Meteoric magnesium ions in the Martian ... - Wiley Online Library

JOURNAL OF GEOPHYSICAL

RESEARCH, VOL. 105, NO. El, PAGES 1695-1707, JANUARY 25, 2000

Meteoric magnesiumionsin the Martian atmosphere William Dean Pesnell Nomad Research,Inc., Bowie, Maryland

JosephGrebowsky NASA/Goddard SpaceFlightCenter,Greenbelt, Mm.'yland

Abstract. From a thoroughmodelingof the altitudeprofileof meteoricionizationin the Martian atmosphere we deducethat a persistentlayer of magnesiumionsshouldexistaroundan altitudeof

70 km.Onthebasisof theestimated meteorold massfluxdensity, a peakiondensity of • 104ions cm 3 ispredicted. Allowingfortheuncertainties in all of themodelparameters, thisvalueis probablywithin an orderof magnitudeof the correctdensity.Of theseparameters, the peakdensity is mostsensitiveto the meteoroldmassflux densitywhichdeterminesthe sourcefunctionfor Mg

fromtheablatingmeteoroids. Unliketheterrestriai case,wherethemetallicionproduction is dominated by charge-exchange of thedeposited neutralMg withtheambientions,Mg4 in the Martianatmosphere is producedpredominantly by photoionization. The low ultravioletabsorption of the Martian atmospheremakesMars an excellentlaboratoryin whichto studymeteoricablation. Resonancelinesin the ultravioletthatcannotbe seenin the spectraof terrestrialmeteorsmay be visibleto a surfaceobservatoryin the Martian highlands.

1. Introduction

19841, Uranus [Strobel et al., 1991], and Neptune [Lyons, 1995] also show tracesof narrow, low-altitude ionosphere Althoughthe meteoricinflux into planetaryatmospheres layersbelow the main ionospherepeak that could be meteis a well-modeled process [Opik, 1958;Lebedinets et al., oric in origin. A continuousinthll of interplanetaryparticles 1973], few measurements of the ionosphericconsequences is commonto all planetsandwill releasethe metallicspecies of this depositionare availablebecausethe maximum ionnecessaryto form similar layersin thoseatmospheres, inizationoccursin a regionof the ionosphere difficultto reach cluding Mars. However, Mars may have only a single layer by spacecraft. In situ measurementsfrom soundingrock- with a relatively large altitude extent. Narrow layers on ets in the terrestrialionospherealways detect metallic ion Earth [Mathews,19981andprobablyon the giantplanetsare layersbelow the main ionosphericpeak [Grebowskyet al., producedby verticalwind shearsin the presenceof strong 19981suchasillustratedin Figure1. Ground-based observa- •nagneticfields, conditionsthat are absenton Mars. There tionsoftenseelayersin this regionas narrow,time-varying, are, as of yet, no positivedetectionsof low-altitude ionosporadic-E events (seereviewbyMathews [1998]).Within sphericstructuresin the Martian atmospherethat could be thesesporadic-Elayers,metallic ions of extraterrestrialoriattributedto meteorolds.This may reflect an observational gin canbethe dominantion species.The low-altitudemetal- limitation,or, if the layersare absent,what is happeningto lic ion layersare but one plasmamanifestationof the metethe incominginterplanetarymaterialmustbe determined. oric inputinto the lowerterrestrialionosphere.TheseterresWhile extensiveradio occultationelectronprofilesand trial metallicion layerscouldprovideelectriccurrentchandetailedtheoreticalmodelionospheres are publishedfor the nels and affect the ionosphericdynamoand electric field Martian atmosphereabove 100 km [Zhang et al., 1990a, b], plasmadynamics[Sz.usz. cz.ewicz.et al., 1995]. Metallic ions the regionbetweenthe surfaceand 100 km hasbeenlessinhave long lifetimes and are transportedfar from their retensivelystudied. Vertical profilesof the daysideelectron gion of origin by neutral winds and ionosphericE fields. density obtainedfrom spacecraftradio occultationsagree They can be usedas tracersto track ionosphericand therquitewell in magnitudewith modelsof theupperionosphere mospheric m•)tions, and,underfine-scale LIDAR investiga[Fox et al., 1996; Haider, 1997; Shinagawa and Cravens, tions, providea uniquelaboratoryfor studyingion-neutral 1989] althoughthe observations showcomplexstructures chemistryprocesses [Alperset al., 1993]. that have not yet been modeled.The models,in agreement Radio occultationmeasurements of the ionospheres of with the few in situ measurements, have a prominentmain Venus [seeFigure I and Kliore et al., 1979], Jupiter[see layer consisting of O• changing to a bottomside ofO• and Figure I and Hinson et al., 1998], Saturn [Atreya et al., Copyright2000 b'ytheAmericanGeophysical Union. Papernumber1999JE001115. 0148-0227/00/1999JE0011

t 5509.00

NO-•. Below 100 kin, theoreticalstudiesby Aikin [1968] and Whittenet al. [1971a, b] predicteda minimum in the electrondensityat the altitudethatcoincideswith the metallic ion layer we are goingto discuss.Below40 kin, those samemodelspredictan ionosphere thatis generatedby cos-

1695

1696

PESNELL

AND GREBOWSKY:

MARTIAN

METEORIC

MAGNESIUM

ION LAYERS

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Figure 1. (a) Venus[Reprintedwith permissionfrom Kliore et al., 1979. Copyright 1979 American Associationfor the Advancementof Science],(b) Earth [Kopp, 1997], (c) Mars [Halder, 1997], and (d) Jupiter[Hinsonet al., 1998]. In situ terrestrialion spectro•neter observations near90 km alwaysdetect meteoricions. Radio occultationmeasurements for Venusand Jupiter show multiple layers for which the lowestlayerscouldbe meteoricin origin. Mars data showno suchobviousstructure,indicatingthat eithermeteoroldionizationwasnotprominentor wasat altitudeswhereit is difficultto extracttheelectron densityinformationfrownthe radiooccultationdata.

•nic rays and, after some chemicalrearrangement,domi-

pointedout that morecometarystreamsareexpectedto en-

nated byO•-CO2,H30+, andCO;,forn - I, 3,and4.

counter Mars than do Earth. Kresdk [1993] and McBride

Models of the terrestrial meteoric ionization environment

andHughes[I 992] estimated thatthepopulation densityof

haveonly recentlybegunto morethansuperficiallyexplore suchstrea•ns is greaterat Mars thanfor any otherplanet• the metallicion layers.The structurein the measureddistri- Hence•neteoricionizationin the Martian aUnosphere could butionsfar exceedsthoseproducedby availablelocal ti•ne be particularly dependent on showeractivity,especiallyat indicatethatthe main modelingof the ion-neutralchemistryandelectrodynamics nightwhereoccultationmeasurements of the Jnainmetallicion layer [McNeil et al., 1996]. Never- ionospheric layer frequentlyis undetectable [Zhanget al., theless,the mostcompletemodelingof the effectsof mete- 1990a, b l. An investigationof the meteoric ionization in the Maroric depositionhasbeendonefor Earth. Other planetsfor which meteoricdepositioneffectsand ionizationhavebeen tianatmosphere isrelevantto theseveralcurrentandplanned investigatedincludeNeptune [Lyons, 1995; Moses, 1992], missionsto that planet. Further,a comparisonof the meJupiter[Kim et al., 1998; Grebowsky,1981], andMars [Sha- teoric ionization interactions for Mars with the established .frir, 1967;Flynnand McKay, 1990;Davis, 1993;Adolfsson knowledge of the interaction at Earth,will providea good frmneworkfor studyingthe relativeimportanceof eachof et al., 1996|, as well as a moonof Saturn,Titan lip, 1990]. Adolfssonet al. [1996] highlightedsomeof the proper- the possiblemeteoricionizationprocesses andfor isolating tiesof meteoroldimpactswith the Martianatmosphere and thosecouplingprocessesthat requireadditionalresearch. of meshowedthattheincomingfluxesandspeedswill be lessthan Althoughwe will locusontheplasmaconsequences primarilybecause thoseat Earth but that the opticalsignaturesof the meteors teoricimpactsin theMartianatmosphere, perturbation isthemosteasilymeasured propwould be visible from the surface. In that paper it is also theionosphere

PESNELL AND GREBOWSKY:

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1697

erty, the analysiswill also explorethe neutralatom depo- rial remainsin the solidphaseat the end of the ablationprosition profilesand the propertiesof the interplanetarydust cess.For example,McNeil et al. [1996] usedthe massflux distributionof Hughes [1992] but reducedit by a factor of particlemassdistribution.

4 to findagreement withtheobserved terrestrial Mg-[ den2. Micrometeoroid Meteorolds

Environment

at Mars

and the more massive asteroids have bom-

bardedthe Earth and other planetssincethe larger bodies were formed. In the currentsolarsystem,impactsof large, crater-producingmeteoroldson planetsare rare eventsthat haveonly an ephemeraleffect on the atmosphereand ionosphereduringtheir initial passage.However,smallerinterplanetaryparticlescontinuouslypelt the planets,depositing the bulk of the materialfrom interplanetaryspace(the case of the Earth is discussedby Chyba et al. [1994]) and maintaining a persistentconcentrationof metallic atoms in all planetaryatmospheres. Two main populationsof meteorswere distinguishedby their velocitiesas observedwith Earth-basedradarand photometric observations. The more easily observedare the shower(or stream) particles,whose relative velocitiescan reachvaluescorresponding to the sum of the Earth'sorbital velocity and the parabolicorbital velocity of a retrograde particleat the samedistancefrom the sun,to which is added

theescape velocity[Opik,1958].Thelargerelativevelocity producesa largeamountof impactionization,increasing boththe reflectedsignalsin a radarobservationandtheemitted radiationfor the photometricmeasurements. Theseparticlestendto occurat fixed timesof the year and eachstream appearsto come from a radiantpoint in the sky. A more

continuous component, commonlyreferredto as the "sporadic"component,is characterized by averagevelocitiesof the orderof escapevelocityof the planetplusa portionof the orbitalvelocity. The term "sporadics"derivesfrom the randomdistributionon the skyof the meteorscausedby this incomingmaterial.That theydominatethe massdistribution of micrometeoroids wasdeterminedafter they were named. A third,higher-speed populationof micrometeoroids, whose velocitiesindicatetheyoriginatedoutsideof thesolarsystem [Tayloret al., 1996],is alsoknownto existbuthasinsignifi-

sity. Similarly, Carter and Forbes[1999] retainedonly 5% of the depositedmeteoricmaterialin the gasphasefor their

Fe• calculations. Giventhe agreement with measurements whenthe Dohnanyi [1972, 1973] massflux modelwas used to computethe terrestrialion profile, and the uncertaintyin the fraction remainingin the gasphasewhen othermassflux •nodeis are used, we continue to use it and consider all of the

materialto be ablatedinto the gasphase(with the exception of the unablatedremnant mass.)

Interplanetarymicrometeoroids in the sporadicpopulation are continuouslyreplenishedby collisionsin the asteroid belt [Kortenkampand Dermott, 1998; Dermott et al., 1984]. They show only small seasonalflux variationsat the Earth and currentlydominatethe flux of material into the planetary atmospheresof the inner solar system. This populationof micrometeoroids will alsodominatethe ionosphericeffects,after the ablationproductsdiffusethrough the atmosphereand are ionized. At fixed periodsduring the year, when a planet passes throughthe duststreamleft alongthe orbit of a shortperiod comet, meteor showersare superimposedon the sporadic backgroundinflux. Meteor showersproduceshort density enhancementsin an ionosphere,the impactof which is still not quantitativelyunderstoodat the Earth muchlessfor any otherplanet. Grebowskyet al. [1998] founda trendfor a factor of 2-3 increasein the peak magnitudeof the terrestrial metallicion concentrations during showerperiods.Models of the effect of the showerinput on the metallic ions show that thoseeffectsare not long lastingcomparedto the sporadic background[McNeil, 1999; Grebowskyand Pesnell, 19991.

The relativeimportanceof sporadicand showerinfluxes variesfrom Earth to Mars becauseof the dependenceof the interplanetaryparticledistributionson hellographicdistance and becauseof each planet'sdiffering gravitationalfocusing. Studiesof IRAS data show that there are more comets cant masscomparedto the othertwo and was not considered with periheliawithin 1.5 AU of the sunthan within 1 AU of in our study. the sun [Kresc•k,1993]. Short-periodcometsin the Jupiter In the caseof the Martian atmosphere, sporadicmeteors family, with periods< 20 years,are4 timesaslikely to cross would be causedby micrometeoroids with velocitiesof the Mars' orbit as they are Earth's. Hencemeteorshowersmay orderof 10kms-• [FlynnandMcKay,1990].Thedistribu- play a more importantrole on Mars thanEarth. tion of the incidentmassflux of theseparticlesasa function The valueof usingMars asa uniqueobservationpostfor of the massof the particlesemployedin thisstudyis from studyingmeteorsneedsto be closelyexaminedin the future Dohnanyi[1972, 1973]. When this distributionwas usedfor becauseof the higherprobabilityof meteorshowersat Mars a terrestrialcalculation,goodagreementwasfoundbetween thanat Earth [KrestSk,1993; McBride and Hughes, 1992]. In thecalculated andaveraged measured Mg+ altitudeprofile thispaper,however,emphasisis on thesporadicbackground [Grebowskyand Pesnell, 1999]. The total mass flux onto co•nponentof the meteoroldflux as it containsthe bulk of theEarthfromthismassfluxdistribution is 6.2 x 103kg the massenteringthe Martian atmosphereand suppliesthe d- •, somewhat lessthanthemass fluxestimated byHughes majorityof the metallicions. With the goal of derivingthe [ 1992], Leinert and Grtin [ 1990], Divine [ 1993], or Loveand averagestateof the meteorion layer, we will use a steady Brownlee[1993]. Using one of the other massflux models state model, in which the effects of a short-lived shower are would requireassumingsomefractionof the ablatedmate- not important.

1698

PESNELLAND GREBOWSKY:MARTIAN METEORICMAGNESIUM ION LAYERS

3. Martian Atmosphere

sphere, where 02 isthedominant absorber neartheMgI ion-

A variety of atmosphericmassdensityprofilesfor the CO2-dominatedMartian atmospherehas been published. For the meteorablationstudy,we usedthe densityandtemperatureprofile from the 1974 revisionof the Mars Atmosphere[NASA, 1974] becauseof its accuratetabulationof temperatureas a function of altitude. A meteor ablation calculationwasalsoperformedusingthe Viking entrydata atmosphere[Seiffand Kirk, 1977; Seiff, 1982], but this resultedin only small variationsin the ablationprofiles.This is becauseall of theatmosphere modelshavesimilarvertical

ization edgeat 1610A, it hasminor consequences atMars; CO2istheprimary absorber intheMartianatmosphere. The low absorption crosssectionof CO2 allowsthisradiationto

penetrate below100kmandphotoionize magnesium atoms

untilalmost 60 km(Figure3). At Earththesewavelengths areabsorbed abovethe mainmetaldeposition regionand playonlya minorrolein creatingterrestrial metallicions.

4. Physicsof MeteoroidImpacts

A meteorold rammingintoa denseatmosphere heatsas the kinetic energy of impacting atmospheric gas is converted oric ablation.The tidal enhancements propagating through energy. Thisproduces melting andevaporation the Martian atmosphere,notedin the Viking data by Adolf- intointernal of surface material, as well as sputtering material off of the sson et al. [1996], do not affect the ablation results as the

column mass densities near the altitude of maximum

mete-

particle [Opik,1958]. Thenetprocess oflosing surface madiffusionof the meta!licsthroughthe atmospheresmooths terial is called ablation. out thosesmall perturbations.Hencethe unperturbedmass Equations describing thedeposition of meteorold atoms densitymodelatmospherealoneis usedto calculatethe abor molecules, the precursors to meteoric ions, quantify the !ationdepositionratesas a functionof altitude. acceleration, mass loss and heating of the meteorold along Oncethe localmetallicatomdepositionprofilesarecaltheatmosphere. Themomentum equation culated,a modelatmosphere of the gastemperature T, and itspaththrough describing thedecrease inrelative impact speed, v,withtime, t, owing to the drag of the impacting atmospheric gas(with of altitudeis requiredto treatthe diffusionandchemistry

the concentrationsof CO2, N2, 03, 02, and O as a function

of the depositedmetals.Theseare the majorneutralatmosphereconstituents relevantto the problem.We adoptedthe ozoneprofilefrom KongandMcElroy[ 1977].The O profile is basedontheprofilesby Rodrigoet al. [1990],integrated usinga molecularandeddydiffusionmodel.The modelatmosphere is summarized in Figure2. Followingthediscussionin Rodrigoet al. [1990], our eddydiffusioncoefficient

massdensityp,) on the meteoroidis: d¾

md--• - - FPa•rr2v2'

(1)

Although meteoric particles canhavecomplicated shapes, theyaremost simply, andtypically, approximated asspherical,sothatthemass ofthemeteorold isrelated totheparwas set toK=2.5x 107cm2s -I. ticleradius (r) anddensity (Pro) bym- (4•/3)pm r3. The Solarradiation with• < 3000A, whichisresponsiblemass density Pmwaschosen tobe3.2gcm-3,characterizing for ionizingsomeof the metallicatoms,is absorbed by the theasteroidal origin ofthesporadic background component. gasesin theMartianatmosphere.Unlike theterrestrialatmo- Thedragcoefficient F wassetto0.75,theaverage of the 120

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106

108

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Number Density (cm-a)

Figure2. Middaymodel ofMartian neutral atmosphere.

1016

PESNELL

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GREBOWSKY:

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120

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80

•

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Figure3. Relative absorption bytheMartianatmosphere attheionization edgeofMg I, 1610A. A value of (51610 -- 1.5x 10-19wasadopted asthecross section [Innetal., 1953].

rangeof valuesin Hughes[ 1992] andusedfor theterrestrial tional) heating, radiative cooling/heating,and evaporative calculationsof Grebowskyand Pesnell[1999] andMcNeil et al. [I 996]. Allowing for an obliqueincidenceangle(0i), the altitudeof an infallingbodychangesas dz. dt

cooling. Thesymbols areC.¾h m I x 107 ergsg-• K-•, the specificheat at constantdensity;g- 1, the radiativeefficiencyof the (assumedblackbody) meteoroid;{5- 5.68 x

10-5 ergscm-2 s-l K-4 theStefan-Boltzmann constant; = --VCOS0/.

(2)

For this studywe assumedan incidenceangle0i - 45¸. Ablation of material from a meteoroidoccursvia sputteringandevaporation'

T,,q,the equilibrium temperature of meteoroid outsidethe atmosphere(K); and A, the heat transferefficiency. It is assumedthatthe ablatingbodyis uniformlyheatedthroughout.

If a balancebetweenram heatingand evaporativecool-

ing is established by ignoringthermalradiationandthe time v3 4•r2CI dm _ _A.¾p,,nr2 exp(-C2/T) (3) dependencein (4), the sputteringand evaporativemassloss dt 2Q TI/2

equationcan be replacedby a singleablationterm with A.¾ replacedby A. When usedwith constantvaluesof Q, A, and F, (1) and (2) would then providea closedsolutionfor the massof the meteoroldas a functionof velocity [Hughes, 1992]. Althoughwe do not use this approximation,it was tent heat of ablation [Lebedinets et al., 1973]. Values of usedto verify the numericalintegrationsof (1)-(4). C• -6.92 x 10Iø g cm-2 s-• K•/2, C2- 57,800K, and Equations(1)-(4)describe the ablation of a meteoroid Q- 7 x 10Iø ergsg-I, appropriate for stonymeteoroidsin any planetaryatmosphere.The massdensityprofile for [Lebedinetset al., 1973], were usedin our calculations.The the atmosphereand the propertiesof the interplanetarydust heatequationis usedto determineT by relatingthe change distributionuniqueto eachplanetwill determinethe differin internalenergyof the meteoroidin response to the kinetic encesin the altitudeprofile of the massdepositionrate from energydepositedby the neutralatmosphere,thermalradia- planetto planet. Local depositionratesof individualmetetion from the meteoroid,and lossof meteoroldheatthrough oroidneutralatomspeciesasa functionof altitudeare calcuablation.As describedby Lebedinetset al. [ 1973], thisequa- latedby solving(1)-(4) andintegratingthe individualdepotion is sition curvesover the velocity and massdistributionscharacterizingthe incomingmeteoroids.An adaptivestepsize Runge-Kuttaalgorithmis usedto integratethe equationsin time alongthemeteoroid'strajectory,ensuringthattherapid accelerationregion will be accuratelyresolved. TrajectoT1/2 exp( , ries in the Martian atmospherewere startedat z.- 500 km. wherethe termson the right representram pressure(or fric- Initialvelocities werechosen aseither10 km s-I repre-

where A.¾_< 1 is the sputteringefficiency,T is the temperature of the particle,Ci and C2 are evaporationconstants that describethe vaporpressureof the outgassingmaterial, and Q is the averagesurfacebindingenergyor averagela-

C.¾hm dt = 4•r2[A-A¾ g ' pav3 - (5œ(T 4- T•) dT

QCi-C2/T)] (4)

1700

PESNELL AND GREBOWSKY:

MARTIAN

METEORIC

MAGNESIUM

ION LAYERS

2OO

150

,-•

100

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10 km/

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10-10 10-8 10-6 10-4 Neutral and Ion DepositionRates (cm

10-2

100

Figure 4. ComputedMartianmassdeposition ratesof atomicmagnesium atoms(solidlines)calculatedtbr

10kms-• input speeds characteristic ofthesporadic meteorold infallandfor50kins--•typical forahigh speedstream.The massflux distribution of Dohnanyi[1972, 1973]wasassumed for bothpopulations. The localionizationdeposition ratedueto the impactionizationof the neutralMg collidingwith the atmospheric molecules isalsoplottedasdashed lines.High-velocity streams canhaveiondeposition rates comparable to theneutralrates.Fora planetary regionin whichtheambientionosphere is weakandhence the chargeexchangemeteoricion sourceis not significant,the presence or absenceof a showerstream will havea majorimpacton thelevelsof ionizationpresent.

sentative of thetypicalentryspeedsintotheMartianatmo-

The neutralMg productionrateas a functionof altitude

sphere of thesporadic population [FlynnandMcKay,1990], is usedas inputto a one-dimensional verticalneutraldiffuor50kms-• asanexample ofthevelocity ofahigh-speed sionequation derivedfromthecontinuity equation.Once shower stream (seediscussion insection 2.) MassdepositionanMg profileisestablished, chemical equilibrium isusedto profileswerecalculated for spherical masses between10- • calculate whatMg compounds arepresent at eachaltitude. and10g. Smallermasses contribute littleto thedeposition profilewhilelargermasses impacttooinfrequently toaffect averageconditions.

5. Meteoric Ionization and Ion-Neutral

Thecomposition of thereleased material depends onthe Chemistry meteoroid typeandrefractory properties. Thecomposition Giventhehigh-impact velocities of theincoming mateof terrestrialmeteoritesis eithermetalor stonewith the latter rial,thekinetic energy of impact is sufficient to stripouter morecommon.We haveselected theMg+ ionto character- electrons off the atomsvia impactionization.For the av-

izethemeteoric ionization distribution onMars,asMg is eragesporadic meteoroid at Earth(withaverage incoming speeds • 15-20km s-!) thisimpactionization source is [Mason,1971]. A weighted average of thevaluesby Ma- weakcompared to theothersources, although it doesproson [1971, Table I] gives 14.3% of the massof chondritic ducetheionized trailswhichcanbedetected byradar.Howone of the dominantelementsfound in terrestrialmeteorites

meteoritesasMg. The near-Earthparticlemassflux distributionof Doh-

nanyi [1972, 1973] was assumedto be valid at Mars and

ever,duringretrograde meteorshowers at theEarth,thein-

coming speeds canapproach 72 kms-•. Since theimpact ionizationcrosssectionfor stonymeteorolds increases as

used tointegrate thecomputed ablated linedensity foraspe- v3'5[Lebedinets etal.,1973], thiscanbeasignificant source

cificmassintolocalMg deposition rates,whichareshown of ionization. Oneterrestrial example is theLeonidshower inFigure4. Masses near10-7 g werethedominant contrib- with its anticipated dramaticdisplayin 1999[Jenniskens, utorstothemeteoric massdeposition of Mg. Differences in 1996].Figure4 shows theincrease of therelativeimportheeffectsof gravitational focusing of meteorolds andvaria- tanceof impactionization for a high-speed meteorshower tionsof thenumber fluxdistribution withposition in theso- at Marscompared to thesporadic background. In thecur-

larsystem produces anestimated factor of • 10uncertaintyrent studywe ignorethe showereffect and concentrateon thedominant mass injection bythesporadic background.

in ourresults[FlynnandMcKay,1990].

PESNELLAND GREBOWSKY:MARTIAN METEORIC MAGNESIUM ION LAYERS

1701

Table 1. Reactions and Reaction Rates

#

Reaction

Ratea

NIb Mg+ 02+ CO2--*MgO2 + CO2 N2 MgO2+ O •MgO + 02 N3 MgO+ O--•Mg+ 02 N4 Mg+03-•MgO+02 N5b MgO+ CO2+ CO2-• MgCO3 + CO2 N6 MgCO3 + O-•MgO2+ CO2 N7 MgO+ 03 --•MgO2 + 02 N8 MgO2+ H -• MgOH+ O N9 MgOH+ H -•Mg + H20 N10b MgO+H20+CO2-•Mg(OH)2 +CO2 Nil Mg(OH)2 + H-•MgOH+ H20 I1 Mg+hv--•Mg{ +e I2 Mg+O• --•Mg 4 +O2 I3 Mg+ NO+ .•Mg+ + NO I4 Mg+O• •Mg 4•+O I5 Mg++e- --•Mg+hv I6b Mg4 + 02+ CO2-• MgO•+ CO2 I7 Mg+ + 03 -•MgO4 + 02 I8 MgO4 + 03 -•Mg4 + 202 I9 MgO ++e- -•Mg+O II0 MgO+ +O-•Mg • +02

Ill II2

MgO•+e- --*Mg+O2 MgO•+ O-•MgO4-+ 02

Reference

I I x 10-30 exp(-2790/T) 5 x 10-•ø exp(-940/T) 2.2x 10 l0 (T/200)•/2 2.3x10 •0 exp(-139/T) 4.6x 10-27 I x 10 •3 2.2x 10 l0 exp(-548/T) I x 10 9 exp(-1000/T) 4 x 10 • exp(-550/T) 3.9x 102,s (T/200)3.7 5 x 10 •0 exp(-2500/T) 4.0xi0 7 1.2x109 8.1x 10 l0 1 x10--9 4 x10 •2 9 x l0 3{) (T/200)• 7 x 10-•0 8 x 10-10 2 x10..... 7 (T/200)1/2 1 x 10-•0

113 Mg•++N2 +N2--•MgN•+N2 114 MgN•,+ e- -• Mg+N2 I15 Mg• +CO2+CO2-•Mg+.CO2+CO2 116 Mg4-.CO2 +CO2-•Mg+ +CO2+CO2 I17 Mg+.CO2+e - -•Mg+CO2

2x10-7 (r/200)--!/2 I x 10 •0 I x 10-30 3 x 10-7 1 x 10-30 I x 10-l0 3 x 10-7

References areasfollows:1, PlaneandHelmet[1995];2, HelmetandPlane[1993];3, McNeil et al.

[ 1996]'4, Swider[ 1969];5, Roweetal. [ 1981]; 6, estimated anddiscussed inthiswork.

aUnits ares ! forphotolysis, cm3s-! forbinary, andcm6s ! forternary reaction rates. bThese three-body ratesweremultiplied by 2.3 [Lindner,1988].

of terrestrial meteoric After a few collisionswith atmospheric particles,the beenexplicitlyincludedin all studies meteoricatomsslow to becomeminor atmosphericspecies ions[e.g.,McNeil et al., 1996;Carterand Forbes,1998]. is poorlyunderstood subject todiffusion, chemistry, andneutral winddrag.Once Sincetheeffectsof thesetwoprocesses

in equilibrium withtheatmosphere, neutral metallicatoms areionized bysolarradiation, charge exchange withambient ionospheric ions,orenergetic charged particles. Thedominantionization process for Marsissolarphotoionization, as themeteor deposition occurs below100km,lowerthanthe

on the Earth, where extensivemeasurementsexist, we felt it

waspremature to includethemin ananalysis of theMartian atmosphere wheretheionosphere is lesswellcharacterized. Effects of condensation have been included in some terres-

trial studiesby assumingthat only a fractionof the metebaseof the nominalambientMartian ionosphere, the peak oroidmassremainsin thegasphaseafterbeingablated.For that25% of theabof whichistypicallynear125km(in sunlight, [seeZhanget example,McNeilet al. [1996]assumed al., 1990a]).Belowthemetallicionlayeris a neutralatomic lated massremainsin the gaseousstate,while Carter and layer,andbelowthatmolecular metallic compounds. The Forbes[1998] assumed5% of the massis availablefor the detailed chemistry of theconversion fromatomicto molec- gas-phasechemistry.In light of the uncertaintyin the fracularcompounds is dependent onreactions withsomewhattion of the meteoroldmassthat remainsin the gas phase, uncertain rates,buttheion layeris controlledby fairly sim- we usea massflux distributionthat givesgoodresultsat the Earth whenall of the ablatedmassremainsin the gasphase plechemistry notaffected bythese uncertainties. Condensationof ablated atoms and moleculesinto dust for this studyof the Martian atmosphere. The calculationof the vertical profile of the chemical as well as attachmentof ionsontodustwere not considered. partitioning of Mg compoundsincludesall of the ion and Theseprocesses, although potentially important, havenot

1702

PESNELL AND GREBOWSKY: MARTIAN

neutralreactionsfor the dry reactionnetworklisted in Table I. Aside from the substitutionof CO2 for N2 as the third

METEORIC MAGNESIUM

ION LAYERS

A vertical profile of Mg in diffusive equilibrium was found usingthe model of the Martian atmosphereshownin

2. Aneddydiffusion coefficient K = 2.5x l07cm2 bodyin threebodyreactions, mostof thesereactions arethe Figure et al., 1990]wasusedto characterize thetursameasthosegoverningMg chemistryin the terrestrialat- s i [Rodrigo mosphere lGrebowsky and Pesnell,1999]. Followingthe bulentmixing processes.A moleculardiffusioncoefficient discussionof Lindner [1988], the three-bodyreactionrates

D = 6.6x 10•6T 3/4cm2 s--• [Grebowsky andPesnell, 1999]

weremultipliedby 2.3 to accountfor the increased reaction wasusedto characterizethe moleculardiffusionof Mg. The wasfoundusinga rateswhenCO2 replacesN2 as the buffergas. In addition, verticalprofileof theMg+ concentration equithecomplex Mg4.CO2wasincluded inthereaction network completesetof ion-neutralreactionsin photochemical (I15-I17). Thiscompound is formedby a three-body attach- librium. The result is the predictionof a persistentlayer of the peak ment (I15) and destroyedby detachment(I16) and disso- Mg *, well belowthe baseof the usualionosphere, ciative recombination(I17). The reactionratesfor I15-I17 listed in Table 1 are basedon typical termolecularinterac-

concentration ofwhichisoftheorderof 104ionscm-3 (Figure 5).

The chemicalpartitioningpresentedin Figure5 can be understood by discussingseparatelythe neutralmolecular atomicMg is includedwithaccounttakenof theabsorption region (30-60 km) and the ionized atomic region (above 60 km). The molecularreactionsof N I-N7 are linearin Mg, of solar radiationwith altitude (see Figure 3). Althoughwatervaporis seenin thelowerMartianatmO- and ratios can be used to undekstand the calculated concen-

tions,andthe sensitivity of theMg+ altitudeprofileto reaction rates I 15 and I 16 was examined. Photoionization of

sphere, themainmeteoricionlayerisathighaltitudes where trations in the molecular region. For example both watervaporis absent;hencea dry chemistry(reactions N1- [MgO2]/[MgO] and [MgCO3]/[MgO] are > I below 80 km N7 and I 1-112)was usedin this study.Includinghydrogen altitude,whichagreeswith thesmallamountof MgO present chemistry wouldchangethe ultimatemolecularsinkof Mg in the results.For a temperatureof 150 K and,assumingthat belowaltitudesof 40 km but leavethe atomicion layersrel- [O1/[CO2]