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 , are suspect. Johnson (1967a) has drawn attention to the possible importance of eddy conductivity as a mechanism for heat loss in the Martian . 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 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,

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).

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