DavidCatlingDept.ofEarthandSpaceSciences/AstrobiologyProgram

UniversityofWashington,Sea@le

+NASAEarth&SpaceScienceFellowshipstudent,JoshKrissansen-To=on

Metricstofindlifefromremotesensingdata&theirlimitaDons

Part1:Background:ideasoflookingforliferemotelyPart2:A)Lookingatthermodynamicdisequilibrium(inGibbsenergy/moleair)inSolarSystematmospheresasametricforlifeB)thekineDcsofdisequilibriumPart3:Wherewe’regoingfromhere

Outline

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?

Part1Chemicaldisequilibriumasasignoflife?“Kine&cinstabilityinthecontextoflocalchemicalandphysicalcondi&ons…”JoshuaLederberg(1965)NatureMorethanone:“Searchfor…compoundsintheplanet’satmospherethatareincompa&bleonalong-termbasis”JamesLovelock(1965)Nature“gaseousoxygen…andatmosphericmethaneinextremethermodynamicdisequilibrium...arestronglysugges&veoflifeonEarth”CarlSagan(1993)Science

Part1Chemicaldisequilibriumasasignoflife?“Kine&cinstabilityinthecontextoflocalchemicalandphysicalcondi&ons…”JoshuaLederberg(1965)NatureMorethanone:“Searchfor…compoundsintheplanet’satmospherethatareincompa&bleonalong-termbasis”JamesLovelock(1965)Nature“gaseousoxygen…andatmosphericmethaneinextremethermodynamicdisequilibrium...arestronglysugges&veoflifeonEarth”CarlSagan(1993)Science

Part1Chemicaldisequilibriumasasignoflife?“Kine&cinstabilityinthecontextoflocalchemicalandphysicalcondi&ons…”JoshuaLederberg(1965)NatureMorethanone:“Searchfor…compoundsintheplanet’satmospherethatareincompa&bleonalong-termbasis”JamesLovelock(1965)Nature“gaseousoxygen…andatmosphericmethaneinextremethermodynamicdisequilibrium...arestronglysugges&veoflifeonEarth”CarlSagan(1993)Science

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)

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

Atmosphericdisequilibriumasabiosignatureonexoplanets?

9

Part1

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

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

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

AssumingesDmatesofgasabundances…e.g.,

Invertbandsofwell-mixedgas(i.e.,CO2@4.3μm;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,O2band@0.76μm=>columnO2IbelieveN2wasassumedbySagan+(1993).

H2O H2O

H2O

H2O H2ON2OCH4

CH4

N2O

H2O

CO2

CO2

CO2

CO2H2O

O3

PART2A:

Doesthermodynamicdisequilibrium(inGibbsenergy/moleair)inSolarSystematmospheresactasametricforlife?

CanquanDfychemicaldisequilibrium

15

Observedatmosphere

Atmosphereifitwereinchemicalequilibrium

Part2A

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

( ,P) ( ,P)

Available energy,( ) ( )

0.001

Δ = −

Δ =T TG G obser G equil

G J /mol

Part1

Jupiter

( ,P) ( ,P)

Available energy,( ) ( )

136

Δ = −

Δ =T TG G obser G equil

G J /mol

Mars

O2 + 2CO! 2CO2

Part2A

Venus

( ,P) ( ,P)

Available energy,( ) ( )

0.06T TG G obser G equil

GΔ = −

Δ = J /mol

Venus

( ,P) ( ,P)

Available energy,( ) ( )

1.5

Δ = −

Δ =T TG G obser G equil

G J /mol

Earth(atmosphereonly)

Part2A

2O2 +CH4 ! CO2 + 2H2O

Typical surface of Mars

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

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

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

Earthhaslargestdisequilibriuminthesolarsystem

Part2A

Earthhaslargestdisequilibriuminthesolarsystem

OnlyonEarthisavailableenergy≈thermalenergyofair

Part2A

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

SensiDvitytest Part2A

SensiDvitytest Part2A

SensiDvitytest Part2A

SensiDvitytest Part2A

SensiDvitytest

(extraslides)

Part2A

Whatdoesdisequilibriummean?-  Some[mesthermodynamicdisequilibriummeanslife.

CO2 + H2O→O2 + CH2O

2O 2N

3NO-

Biologicalnitratereduc[on(denitrifica[on)

Oxygenicphotosynthesis

Respira[on

Biology:fixa[on+nitrifica[on

Part2A

-  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

( ,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

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

PART2B(brief)KineDcmetrics

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))

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.

KineDcs=>SourcefluxesBo@omlines:1)  Wegetasourcefluxfrombalancingasink

flux,whichrequiresaknowledgeoftheatmosphericchemistrybeyondthatforthermodynamicequilibriumfrombulkgases

2)  Abio[cvs.bio[csourcefluxiss[llaques[onofdegree:

-  plausiblebiomass?-  implausibleabio[cflux?

Part3:Futurework1)Currentto-dolist:morediseqmcases;kine)cmetrics-deducingunusualgasfluxes,givenexpectedlifeDmes-waystoquanDfythis;metrics2)ThefullshebangwithaBayesianapproach:spectrumàinversionàgases+/-errorsàmetrics+/-errorsàprobabilityofwhetherlifeispresent=ProjectofVPL/UWastrobiologyPhDstudentJoshKrissansen-To@on:

Summary-EarthhasthelargestdisequilibriumintheSolarSystem,whichisbiogenic.-TheotherSolarSystemplanetshavesmallerdisequilibriamaintainedbyabio[cprocesses.-Forexoplanets,thermodynamicdisequilibriumcouldbecomputeddirectlyfromobserva[onswithoutassump[onsaboutgasfluxes.Animperfecttoolbutoneforthetoolbox;key(dis)equilibriainformobs.-Kine[cmetricsareanotheravenue:meansassessingplausibilityofbiogenicvs.abio[cgasfluxes

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: