The CH4 Density in the Upper Atmosphere of Titan

Icarus 158, 191–198 (2002) doi:10.1006/icar.2002.6861

The CH4 Density in the Upper Atmosphere of Titan L. M. Lara, M. Banaszkiewicz,1 R. Rodrigo, and J. J. Lopez-Moreno Instituto de Astrof´ısica de Andaluc´ıa, CSIC, P.O. Box 3004, 18080 Granada, Spain E-mail: [email protected] Received May 14, 2001; revised October 24, 2001

Previous modeling by Banaszkiewicz et al. (2000a,b) showed that the CH4 thermospheric mixing ratio on Titan could vary as much as 35–40% due to ion–neutral chemical reactions. A new vertical methane profile has been computed by simultaneously modifying the stratospheric methane mixing ratio and the K(z) previously considered by Lara et al. (1996) and Banaszkiewicz et al. (2000a,b). A satisfactory fit of the methane thermospheric abundance and stratospheric mixing ratio of other minor constituents is achieved by placing the homopause at ∼1000 km and increasing the methane stratospheric mixing ratio (qCH4 ) up to 3.8%. The new proposed eddy diffusion coefficient steadily rises from 1 × 107 cm2 s−1 at 700 km to 1 × 1010 cm2 s−1 at 1500 km, whereas the stratospheric values are in the range (4–20) × 103 cm2 s−1 . Other likely ionization sources that can influence the methane distribution are (i) a metallic ion layer produced by micrometeoroid infall and (ii) frequent X-rays solar flares. Analysis of the effects of these ionization sources on the methane distribution indicates that, unlike previously assumed, CH4 can suffer considerable variations. These variations, although proved in this work, must be cautiously regarded since several assumptions have to be made on the rate of N2 and CH4 ionization by the processes previously mentioned. Hence, these results are only indicative of methane sensitivity to ionospheric chemistry. c 2002 Elsevier Science (USA) Key Words: Titan; CH4 thermospheric abundance; atmospheres; ionospheres.

1. INTRODUCTION

During the Voyager 1 encounter with Titan, the UVS (ultraviolet spectrometer) performed a solar occultation experiment which measured the differential absorption of sunlight as a function of altitude and wavelength in Titan’s upper atmosphere (Broadfoot et al. 1981, Smith et al. 1982, Strobel and Shemansky 1982). At the time of the encounter, Titan was well inside the Saturnian magnetosphere, but this situation changed during the Voyager 2 encounter when the intensities of the N2 airglow were typically a factor of 2 higher (Sandel et al. 1982), but no noticeable differences in the CH4 thermospheric abundances were reported. In spite of many modeling efforts, the attempt to obtain a CH4 density profile that links all observational points has been, so far, 1

Also at Space Research Centre, Bartycka 18a, 00816 Warsaw, Poland.

unsuccessful. As discussed in detail by Lara et al. (1996), there are two parameters in the neutral atmosphere models that determine the shape of the CH4 density profile: (i) the mixing ratio at the lower boundary (placed at 40 km in the modeling), and (ii) the eddy K profiles. The first parameter is not very well known, but can be limited to the range 1.5–3.4% (Lellouch et al. 1989) or even up to 4.4% after a reanalysis of the IRIS data by Courtin et al. (1995). The second one varies by orders of magnitude from one model to another. The differences are the largest ones in the upper part of the atmosphere. The choice of each parameter not only influences the CH4 mixing rate but also changes the profiles of other constituents and brings them to a better or a worse agreement with observations. Therefore, the process of finding the optimal parameters is a difficult one, especially because not all of the observational data can be accurately reproduced with a single parameter and becasue selecting a “unique” K (z) requires an arbitrary compromise. Lara et al. (1996) recommended the model in which the CH4 mixing ratio at 40 km is equal to 1.7% and the eddy diffusion coefficients increases from 1 × 107 cm2 s−1 at 700 km to 5 × 107 cm2 s−1 at 1430 km. Recently, a coupled model of the atmosphere and the ionosphere of Titan was published (Banaszkiewicz et al. 2000a, 2000b) from which it has become obvious that ions can play an important role in decreasing the density of CH4 at high altitudes. The typical change of CH4 mixing ratio due to interaction with ions is ∼35% at 1000–1400 km. This surprising result was not clearly enough described in the mentioned papers and, as has been pointed out to us (R. Yelle, private communication, 2000), it deserves a more careful treatment as presented in this paper. The neutral–ion interaction is not limited to the main ionospheric region, where the dominating ionospheric processes are: photoionization and ionization by magnetospheric electrons and photoelectrons. Hence, it is interesting to look for the effects connected with other ionization sources, such as micrometeoroids (Molina-Cuberos et al. 2000) or solar flare X-rays (Banaszkiewicz and Zarnecki 1999). Furthermore, the role of temporal changes in the ionosphere that are introduced, for instance, when Titan exits from and returns to the magnetosphere of Saturn to/from the solar wind should be analyzed more carefully. With the newly gained knowledge of the strength of the ionosphere–atmosphere coupling, we can come back to the

191 0019-1035/02 $35.00 c 2002 Elsevier Science (USA)  All rights reserved.

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question of the agreement between CH4 observations and theoretical models. In this paper we try to improve the situation by first introducing the coupling of neutral atmosphere with the ionosphere of Titan and then by changing the turbulent diffusion coefficient mainly above 700 km in order to get an overall agreement between observations and theoretical predictions. 2. MODEL

The neutral model was extensively introduced by Lara et al. (1996), whereas the ion–neutral coupled model was recently published by Banaszkiewicz et al. (2000a, 2000b). Among other results, the most surprising outcome of the chemical coupling between ions and neutral species is the variation of CH4 concentration in the upper atmosphere. The methane number density is a factor of 1.4 smaller when chemical terms due to ion–neutral reactions are taken into account. The column integrated chemical loss by ionospheric chemistry represents 25% of the total loss rate. This increased loss is not balanced by an enhancement in the production rate by ion–neutral chemistry in order to maintain the same flux at each atmospheric level as is the case when ionospheric chemistry is not considered. It has been long held that CH4 is largely unaffected by neutral chemistry, including photolysis. A simple dimensional analysis for a neutral atmosphere shows that diffusion is more than three

orders of magnitude more efficient than the estimated photodissociation and will replace any photolyzed molecules before they are missed. This implies that methane distribution should be very close to diffusive equilibrium. In view of the results presented by Banaszkiewicz et al. (2000a, 2000b), the widely accepted idea that Titan’s methane distribution is unaffected by neutral chemistry, and thus its vertical profile is purely a balance between turbulent and molecular diffusion, must be reconsidered and thoroughly analyzed. In order to verify that CH4 thermospheric distribution is sensitive to chemical processes, for simplicity we have first considered those chemical schemes involving only neutral compounds and different eddy diffusion coefficients. Figure 1 shows three examples in which the choice of the K (z) clearly determines by how much methane distribution is affected by neutral chemistry. The three panels in Fig. 1 contain the methane vertical profile obtained by running our neutral photochemical model under several assumptions. In every plot, the three lines refer to cases where we have (i) switched off the methane photodissociation, JCH4 = 0.0 (note that CH4 can still be catalytically destroyed by reaction with 1 CH2 , C2 H, C4 H, C2 , O(1 D) and OH), (ii) considered photodissociation and catalytic destruction of CH4 , and (iii) taken into account only transport processes as turbulent and molecular diffusion (i.e., switching off the methane chemistry), whereas the differences only lay in the different K (z) (Toublanc et al. 1995, Steiner and Bauer 1990, and Strobel et al. 1992,

FIG. 1. Methane vertical profiles obtained by running our neutral photochemical model when using the eddy diffusion coefficient in Steiner and Bauer (1990) (KS&B ), Toublanc et al. (1995) (KToub ), and Strobel et al. (1992) (KStro ), respectively. Solid line: CH4 chemistry is ON; dashed line: CH4 chemistry is OFF; and dash–dotted line: methane photodissociation is switched off (i.e., JCH4 = 0.0).

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TABLE I Column Integrated Production and Loss for a Stratospheric Methane Mixing Ratio of 3.8 × 10−2 Eddy diffusion coefficient Column

K Stei

K Stro

K Toub

K Lara

K This work

P Ltotal LJCH4 Lcatal P-Ltotal

2.19(7) 1.05(10) 2.74(9) 7.74(9) −1.05(10)

Neutral chemistry ON 6.44(8) 2.61(7) 8.01(9) 1.05(10) 2.65(9) 2.66(9) 5.38(9) 7.79(9) −7.37(9) −1.04(10)

3.73(7) 1.00(10) 2.62(9) 7.45(9) −1.00(10)

1.08(8) 9.74(9) 2.51(9) 7.23(9) −9.63(9)

P Ltotal P-Ltotal

1.05(7) 1.09(9) −9.90(9)

JCH4 OFF 2.00(7) 6.94(7) 1.46(8) 6.31(8) −1.25(8) −5.61(8)

3.45(7) 2.77(8) −2.42(8)

3.44(7) 2.49(8) −2.14(8)

P Ltotal Lneutrals Lions P-Ltotal

5.95(8) 1.41(10) 1.01(10) 3.94(9) −1.35(10)

Neutral + ion chemistry ON 3.40(8) 5.35(8) 4.73(8) 1.17(10) 1.39(10) 1.35(10) 8.63(9) 1.02(10) 9.96(09) 3.13(9) 3.73(9) 3.53(9) −1.14(10) −1.34(10) −1.30(10)

3.65(8) 1.30(10) 9.88(9) 3.15(9) −1.27(10)

Note. Read 5(5) as 5 × 105 . K Stei , K Stro , K Toub , K Lara , K This work refer to the eddy diffusion coefficients in Steiner and Bauer (1990), Strobel et al. (1992), Toublanc et al. (1995), Lara et al. (1996), and the one proposed in this work, respectively.

respectively). Table I contains the column integrated production and loss rates for each of the cases considered above. The results clearly show the smaller the K (z) above 600 km and the higher the catalytic destruction of CH4 compared to photodissociation, the more sensitive to neutral chemistry is the methane thermospheric abundance. Note that catalytic destruction of methane is as much as a factor of ∼3 larger than the direct CH4 photodissociation (75% of the total methane destruction is due to catalytic cycles). In the case of high turbulence, as indicated by Strobel et al. (1992), (i) chemistry acting on CH4 can still determine its vertical abundance, and (ii) the catalytic destruction of methane below ∼700 km is “only” twice that due to exclusively photodissociation. Furthermore, regardless of the eddy diffusion coefficient, the methane mixing ratio profile obtained when its photodissociation is switched off (CH4 only lost by catalytic processes) and when no methane chemistry is considered are very similar with differences ≤2.5% (see Fig. 1). This result arises from the fact that methane photodissociation is a key process for methane photolytic loss, that is, by making JCH4 = 0.0, not only the direct CH4 destruction by solar photons is ensured, but also the rate (cm−3 s−1 ) of reactions between methane and radicals is noticeably decreased. Therefore, these results change the long held views that methane vertical distribution is not affected by chemistry and that any chemical process acting above 600 km can indeed determine the methane abundance at much higher levels. The assumption that P − nl could be ignored in the continuity equation and therefore, the derivative of the flux should be zero (i.e., flux should be constant in the homosphere) is not accu-

rate. Deduction of K (z) and CH4 profile assuming that methane loss occurs entirely above the upper boundary of the model can introduce some errors in the results, since in fact, significant methane destruction occurs down to 700 km and below this level. The fact that the distribution of methane (or any other longlived species) is sensitive to the considered chemical scheme can be also viewed by analytically solving the flux equation under some (realistic) assumptions (see Steiner and Bauer 1990). The general solution of 

1 ∂n i 1 + αi ∂ T 1 i = −n i Di + + n i ∂z T ∂z Hi   1 ∂T 1 1 ∂n i − ni K + + n i ∂z T ∂z H



(1)

in an isothermal atmosphere, with an exponential variation of Di and K with altitude, is  1−si i P H P −h  n i = Ae−h 1 + eh (1−κ) 1−κ + e , K 0 (1 − si )

(2)

where h is the geopotential altitude above the homopause in units of the scale height of the background atmosphere, κ is obtained from the parameterization of the eddy diffusion co efficient (K = K 0 eκh , 0 ≤ κ < 1), si = HHPi (Hi and H P being the individual and atmospheric scale height), A is a constant of integration, and i P is the flux referenced to the planetary surface. For a hydrostatic atmosphere, the vertical distribution of the background gas is [N2 ] = [N2 ]0 e−h , and therefore, the mixing ratio of the different components is fi =

 1−si A  i P H P 1 + eh (1−κ) 1−κ + e−h . [N2 ]0 K 0 [N2 ]0 (1 − si )

(3)

In the previous equation, all of the terms are known except the 1 constant of integration A. If we compare the mixing ratio f CH 4 1 2 when CH4 photodissociation is off (i.e., CH4 ,P ) with f CH4 from the nominal case (i.e., 2CH4 ,P ), we readily obtain that 2 f CH 4 1 f CH 4



=1+

CH4 ,P e−h H P , 1 K 0 [N2 ]0 (1 − si ) f CH 4

(4)

where CH4 ,P = 2CH4 ,P − 1CH4 ,P . If methane photodissociation is not considered in the model, 1CH4 ,P > 2CH4 ,P (note that CH4 ,P < 0) and thus, the second term on the right-hand side of 2 1 < f CH . From Eq. (4), it can Eq. (4) is negative, resulting in f CH 4 4 be also deduced that the higher the turbulence (i.e., higher K 0 or higher location of the homopause), the smaller the differences in f CH4 for different methane chemical schemes. At first, it seems that the loss rates due to ions should be smaller than the neutral loss rates and therefore ion chemistry should not affect the CH4 density. Few arguments on this issue were given by Banaszkiewicz et al. (2000a) and therefore

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FIG. 2. Methane loss rate in our nominal model; that is, neutral and ionospheric chemistry is considered, and the K (z) is the newly proposed in this work. (a) Specific loss rate (li ). Solid line: specific loss due to ionospheric chemistry; dashed line: specific loss due to direct methane photodissociation; dash–three dotted line: specific loss due to neutral chemistry; and dotted line: specific loss due to neutral catalytic cycles (JCH4 = 0.0). (b) [CH4 ] × li . Solid line: specific loss due to ionospheric chemistry; dashed line: specific loss due to direct methane photodissociation; dash–three dotted line: specific loss due to neutral chemistry; and dotted line: specific loss due to neutral catalytic cycles (JCH4 = 0.0).

their statements could be misleading. The ion–neutral reactions are much more effective in removing CH4 at altitudes above the ionospheric peak than photodissociation, as can be seen in Fig. 2. The photodissociation coefficient is about 2.5 × 10−8 s−1 + at 1000 km, while the methane reactions with CH+ 3 and N2 −8 −1 amount to a loss coefficient 8 times larger (i.e., 20 × 10 s ) + at the same height (note that CH+ 3 and N2 number densities are −3 about 90–100 cm , and the reaction rate is 1.1 × 10−9 cm3 s−1 , as taken from Anicich and McEwan 1997). After integration over altitude, the overall effect is much smaller than that estimated at the peak, but still a change of 25–40% might be observed (see Table I). Any substantial change in the photo-

chemical scheme introduced, for instance, by including ions, is able to affect the slope (and the values) of the density profile [see Eqs. (3) and (4)]. 3. RESULTS AND DISCUSSION

For a methane tropospheric mixing ratio of 1.7% (as in Lara et al. 1996 and in Banaszkiewicz et al. 2000a, 2000b), it seems quite difficult to reproduce the CH4 thermospheric observations by only modifying K (z). The observed CH4 mixing ratio at 1400 and 1125 km suggests strong turbulent processes forcing the relative abundance of methane to slowly increase from

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CH4 DENSITY IN TITAN’S UPPER ATMOSPHERE

the homopause. If we place the homopause at ∼800 km, as in our previous models, the CH4 observations at 1000 km are too high when compared with the predicted abundances. A better agreement between the whole set of methane observations and predictions by the one-dimensional model can be achieved by simultaneously modifying the stratospheric methane mixing ratio and the eddy diffusion coefficient. We note that the stratospheric CH4 mixing ratio is still not well constrained. Lellouch et al. (1989) indicated that any value between 0.5 and 3.4% can satisfy Voyager infrared and radio-occultation measurements. From a reanalysis of the IRIS spectra, Courtin et al. (1995) have suggested a possible supersaturation of CH4 in the troposphere, which widens even more (up to 4.4%) the range of allowed CH4 . None of the published eddy diffusion coefficients (Tanguy et al. 1990, Strobel et al. 1992, Toublanc et al. 1995, Lara et al. 1996, Hidayat et al. 1997) gives a simultaneous and satisfactory fit to the methane, minor hydrocarbon, and nitrile observations. All those values of K (z) placing the homopause below 800 km (see Fig. 3) with a methane stratospheric mixing ratio ∼3% give rise to very high methane thermospheric abundance, although the minor hydrocarbon and nitrile mixing ratios closely match the observed values. On the other hand, the eddy diffusion coefficient proposed by Strobel et al. (1992) reproduces well the available CH4 observations in the thermosphere, but it severely underestimates the stratospheric mixing ratio of hydrocarbons and nitriles.

In an attempt to clarify these problems, we have investigated models with 3 × 10−2 < qCH4 < 4.4 × 10−2 and different K (z) constructed from the constraints previously mentioned (high location of the homopause, low stratospheric turbulence). Variations in the stratospheric mixing ratio as well as in the K (z) will also induce strong variations in the stratospheric abundance of other minor compounds (hydrocarbons, nitriles, and oxygen compounds). Therefore, besides matching the observed CH4 thermospheric profile, the predictions for other minor compounds must remain as close as possible to those presented by Lara et al. (1996) or to IRIS observations. As a result, the best overall agreement between observations and theoretical predictions (see Fig. 4 and Table II) is obtained when qCH4 = 3.8 × 10−2 , the homopause is placed at ∼1050 km (as Strobel et al. 1992 already suggested), and the K (z) stratospheric values are rather similar to those presented in Lara et al. (1996). The eddy diffusion coefficient obtained in this way is plotted in Fig. 3 and is indeed a function of the K (z) proposed by Strobel et al. (1992) and that by Toublanc et al. (1995) below 700 km. Above that level we adopted the one by Strobel et al. (1992), that is,  K (z) =

[K S × K T ]1/2 K (z) = K S

z ≤ 700 km z > 700 km,

FIG. 3. Different K (z) profiles proposed for Titan’s atmosphere. The K (z) labeled as This model is the one best reproducing the available observations of the minor compounds on Titan. This newly proposed eddy diffusion coefficient coincides with the one by Strobel et al. (1992) above 700 km (dash-dotted line), and it is a function of the latter and the one by Toublanc et al. (1995) below that altitude (dotted line) (see Section 3). The CH4 molecular diffusion coefficient is also plotted for comparison.

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4. SENSITIVITY OF THERMOSPHERIC CH4 TO OTHER IONIZATION SOURCES

In a recent paper, Molina-Cuberos et al. (2000) considered micrometeoroids as an additional important source of ions in Titan’s atmosphere. In their model, they employed meteoroids composed of only four chemical elements: Fe, Mg, Si, and O. They obtained a dense layer of metallic ions which, in the extreme case, reached a density of more than 104 cm−3 at its peak of about 800 km and extended between 600 and 1100 km. Although the obtained concentration is probably too high and was not observed by Voyager (Bird et al. 1997), the presence of such an ionospheric layer is very probable. The metallic ions do not easily exchange their charge with neutral molecules, such as N2 + and CH4 , to form corresponding layers of N+ 2 and CH3 that could, eventually, influence the CH4 density. There is only an indirect conjuncture put forward by Jones (1997) that suggested the existence of N+ 2 ions associated with metallic ion layer of micrometeoriod origin in the atmosphere of Earth. To avoid any speculations, we prefer to check the possible importance of + an hypothetical layer of N+ 2 on CH4 , assuming that N2 amounts to 0.1 to 1% of the total electron density in the layer produced by meteoroids. Those additional N+ 2 ions interact with CH4 + via the reaction N+ + CH → CH + N2 + H, and in that way 4 2 3 effectively decrease the number density of methane. These N+ 2 ions associated with metallic ion layer have dramatic effects on the CH4 number density profile decreasing it by a factor of 3. The disagreement between observations and theoretical results is of high importance at every thermospheric level (see Fig. 4). To better quantify the effect of these metallic ions, we have also considered that the N+ 2 produced by micrometeoroids can be lower by a factor of 0.1. In that case, the resulting CH4 number density profile is brought into good agreement with observations.

FIG. 4. Comparison among different methane mixing ratio profiles obtained under different assumptions on the ionization processes, K (z) and qCH4 stratospheric mixing ratio. Thick solid line: results from Lara et al. (1996) (i.e., qCH4 = 1.7% at the stratosphere). Solid line: stratospheric qCH4 = 3.8%, new K (z) and ionospheric chemistry by magnetospheric electrons is switched off. Dashed line: qCH4 = 3.8%, new K (z) and ionization by magnetospheric electrons is switched on. Dotted line: qCH4 = 3.8%, new K (z) and ionization by magnetospheric electrons and micrometeoroids are considered. Dash–dotted line: qCH4 = 3.8%, new K (z) and ionization by magnetospheric electrons and by solar X-rays flares are taken into account. Observational values (and their error bars) by Smith et al. (1982) are shown as asterisks.

where K S (z) and K T (z) refer to the eddy diffusion coefficient in Strobel et al. (1992) and Toublanc et al. (1995), respectively. Note that the K (z) proposed by Lara et al. (1996) can be obtained from K S (z) and K T (z) as 

 1 K (z) = exp [2 log(K T ) × log(K S )] . 3

TABLE II Results of the Model for the Proposed Eddy K Profiles Species

Altitude

C2 H2

>825 ∼725 130

C2 H4

125

C2 H6

130

CH3 C2 H C3 H8

105 125

C4 H2

110

HCN

110

CO CO2 H2 O

170 300 Stratosphere 120 400

Observed value Lara et al. (1996)

This work

∼1–2% ∼0.1–0.3% 2.2+0.7 −0.9 (−6)

0.1% 0.07% 3.0(−6) 8.3(−8)

1.17(−7)

1.3+0.5 −0.7 (−5)

8.7(−6)

8.3(−6)

4.4+1.7 −2.1 (−9) 7 ± 4(−7)

2.3(−11) 1.0(−7)

1.1(−11) 5.9(−8)

1.4+0.6 −0.7 (−9)

4.7(−9)

9.6(−9)

1.2(−7)

4.6(−8)

5.8(−7) 5.9(−6) 5.0(−5) 1.0(−8)

2.3(−7) 3.0(−6) 5.1(−5) 2.1(−9)

2.0(−8)

9.9(−9)

9+3 −5 (−8)

1.6+0.4 −0.6 (−7)

3.3(−7)b /2(−7)c 5.0(−6)b /4(−7)c 5(−5) 1.4+0.3 −0.5 (−8) 8+6 −4 (−9)

Note. Read 5(−5) as 5 × 10−5 .

0.1–0.2% 0.01% 4.1(−6)

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CH4 DENSITY IN TITAN’S UPPER ATMOSPHERE

A similar approach can be also applied to the sporadic ionospheric layer formed by X-ray photons during solar flares (Banaszkiewicz and Zarnecki 1999). In this case, a high electron density of about 100–1000 cm−3 is produced during the flare event that lasts 20 to 60 min. Later, the density decreases exponentially to reach 20–50 cm−3 after 24 hours, the final number depending on the effective loss rate (i.e., recombination rate of species with longest lifetime, that is, HCNH+ ). During solar maxima, when flare events are frequent (more than one event per day), one can therefore expect a quasi constant maximum electron density of about 50–100 cm−3 at the peak located at 550–720 km, depending on the flare spectrum. Those of the resulting ions that can react with CH4 represent only a fraction of the total ion content. The quantitative assessment requires a model at least as complicated as the one that describes the main ionospheric layer; hence, we only determine a plausible upper limit of the effect, assuming that the maximum mixing + ratio of N+ 2 and CH3 represents 10% of the electron density. To compare, in the main ionospheric layer, at the height of their maximum abundance, those ions amount to about 5% of the total ion number density. For our modeling, we take the ion production rate as a function of altitude that corresponds to the flare type X1 and results in a peak at 675 km. Assuming that the N+ 2 density at the peak is 10 cm−3 , it is straightforward to obtain the density profile for the range of altitudes 300–1450 km using the production rate profile. The effect of this additional ionization by solar flares on CH4 density is shown in Fig. 4. The methane mixing ratio noticeably decreases above 400 km; however, we note that the most important uncertainty in these CH4 variations produced by X-rays solar flares comes from the assumption that + N+ 2 and CH3 amount to 10% of the electron density. Therefore, the methane profile plotted in Fig. 4 should be considered as a very lower limit. Finally, we address the problem of time variations of the main ionospheric layer resulting from Titan’s excursions outside Saturn’s magnetosphere on part of its orbit. This phenomenon occurs when the solar wind pressure is large and when it compresses the magnetosphere to the extent that its “nose” is inside Titan’s orbit. Titan enters then the region of shocked, warm, and dense solar wind plasma in which the average energy of electrons is a few eV, i.e., much lower that the mean energy (200 eV) of electrons in the magnetosphere of Saturn. In the first approximation, we can assume that the ionization by magnetospheric electrons is switched off completely and is not replaced by any other factor. Since the magnetospheric electrons and photoelectrons amount to 20–25% of the total ionization rate at Titan and since the introduction of ionospheric chemistry induces CH4 variations in a 35–40% (Banaszkiewicz et al. 2000a), the decrease of methane thermospheric mixing ratio when Titan is outside Saturn’s magnetosphere is ∼7–10%. Indeed the detailed calculations confirm this estimate (Fig. 4). The obtained variation will occur only if the characteristic time of CH4 variations is smaller than the interval during which Titan is outside the magnetosphere of Saturn (several days;

i.e., ∼105 s). The chemical lifetime of CH4 at 1000 km is long (>107 s); however, the dynamical (diffusion) time scale is about 3 orders of magnitude smaller. Hence, any local changes of CH4 density will be quickly distributed in the atmosphere by diffusion, and the methane distribution will not be seriously affected along Titan’s orbit. Furthermore, note that the CH4 variations due to magnetospheric and secondary electrons are rather small (see Fig. 4) and that both profiles fit well to observational data. 5. CONCLUSIONS

Although the solution to the problem of fitting CH4 thermospheric abundances and the IRIS observations of other minor components in Titan’s atmosphere does not have a unique solution, reproducting of the methane thermospheric profile requires (i) an effective turbulence in the thermosphere placing the homopause at ∼1000 km and (ii) a stratospheric CH4 mixing ratio >3.5 × 10−2 . Moreover, even if the homopause were placed very high in the atmosphere, low stratospheric methane abundances would not allow a good match between UVS observations and model predictions. To also reproduce the IRIS observations (Coustenis et al. 1989), this effective turbulence in the high atmosphere must be balanced by low values of the eddy diffusion coefficient in the mesosphere and stratosphere. The thermospheric methane abundance is sensitive to ionospheric chemistry. Besides the ionization induced by EUV radiation, photoelectrons, saturnian magnetospheric and secondary electrons, and other ionization sources in the mesosphere and thermosphere, such as micrometeoroids and X-Rays solar flares, respectively, will give rise to lower CH4 abundances at those atmospheric levels. Large uncertainties in the N+ 2 ions associated with metallic ion layer of micrometeoroid origin and in the + amount of N+ 2 and CH3 produced by X-rays solar flares still remain. Therefore, the mentioned CH4 variations due to these processes must be cautiously considered and regarded as very upper limits. Although the effect of ionization by saturnian magnetospheric and secondary electrons is clear from the results presented here, the fact that Titan is out of the saturnian magnetosphere for a few days and that the lifetime for molecular diffusion is shorter implies that the effect of Titan’s excursions into/out of Saturn’s magnetosphere on the CH4 mixing ratio profile is not clearly manifested. Should it be manifested, these variations are not large enough to disagree with UVS observations. This seems to confirm the information obtained from the Voyager 1 and 2 encounters with the satellite: CH4 thermospheric abundance appeared to be the same in November 1980 and August 1981, when Titan was inside and outside the saturnian magnetosphere, respectively. A recent reanalysis of the Voyager 1 UVS data by Vervack et al. (submitted paper, 1999) seems to indicate that CH4 thermospheric abundance is lower than the previously published results by Smith et al. (1982). The nominal methane profile derived by Vervack et al. is well retrieved below 1050 km. At

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850 ≤ z ≤ 975 km, the methane number density obtained by Lara et al. (1996) is quite in agreement with these new observational results. However, some discrepancies arise above that level, and the apparent decrease of the observed CH4 profile at higher altitudes is more pronounced than that obtained by Yung et al. (1984), Toublanc et al. (1995), or Lara et al. (1996). The results presented here firmly indicate that ionospheric chemistry can indeed influence the methane distribution at very high atmospheric levels and be responsible for such an observed decrease. As Vervack et al. mention, there is a puzzling discrepancy between the ingress and egress profiles. The ingress occultation occurred on the dusk terminator, and photodissociation might have risen to a lower CH4 number density than for the egress. However, just the opposite is observed and as these authors mention, it is possible that magnetospheric interaction with Titan’s atmosphere has introduced longitudinal variation. The possibility of a substantial diurnal or longitudinal variation is also entertained, but we leave it for future modeling. ACKNOWLEDGMENTS The work performed by M.B. was partially funded by the project 2P03D 024 18p01 of the Polish Committee for Scientific Research (KBN). This research has been also supported by the Spanish Ministerio de Ciencia y Tecnolog´ıa under Contracts PNE-001/2000-C-01 and PNE-002/2000-C.

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