~0 >80%Biology;+volcanic
OCS 10-10 ~0 Biology+photochemistry
SO2 10-11–10-10 ~0 Volcanic+photochemistry
CH4
1.8×10-6
10-145 >90%biology;+geothermal
NH3 10-10–10-9 10-60 Biology
O3 10-8–10-7 3x10-30 Biology+photochemistry
N2O 3×10-7 2x10-19 Biology(+minorabio[c)
Note:NumberofmoleculesinEarth’satmosphere~1044
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Bringingallgasestoequilibrium(Gibbsenergyminimiza[onformalsolu[on):
Gas Abundance Equilibriumabundance
Source DetectableinGalileoNIRspectraofEarth?
CH4
1.8×10-6
10-48 >90%biology;+geothermal
O3 10-8–10-7 2x10-30 Biology+photochemistry
N2O 3×10-7 3x10-20 Biology(+minorabio[c)
Note:NumberofmoleculesinEarth’satmosphere~1044
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AssumingesDmatesofgasabundances…e.g.,
Invertbandsofwell-mixedgas(i.e.,[email protected]μm;4.8μm)
Temperature-pressureprofile
CH4,N2O,H2O,O3abundances,albeitnon-uniquever[cal
distribu[on
Radia[vetransfermodel+retrieval/fit
Logflu
x(m
W/cm
2 /μm
/sr)
wavelength(μm)GalileoNear-IRMappingSpectrometer(Drossart+,1993)NIMS’long-visible,[email protected]μm=>columnO2IbelieveN2wasassumedbySagan+(1993).
H2O H2O
H2O
H2O H2ON2OCH4
CH4
N2O
H2O
CO2
CO2
CO2
CO2H2O
O3
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PART2A:
Doesthermodynamicdisequilibrium(inGibbsenergy/moleair)inSolarSystematmospheresactasametricforlife?
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CanquanDfychemicaldisequilibrium
15
Observedatmosphere
Atmosphereifitwereinchemicalequilibrium
Part2A
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CanquanDfychemicaldisequilibrium
16
Observedatmosphere
Atmosphereifitwereinchemicalequilibrium
Wequan[fydisequilibriumasthechangeinGibbsenergyofthesystemduringreac[ontoequilibrium:
Available energy, ΔG = G(T ,P) (observed)−G(T ,P) (equilibrium)
AppliedtoSolarSystematmospheres….
G(T ,P) = ni(Gi
°(T ,Pr ) + RT ln(
Pγini
ntot
))i∑
molesgasi,ac[vitycoeff.γi
totalmolesair;doitfor1
Part2A
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( ,P) ( ,P)
Available energy,( ) ( )
0.001
Δ = −
Δ =T TG G obser G equil
G J /mol
Part1
Jupiter
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( ,P) ( ,P)
Available energy,( ) ( )
136
Δ = −
Δ =T TG G obser G equil
G J /mol
Mars
O2 + 2CO! 2CO2
Part2A
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Venus
( ,P) ( ,P)
Available energy,( ) ( )
0.06T TG G obser G equil
GΔ = −
Δ = J /mol
Venus
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( ,P) ( ,P)
Available energy,( ) ( )
1.5
Δ = −
Δ =T TG G obser G equil
G J /mol
Earth(atmosphereonly)
Part2A
2O2 +CH4 ! CO2 + 2H2O
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Typical surface of Mars
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Typical surface of planet Earth
(2012: 13°S, mid-Atlantic, 3.8 km depth of water )
Photo on way to Ascension Island from St. Helena: David Catling
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Earth(atmosphere-oceanfluidenvelope)
Gilbert Lewis (1923): “starting with air and water…nitric acid should form. It is to be hoped that nature will not discover a catalyst for this reaction, which would… turn the oceans into dilute nitric acid”.
Available energy,2326Δ =G J /mol
Part2A
Atmosphericspecies
Aqueousspecies
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Available energy,2326Δ =G J /mol
Earth(atmosphere-oceanfluidenvelope)Atmosphericspecies
Aqueousspecies
2N2(g)+5O2(g)+ 2H2O(l)! 4H+ (aq)+ 4NO3− (aq)
Gilbert Lewis (1923): “starting with air and water…nitric acid should form. It is to be hoped that nature will not discover a catalyst for this reaction, which would…turn the oceans into dilute nitric acid”.
Part2A
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Earthhaslargestdisequilibriuminthesolarsystem
Part2A
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Earthhaslargestdisequilibriuminthesolarsystem
OnlyonEarthisavailableenergy≈thermalenergyofair
Part2A
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IsthispracDcalforexoplanets?-Forexoplanets,thermodynamicdisequilibrium(inprinciple)couldbecomputedfromobserva[onswithoutassump[onsaboutgasfluxes.-Bulkabundance,oceans,andtotalpressureareobserva[onalchallenges,buthavebeenconsidered:
-N2fromN2-N2dimerabsorpDon,4.3μm(Schwieterman+2015).-Oceanpresencefromspecularglint+H2O-richspectra(e.g.,LackofglintonMars:Phillips(1863)Proc.RoySoc.Lond.;Sagan+1993;Robinson+2010;2014)-PressurefromO2-O2dimer,1.06&1.27μm(Misra+2014).
-Sensi[vityteststodifficult-to-observevariablesinthecalcula[onshowrelaDveinsensiDvity
Part2A
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SensiDvitytest Part2A
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SensiDvitytest Part2A
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SensiDvitytest Part2A
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SensiDvitytest Part2A
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SensiDvitytest
(extraslides)
Part2A
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Whatdoesdisequilibriummean?- Some[mesthermodynamicdisequilibriummeanslife.
CO2 + H2O→O2 + CH2O
2O 2N
3NO-
Biologicalnitratereduc[on(denitrifica[on)
Oxygenicphotosynthesis
Respira[on
Biology:fixa[on+nitrifica[on
Part2A
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- Some[mesthermodynamicdisequilibriummeanslife.
- Butsome[mesthermodynamicdisequilibriummeanstheabsenceoflife(anD-biosignature).
Largeavailableenergy=an“uneatenfreelunch”->nolifeexistsor“noonehome”
CO2 + H2O→O2 + CH2O
2O 2N
3NO-
Biologicalnitratereduc[on(denitrifica[on)
Oxygenicphotosynthesis
Respira[on
Biology:fixa[on+nitrifica[on
Whatdoesdisequilibriummean?Part2A
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( ,P) ( ,P)
Available energy,( ) ( )
136
Δ = −
Δ =T TG G obser G equil
G J /mol
Aprimi[vemetabolism:2 2 2CO H O CO H+ → +
MuchCO=>nolifetodayonthesurfaceofMars.(Weissetal.2000;Zahnleetal.2011).Wecalculate:≤100bluewhalesequiv.CO-eaDngbiomassinsubsurface.
Mars Part2A
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Whatdoesdisequilibriummean?- Some[mesthermodynamicdisequilibriummeanslife.
- Butsome[mesthermodynamicdisequilibriummeanstheabsenceoflife:anD-biosignature.Largeavailableenergy=an“uneatenfreelunch”->nolifeexists.
- Conclusion:asinglenumbermetriclikeavailableenergyhastobeconsideredjudiciously–incontext.
CO2 + H2O→O2 + CH2O
2O 2N
3NO-
Biologicalnitratereduc[on(denitrifica[on)
Oxygenicphotosynthesis
Respira[on
Biology:fixa[on+nitrifica[on
Part2A
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PART2B(brief)KineDcmetrics
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LifeDmes,sources,sinksConcentra[onC;
Part2B
abundance lifeDme(years)
Source(Tmol/yr)
AbioDcfracDon
CH4 1.8ppmv 10 30 <10%
N2O 0.3ppmv 120 ~1 negligible
∂C∂t
= −λt⇒C = C0 exp(−λt) =>Life[me=1/λ
λcalculatedfromphotochemicalsinks.Examplesbelow.
Toes[matesource=sink,needtoknow:1) CH4+OH lossofCH42) MainlyN2Ophotolysislossinstratosphere(plussomeN2O+O(1D))
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O3
O(1D)excited
H2O(g)
OH‘detergent’
O(3P)groundstate
O2
Muck:NH3,CO,H2S,SO2CH4,organicsNO2
hvhv
oxidaDon
Keys:O3andH2O(g)generateOHradicals;O3photol.<340nm->O(1D)
H2O+O(1D)èOH+OH OH oxidizes muck => air transparent to visible (no significant haze at alt.)
O2,N2NO2 NO
ToknowOHsinkonCH4needtoknow…
rain
Schema[c:Essen[alsofEarth’stroposphericchemistry:Seereview:D.C.Catling(2015)PlanetaryAtmospheresinTrea&seonGeophysics(2ndEd.),vol10,Elsevier,NewYork,429-472.
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KineDcs=>SourcefluxesBo@omlines:1) Wegetasourcefluxfrombalancingasink
flux,whichrequiresaknowledgeoftheatmosphericchemistrybeyondthatforthermodynamicequilibriumfrombulkgases
2) Abio[cvs.bio[csourcefluxiss[llaques[onofdegree:
- plausiblebiomass?- implausibleabio[cflux?
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Part3:Futurework1)Currentto-dolist:morediseqmcases;kine)cmetrics-deducingunusualgasfluxes,givenexpectedlifeDmes-waystoquanDfythis;metrics2)ThefullshebangwithaBayesianapproach:spectrumàinversionàgases+/-errorsàmetrics+/-errorsàprobabilityofwhetherlifeispresent=ProjectofVPL/UWastrobiologyPhDstudentJoshKrissansen-To@on:
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Summary-EarthhasthelargestdisequilibriumintheSolarSystem,whichisbiogenic.-TheotherSolarSystemplanetshavesmallerdisequilibriamaintainedbyabio[cprocesses.-Forexoplanets,thermodynamicdisequilibriumcouldbecomputeddirectlyfromobserva[onswithoutassump[onsaboutgasfluxes.Animperfecttoolbutoneforthetoolbox;key(dis)equilibriainformobs.-Kine[cmetricsareanotheravenue:meansassessingplausibilityofbiogenicvs.abio[cgasfluxes
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1)J.Krissansen-To=on,D.Bergsman,D.C.Catling,Ondetec[ngbiospheresfromthermodynamicchemicaldisequilibriuminplanetaryatmospheres.Astrobiology16,39-67,2016.2)J.Krissansen-To=on+,IsthePaleBlueDotunique?Op[mizedphotometricbandsforiden[fyingEarth-likeplanets.Astrophy.J.817,31,2016.Shamelessplugforresearcher-levelbook,comingsoon:D.C.Catling&J.F.Kas[ng(2017)AtmosphericEvolu)ononInhabitedandLifelessWorlds,CambridgeUniversityPress.~500pages,~2600references.
Somereferences: