Available online at www.sciencedirect.com

Planetary and Space Science 51 (2003) 239–249

www.elsevier.com/locate/pss

Meteoric ions in the atmosphere of MarsGregorio J. Molina-Cuberosa;b;∗, Olivier Witassec, Jean-Pierre Lebretonc, Rafael Rodrigob,

Jos.e J. L.opez-Morenob

aGrupo de Electromagnetismo Aplicado, Dep. F��sica, Facultad de Quimica, Universidad de Murcia, E-30100 Murcia, SpainbInstituto de Astrof��sica de Andaluc��a, CSIC, P.O. Box 3004, E-18080 Granada, Spain

cResearch and Scienti-c Support Division of ESA, ESTEC, P.O. Box 299, 2200 AG Noordwijk, The Netherlands

Received 20 February 2002; received in revised form 2 October 2002; accepted 14 October 2002

Abstract

The e3ect of the metallic concentration in meteoric material in the Martian ionosphere has been investigated with the scope of exploringthe possible existence of meteoric ionospheric layers as it occurs on Earth. We have calculated the ion and neutral deposition of the mostabundant metals in meteoroids, iron, magnesium and silicon, and modeled their ion-neutral chemistry in order to estimate the concentrationof neutral and ionic metals and electrons. We found that a low ionospheric metallic layer due to meteoric ablation is created at around80 km, well below the main maximum at 125–135 km produced during the daytime by photoionization and photoelectron impact. Daytimeand nighttime models have been developed and the e3ect of solar activity, and seasonal variations has also been explored. This meteoriclayer is mainly formed of Mg+ and Fe+ with a magnitude of the order of 104 cm−3 at noon and decreases by two orders of magnitudeduring the night. The study of this lower part of the Martian ionosphere is very important in the context of the future missions, especiallyregarding the HF radio wave sounding technique that will be used onboard Mars Express and Nozomi spacecraft in 2004–2005.? 2003 Elsevier Science Ltd. All rights reserved.

Keywords: Meteorites; Ionosphere; Mars; Magnesium; Iron

1. Introduction

Metallic ions coming from the ablation of extraterrestrialdust form continuous ionospheric layers in the Earth’s atmo-sphere (Hughes, 1978). These layers have been extensivelyobserved from ground and space. Metal ions, predominantlymagnesium and iron, and to a lesser extent, sodium, alu-minum, calcium and nickel, have been found in the lowerionosphere of the Earth (see Kopp, 1997 and referencesherein). Nowadays theoretical models can aptly model theconcentration of the most abundant metals as well as its lat-itude and temporal variability. Models indicate that above95–100 km, the metals except silicon should occur predom-inantly in the form of free ions (Kopp, 1997).

The presence of interplanetary dust at larger dis-tances from the Sun than Earth has been conBrmed by

∗ Corresponding author. Tel.: +34-968-367390; fax: +34-968-364148.E-mail addresses: gregomc@um.es (G.J. Molina-Cuberos),

owitasse@rssd.esa.int (O. Witasse), Jean-Pierre.Lebreton@esa.int(J.-P. Lebreton), rodrigo@iaa.es (R. Rodrigo), lopez@iaa.es(J.J. L.opez-Moreno).

measurements of Pioneer 10 and 11 spacecraft. The dataobtained by the penetration detector on board Pioneer 10and 11 show similar behavior in spite of the di3erences onthe threshold mass, 10−9 and 10−8 g, respectively, whichmeans that the particle size distribution does not changestrongly with the distance to the Sun (Cuzzi and Estrada,1998). Humes (1980) analyzed the results of the pene-tration detector of Pioneer 10 and found that the spatialdensity of 10−9 g meteoroids is essentially constant be-tween 1 and 18 AU. Therefore, meteoroids could also a3ectthe ionospheric structure of other planets as they do onEarth.Radio occultation measurements performed at Venus and

the giant planets show a daytime ionosphere with a mainpeak produced by the ionization due to EUV solar radia-tion. Below this peak one or more secondary ionosphericlayers appear, that could be due to the meteoric deposition(e.g. Kliore et al., 1979; Grebowsky, 1981; Hinson et al.,1998).The e3ect of meteoroids on the ionospheric structure has

been studied for the atmospheres of Jupiter (Kim et al.,2001), Saturn (Moses and Bass, 2000), Neptune (Lyons,1995) and the Saturnian moon Titan (Molina-Cuberos et al.,

0032-0633/03/$ - see front matter ? 2003 Elsevier Science Ltd. All rights reserved.doi:10.1016/S0032-0633(02)00197-6

240 G.J. Molina-Cuberos et al. / Planetary and Space Science 51 (2003) 239–249

2001). In the case of Mars, there is no clear report in theliterature of the detection of a secondary ionospheric layer ofmeteoric origin, despite the number of missions. Pesnell andGrebowsky (2000) modeled the meteoric ablation at Marsand calculated the density of meteoric magnesium ions forincoming velocities of 10 and 50 km s−1. They predict apersistent layer of Mg+ ions of the order of 104 ions cm−3

at 80 km, while the main ionosphere peak reaches about 1–2×105 cm−3 at 125–135 km. Considering the uncertaintiesin all the input model parameters, theMg+ ion density wouldbe at least 103 cm−3.The presence of such a layer in the atmosphere of Mars

could have some implications on the operational scenarioof one of the instrument on board the ESA Mars-Expressmission in 2004, as outlined by Witasse et al. (2001). Theradar, called MARSIS, is devoted to the detection of sub-surface water and operates with HF radio waves. At thesefrequencies, a strong radio wave absorption due to this layer(50 dB at 5 MHz, 360 dB at 1:8 MHz) may be present,which is not compatible with the normal operating mode.This e3ect may also a3ect the HF radar altimeter onboardNozomi. Thus, there is an urgent need to know more aboutthe meteoric layer: Does it really exist? If the answer ispositive, then what is the altitude and the magnitude of thepeak? Furthermore, what is the variation of these parame-ters with the solar illumination, the season, the distance tothe sun? On the other hand, what is the inKuence of meteorshowers?Pesnell and Grebowsky (2000) began to provide some of

the answers to these questions by modelling the magnesiumion density proBle. The aim of this study is to develop a morecomprehensive model of the ionosphere of Mars between60 and 100 km height. We have developed a steady statemodel which includes the chemistry of magnesium, iron andsilicon. We have applied it to daytime and nighttime con-ditions. The expected variability due to solar activity vari-ation is also investigated, through the UV photoionizationprocess.

2. Meteoric ablation

Collisions in the asteroid belt produce a continuous Kuxof interplanetary material which dominates the Kux of mi-crometeoroids into planetary atmospheres of the inner solarsystem. This source also produces most of the total amountof metallic ions in the Earth’s ionosphere.Meteor showers, that are produced when a planet crosses

the dust stream left along a comet orbit, can increase themetal ion concentration during short periods of time. Kopp(1997) found an increase of a factor of 2–3 in the metallicconcentration in Earth’s atmosphere during shower periods.This increase can rise up to one order of magnitude duringstrong meteor shower. In spite of this spectacular increase ofthe amount of metal during a shower, the net mass inKux tothe Earth from each meteoric shower is only a small fraction

of the total annual Kux due to the continuous background(Hughes, 1978). At Mars this fraction is expected to behigher (Pesnell and Grebowsky, 2000).Christou and Beurle (1999), in a search for individual

comets or asteroids that are likely to generate observablemeteor showers in the Martian atmosphere, found thatthe number of objects approaching to within 0:2 AU tothe Martian orbit is approximately twice the one for Earth.The smaller relative velocities in the case of Mars compli-cates the production of meteors in the upper atmosphere ofthe red planet.Adolfsson et al. (1996) showed that meteoroids with in-

coming velocities higher than 30 km s−1 would producesimilar maximum luminosities in the atmospheres of Earthand Mars. The height of maximum meteor intensity wasfound at altitudes lower than in the case of the Earth, be-tween 50 and 90 km on Mars and between 70 and 100 kmon Earth.In spite of the fact that a meteor shower may have an

inKuence on the distribution of atmospheric metal ions, es-pecially during short times period, we have only consideredhere the continuous component of interplanetary dust.We have made use of the interplanetary model by GrMun

et al. (1985) �Earth, characterized by an isotropic Kux witha mean velocity at 1 AU of 20 km s−1, and scaled to theMartian orbit according to �Mars = �Earth×D−1=2

Mars (Bauer,1973), where DMars is the planetary distance from the sun inAU. In addition, the meteor Kux near a planet is enhanceddue to the gravitational e3ect by a factor of G=1+ v2∞=v20,where v∞ and v0 are the escape and incoming velocity,respectively (Bauer, 1973). Considering the distance andgravity factors, the mean velocity of meteoroids penetratingMars’ atmosphere is around 18 km s−1.

The collisions of meteoroids with atmospheric con-stituents slow down the particles, remove part of the massand heat the surface producing an additional loss of mass byablation. The kinetic energy lost by the meteoroid is trans-formed to radiation of energy, the particle heating and lossof heat through ablation. The dynamic of the meteoroidsin the atmosphere of Mars was calculated by adapting themodel developed by Lebedinets et al. (1973) for Earth’satmosphere to Martian conditions. This model was alsoapplied to the Saturnian moon Titan in order to study theion metal layers in its ionosphere (Molina-Cuberos et al.,2001). The treatment of the meteoroid ablation usedhere has been reported in detail in the above refer-ences. Here we shortly present the equations describingthe time-evolution of a meteoric particle crossing theatmosphere.The interaction of a small solid particle with a planetary

atmosphere is described by the following system of equa-tions.The equation of motion

cos �dvdz

=A�v

2=3m1=3 ; (1)

G.J. Molina-Cuberos et al. / Planetary and Space Science 51 (2003) 239–249 241

the ablation equation

cos �dmdz

=−4AK1m2=3

2=3vT 1=2 e−K2=T − �SA�m2=3v2

2Q2=3 (2)

and the energy equation

cos �dTdz

=4A�v2

8C 2=3m1=3 (� − �S)− 4A�T 4

C 2=3vm1=3

− 4AK1QC 2=3T 1=2m1=3v

e−K2=T ; (3)

where � is the entry angle, v the meteoroid velocity, Tthe temperature, m the mass, the drag coeOcient, A theshape factor, � the atmospheric mass density, the mete-oroid mass density. K1 and K2 are constants describing thedependence of the evaporation rate on temperature. �S isthe sputtering coeOcient, Q the energy of evaporation of1 g of the meteoroid, � the heat transfer coeOcient, � theStefan–Boltzmann constant and C the heat capacity of themeteoroid substance.Eqs. (1)–(3) were solved by a third order Adams–

Bashforth method, in a 50 m grid for stony meteorites com-ing with initial temperature T0=216 K, =1, A=1:21, =2:5 g cm−3, C1=6:92×1010 g cm−2 s−1, C2=5:78×104 K,�S =6×10−6 exp(T=290), Q=6×1010 erg g−1, �=1, andC = 107 erg g−1 K−1. We assume a mean incidence angleof � = 45◦, the e3ect of non-vertical entry is to elevate thealtitude at which ablation occurs. The meteoroid density wastaken from GrMun et al. (1985). The remainder of parameterscame from Lebedinets et al. (1973) and Grebowsky (1981).Cosmic dust is mainly composed of carbonaceous chon-

drites, that have a relative concentration of Mg = 6:1%,Si=5:7% and Fe=5:1% (Anders and Ebihara, 1982). Otherless abundant metals as Na, Al, Ca, and Ni, were not con-sidered here because their relative concentration in meteoricmaterial is one order of magnitude lower and, therefore, theyhave a small inKuence on the total electron concentration,as it really occurs on Earth.Once the ablation of meteoroids is computed, the linear

density of the individual ion species is

�i =−pi�i

mivdmdt

; (4)

where pi is the ratio of atom i type to the total, mi is theatomic mass, and �i is the ionization probability, which de-pends on the produced ion and meteoroid velocity. �i wascalculated by using an analytical expression value for mi-crometeoroids with v0 ¡ 35 km s−1 for which no secondaryionization or recombination takes place (Jones, 1997):

�i = ki×(v − v∗i )2v0:8; (5)

where ki is a value experimentally determined: kFe =34:5×10−6, kMg = 9:29×10−6, and kSi = 18:5×10−6; v isin km s−1 and v∗i is the minimum velocity of the meteoroidthat can produce ionization: v∗O=16:7, v∗Fe =9:4, v∗Mg=11:1,and v∗Si = 11:0 km s−1 (Jones, 1997). The production rate

of i type ions (Pi) is calculated as

Pi =∫ max

min�if(m) dm; (6)

where f(m) is the Kux per unit of micrometeoroidal mass.We have used the density and temperature proBles corre-

sponding to Viking 1; lander and Moriner 6 spacecraft geo-physical condition (Witasse, 2000). These data are providedby the Mars Thermosphere General Circulation Model (e.g.Bougher et al., 1990). It is a 3-D upper atmosphere modelthat addresses the Mars thermosphere structure and dynam-ics above 70 km altitude. We have tested our model usingthe density and temperature proBles for both geophysicalconditions. Negligible variations in the meteoric ablationhave been found.Fig. 1 shows the neutral and ion deposition rate of mag-

nesium, iron and silicon. The altitude of the maximum de-position is around 80 km, which is compatible with the cal-culation obtained by Pesnell and Grebowsky (2000) for Mgand incoming speeds of 10 km s−1 at 70 km and 50 km s−1

at 95 km. The e3ect of increasing the velocity is to raise thealtitude at which ablation occurs and to increase the numberof atoms ionized. The magnitude of the Mg peak obtainedhere, 2×10−2 cm−3 is a factor of 4 smaller than the one cal-culated by Pesnell and Grebowsky (2000) due to di3erencesin the input Kux.We have also found that the ablation proBle peak height

does not strongly a3ect the resulting concentration proBles,as it is also the case in the atmosphere of the Earth (McNeilet al., 1996).

3. Densities calculations

The mathematical transport equations used in the modelare those developed by Stubbe (1973) to study the ionicconstituents in the Earth’s atmosphere. Later on, Rodrigoet al. (1986, 1990) applied them to the study of the neutralatmospheric composition in Earth and Mars, respectively.The concentration of each neutral and ion species is cal-

culated from the continuity and momentum equations that,assuming steady state, may be expressed as

Pi − nili =@@z

nivi (7)

vi =−Di

(1ni

@ni

@z+

1Hi

+1T

@T@z

)

−K(1ni

@ni

@z+

1H

+1T

@T@z

); (8)

where i denotes the ith constituent, z the altitude, ni theconcentration, Pi the production, li the speciBc loss and Tthe temperature. vi is the mean vertical velocity, Di and Ki

are molecular and Eddy di3usion coeOcients, Hi and H arethe individual and atmospheric scale heights.

242 G.J. Molina-Cuberos et al. / Planetary and Space Science 51 (2003) 239–249

Fig. 1. Neutral and ionic deposition rates of magnesium, iron, and silicon in the Martian atmosphere due to an incoming meteoroid Kux with the followingcharacteristics: vO = 18 km s−1, mass Kux distribution by GrMun et al. (1985), carbonaceous chondrites.

The atmospheric composition was taken from the modelsby Rodrigo et al. (1990) and Nair et al. (1994) and we haveassumed that the chemistry derived by the presence of metalsdoes not a3ect the concentration of the major species.Vertical transport of metallic species is produced by tur-

bulent and molecular di3usion, the former being more ef-fective at lower levels. The molecular di3usion coeOcientsDi were calculated following the di3usion theory (see, e.g.Chapman and Cowling, 1970). The turbulent di3usion isthe least known factor in the modeling of the atmosphere.It is typically parameterized by means of an Eddy di3usioncoeOcient K . Rodrigo et al. (1990) calculated the K pro-Ble to reproduce the experimental concentrations of O andCO proBles, which are strongly a3ected by turbulent trans-port. The K coeOcient obtained below 120 km ranges from2×107 to 6×107 cm2 s−1.For the Earth, meteoric metals have been modeled in de-

tail, the chemistry and temporal and spatial variations havebeen extensively treated in the literature (Swider, 1969;Aikin and Goldbert, 1973; Kopp, 1997). Temporal and spa-tial variations of Mg+, Fe+ and Si+ have been extensivelytreated in the literature and the chemistry is well known(Kopp et al., 1995; Joiner and Aikin, 1996; McNeil et al.,1996; Helmer et al., 1998; Carter and Forbes, 1999). Wehave made use of this terrestrial knowledge and applied itto Mars because the chemistry of metallic ions and neutralsis quite similar in the atmosphere of both planets, makingCO2 to play the role of the third body in Mars instead ofN2 on Earth. On the basis of the Earth’s chemical scheme

Fig. 2. Schematic diagram showing the chemistry of magnesium species.Rate coeOcients for these reactions and the ion-electron recombinationrate are from McNeil et al. (1996) and Pesnell and Grebowsky (2000).The photoionization rate of magnesium is calculated from the relativecross section and solar radiation spectrum.

of Fe, Mg and Si, new reactions have been added, in orderto include CO2 as the major species.Figs. 2 and 3 show the chemical scheme of Fe and Mg, re-

spectively. Table 1 shows the reaction rate used in this work,where the reactions were mainly compiled from McNeilet al. (1996), Helmer et al. (1998), Carter and Forbes (1999)and Pesnell and Grebowsky (2000). Lindner (1988) pro-posed to multiply three-body reaction rates by a factor of2–3 in order to take into account the higher rate experimen-tally found when CO2 replaces N2 as the third body. The

G.J. Molina-Cuberos et al. / Planetary and Space Science 51 (2003) 239–249 243

Fig. 3. Schematic diagram showing the chemistry of iron species. RatecoeOcients for these reactions and the ion-electron recombination rateare from Helmer et al. (1998) and Carter and Forbes (1999). The ironphotoionization rate is calculated from the relative cross section and solarradiation spectrum.

results obtained here would not signiBcantly change if thisfactor is used or not, therefore we have not included it.The neutral reactions leading to the removal of magne-

sium and iron atoms are:

Mg + O3→MgO+ O2;

Mg + O2 +M→MgO2 +M;

Fe + O3→FeO + O2;

Fe + O2 +M→FeO2 +M:

The oxidation by ozone is the most eOcient mode of convert-ing atomic metals into neutral oxides, while the three bodyassociation with O2 occurs at a very low rate. Once MgOand FeO are formed, three body association of CO2 pro-vides MgCO3 and FeCO3, the more stable neutrals. Metalliccarbonates are removed by O reactions:

MgO2CO2→ MgOCO3O→MgO2; (9)

FeO2CO2→ FeOCO3O→FeO2: (10)

Mg and Fe follow processes quite similar, but with di3er-ent rates. The rate of producing MgO2 from reactions of Owith MgCO3 is several order of magnitude higher than theprocess with iron. Therefore MgO2 is very abundant below80 km and FeO2 not.Fe+ and Mg+ can undergo electron recombination or be

converted to oxygenated ions. These molecular ions reactwith electrons more quickly than atomic ions, producingthe neutral atoms, or with atomic oxygen to recycle themetallic ions. At higher pressures, molecular association bythree body reactions produce molecular ions which undergomolecular dissociative recombination to form Fe and Mg.Silicon chemistry is simpler, neutral Si is removed by

SiO2→SiO (11)

Table 1Units of cm6 s−1 for termolecular, cm3 s−1 for bimolecular and s−1 forphotoionization

Reaction Rate

Fe+h% →Fe+e− ¡4×106 (a)Fe+O2+M →Fe++O2 9:4×10−10 (b)Fe+O2+M →FeO2+M 3:25×−30×exp(−1841=T ) (c)Fe+O3 →FeO+O2 3:44×−10×exp(−146=T ) (c)FeCO3+O →FeO2+CO2 2:0×1010×exp(−3249=T ) (c)FeO+O →Fe+O2 1:8×10−10×(T=200)1=2 (c)FeO+CO2+M →FeCO3+M 2:0×10−28×(200=T )3=2 (c)FeO+O2+M →FeO3+M 6:5×10−30×(200=T )3=2 (c)FeO+O3 →FeO2+O2 2:2×10−10×exp(−553=T ) (c)FeO2+O →FeO+O2 5:0×10−10×exp(−998=T ) (c)FeO3+O →FeO2+O2 2:5×10−10×(T=200)1=2 (c)Fe++N2+M →Fe+N2+M 2:5×10−30 (b)Fe++O2+M →Fe+O2+M 1:0×10−30×(200=T ) (c)Fe++O3 →FeO++O2 7:0×10−10 (c)Fe+N2+O →FeO++N2 1:0×10−10 (b)FeO++O →Fe++O2 1:0×10−10 (c)Fe+O2+O →Fe+O+O2 1:0×10−10 (c)Fe++e− →Fe+h% 9:5×10−12 (d)FeN+

2 +e− →Fe+N2 2:0×10−6 (b)FeO++e →Fe+O 1:0×10−7×(200=T )1=2 (c)Mg+h% →Mg++e− ¡ 1:3×10−6 (a)Mg+O+

2 →Mg++O2 1:2×10−9 (f)Mg+O2+CO2 →MgO2+CO2 1:1:×10−29×exp(−2790=T ) (f)Mg+O3 →MgO+O2 2:3×10−16×exp(−139=T ) (f)MgCO3+O →MgO2+CO2 1:0×10−13 (e)MgO+2CO2 →MgCO3+CO2 4:6×10−27 (f)MgO+O →Mg+O2 2:2×10−10×(T=200)1=4 (f)MgO+O3 →MgO2+O2 2:2×10−10×exp(−548=T ) (f)MgO2+O →MgO+O2 5:0×10−10×exp(−940=T ) (f)Mg++2CO2 →Mg+CO2+CO2 1:0×10−30 (f)Mg++2N2 →Mg+N2+N2 1:0×10−30 (f)Mg++O2+CO2 →MgO+

2 +CO2 9:0×10−30×(200=T ) (f)Mg++O3 →MgO++O2 7:0×10−10 (f)Mg+CO2+CO2 →Mg++2CO2 1:0−10 (f)MgO++O →Mg++O2 1:8×10−10 (e)MgO+O3 →Mg++2O2 8:8×10−10 (e)MgO+

2 +O →MgO++O2 1:0×10−10 (e)Mg++e− →Mg+h% 4:0×10−12 (g)Mg+CO2+e− →Mg+CO2 3:0×10−7 (f)Mg+N2+e− →Mg+N2 3:0×10−7 (f)Mg+O+e− →Mg+O 2:0×10−7×(200=T )1=2 (f)Mg+O2+e− →Mg+O2 2:0×10−7×(200=T )1=2 (f)

References: (a) Calculated here, see Fig. 4; (b) Carter et al. (1999);(c) Helmer et al. (1998); (d) Nair et al. (1994); (e) McNeil et al. (1996);(f) Pesnell and Grebowsky (2000); (g) Swider (1969).

and Si+ ions by

Si+H2O→SiOH+O3→SiO2H+; (12)

where the coeOcient rate for these processes and for theelectron recombination rate of Si+, SiOH+ and SiO2H+ canbe found in Kopp et al. (1995).Magnesium and iron ions are produced by direct mete-

oric ionization, photoionization and charge exchange withatmospheric ions, mainly O+

2 . The metal photoionizationrates are calculated using the cross sections given in Verneret al. (1996), and we used the solar radiation spectrum in

244 G.J. Molina-Cuberos et al. / Planetary and Space Science 51 (2003) 239–249

Fig. 4. Charge exchange and photoionization rate of Mg and Fe in the Martian atmosphere. Photoionization rates are calculated when the planet is atperihelion and during the maximum on solar activity (maximum photoionization) and in the case of aphelion and minimum on solar activity (minimumphotoionization).

the 145–165 nm UV range given by Mount et al. (1980)and Rees (1989). The Sun–Mars distance has been takeninto account. The CO2 absorption is included. Fig. 4 dis-plays the production rate of Mg+ and Fe+ by photonioniza-tion and dayside charge exchange rates. Charge exchangedepends on the solar Kux (maximum or minimum) and onthe heliocentric distance.Charge exchange with ions is the main source for the pro-

duction of metallic ions. The electron and ion density proBlesare taken from the EUV and cosmic rays ionization modelsby Molina-Cuberos et al. (2002) (below 100 km) and fromWitasse (2000) (at higher levels). O+

2 is expected to be themost abundant non-metal ion above 70 km, with a concen-tration increasing with altitude, from some tens of atoms percubic centimeter to the maximum peak at 125–135 km of1–2×105 cm−3. Hydronium hydrates H3O+(H2O)n are themajor ions below 70 km.In order to calculate the concentration of metallic species

during daytime, we make use of initial electron and O+2

density proBles from the above references. We considered aKux of metals as shown in Fig. 1. The coupled set of Eqs. (7)was solved by means of a modiBed Gaussian algorithm. Weassume photochemical equilibrium as the lower boundarycondition and di3usive equilibrium for the upper boundary.We have simulated the nighttime conditions by assuming

that direct meteoric ionization is the main source for the pro-duction of metallic ions below 100 km. UV photoionizationdoes not occur, and charge exchange processes are assumed

to be negligible. The ionization probability depends on thevelocity of incoming meteoroids, for input speed less thanaround 10 km s−1, direct meteoric ionization will not occur,and probably the most important source of source could becosmic rays.

4. Results and discussion

Despite the Si 5.7% concentration in the meteoric mate-rial, the concentration of the silicon ions is less than 1 cm−3,much smaller than Mg+ or Fe+. Si is quickly removed byreactions with O2, Eq. (11), and therefore the production ofSi+ by charge exchange and photoionization are negligible.Also, the channel (12) leads to the production of SiOH+

and SiO2H+, that are very quickly removed by electron re-combination. Therefore we paid more attention to the mag-nesium and iron ions.

4.1. Mg-type species

Figs. 5 and 6 display the concentration of Mg-type speciesduring daytime and nighttime, respectively. During day-time, we found the same general trends as in Pesnell andGrebowsky (2000). The Magnesium peak density reaches2×104 cm−3 at around 85 km height, while the Mg+ peakreaches 104 cm−3 at the same altitude. This peak is 10 kmhigher in altitude and much broader than the calculated by

G.J. Molina-Cuberos et al. / Planetary and Space Science 51 (2003) 239–249 245

Fig. 5. Diurnal concentration of Mg-type species for ionization rate shown in Fig. 1 and chemical scheme of Fig. 2.

Fig. 6. Same as Fig. 5 for nocturnal case.

Pesnell and Grebowsky (2000). Mg+N2 density is about20 cm−3 at 70 km.Pesnell and Grebowsky (2000), by calculating the relative

absorption of the Martian atmosphere at the ionization edge

of MgI, claimed that Mg+ is predominantly produced byphotoionization. We have found that, during the daytimeand minimum solar activity, the metallic ion production isdominated by charge-exchange of the deposited neutral Mg

246 G.J. Molina-Cuberos et al. / Planetary and Space Science 51 (2003) 239–249

with the ambient ions, mainly O+2 , like in the terrestrial case.

At 85 km the photoionization of Mg produces around 0.5%of the total Mg+, while direct meteor ionization is around0.05%.Concerning the neutral Mg species, MgCO3 and MgO2

prevail below the Mg peak, which is in accordance withthe daytime and nighttime model by McNeil et al. (1996)on Earth and the daytime Martian model by Pesnell andGrebowsky (2000). MgO exhibits a layer with 40 cm−3 at80 km.The main di3erence between day and night is the strong

decrease of the Mg+ concentration. The peak density de-creases down to 50 cm−3 at 83 km height. The reason forthis decrease is obviously the absence of UV solar radiationand of the charge exchange mechanism. Therefore, directmeteoric ionization is the main nocturnal source of Mg+.Another interesting di3erence between day and night

is the greater density of Mg at night, which reaches3×104 cm−3. The reason is the absence of the UV pho-toionization and charge exchange processes.We have searched for other nocturnal sources of ioniza-

tion available onMars at the meteoric layer. In the case of theterrestrial atmosphere, a signiBcant ionosphere remains dur-ing the night because of the starlight ionization (Titheridge,2000). Starlight (911–1026 QA) is absorbed in a fairly nar-row height range near 105 km on Earth which correspondsapproximately to an altitude of 90 km in the Martian atmo-sphere. We have calculated the contribution of starlight inthe nocturnal ionosphere of Mars and found that it is negli-gible because the concentration of O2, the only atmosphericconstituents able to be ionized by starlight, is a factor of 175smaller than on Earth. On Earth, there is also an eOcienttransport of ionization from day to night that maintains thenightside ionosphere. The inclusion of this term needs thedevelopment of a 3-D model that is not considered here.

4.2. Fe-type species

Figs. 7 and 8 display the concentration of iron speciesduring daytime and nighttime, respectively. The daytime Feconcentration reaches 5×103 cm−3 at 80 km, while the Fe+

reaches 9×103 cm−3 at 83 km. The minor ions Fe+N2 andFeO+ are also expected, but with a concentration lower than1 cm−3. FeCO3 is the major species below 80 km. As forMg+, the same di3erence in the Fe+ concentration from dayto night time is observed. Again the reason is that the mainsource for the metallic ion is the charge exchange with O+

2 ,and the density of this ion is negligible during nighttime.Fe+ peaks at lower altitude than Mg+ due to its higher mass,which makes the transport towards the groundmore eOcient.At 80 km, Fe+ is produced by charge exchange (99%) anddirect meteoric ionization (1%), while the contribution ofFe photoionization to the total Fe+ production is negligible(lower than 0.5%).On Earth, models predict Fe (Helmer et al., 1998) and Mg

(McNeil et al., 1996) neutral layers below the ionic ones.We

have not found this on Mars due to the lower concentrationof O3 and O2 in the Martian atmosphere compared to that onEarth, which makes the process of Fe+ and Mg+ conversionto Fe and Mg less eOcient:

Fe+O3→FeO+ e→Fe; (13)

Fe+O2+M→ FeO+2

e→Fe; (14)

Mg+O3→MgO+ e→Mg; (15)

Mg+O2+M→ MgO+2

e→Mg: (16)

4.3. Solar cycle variability and e?ect of the Sun–Marsdistance

The results displayed in Figs. 5–8 have been obtainedfor solar minimum conditions and when the planet is ataphelion. We investigate here the solar activity variabilityand the e3ect of the distance to the Sun, through the changesin the photoionization process.When the solar activity is higher and the Sun–Mars dis-

tance is smaller, the solar UV Kux increases. As a conse-quence, the Mg and Fe photoionization rates are higher, asseen in Fig. 4. The metallic ion densities have been calcu-lated by using two di3erent rates (solar maximum/perihelionand solar minimum/aphelion). We decided here to look atthe behavior of the electron density, which is computed byadding the di3erent ion densities.The results are plotted in Fig. 9. We added the nighttime

electron density, which has obviously no solar activity anddistance to the sun dependencies. On the dayside, the elec-tron density peaks at around 85 km altitude. The e3ects ofboth the solar activity and of the Sun-planet distance is thusa total increase of about 10%. This increase may be consid-ered as small compared with the di3erence in the ionizationrates shown on Fig. 4. However, the metal photoionizationdoes not represent the main ion production source, contraryto charge exchange. Actually there is a lack in the modellingof the O+

2 density variability with the solar activity below100 km altitude. It is therefore not possible to study here thevariability of the charge exchange mechanism. As expected,during nighttime, the density has drastically dropped downto 230 cm−3 at 80 km.

5. Conclusions

In this work we have developed a model of the ionosphereof Mars between 60 and 120 km, both for daytime and night-time conditions. We have considered the metallic ion of me-teoric origin, that are Mg+, Fe+ and Si+. The neutral par-ent Mg, Fe and Si and the species MgO, MgO2, MgCO3,FeO, FeO2, FeO3, FeCO3, and SiO are also included in thecalculations. The resulting densities have been computedby taking into account the following processes: ablationof meteoroids, metals di3usion in the atmosphere, neutral

G.J. Molina-Cuberos et al. / Planetary and Space Science 51 (2003) 239–249 247

Fig. 7. Diurnal concentration of iron species for ionization rate shown in Fig. 1 and chemical scheme of Fig. 3.

Fig. 8. Same as Fig. 7 for nocturnal case.

photoionization by ultraviolet photons, and the chemistry ofions and neutrals including charge exchange.Most of the calculations have been performed for solar

minimum conditions, and the planet at aphelion. The day-time Mg+, Fe+ and Si+ densities reach respectively 104,

5×103, and less than 1 cm−3. During nighttime, the Mg+,Fe+ densities drop down to about 50 and 2×102 cm−3,respectively. Three ionization sources for metalic ions havebeen considered, i.e. direct ionization, photoionization andcharge exchange. We found that charge exchange with

248 G.J. Molina-Cuberos et al. / Planetary and Space Science 51 (2003) 239–249

Fig. 9. Electron concentration in the Martian atmosphere due to meteoric ablation: (solid line) maximum on solar activity and planet at perihelium;(dotted line) minimum on solar activity and planet at aphelium; and (dashed-line) nighttime.

non-metal ions, mainly O+2 , represents around 99% of the

total production of metallic ions. Concerning the Magne-sium ions, the same trend as in Pesnell and Grebowsky(2000) has been found. The e3ects of the solar cycle and ofthe distance between Mars and the Sun have been investi-gated through the photoionization process. We have foundan increase of about 9% in the electron density betweensolar maximum and planet at perihelion to solar minimumand planet at aphelion.This work is also related to the HF sounding that will

be performed onboard Mars-Express (1.8–5 MHz range)and Nozomi (8–10 MHz range). The results obtained herestrongly conBrm the conclusion of Witasse et al. (2000).During the daytime a strong HF radio wave absorption isexpected. The new daytime electron density proBle impliesa one-way attenuation of 70 dB at 5 MHz and of 21 dB at9 MHz, compared to the previous numbers 56 and 18 dB,respectively (Witasse et al., 2000).The study of the ionosphere induced by the meteoric ac-

tivity is a very interesting and promising Beld of research.Future work will include investigation of the variability ofthe charge exchange process, e3ect of a meteor shower, ef-fect of the magnetic Beld, and detectability of this secondaryionospheric layer.

Acknowledgements

We appreciate very much the detailed review and con-structive criticisms and comments made by E. ChasseBRere

and J.M. Grebowsky. G.J. M.-C. thanks Caja Murcia for aresearch grant. O.W. thanks S.W. Bougher and S. Engel forproviding neutral atmosphere data for Viking 1 Lander andMariner 6 orbiter geophysical conditions.

References

Adolfsson, L.G., Gustafson, B.A.S., Murray, C.D., 1996. The Martianatmosphere as a meteoroid detector. Icarus 119, 144–152.

Aikin, A.C., Goldbert, R.A., 1973. Metallic ions in the equatorialiononosphere. J. Geophys. Res. 78, 734–745.

Anders, E., Ebihara, M., 1982. Solar-system abundances of the elements.Geochim. Cosmochim. Acta 46, 2363–2380.

Bauer, S.J., 1973. Physics of Planetary Ionosphere. Springer, Berlin.Bougher, S.W., et al., 1990. The Mars thermosphere. II-General circulation

with coupled dynamics and composition. J. Geophys. Res. 95,14811.

Carter, L.N., Forbes, J.M., 1999. Global transport and localized layeringof metallic ions in the upper atmosphere. Ann. Geophys. 17,190–209.

Chapman, S., Cowling, T.G., 1970. The Mathematical Theory ofNon-Uniform Gases, 3rd Edition. Cambridge University Press, NewYork.

Christou, A.A., Beurle, K., 1999. Meteoroid streams at Mars: possibilitiesand implications. Planet. Space Sci. 47, 1475–1485.

Cuzzi, J.N., Estrada, P.R., 1998. Compositional evolution of Saturn’srings due to meteoroid bombardment. Icarus 132, 1–35.

Grebowsky, J.M., 1981. Meteoric ion production near Jupiter. J. Geophys.Res. 86, 1537–1543.

GrMun, E., Zook, H.A., Fechtig, H., Giese, R.H., 1985. Collisional balanceof the meteoritic complex. Icarus 62, 244–272.

G.J. Molina-Cuberos et al. / Planetary and Space Science 51 (2003) 239–249 249

Helmer, H., Plane, J.M.C., Qian, J., Gardner, C.S., 1998. A modelof meteoric iron in the upper atmosphere. J. Geophys. Res. 103,10,913–10,925.

Hinson, D.P., Twicken, J.D., Karayel, E.T., 1998. Jupiter’s ionosphere:new results from Voyager 2 radio occultation measurements. J.Geophys. Res. 103, 9505–9520.

Hughes, D.W., 1978. Meteors. In: McDonnell, J.A.M. (Ed.), CosmicDust. Wiley, Chichester, pp. 123–184.

Humes, D.H., 1980. Results of Pionner 10 and 11 meteoroid experiments:interplanetary and Near-Saturn. J. Geophys. Res. 85, 5841–5852.

Joiner, J., Aikin, A.C., 1996. Temporal and spatial variations in the upperatmospheric Mg+. J. Geophys. Res. 101, 5239–5249.

Jones, 1997. Theoretical and observational determinations of the ionizationcoeOcient of meteors. Mon. Not. R. Astron. Soc. 288, 995–1003.

Kliore, A.J., Patel, I.R., Nagy, A.F., Cravens, T.E., Gombosi, T.I., 1979.Initial observations of the nightside ionosphere of Venus from PioneerVenus orbiter radio occultations. Science 205, 99–102.

Kim, Y.H., Pesnell, W.D., Grebowsky, J.M., Fox, J.L., 2001. Meteoricions in the ionosphere of Jupiter. Icarus 150, 261–278.

Kopp, E., 1997. On the abundance of metal ions in the lower ionosphere.J. Geophys. Res. 102, 9667–9675.

Kopp, E., Balsiger, F., Murad, E., 1995. Silicon molecular ions in theD-region. Geophys. Res. Lett. 22, 3473–3476.

Lebedinets, V.N., Manochina, A.V., Shushkova, V.B., 1973. Interaction ofthe lower thermosphere with the solid component of the interplanetarymedium. Planet. Space Sci. 21, 1317–1332.

Lindner, B.L., 1988. Ozone on Mars: the e3ect of clouds and airbornedust. Planet. Space Sci. 36, 125–144.

Lyons, J.R., 1995. Metal ions in the atmosphere of Neptune. Science 267,648–651.

McNeil, W.J., Lai, S.T., Murad, E., 1996. A model for meteoricmagnesium in the ionosphere. J. Geophys. Res. 101, 5229–5251.

Molina-Cuberos, G.J., Lammer, H., Stumptner, W., Schwingenschuh, K.,Rucker, H.O., L.opez-Moreno, J.J., Rodrigo, R., Tokano, T., 2001.Ionospheric layer induced by meteoric ionization in Titan’s atmosphere.Planet. Space Sci. 49, 143–153.

Molina-Cuberos, G.J., Lichtenegger, H., Schwingenschuh, K.,L.opez-Moreno, J.J., Rodrigo, R., 2002. Ion-neutral chemistry model ofthe lower ionosphere of Mars. J. Geophys. Res. 105 (E5), (3)1–(3)7.

Moses, J.I., Bass, S.F., 2000. The e3ects of external material on thechemistry and structure of Saturn’s ionosphere. J. Geophys. Res. 105,7013–7052.

Mount, G.H., Rottman, G.J., Timothy, J.G., 1980. The solar spectralirradiance 1200–2550 QA at solar maximum. J. Geophys. Res. 85, 4271.

Nair, H., Allen, M, Anbar, A.D., Yung, Y.L., Clancy, R.T., 1994. Aphotochemical model of the martian atmosphere. Icarus 111, 124–150.

Pesnell, W.D., Grebowsky, J., 2000. Meteoric magnesium ions in theMartian atmosphere. J. Geophys. Res. 105, 1695–1707.

Rees, M.H., 1989. Physics and Chemistry of the Upper Atmosphere.Cambridge University Press, Cambridge.

Rodrigo, R., L.opez-Moreno, J.J., L.opez-Puertas, M., Moreno, F.,Molina, A., 1986. Neutral atmospheric composition between 60 and200 km: a theoretical model for mid-latitudes. Planet. Space Sci. 34,723–743.

Rodrigo, R., Garc.Va- .Alvarez, E., L.opez-Gonz.alez, M.J., L.opez-Moreno,J.J., 1990. A nonsteady one-dimensional theoretical model of Mars’neutral atmospheric composition between 30 and 200 km. J. Geophys.Res. 95, 14,795–14,810.

Stubbe, P., 1973. The thermosphere and the F-region: a reconciliationof theory with observations. Sci. Rep. 418, Ionosphere ResearchLaboratory, Pennsylvania State University, University Park.

Swider, W.J., 1969. Processes for meteoric elements in the E region.Planet. Space Sci. 17, 1233–1246.

Verner, D.A., Ferland, G.J., Korista, K.T., Yakovlev, D.G., 1996. Atomicdata for astrophysics. II. New analytic Bts for photoionization crosssections of atoms and ions. Astrophys. J. 465, 487.

Witasse, 2000. Mod.elisation des ionosphReres plan.etaires et de leurrayonnement: la Terre et Mars, Ph.D., Universit.e Joseph FourierGrenoble.

Witasse, O., Nouvel, J.-F., Lebreton, J.-P., Kofman, W., 2001. HF radiowave attenuation due to a meteoric layer in the atmosphere of Mars.Geophys. Res. Lett. 28, 3039–3043.