19 67 Ap J. . .150.1125M 9-3 + 5-3 1 ultraviolet solarradiationandallowedforredistribution ofenergybyinfraredradiation low particledensities(^10cm)at125km.This resultdoesnotappearconsistent be concernedprimarilyinthispaperwithinterpretationoftheionosphericmeasure- upper atmospherethat400°Kshouldrepresenta lower limittotheexospherictempera- with anyavailablethermalcalculations(asdistinct fromheuristicmodels)forthelower McElroy (1966).Theirobjectionsweretwofold.First, theF2modelsrequireextremely atmosphere. Furthermore,theyarguedonthebasis ofdetailedthermalstudiesthe of 85°KwasderivedfortheMartianexosphere. ments. tures onMars.Theirmodelconsideredheatingofthe upperatmospherebyabsorptionof chemistry. Iftheionicmassisknown,observedscaleheightaboveF2peakgives the ionizationmaximumisformedundercombinedeffectsofdiffusionandphoto- type. Thisinterpretationwassuggestedbyanalogywiththeterrestrialionospherewhere in earlyafternoon.Thealtitudeoftheobservedionizationpeakwasunexpected,as sible toinferdensitiesanddensitygradientsfortheneutralatmosphereatlowaltitudes as aconsequencetheupperatmospheremaybedeficientindissociationproductssuchCO,O2,and0. the NationalScienceFoundation. the plasmatemperature.Itwasassumedthat0is thedominantionandatemperature the smallvalueobservedforscaleheightofelectrondensityabovepeak.Weshall electron densityoforder10cmwasobservedatanaltitude125kmoverElectris agree generallywithrecentgroundbasedspectroscopicstudies(BeltonandHunten and, athighaltitudes,similarinformationaboutambientionization. Earth asthespacecraftpassedbehindMars.Fromobservedphaseshiftsitwaspos- Fjeldbo, andDrake1965),phaseshiftsofthetelemetrycarrierwaveweremeasuredon of reactionsinvolvingItissuggestedthatassociation(X/?)andCOmayoccurrapidly, 400° KrepresentsareasonablelowerboundtotheexospherictemperatureonMars. The AstrophystcalJournal,Vol.150,December1967 does notprovideanimportantmodeofheatlossfromtheMartianthermosphere.Itisconcludedthat model isderivedandshowntocomparefavorablywithobservation. with thisinterpretationItcannot,however,becategoricallyexcludedattime.AdetailedEregion bility ofanPIinterpretationisexplored,andattentiondirectedtoimportantdifficultiesassociated recent MarinerIVoccultationexperiment,isdeveloped.Itarguedthattheionospheredetectedinthis 1966a) interpretingtheMarinerdataassumedthationosphericpeakisofF2 experiment shouldnotbeinterpretedasanF2layeranalogoustotheterrestrialionosphere.Thepossi- 1966). Theionosphericmeasurements,however,revealedseveralsurprises.Apeak © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem Some argumentsagainstanF2interpretationwere presentedbyChamberlainand The firstpapers(Klioreetal.1965;JohnsonFjeldbo,andEshleman Derived valuesforsurfacepressureonMarsareintherange4to7millibars,and * KittPeakNationalObservatoryContributionNo.257. In therecentMarinerIVoccultationexperiment(Kliore,Cain,Levy,Eshleman, The photochemistryofCO2isdiscussedbrieflyandattentiondirectedtothepossibleimportance The accuracyofthermalcalculationsisdiscussed.Inparticularitarguedthateddytransport f OperatedbytheAssociation ofUniversitiesforResearchinAstronomy,Inc.,undercontract with A comprehensivemodelfortheMartianupperatmosphere,consistentwithresultsobtainedin THE UPPERATMOSPHEREOFMARS* Kitt PeakNationalObservatory,!Tucson,Arizona Michael B.McElroy Received April11,1967 I. INTRODUCTION ABSTRACT 1125 1126 MICHAEL B. McELROY Vol. 150 and molecular conduction. A similar study has recently been published by Fjeldbo et al. (19666). We shall discuss thermal calculations of the upper atmosphere in § II and argue that the calculations of Fjeldbo et al., which yield low temperatures for the Martian exosphere, are suspect. Johnson (1967a) has drawn attention to the possible importance of eddy conductivity as a mechanism for heat loss in the Martian thermosphere. In his analysis the convective heat flux is derived from the coefficient of eddy diffusion. Here we dispute that this pro- cedure is correct and argue in § III that eddy transport of mass and energy are not re- lated in a simple way for Mars. In particular, mass mixing may occur to great heights on Mars without significant transport of heat. The objections to an F2 model for the Martian ionosphere depend primarily on calcu- lations of temperature profiles for the neutral gas. If the observations do indeed refer to an F2 layer, the topside scale height should reflect not the neutral temperature but a mean of electron and ion temperatures. At the F2 peak on Earth, the electron tempera- ture exceeds the gas temperature by a factor of about 2.5 (Evans 1965; Dalgarno, McElroy, and Walker 1967). If a similar situation prevailed on Mars, this would indicate neutral temperatures much less than values discussed earlier in connection with F2 models and would intensify already serious difficulties of reconciling F2 models with theory. The relationships between temperatures of electrons, positive ions, and neutral particles in the hypothetical Martian F2 layer are discussed in § IV. It will be shown that, unless electron heating rates on Mars are very much less than values obtained by scaling from Earth, the F2 model requires unacceptably small values for the neutral temperature. Alternative models for the Martian ionosphere are discussed in § V. Chamberlain and McElroy (1966) adopted a model for the lower atmosphere which contained appreciable amounts of N2 and, on the basis of computed number densities at 125 km, proposed an E region interpretation for the observed ionosphere. Donahue (1966a), however, specu- lated that the ionosphere might be consistent with an FI interpretation if the lower atmosphere contained essentially pure C02. Even in this extreme situation, we find that calculated number densities at 125 km favor an E rather than FI interpretation of the Mariner data. An illustrative E region theory is developed and shown to compare favorably with observation. The theory is consistent with recent laboratory data on important reaction rates. A major uncertainty in the model, however, concerns the rate for dissociative recombination of C02+, and we emphasize that measurements of this quantity are urgently required. II. THERMAL STUDIES OF THE UPPER ATMOSPHERE Pre-Mariner studies of the Martian upper atmosphere (Chamberlain 1962; McElroy, L’Écuyer, and Chamberlain 1965) indicated exospheric temperatures of order 700° K. These studies supposed C02 was dissociated in the thermosphere and did not therefore consider the possible importance of C02 as a thermostat. Chamberlain and McElroy (1966) allowed for this possibility and attempted to derive a lower limit to the exospheric temperature. Their result (400° K) is consistent with earlier studies but disagrees with recent calculations reported by Fjeldbo et al. (19666). We shall explore in this section possible reasons for the discrepancy. Our discussion will emphasize models with minimal dissociation; evidently models with significant dissociation are not consistent with the Mariner observations. In both calculations, the effects of eddy conductivity were ignored. Heat was supplied to the upper atmosphere by absorption of solar ultraviolet radiation, transferred by molecular conduction, and lost primarily by infrared radiations from C02. The tempera- ture was obtained by numerical integration of the heat equation d_ =o (i) dz 0 "^2) ^ ^ ^ e\ -
© American Astronomical Society • Provided by the NASA Astrophysics Data System 19 67 Ap J. . .150.1125M 163-1 163-1 1 No. 3,1967ATMOSPHEREOFMARS1127 pause. for theratecoefficientdescribingvibrationalquenching ofC0.Thisfunctionalde- CO2. Inarrivingatalowerlimittotheexospherictemperature,Chamberlainand energy depositedaslocalheat,mesopause (atwhichthetemperaturegradientiszero).Theexistenceofa value ofexospherictemperature. estimated andtheexospherictemperatureisconsequentlyevenlargerthanthatob- model consideredbyFjeldboetal.;hence,itisdifficulttoaccepttheirasphysically provided by15-/¿radiationtransferredfromtheplanetarysurfacethroughlower requires, however,anenergysourceintheloweratmosphere.Inpracticethisis merical technique,couldhavedistinguishedrealfromspurioussolutionstotheheat downward fromthethermopause.Numberdensitieswereratherarbitrarilychosenat profiles fortheupperatmosphere.Thebarometricandheatequationswereintegrated have beentakentoinsurethenumericalaccuracyofcalculationsdescribedby plied toEarth’satmosphereabove60km.Wefeel,therefore,thatadequateprecautions however, withChamberlainandMcElroy.Theprogramhasalsobeensuccessfullyap- from thoseemployedinearlierstudies.TheresultsfortheMartianexosphereagree, planetary atmospherefromgroundlevelup.Thenumericaltechniquesdifferradically by differencesintreatmentofeitherradiativelossorthermalconductivity.Themost in theatmosphere.Sinceapproximately35eVarerequiredtoproduceanionpair,this km. Hespeculatesthatasimilareffectmayoccur onMarsandconsequentlytheexo- instability. Furthermore,itisnotobvioushowFjeldboetal.,evenwithastablenu- computations. Thebehaviorobservedatlowaltitudesforanexospherictemperatureof reasonable possibilitywouldappeartobenumericalerror. tained usingthemoreaccurateexpressionforrj. readily estimateeddyfluxesofheat.Specificallyhe assumesthattheeddyfluxofheatis advective transportofheatmightbeneglected.However,massmixingisobservedup meaningful. when asmallchangeinstartingconditionsintroduceslargeeffectthesubsequent od employedbyFjeldboetal.tosolvetheheatequation.Instabilityisusuallyrecognized estimate appearsconservative.Furthermore,thedifferenceisnotapparentlyintroduced about 130°Kwouldappearthereforetoprovideanexcellentexampleofnumerical thermopause. TheseparatecurvesintheirFigure4reflectvariationstheassumed the thermopauseandtemperatureswerepresentedasfunctionsofdistancebelow Chamberlain andMcElroydeliberatelyadoptedalowerboundfortheultraviolet heat shouldbemuchless than thecoefficientfortransportofmass.Thisresult isaconse- spheric temperaturemayindeedbemuchlessthan 400°K,thelowerlimitquotedby atmosphere. Thiseffectdoesnotappeartohavebeenincludedinthemathematical and shallarguethat,atleastforMars,massmixing mayoccurwithoutsignificantex- given bypcK{dB/dz),whereKistheeddydiffusion coefficientandQrepresentspoten- conductivity providesthedominantmodeofheat transportbetweenabout60and120 equation. Apparently,oneoftheassumedboundaryconditionswasexistencea Chamberlain andMcElroy. 1128 MICHAELB.McELROYVol.150 because ofthegreaterabundance ofinfraredactivemolecules.Theexpression proposed change ofheat.Inparticularweshalldemonstrate thattheeffectiveKfortransportof tial temperature.Weshalldiscussbrieflyinthis section thephysicsofeddytransport to analtitudefromabout115kmonEarthandJohnson (19676)hasarguedthateddy quence ofatendencyeddies torelaxradiativelyandassumethesametemperature as the meanfieldinwhichthey move.TheeffectismoreimportantforMars thanEarth Chamberlain andMcElroy(1966). p © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem It isdifficultthereforetodiscoveranyphysicalexplanationforthediscrepancy. We haverecentlydevelopedanelaborateprogram,designatedtotreatageneral We believethattherearegoodreasonstoquestionthenumericalstabilityofmeth- His analysisassumesthatiftheeddydiffusion coefficient isknown,thenonecan In thethermalstudiesdiscussedin§IIitwasimplicitlyassumedthatconvectiveand Fjeldbo etal.employedarathercuriousnumericalmethodtoderivetemperature III. EDDYTRANSPORTINTHEUPPERATMOSPHERE 19 67 Ap J. . .150.1125M 11-3 72-1 2 spectively; rjdenotestheratecoefficientforvibrational quenchingofCO2;andhv pendence forrjpresented in§II,wefind specifies theenergyofemittedphoton.Substituting theexplicittemperaturede- where nÇCO*)andndenotenumberdensitiesof C0andtotalnumberdensities,re- Thus wecansimplyassociateatimeconstantrwithradiativerelaxationbysupposing where Rdenotestheradiativeenergylossduetoemissionof15-/¿radiationbyCO2. may bedescribedbyatemperatureTandthatrepresentativeeddyhasanassociated molecular conduction.Thetimeconstantsfortheseprocesseswill,ingeneral,differfrom equations wecanreadilyshowthattheradiationrelaxationofperturbationisde- in bothcasestendtoequilibratewithitssurroundings.Byconsideringtherelevantheat up todensitylevelsoforder10cmonMars.Atthisaltitudemoleculardiffusion that theperturbarion€variesasexp(—t/r).Itisgivenby scribed bytheequation we deriveatimeconstantoforder1dayforeddytransportmolecules.This would becharacterizedbyacoefficientofabout2.5X10cmsecandweshallassume We shallconsideranextremesituationwhereeddymixingdominatesmoleculardiffusion diffuse adistanceoforderH,thelocalscaleheight,andisgivenapproximatelybyH/K. than thebackgroundgas.Ifperturbationisnegativeitwillradiatelessrapidlyand temperature Te.litheperturbationeispositive,eddywillradiatemorerapidly constant mustbecomparedwiththetimedescribingthermalrelaxationof diffusion coefficientKisknown.Conventionallyitdefinedasthetimerequiredto it mustproceedrapidlycomparedwithcompetingprocessessuchasradiationand upper atmosphere.Eddiesacttotransportatomicspeciesdownwardandreplacethem mixing time. eddy withthemeanfieldinwhichitmoves. that timeconstantsforeddyandmoleculardiffusionareequal.Withthisassumption important eddytransportofheat.Inanequilibriumsituationeddiesdonoteffectany where eddiesconservetheirthermalidentityovertimeperiodsoftheordermass by Johnson(19676)todescribeeddytransportofheatwouldbevalidonlyinthelimit those associatedwithchemistryandmoleculardiffusion. chemistry andmoleculardiffusion.Similarlyifeddytransportofheatistobeimportant occur withinatimeperiodshortcomparedtoperiodsassociatedwithatmospheric by moleculesfromtheloweratmosphere.Forcompletemixing,eddytransportmust diffusion, bothofwhichleadtoapreponderanceatomicovermolecularspeciesinthe net transportofmass.Theyact,however,tosmoothoutchemicalinhomogeneitiesin No. 3,1967ATMOSPHEREOFMARS1129 the atmosphere.Theseinhomogeneitiesoccurfortwoprincipalreasons,chemistryand t 2 © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem In theupperatmosphere,radiativecoolingbyCO2 maybeadequatelydescribedby In ordertoderiveanappropriaterelaxationtimewesupposethatthebackgroundgas The timeconstantformoleculardiffusionmaybereadilyestimatedifthe It isarelativelysimplemattertoshowthatmassmixingdoesnotnecessarilyimply pcj= -[R(T+i,z)-R{T,z)]c^-t~, vt T 24 R(T,z){(27.6/T*/3)+hv/kT} R(T,z) =n(C0)nr¡hvexpÇ— 2t (T Pp T ~dR/dT' _ PCp 19 67 Ap J. . .150.1125M 2 + + 103 3 -3 -31 min. Evidentlythisvalueisverymuchlessthanthetimerequiredtotransportmassa great heightsonMarswithoutsignificanttransportofheat. distance oforder1scaleheight.Weconcludethereforethatmassmixingmayoccurto height Hwouldbegivenby to assumethesametemperatureasbackgroundgaswithatimeconstantoforder45 1130 MICHAELB.McELROYVol.150 ultraviolet solarphotonsandmanyinitiallypossessconsiderablekineticenergy.They With theseassumptions,temperaturesoforder85°KwerederivedfortheMartian for thelowtemperaturesofinterest inthisdiscussion,electroncollisionswithOandCOmay beneglected. slow byavarietyofcollisionprocessesincludingCoulombiccollisionswithambient be identifiedasanF2layer. where Tand7\-representelectroniontemperatures,respectively,Mdenotes of theunfavorablemassratiothisprovessisslowcomparedwithenergytransferbe- electrons. Thisprocessrepresentsanimportantselectiveheatsourcetotheambient exosphere. Weshalldiscussinthissectiontherelationshipbetweentemperaturesof electrons, ionsandneutrals.UnderthesecircumstancestheMartianionospherecannot argue that,unlessimprobableassumptionsaremadeconcerningthemagnitudeofelec- tron temperatureshouldexceedtheneutralandiontemperatures.Indeed,weshall electrons, ions,andneutralsarguethat,forMarsaswellEarth,thedaytimeelec- the massofmostabundantion.PreviousdiscussionsF2layeronMarsassumed scale height(29km)providesathirdequation, where w(O)denotesnumberdensityofneutraloxygen atomsandTindicatesneutral tron heatingrates,theredonotexistanycrediblesolutionstotheheatequationsfor that electronandiontemperatureswereequalidentifiedthedominantas0. density isaslow10cm” . constituents. FollowingpreviousdiscussionsoftheF2model,weassumethatatomic the iontemperature.Inequilibrium(Dalgarno,McElroy,andMoffett1963) electrons. tween electrons.Oneexpects,therefore,thattheelectrontemperatureshouldexceed will bedescribedbytheequation(Brace,Spencer, andDalgarno1965) oxygen isthedominantconstituentatF2peak.Thenthermalbalanceiniongas and nrepresentsthenumberdensity(cm)ofambientelectrons. where Qdenotestherate(eVcmsec“)atwhichenergyissuppliedtoelectrongas temperature. e n e 113 2 3 © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem Thus atanaltitudecorrespondingtoneutraldensitiesof10cm“,eddieswilltend If theobservedionosphereonMarswereindeedanF2layer,thentopsidescale Photoelectrons areproducedintheupperatmospherebyabsorptionofextreme The ambientelectronscoolbyCoulombiccollisionswithpositiveions,andbecause Thetimeconstantsforthermalrelaxationandmassmixing arenotcomparableuntilthetotal We havenowspecifiedtwoequationsrelatingT , Ti,andT.Theobservedplasma Positive ionsareheatedbycollisionswithelectronsandcoolneutral Thisformulaconsidersenergy lossbyelectroncollisionswithoxygenions.Wehaveverified that, e n 5 xio-%e(r-T)T-w=8.6XlO^MOXT; -T) cxe n IV. IMPLICATIONSOFTHEF2HYPOTHESIS <2 =5X10-W(r-T)T-w, e% T +T,=195, e kjTe+Ti) M+g 19 67 Ap J. . .150.1125M 1 931 31 3 31 31 functions ofQ.Itisevidentthatacceptablephysicalsolutions(i.e.,positivetempera- for allthreetemperatures. No. 3,1967ATMOSPHEREOFMARS1131 much largererrorsforMars,andwefeelthereforethattheelectronheatingrateonMars and, inprinciple,iftheelectronheatingrateQisknown,wecansolvetheseequations height mustbegreaterthan29km.Ifweinsistthattheneutraltemperatureexceeds A detailedcalculation,usingtechniquesdevelopedfortheterrestrialionosphere,gives sec“. oxygen densityof10cm“.IfQexceeds310eVsec“,thentheionosphericscale tures) requireheatingratesQlessthan310eVcm“sec“.Wehaveassumedanatomic 85° K,thelowestvaluequotedinliterature,thenQmustbelessthan55eVcm“ curate towithinperhapsafactorof2forEarth.Itwouldbedifficultaccommodate significantly exceeds55eVcm“sec“. theory andobservation(Dalgarnoetal.1967)suggeststhatcalculatedQvaluesareac- a valueof1000eVcm“sec“fortheelectronheatingrateonMars.Comparisonbetween T. +Ti=195. (Tn) atthehypotheticalF2peak areshownasfunctionsofelectronheatingrate(Q),under theconstraint © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem Temperatures ofelectrons,ions,andneutralparticlesarepresentedinFigure1as It isinstructivetoconsiderpossiblerangesforQatthehypotheticalMartianF2layer. Fig. 1.—ImplicationsoftheF2 hypothesis.Temperaturesofelectrons(T<),ions(Ti), and neutrals 19 67 Ap J. . .150.1125M -1 5/2 12-3 more thananorderofmagnitudelessvaluescomputedusingtechniquesdeveloped hypothetical F2peakislessthan1000°Kkm.ItobviousfromtheMarinerIVdata however, variesasr,andconductionwillthereforeberelativelyinefficientforthe difficulties thishypothesisencountersinsatisfyingmodelcalculations.Inthenext fied asanF2layer.Theargumentspresentedinthissectioncompoundalreadyserious for andsuccessfullyappliedtoEarth,theobservedMartianionospherecannotbeidenti- small temperaturesofinterestinthisdiscussion.Itisasimpleexercisetoshowthat energy-loss mechanismfortheelectrons.Theconductivitycoefficientanelectrongas, limiting caseofpureCO2.Evenforthisextremesituationwefindthatcalculated section wediscussalternativetheoriesfortheMartianionosphere. conduction maybeneglectedsolongasthegradientofelectrontemperatureat observation. Throughoutthediscussionweemphasizemodelswhichgivelowest and McElroy(1966).Weshallexaminehissuggestioncriticallybyconsideringthe contained anappreciablylargerfractionofCO2thanthatconsideredbyChamberlain conditions theyadoptedtheonlycrediblemodelavailableforloweratmosphere. specified atsomealtitude,preferablyabovethemesopause,andtosupplytheseboundary heating byabsorptionofnear-infraredandultravioletsolarradiationsallowedfor possible densityat125km.Thus,wehaveexcludedmodelswithanyappreciabledis- redistribution ofheatbymolecularconductionandradiativetransferat15Complica- surface pressureof6.3millibars.Theprogramusedtoderivetheseresultsconsidered data. AdetailedEregionmodelwillbedevelopedandshowntocomparefavorablywith number densitiesat125kmfavoranEratherthanFIinterpretationoftheMariner ionosphere asanEregionproducedbyabsorptionofsoftsolarX-rays. order 5X10cmat125kmandwereobligedthereforetointerprettheMartian This model,computedbyPrabhakaraandHogan(1965),assumedasurfacepressureof atmosphere ofMars.Theirprogramrequiredthatnumberdensitiesandtemperaturesbe that gradientsofthismagnitudedonotoccur. Hall, andSchmidtke(1965).Absorptionphoto-ionization cross-sectionswereob- shapes withpressureandtemperaturewereadequatelydescribedbythetheory,whose sociation belowthisaltitude. For thisatmosphereChamberlainandMcElroy(1966)calculatedanumberdensityof simple FIinterpretationoftheMarinerdata.For thisdescriptiontoapply,wewould tions duetodeparturesfromlocalthermodynamicequilibriumandvariationsofline 1132 MICHAELB.McELROY perature, etc.)inourprogram. WeareunablethereforetosubstantiateDonahue’s sug- require thatmaximumionizationoccurat125km. Wehavenotbeenabletoachieve details willbediscussedelsewhere. 10 millibarsandfractionalabundancesof4456percentforCO2N2,respectively. this resultbyreasonablevariationoftheseveralparameters (composition,surfacetem- tions presentedinFigure2,aregiven3. tained byaveragingthebestavailablelaboratorydata (Hintereggeretal.1965;Cookand 6 Ching 1965).Ourresults,forasolarzenithangleof 70°andtheneutralparticledistribu- gestion thattheionospheric layershouldbeinterpretedasanFIpeak. (X <1025Â)intosixty-eightspectralintervalsin the mannerdescribedbyHinteregger, © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem In theaboveanalysiswehaveignoredpossibleimportanceofconductionasan In summary,therefore,wehaveshownthat,unlesselectronheatingratesonMarsare Donahue (1966a)hasarguedthatanFImodelshouldprevailiftheloweratmosphere To obtainionproductionrateswedividedtheionizing portionofthesolarspectrum Chamberlain andMcElroy(1966)didnotattempttocomputemodelsforthelower Evidently theionproductionratespresentedin Figure 3arenotconsistentwitha Figure 2presentsamodelatmospherecomputedforessentiallypureC0,with Following asuggestionby Dr.Eshleman,wehavealsoconstructedamodel atmos- 2 V. ALTERNATIVEMODELSFORTHEMARTIANATMOSPHERE 19 67 Ap J. . .150.1125M Fig 2. bars wasadoptedandtheatmosphereassumedtobecomposedofessentiallypureCO2atallaltitudes. © American Astronomical Society Fig. 3.—Ionproductionratesforasolarzenithangleof70° andtheatmosphericmodelpresentedin Fig. 2.—DensitiesandtemperaturesinamodelMartianatmosphere.Asurfacepressureof63milli- 100 200300 Provided bytheNASA Astrophysics DataSystem 19 67 Ap J. . .150.1125M -43 43 phere forwhichthetemperaturewasfixedat160°Kbelow30km.Weadoptedasurface phere arepresentedinFigure5. pressure of5millibarsandassumedpureCO2atallheights.Evenforthisextremesitua- separation above128km.Attheturbopauseweadoptedfractionalabundancesof phere theFIpeakwouldoccuratabout135km. assumed thatCO2andN2werethedominantconstituentsallowedfordiffusive tion, wewereunabletoobtainanionizationmaximumaslow125km.Forthisatmos- 1134 MICHAELB.McELROYVol.150 10,10~, and10“for0CO,0,respectively.Ionproductionratesthisatmos- present withfractionalabundancesof10~,10“andrespectively. and 56percent,respectively.Theturbopausewaslocatedat128kmwhereO2,CO,Owereassumed bars wasadopted.TheloweratmospherecomposedofCO2andN2withfractionalabundances44 interpretation thattheupperatmospherecontain relativelysmallamountsofOandO2. reactions involvingtheoxygenmetastabletermO^D). were ratherarbitrarilychoseninthisanalysis.It is,however,essentialtoanEregion followed by Quite possiblyjustsuchamechanismdoesindeed exist,proceedingthroughchemical This wouldbethecaseifthereexistedafastassociation mechanismforCOandO. 2 1964; Slanger1966;YoungandUng1966)is © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem Figure 4depictsanalternativemodelfortheMartianatmosphere.Inthiscasewe The locationoftheturbopauseandfractionalabundances ofdissociationproducts Fig. 4.—DensitiesandtemperaturesinamodelMartianatmosphere.Asurfacepressureof10milli- A possiblereactionpath,suggestedbylaboratory studiesofphotolysis(Warneck 100 200300400 1 ^ +C0-^0(P) 2 OOD) +C02->C0, 3 CO3 +CO—>2C0. 2 TEMPERATURE (°K) 19 67 Ap J. . .150.1125M 63-181 + + + 5317 4 The intermediatecomplexCO3isprobablyunstablebutshouldinsureaveryrapidrate photochemical equilibrium.Someofthemoreimportantchemicalreactionsandasso- No. 3,1967ATMOSPHEREOFMARS1135 for three-bodyassociationofO^D)andCO. results weassumedamodesttemperaturedependenceforreactions(10)and(11). 44 percentCO2,56N2atmospherearepresentedinFigure6.Toderivethese order 10“cmsecand10~sec“at130°Ktotheroomtemperaturevalues ciated ratecoefficientsadoptedinthisstudyaresummarizedTable1.Resultsforthe Specifically weassumedthattheassociatedratecoefficientsincreasedfromvaluesof perhaps fortuitous,thatthemodelpredictsasecondary ionizationledgeatapproximate- ment withobservation.AnF2peakdoesnotoccur becauseoftherapidreaction0 laboratory data(Norton,Ferguson,Fehsenfeld,and Schmeltekopf1966). potential surfaces(Batesand Dalgamo1962). Evidently ourresults,particularlyabovetheionization maximum,areinexcellentagree- ner data.Itisofferedmerelytoillustratethepossibility ofanEregioninterpretation. and C0.Belowthepeakoccultationdataare lessreliable.Itisintriguing,though 02 recombinationcoefficientbelowarout200°K andtotheC0valueaboveabout Fig. 4. 200° K.Theratesadoptedforallotherreactions are inexcellentagreementwithrecent or remainconstantwithtemperature. Theappropriatebehaviorisdeterminedbythedetailed shapeof 10“ cmsec“and10”indicatedinTable l.Ourresultsaresensitivetothe 2 2 4 © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem The electrondensitymaybeobtainedbysolvingasetofcoupledequationsdescribing The modeldepictedinFigure6isnotproposedasa uniqueinterpretationoftheMari- Fig. 5.—Ionproductionratesforasolarzenithangleof70°andtheatmosphericmodelpresentedin Itshouldberecognizedthat ratecoefficientsfordissociativerecombinationmayincrease, decrease, IOO 200300400500600700800900 ION PRODUCTIONRATE(cnî3sec“l) 19 67 Ap J. . .150.1125M + was 70°.Computedelectrondensities(solidcurve)arecompared withobservationsfromMarinerIV, constants summarizedinTable1. indicated by*.Thefigure,whichalsodepictscalculateddensities ofC0and0,wasderivedusing the modelatmospherepresentedinFig.4,ionproduction ratespresentedinFig5andchemicalrate 2 © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem Fig. 6.—ResultsofadetailedcalculationfortheMartian ionospherewhenthesolarzenithangle + + (14) N (13) NO+ (12) CO+ (10) co (11) o+ 2 2 2 (9) (8) (7) C0+ (6) N+ (5) N+ (4) N+ (3) 0+ (2) 0+ (1) 0+ 2 2 2 ccv C0+ + e + e + e + e + 0 + o + co + 0 + e■ + o + co + n + 0 + co 2 2 2 2 2 2 2 2 Photochemistry intheIllustrative •N •n ■ n •0+ •0 •CO 2 2 0 N N c CO co CO CO 2 Model oftheERegion + + + + Reaction + + + N. + 0 + 0. + 0 + 0. + 0. + 0.. + co + N0+ + o + C0 + N. +0 + o. 2 2 2 2 2 2 2 TABLE 1 10 7 7 10 7 10 10 7 5 9 11 10 12 9 Rate Coefficient 2 OXIO" 1 0XIO“ 1 0XIO" BOX IO“ 1 0XIO" 2 5XIO" 1 0XIO" 1 0XIO“ 1.0 XIO" 1 Xio- 3 5X10- 1 0Xio- 1 8X10- 1 2X10- -1 (cm3 sec) at 300°K 19 67 Ap J. . .150.1125M + 5 -6 3 9-3 + 531 + 1+ + 63-1 -63 urgently required.Weshouldpointout,however,thatdissociativerecombinationcanbe by absorptionofhardX-raysandwouldbeanalogoustotheterrestrialDregion. ly thesamealtitudewhereMarinerdetectedasimilareffect.Itisproducedinourmodel recombination ofC02.Measurementsforthisreactionarelackingandobviously mon. picture are:(1)calculatedmodelsfortheloweratmospherearenotconsistentwith standing oftheMartianupperatmosphere.Specifically,webelievethationosphere a veryrapidprocessandratecoefficientsaslargeseveraltimes10arenotuncom- level, whereasdetailedcalculations,andexperiencewithEarth’sionosphere,suggest detected byMarinerIVshouldnotbeinterpretedasanF2layer.Theobjectionstothis cold exosphere,whereasourcalculationsindicatethattheexospherictemperature model wasdevelopedandshowntocomparefavorablywithobservation.This value forthecorrespondingreactiononEarthbyafactoroforder10, appreciable differencebetweentemperaturesofelectrons,ions,andneutralsatthis should exceedabout400°K;and(3)theF2hypothesiscannotbereconciledwithany the requiredneutraldensityof10cmat125km;(2)F2interpretationrequiresa demonstrate thatanEregiontheorycanbeconstructedwhichaccuratelyreproduces considerations wouldreconciletheF2hypothesiswiththeory.AnillustrativeEregion larger sincetheoccultationtechniqueselectselevatedsurfacefeatures.Neitherofthese may beimportantforMars.Weassumedalsothatthereportedaltitude(125km) coefficient fortheimportantchemicallossprocess(0+CO2)onMarsexceeds that pronounceddifferencesshouldindeedexist. sections forCO2wereequalbelow600Â.Atlonger wavelengthsthereareindications appropriate. Inouranalysisweassumedthatphoto-ionization andabsorptioncross- for whichwerequiredavalueoforder10“cmsec“at300°K.If,however,thisrate sible Eregioninterpretationsofthedata.Weofferthisparticularmodelmerelyto several unknowns(composition,chemistry,etc.)wecouldconstructaninfinityofpos- should not,however,beconsideredinanysenseunique.Obviouslyvaryingthe of thermaltidesinourmodels,althoughLindzen(1967)hasarguedrecentlythatthey it isdifficulttoacceptasimpleElinterpretation,althoughwearereluctantexclude ditional laboratorydataarerequiredandshould clarify manyoftheoutstandingdiffi- sistent withrathersmallerratesfordissociativerecombination ofC02.Evidentlyad- the Eregiontheoryremainsalikelycandidate.Onbasisofourthermalcalculations, the ionizationmaximumwasaccurate.Therealheightcould,however,besomewhat this possibilitycategorically.Wedidnot,forexample,considerthepossibleimportance sec" forNat300°K. computational techniqueswereappliedtoEarth’s atmosphereandprovedadequate. culties. curred alsobelow600Â,itispossiblethatthisE regioninterpretationmightbecon- that theionizationefficiencymaybeconsiderably lessthanunity.Ifsuchaneffectoc- in themodeldevelopedhereconcernsratefordissociativerecombinationofCC>2 the observedtopsidescaleheight. ticular wearguedthateddyconductivityisprobably notimportantforMars.Our coefficient weresomewhatsmaller,itisstillpossible thatthisEregionmodelcouldbe D regionmaybeaslarge 2 XÍ0“cmsec.Biondi(1964)hasreportedalowerlimit of10cm 4 5 + © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem The mostcriticalassumptioninourchemicalmodelconcernstheratefordissociative We haveattemptedtopresentacriticalassessmentofthecurrentlevelunder- We havediscussedalternativeexplanationsfortheMarinerdataandconcludedthat It iseasytounderstandwhyanF2layershouldbesuppressedonMars.Therate All Eregiontheoriesencounterparticulardifficulties.Themostcriticalassumption We havediscussedinsomedetailtheaccuracyof ourthermalcalculations.Inpar- Forexample,Donahue(1966ft) hasarguedthattherateforNOrecombinationin terrestrial VI. SUMMARYANDCONCLUSIONS ATMOSPHERE OFMARS1137 1138 Michael b. Mcelroy
The photochemistry of CO2 was discussed briefly and attention was drawn to the pos- sible importance of reactions involving 0(lD). We speculated that recombination of O^jD) and CO might proceed very rapidly on Mars. As a consequence the upper atmos- phere may be deficient in dissociation products such as CO, O2, and O. Finally, we should emphasize that additional observations of the Martian ionosphere would be desirable. It is obviously dangerous to draw any definitive conclusions on the basis of a single observation. Mariner IV might have observed an ionospheric anomaly (such as sporadic E) and the results might not therefore be representative. In common with other writers we have attempted to minimize the number of hypotheses in our theory and have neglected speculative possibilities such as corpuscular impact. The only ionization sources considered in this study were solar ultraviolet radiation and X-rays.
I would like to thank J. W. Chamberlain, R. M. Goody, and D. M. Hunten for their valuable comments and critical interest in this work. Mr. R. E. Bowser provided capable assistance with the numerical aspects of the research. Note added in proof.—C. S. Weller and M. A. Biondi (Phys. Rev. Letters, 1967, 19, 59) have recently reported a measurement of the CC^* recombination coefficient. They find (3.8 + 0.5) X 10~7 cm3 sec-1 at room temperature with an indication that a similar result applies at 210° K. We do not believe that these measurements necessarily exclude the model developed above. For example, it is unlikely that the laboratory and Martian ions are in the same states. Weller and Biondi argue that their C02+ is not in an excited state. + This may not be true for Martian C02 , which is produced in a very different manner by photoionization in a tenuous atmosphere (see §§ V and VI).
REFERENCES Bates, D. R., and Dalgarno, A. 1962, in Atomic and Molecular Processes, ed. D. R. Bates (New York: Academic Press), chap, vii, p. 245. Belton, M. J. S , and Hunten, D. M. 1966, Ap. J, 145, 454. Biondi, M. A. 1964, Ann. de Geophys., 20, 5. Brace, L. H., Spencer, N. W., and Dalgarno, A. 1965, Planet. Space Sei , 13, 647. Chamberlain, J. W. 1962, Ap. /., 136, 582. Chamberlain, J. W., and McElroy, M. B. 1966, Science, 152, 21. Cook, G. R., and Ching, B. K. 1965, Aerospace Corporation Tech. Rept. No. TDR-469(9260-01)-4 (unpublished). Dalgarno, A., McElroy, M. B., and Moffett, R. J. 1963, Planet. Space Sei., 11, 463. Dalgarno, A., McElroy, M. B., and Walker, J. C. G. 1967, Planet. Space Sei. (in press). Donahue, T. M. 1966a, Science, 152, 763. . 1966&, J. Geophys. Res., 71, 2237. Evans, J. V. 1965, J. Geophys. Res., 70, 4365. Fjeldbo, G., Fjeldbo, W. C., and Eshleman, V. R. 1966a, J Geophys. Res , 71, 2307. —. 19665, Science, 153, 1518. Hinteregger, H E., Hall, L. A., and Schmidtke, G. 1965, Space Res., 5, 1175. Johnson, F. S. 1965, Science, 150, 1445. . 1967a, paper presented at the COSPAR 7th Internat. Space Sei. Symp , Vienna (to be published in Space Res. VII). . 19675, paper presented at the COSPAR 7th Internat. Space Sei Symp , Vienna (to be published in Space Res. VII). Kliore, A., Cain, D. C., Levy, G. S., Eshleman, V. R., Fjeldbo, G., and Drake, F. D. 1965, Science, 149, 1243- Landau, L., and Teller, E. 1936, Phys. z. Sowjetunion, 10, 34. Lindzen, R. S. 1967, paper presented at Conf. Atm. of Mars and Venus, Tucson, March. McElroy, M. B., L’Écuyer, J., and Chamberlain, J. W. 1965, Ap. J., 141, 1523. Norton, R B., Ferguson, E. E., Fehsenfeld, F. C., and Schmeltekopf, A. L. 1966, Planet. Space Sei., 14, 969. Prabhakara, C., and Hogan, J. S. 1965, J. Atm. Sei., 22, 97. Slanger, T. G. 1966, J. Chem. Phys., 45, 4127. Warneck, P. 1964, J. Chem Phys., 41, 3435. Young, R. A., and Ung, A. Y.-M. 1966, J. Chem. Phys , 44, 3038. Copyright 1967 The University of Chicago Printed in U S A
© American Astronomical Society • Provided by the NASA Astrophysics Data System