Methane Photochemistry: a Brief Review in the Frame of a New Experimental Program of Titanõs Atmosphere Simulations

ARTICLE IN PRESS

Advances in Space Research xxx (2005) xxx–xxx www.elsevier.com/locate/asr

Methane photochemistry: A brief review in the frame of a new experimental program of TitanÕs atmosphere simulations

C. Romanzin a,*, M.-C. Gazeau a,Y.Be´nilan a,E.He´brard a, A. Jolly a, F. Raulin a, S. Boye´-Pe´ronne b, S. Douin b, D. Gauyacq b

a Laboratoire Interuniversitaire des Syste`mes Atmosphe´riques, Universite´s Paris 12 & 7 – 61, av. du Gal de Gaulle, 94010 Cre´teil Cedex, France b Laboratoire de Photo-Physique Mole´culaire, Universite´ Paris 11 – baˆt. 210, 91405 Orsay Cedex, France

Received 29 October 2004; received in revised form 21 March 2005; accepted 22 March 2005

Abstract

As methane is the main precursor of TitanÕs chemistry, its photolytic behaviour is of great importance for the understanding of TitanÕs atmospheric system. Thus, the main purpose of this paper is to present a critical review about available laboratory methane photolysis results. The way these results are further exploited in photochemical models of TitanÕs atmosphere is also discussed, as those models may lead to diﬀerent and sometimes contradictory conclusions. Following this literature discussions, pertinent aspects of a new experimental project for a re-examination of methane photochemistry in the frame of new TitanÕs atmospheric simulations are also described. Ó 2005 COSPAR. Published by Elsevier Ltd. All rights reserved.

Keywords: Methane photolysis; Titan; Atmosphere; Photochemical models; Atmospheric simulations

1. Introduction In this paper, our interest will focus exclusively on the methane photolytic decomposition. A critical review Titan is the only satellite in the solar system that pre- about methane photolysis at Lyman-a (121.6 nm) will sents a dense atmosphere. Essentially composed of be presented ﬁrst and followed by a discussion on the nitrogen and a few percentage of methane, its compos- way methane photodissociation is described in photo- tion also reveals the existence of a complex organic chemical models. To conclude, new experimental ap- chemistry leading to the formation of numerous nitriles proaches currently developed in our labs and and hydrocarbon molecules among which key com- dedicated to the exploration of methane photochemistry pounds for prebiotic chemistry such as hydrogen will be described. cyanide. The chemical evolution of its atmosphere is essentially driven by the photolysis and the electron impact 2. CH4 photolysis dissociation of nitrogen and methane molecules. So, a good understanding of those primary processes is deci- The electronic absorption spectrum of methane lies in sive to succeed in modeling satisfactorily the chemical the vacuum ultraviolet (VUV) region at wavelengths processes taking place in TitanÕs atmospheric system. shorter than 140 nm, thus Lyman-a is the most intense incident radiation susceptible to be absorbed by methane in the high atmosphere. Therefore experimental * Corresponding author. Tel.: +33 145 171 537. photodissociation data at this wavelength are essential E-mail address: [email protected] (C. Romanzin). for studies aiming at a good atmospheric representative-

2 C. Romanzin et al. / Advances in Space Research xxx (2005) xxx–xxx ness. For this reason, our remarks will concern dissoci- the different fragmentation pathways can be distin- ation by 10.2 eV (Lyman-a) photons or close to only. guished from those studies. Actually, the following paragraphs consist of a compilation and analysis of different results obtained by various 2.1.1. Methylene photofragment, CH2 workers, in order to determine and classify the different Most of the first studies mentioned CH2 as a major photolytic decomposition pathways of methane in the product. Mahan and Mandal (1962) have indeed sug- Lyman-a wavelength region. The electronic transition gested that elimination of molecular hydrogen via (J2) 1 1 expected for such an energy is T2 A1 that corre- was the most important primary photochemical process, sponds to the 3sa1 1t2 electron promotion. The ener- even though they admitted that it was not the only one. getically allowed channels accessible with 10.2 eV Gorden and Ausloos (1967) study strengthened this photons are listed in Table 1. Among those channels, hypothesis, also supported by further arguments estab- two possibilities can be ruled out since they are pre- lished by Laufer and McNesby (1968). These latter suc- cluded because of spin conservation. Thus, seven possi- ceeded in determining the total quantum yield of bilities remained to be considered. molecular hydrogen formation, UH2 , and, the percentage of H2 directly formed by molecular elimination, mol UH2 . According to their results: UH2 ¼ 0.74 and about 2.1. Experiments in photolysis cells 73% of the total molecular hydrogen detected is derived from direct elimination. One can then estimate In most experiments that have been performed in mol UH ¼ 0.58 thus reinforcing the probability for (J2) photolysis cells, a krypton lamp has been used as the 2 and (J5) to occur. So, (J2) and hence CH2 seem to pre- light source (Milligan and Jacox, 1967; Laufer and vail on other channels and photofragments as it has McNesby, 1968; Hellner and Vermeil, 1970; Gorden been underlined by Slanger and Black (1982) (see Table and Ausloos, 1967; Hellner et al., 1971; Rebbert and 2). However, the conclusions of Braun and colleagues Ausloos, 1972/73). The cell filled either with pure CH4 (1967) about the predominance of (J2) are opposite. In or CD4 or with CH4–CD4 mixtures is thus irradiated their opinion, this fragmentation is the less probable un- by the Kr resonance line at 123.6 nm. This emission cor- like (J5) whose non-negligible role has been put forward responds to an energy of 10.0 eV. Other deuterated com- too by Hellner and Vermeil (1970). pounds have been used by some authors (Mahan and Mandal, 1962; Milligan and Jacox, 1967). The conclu- 2.1.2. Methylidyne photofragment, CH sions on primary decomposition processes are based Different works have pointed out the presence of mainly on results obtained from analysis of stable end- CH in methane photolysis experiments. Braun et al. products by means of chromatography or/and mass (1966), in an attempt to investigate the processes tak- spectrometry. Several outstanding points concerning ing place in the VUV flash photolysis have been the first to detect CH radical directly via its C2R+X2P and A2DX2P transitions in absorption. They could not conclude on its origin but they suggested that it Table 1 was produced in a larger amount than CH2. Gorden Energetically allowed channels at 10.2 eV (adapted from Cook et al. and Ausloos (1967) added that the primary dissocia- (2001)) tion channel (J5) must be responsible for a significant fraction of CH radicals rather than a secondary fragmentation of CH2 (Eq. (1)) and CH3 radicals (Eq. (2)): CH2 ! CH þ H; ð1Þ

CH3 ! CH þ H2; ð2Þ However this hypothesis of a subsequent fragmentation of methylene radicals leading to methylidyne (Eq. (1)) has been suggested by Laufer and McNesby (1968) as an additional way to produce CH according to the excess energy for (J2) (see Table 1). Finally, the importance of CH among all the photofragments has been evaluated by Rebbert and Ausloos (1972/73) who Shaded areas indicate that the decomposition pathway considered is measured UCH = 0.059. This low value is in agreement not spin conserving. Note that in the text any reference to (J2) must be with the low probability of (J5) suggested by Slanger understood as (J2a + J2b). and Black (1982) (see Table 2). ARTICLE IN PRESS

C. Romanzin et al. / Advances in Space Research xxx (2005) xxx–xxx 3

Table 2

Relative contributions of the CH4 dissociation product channels after photoexcitation in the Lyman-a wavelength region a b UH UH2 J1 J2a J2b J3 J4 J5 Slanger and Black, 1982 1.16 / – – + + – Mordaunt et al., 1993 scenario 1 1.0 0.24 0.51 0.24 / 0.05 + 0.20 0.00 + 0.00 Mordaunt et al., 1993 scenario 2 1.0 0.51 0.49 0.00 / 0+0 0.28 + 0.23 Heck et al., 1996 (0.77) 0.33 0.66 0.22 / 0 0.11 Brownsword et al., 1997 0.47 0.60 0.38 0.52 / 0.01 0.08 Wang et al., 2000 (0.47) 0.654 0.291 0.584 / / 0.055 0.07 Shaded line indicates that the experiment has been performed in collisional conditions unlike the others and is just mentioned for comparison. a The values for UH correspond to experimental determinations except when the ﬁgures are between brackets, then it corresponds to (J1)+2(J3)+2(J4)+(J5). b The values for UH2 correspond only to (J2a)+(J5) and not to an experimental determination. 3;1 The ﬁgures in italic correspond to: CH4 þ hm ! CH3 þ H ! CH2 þ 2H for (J3) and (J4), and to: CH4 þ hm ! CH3 þ H ! CH þ H2 þ H for (J5). These sequential contributions must be added to the direct contributions.

Considering those results, it appears that even though tially formed following (J3) and (J4) channels. It is CH must be present, its origin is ambiguous. The prob- noticeable too, that Braun et al. (1967) and Rebbert lem is to know whether CH is formed by a primary pro- and Ausloos (1972/73) have been the only ones to take cess (J5) or by a subsequent unimolecular decay of (J6) into consideration, others considering this channel another photofragment like CH2 and/or CH3. However, as unlikely to occur. if we sum UCH and UCH3 (see next paragraph), it appears As a conclusion, one must keep in mind that the re- that those fragments can account only for 16% of the to- sults discussed above were determined in collisional tal branching ratio, so that CH2 must be predominant. conditions and by means of correlations between stable end-products and supposed primary processes. One 2.1.3. Methyl photofragment, CH3 might then have some doubts about the primary nature Braun et al. (1967) were the first to propose a primary of the probed processes because a significant part of dissociation scheme where (J1) was the second most information is lost due to collisional effects. Neverthe- probable dissociation channel, making CH3 the most less, these were the only data available when the first abundant primary fragment after CH. This was sup- models of TitanÕs atmosphere were created (see Section ported by ab initio calculations from Karplus and Ber- 3 for more details). sohn (1969) who pointed out that a C3v deformation type would lead to the fragmentation pathway (J1). Nev- 2.2. Collision free experiments ertheless, the evolution of CH3 radicals is very uncertain since the energy in excess, released in the fragmentation To solve the problems concerning the nature of the after Lyman-a photolysis, is quite important (see Table probed processes and improve our knowledge about 1). Thus, Ausloos and colleagues (Gorden and Ausloos, methane VUV photodissociation, further experiments 1967; Rebbert and Ausloos, 1972/73) evoked the possi- have been carried out. They consisted of irradiations bility of a secondary fragmentation leading to the pro- of pure CH4 and/or pure CD4 at Lyman-a performed duction of CH radicals (Eq. (2)), whereas Laufer and in collision free conditions. In most cases, methane McNesby (1968) suggested that it could lead to CH2 was introduced in the photolysis zone of the experimen- (Eq. (3)). tal device through a jet. The primary processes were probed directly by detection of primary photofragments CH ! CH þ H; ð3Þ 3 2 with techniques such as time of flight spectroscopy Few years later, Hellner and Vermeil (1970) evaluated (TOF) and Doppler line profile measurements. the quantum yield for the CH3 radical production, UCH3 , Mordaunt and colleagues (1993) have been the first to and found it to be negligible (UCH3 ¼ 0.1), in agreement investigate methane photolysis in the conditions de- with LindholmÕs (1970) calculations which estimated the scribed above. They measured and analysed the kinetic formation of methyl radical as highly improbable on the energy distribution of atomic hydrogen formed during basis of symmetry considerations. Further results (Hell- photodissociation (see Table 1 for the corresponding ner et al., 1971; Slanger and Black, 1982), confirmed the channels). They observed two populations of hydrogen low probability for (J1) to occur. atoms: a fast one and a slow one. According to them, To complete this review on methane photodecompo- the fast channel corresponds to hydrogen atoms formed 2 00 sition, one must not forget to quote the determination of concomitantly with CH3ðX A2ÞðJ 1Þ, whereas the slow H the atomic hydrogen quantum yield, UH,bySlanger and atoms are formed via three body fragmentation pro- Black (1982) at Lyman-a. They found UH = 1.16 and cesses such as (J3), (J4)and(J5) (see Table 1 for the cor- reached the conclusion that atomic hydrogen is essen- responding excess energy). They also suggested that a ARTICLE IN PRESS

4 C. Romanzin et al. / Advances in Space Research xxx (2005) xxx–xxx fraction of these slow atoms comes from a subsequent features between the two scenarios being the needed decay of CH3 radicals. The same observation concern- activation energy and the symmetry of the transition ing the energetic distinction between H atoms has also state. In scenario 1, CH3 leads exclusively to the forma- been reported by Heck and coworkers (1996). They eval- tion of CH2 radicals (Eq (3)), while in scenario 2 it leads uated that the fast channel accounts for 87% of the total both to CH and CH2 radicals, (Eqs. (2) and (3)). But, H atoms and considered secondary fragmentation (Eq. independently from the chosen scenario, it appears that (3)) and contribution from (J5), (J3) and (J4) as respon- the simple C–H bond-breaking process (J1) is the pre- sible for the remaining 13%. In addition, they examined dominant channel. Anyway, these scenarios are limiting the H2 distribution after absorption of two UV photons cases, it means that the real situation should be some- in the 210–230 nm range. They found a bimodal distri- where in between the two cases. The same remark ap- bution and suggested that the fast H2 comes from (J2) plies for the values reported by Heck et al. (1996).In dissociation pathway, whereas the slow ones might be fact, they have just established proportionality relations formed through (J5) and by secondary fragmentation between the different channels where (J1) largely pre- of CH3 (Eq. (2)). A more recent study by Wang and vails, thus the set of branching ratios presented in Liu (1998) confirmed the existence of two kinds of H Table 2 has been crudely stated without considering atoms distinguishable according to the fragmentation any contributions from (J3) and (J4) channels. Brown- processes implied in their formation, but these latter sword et al. (1997) combined their new UH value with did not conclude on the concerted or sequential charac- available literature ones to determine the relative contri- ter of those dissociations. Nevertheless they discussed butions of the different channels to methane photodisso- about the radiationless pathways involved in (J1) ciation and concluded that (J2)and(J1) play the major fragmentation. roles in the fragmentation process. But the most com- In order to better understand this photolytic behav- plete determination of quantum yields until now has iour, Mebel and colleagues (1997) have carried out a been performed by Wang and co-workers (2000).By 1 3 comprehensive theoretical study on ( T2) and ( T2) ex- means of multiple experiments, the authors succeeded cited states of methane. They discussed on the different in drawing up a whole set of branching ratios. In addi- dissociation pathways and, according to their results, tion, they brought experimental evidence that CH2 is 2 00 1 formation of fast H atoms and of CH3ðX A2Þ could formed in the (a A1) state, thus confirming earlier theo- be explained either by an internal conversion or by an retical (Karplus and Bersohn, 1969; Lee and Janoschek, intersystem crossing process. Cook and colleagues 1979) and experimental (Masanet and Vermeil, 1975; (2001) performed complementary ab initio calculations Lee and Chiang, 1983) studies. in which they described the possible fragmentation path- Several features can be discussed from previous re- ways from a mechanistic and dynamic point of view. sults. It appears that whereas the fragmentation follow- They also carried out an experimental study and their ing the (J1) pathway was first believed as unlikely, new general conclusions strengthened Wang and LiuÕs re- experimental data revealed that it is in fact one of the marks (1998). two main decomposition processes with (J2) whose With the development of experimental setups ade- importance was already shown in former studies. Con- quate to work in collision free conditions, new determi- cerning methylidyne radical, its presence is not ques- nations of atomic hydrogen quantum yields have tioned but its weak abundance, already noticed by become possible. In fact, it was another outstanding Rebbert and Ausloos (1972/73), tends to be confirmed point of Mordaunt and co-workers (1993) study: they by the different values found for (J5) branching ratio. þ0.6 found UH ¼ 1.00.4, in accordance with Slanger and Another issue is the evolution of methyl radicals since BlackÕs (1982) previous value (UH = 1.16). But, the rein- their secondary fragmentation is often presented as an vestigation of the atomic hydrogen quantum yield by alternative source for CH and CH2 radicals. This Brownsword et al. (1997), performed in very similar sequential decomposition of methane makes its photol- conditions, led to a much smaller value than the one re- ysis even more complex to understand because it be- ported by Mordaunt et al. (1993): UH = 0.47 ± 0.11. To comes difficult to really identify the primary products. explain this difference, Brownsword et al. (1997) attrib- Both primary and sequential processes are important uted a larger importance to (J2) channel leading exclu- and should be taken into account for a realistic simula- sively to H2 than Mordaunt et al. (1993) did (see tion of TitanÕs atmosphere. So far, the only consensus Table 2). established on methane photodissociation at Lyman-a Finally, considering former and new data acquired by concerns the electronic state in which methylene frag- means of these comprehensive studies, several authors ments are produced. Indeed, several studies, Masanet proposed some photodissociation schemes which are and Vermeil (1975), Lee and Chiang (1983), Wang et al. summarized in Table 2. Mordaunt and colleagues (2000), clearly concluded that CH2 is formed in the 1 (1993) proposed two scenarios to rationalize the photo- (a A1) state. We can also point out that there are few lytic decomposition they observed, the distinguishing experimental data deduced from the direct analysis of ARTICLE IN PRESS

C. Romanzin et al. / Advances in Space Research xxx (2005) xxx–xxx 5 hydrocarbon fragments. The main reason is that the number of key rate coefficients. Proper exospheric detection is quite complex but it may be a complemen- boundary conditions, vertical transport and condensa- tary way to fully understand methane VUV photolysis. tion processes at the tropopause have been incorporated into the model. Starting with N2,CH4 and H2O, and including interactions with ultraviolet sunlight, energetic 3. Photochemical models electrons, and cosmic rays, the vertical profiles of the abundance of all the minor compounds detected by Numerous theoretical models have been built to ex- Voyager have been calculated. The authors have then plain and predict the chemical evolution of species in compared the calculated profiles with the observed mix- methane-rich atmospheres. All these models consider a ing ratios (UVS, IRIS and radio-occultation data of straightforward photochemical scheme initiated by Voyager 1 and 2 missions) and have concluded that, ex- methane photolysis followed by radical–radical and rad- cept for C2H4,CH3C2H, C3H8 and C4H2, their model ical–molecule reactions that eventually lead to the for- successfully accounts for all the species. mation of complex hydrocarbons. This paragraph Nevertheless, the scheme for methane photolysis used illustrates how the knowledge of the possible photofrag- by Yung et al. (1984) has been ruled out by the experiment branching ratios has influenced the description of mental study of Mordaunt et al. (1993). As detailed in CH4 chemistry in models. We will focus our review on Section 2.2, these authors have proposed two schemes photochemical models of TitanÕs atmosphere. For the which differ in the importance of CH2 versus CH forma- first time, we give a compilation of all determined pro- tion, CH3 being given in both cases as the main frag- files of computed mixing ratios as a function of altitude ment formed from the 121.6 nm dissociation of CH4. (Fig. 1). Then, we will be able, on one hand, to comment This prediction that methylene radical can be expected on how the theoretical data obtained by the available as the major product has repercussions on models of photochemical models fit the observations (ISO data the outer planets and TitanÕs atmospheres. In the case for hydrocarbons, Coustenis et al., 2003) and, on the of the SaturnÕs satellite, the new scheme for methane other hand, to compare the output of models where CH3 production dominates has been adopted. themselves. Toublanc et al. (1995) built a photochemical model of A model for JupiterÕs atmosphere, mainly composed TitanÕs atmosphere whose main objective was to im- of hydrogen and a few percentage of methane, has been prove the fit of the data obtained by new evaluations first developed by Strobel (1969) and adapted few years of Voyager spectra (Coustenis et al., 1989, 1991) and later to simulate TitanÕs atmosphere (Strobel, 1974). Al- new observations from Earth (Tanguy et al., 1990). De- len et al. (1980) updated the chemical part of this first spite many other improvements, in particular concern- model with the inclusion of the catalytic dissociation ing the radiative transfer in the atmosphere, the model of CH4 by C2H2 and the formation of polyynes as the was not able to explain the mixing ratios observed for precursory molecules for the haze layer on Titan. In C2H4,CH3C2HandC4H2 compounds. the light of the scientific findings from the Voyager mis- An other photochemical model was developed by sion, in particular the predominance of nitrogen, Strobel Lara et al. (1996). This model makes use of the second (1982) constructed an updated model for hydrocarbons scheme for methane photolysis proposed by Mordaunt taking into account, for the first time, the photochemis- et al. (1993) as well as many updated reaction rates. try of N2. Those early models, are incomplete because of Moreover, it includes a more realistic treatment of phys- the fact that methane chemistry was still poorly docu- ical processes that are supposed to occur in the atmo- 1 mented. For this reason CH2 radical was considered sphere (condensation processes at the tropopause, as the main product of the methane photolysis at deposition of water in the atmosphere from meteoritic Lyman-a. ablation, and dissociation of N2 by EUV wavelengths A year later, a one dimensional (1D) model of TitanÕs (k < 80 nm) and by galactic cosmic rays impacts). Most atmosphere was published (Yung et al., 1984). On the species, except C3Hn hydrocarbons, are reasonably well basis of a compilation of experimental studies on meth- reproduced, since their mixing ratios fall within the error ane photolysis at Lyman-a (among others: Laufer and bars of Voyager observations. McNesby, 1968; Rebbert and Ausloos, 1972/73; Slanger In 1996, Romani re-examined the available experi- and Black, 1982), these authors have adopted a scheme mental studies on methane photolysis at Lyman-a. Set- 1 3 leading to CH2, CH2 and CH fragments with respec- ting arbitrarily the (J5) branching ratio to zero, this tive branching ratios of 0.41, 0.51 and 0.08. The branch author proposed a scheme quite similar to the second (J1) (CH4 + hm ! CH3 + H) was, consequently, consid- branching ratio scheme of Mordaunt et al. (1993) ered as negligible. For the first time, the photochemistry (q(J1) = 0.41, q(J2) = 0.38, q(J3) = 0.093, q(J4) = 0.10) of simple molecules containing carbon, hydrogen, nitro- (Table 3). Nevertheless, this result was questioned by gen and oxygen atoms has been investigated using up- Smith and Raulin (1999) theoretical work. Indeed, these dated chemical schemes and new estimates of a authors, took into account the most recent measurements ARTICLE IN PRESS

6 C. Romanzin et al. / Advances in Space Research xxx (2005) xxx–xxx

Fig. 1. Hydrocarbon abundances from various photochemical models. All panels shown in this figure include six different profiles with the associated labels: Yung et al. (1984) (solid line), Toublanc et al. (1995) (dashed line), Lara et al. (1996) (dotted line), Dire (2000) (dot dashed line), Lebonnois et al. (2001) (double dot dashed line) and Wilson and Atreya (2004) (dot segmented line). Reticles represent the stratospheric ISO observations including error bars in abundance and altitude (Coustenis et al., 2003).

Table 3 provided new quantum yields for the ﬁve pathways Theoretical determinations of branching ratio scheme compared to the (q(J1) = 0.41, q(J2) = 0.38, q(J3) = 0.093, q(J4) = 0.10). experimental ones The latter are in good agreement with Brownsword J1 J2 J3 J4 J5 Comments et al. (1997) experimental results (Table 3). Because of Romani 0.41 0.28 0.00 0.21 0.10 cf. Mordaunt these contradictory results both schemes have been tested (1996) et al. (1993) in the ﬁrst 2D latitude–altitude photochemical and trans- Smith and 0.41 0.53 0.00 0.00 0.06 cf. Brownsword port model developed by Lebonnois et al. (2001). This Raulin (1999) et al. (1997) model has been built in order to investigate seasonal variations of the composition of TitanÕs low stratosphere. It of H atoms and H2 yields in the photolysis of methane uses an updated photochemistry based on Toublanc et al. (Heck et al., 1996: q(J1) = 0.66, q(J2) = 0.22, q(J5)= (1995) original work but without oxygen compounds. 0.11; Brownsword et al., 1997: U(H) = 0.47 ± 11) and Whatever scheme is used (Mordaunt et al., 1993 or Smith ARTICLE IN PRESS

C. Romanzin et al. / Advances in Space Research xxx (2005) xxx–xxx 7 and Raulin, 1999), the model gives similar results except In conclusion of this overview of TitanÕs photochem- in the very high atmosphere. The latitudinal variations of ical models, we can only agree with Lee et al. (2000) that most species observed by Voyager 1 are well fitted, never- ‘‘a rigourous test of the theory of hydrocarbon photo- theless, discrepancies are observed for ethylene C2H4, chemistry, and a systematic comparison between these cyanoacetylene (HC3N), diacetylene (C4H2) and cyano- models using a consistent set of photochemical reactions gene (C2N2) and are attributed to problems in the chem- applied to all of the atmosphere of the outer solar sys- ical scheme. tem is still lacking’’. Furthermore, we have seen that The methane photolytic scheme chosen by Dire for none of these models have provided a satisfactory expla- his 2D model (Dire, 2000) is based on the experimental nation for the mixing ratios of the whole set of observed work provided by Heck et al. (1996). Although mixing compounds, and in particular the hydrocarbons re- ratios derived from this model for ethane and propane ported in Fig. 1. agree with Voyager IRIS data, the model overestimates To go further in the knowledge of Titan chemistry, the abundances of acetylene (C2H2) and ethylene (C2H4) sensibility studies are currently performed at LISA in or- and, moreover, of diacetylene (C4H2). The author could der to point out its key parameters (He´brard et al., not explain which parameter (dynamics, photochemical 2005). or diffusion) used in the model was responsible for these discrepancies, except for acetylene and ethylene for which the absence of a chemical reaction may explain 4. New experiments at LISA and LPPM the retrieved high mixing ratios. As the most recent studies have not answered the is- Our teams (LISA and LPPM) are currently develop- sue of methane photolytic product yields at Lyman-a, ing new experimental projects dedicated to re-examine Wilson and Atreya (2000) have used a 1D photochemi- methane photochemistry. With that goal, we are carry- cal model with updated chemistry to investigate the sig- ing out two kinds of photolysis experiments in static nificance of these quantum yield schemes on the conditions and in a gas flow system. hydrocarbon chemistry of TitanÕs atmosphere. Thanks to sensitivity studies, these authors have demonstrated 4.1. Static experiments that no discrimination between schemes is possible by just fitting the observations. Indeed, abundances of sim- Classic photolysis experiments in closed cell are ple hydrocarbons such as acetylene (C2H2) and ethylene performed simultaneously with theoretical models. (C2H4), at the observed altitudes, are not sensitive to the The experimental results are compared with the data choice of methane photolysis scheme. Only C3 molecules obtained with a 0D model using the reaction pathway abundances show substantial changes in their abun- parameters (rate coefficients, branching ratios, photol- dances especially C3H6 and CH3C2H whose changes ysis frequencies, etc.) adapted to the considered per- are even surpassing observational uncertainties. The fectly known physical experimental conditions, authors have concluded that the differences in the abun- namely pressure and temperature. Such a method al- dances among previous photochemical models are lows to test the accuracy of the chemical scheme used. mainly due to the chemistry and the chosen eddy diffu- In the case of methane photochemistry studies, CH4, sion profiles, and not to the methane photolysis scheme. placed in a cell at a pressure of one to few tens of Another issue of Wilson and AtreyaÕs paper (2000) is millibars, is photolysed at Lyman-a delivered by an that their nominal scheme (Romani, 1996) provides an enclosed gas resonance light source filled with an intermediate profile among the tested schemes (Mor- H2/He mixture. The evolution of the irradiated gas daunt et al., 1993; Smith and Raulin, 1999). That is mixture is analyzed by IRTF spectrometry and the the reason why their latest TitanÕs 1D photochemical stable end-products are quantified. The chemical evo- model (Wilson and Atreya, 2004) is based on this lution of the system is then compared with the 0D scheme instead of the one of Brownsword et al. (1997) model that uses a complete description of the CH4 even though the latter has been confirmed by the most dissociation scheme involved (including secondary recent and forefront experiment performed by Wang reactions) (He´brard et al., 2005). Thus, static experi- et al. (2000). Finally, with an improved description of ments can be a very powerful tool to highlight weak- the chemistry (extensive treatment of dissociation pro- nesses in the description of the chemistry in cesses from solar photons and electrons) and the physics photochemical models. From our point of view, com- (parametrization of processes including cosmic rays, parison between models and experiments is an appro- electron impacts and opacity provided by fractal haze priate way to improve our understanding of chemical particles) and with an appropriate eddy diffusion profile, systems, and more accurate chemical schemes can be this model leads to an improved but not fully satisfac- developed through coupling these approaches. How- tory fit with observational data over previous TitanÕs ever, one has to be aware that this objective can only photochemical models. be reached if experiments are performed in such ARTICLE IN PRESS

8 C. Romanzin et al. / Advances in Space Research xxx (2005) xxx–xxx conditions that wall-effects (Smith et al., 1999)and that the chemical system is very complex and discrep- leaks (Vuitton, 2002) are avoided. ancies between observational and theoretical data can- not be attributed to the very primary step of the 4.2. Flow experiments process. As reported above, different schemes for the methane The flow experiments are an alternative to those per- photolysis have been adopted to fit the available observa- formed in photolysis cell as they minimize experimental tions of Titan. However, continuous effort in conducting artefacts evoked above. They differ also from non-colli- laboratory measurements under conditions applicable to sional condition experiments (Section 2.2) as they allow TitanÕs atmosphere (especially low temperature and low a chemical evolution of the system. Indeed, flow experi- pressure measurements) is still necessary. Thus, experiments carried out in a laser photolysis reactor and using ments should be performed in order to test, on one hand, Cavity Ring Down Spectroscopy (CRDS) as detection the reliability of the description of critical reaction path- technique, makes the measure of absolute concentra- ways and, on the other hand, the accuracy of kinetic tions of reactive species within an evolving system possi- parameters (absorption cross-sections, quantum yields ble (Berden et al., 2000). CRDS is an absorption and rate constants). spectroscopy technique in which an increased decay rate A more accurate chemical scheme could be then of laser intensity within a stable optical cavity is induced developed by coupling theoretical and experimental ap- by the presence of absorbing compounds within this proaches. For example, following the photochemical cavity (OÕKeefe and Deacon, 1988; Zalicki and Zare, evolution of a simple gas mixture can provide quantita- 1995). This way, by scanning the wavelength of the tive data which can be directly used to test a restricted probe laser, one can obtain absorption spectra inasmuch part of the chemical scheme used in photochemical mod- as one of the target compounds exhibits a transition in els. This is the method we use in order to determine the the scanned spectral region. At a given wavelength, cor- relative contributions of the different channels to meth- responding to an absorption feature of one of the com- ane photodissociation. In parallel, we are working on pounds, kinetic measurements (concentration versus the direct detection and quantification of fragments is- time) can also be done by collecting the data for a range sued from methane photolysis at Lyman-a and 248 nm of delay intervals (typically 0–1000 ls) between the pho- by laser CRDS spectroscopy techniques (see Section tolysis and probe laser pulses (Atkinson and Hudgens, 4.2.). 1997). Currently, experiments are performed in order These studies are the first step of a new program to probe and compare the CH fragment production named SETUP (a french acronym for Experimental (through its A X transition at 431 nm) from two ener- and Theoretical Simulations Useful for Planetology), getically equivalent photolysis processes, i.e., a single that couples laboratory experiments and theoretical photon process at 121.6 nm (Lyman-a), and a two-pho- studies in order to refine the theoretical description of ton process at 248 nm. the processes involved in TitanÕs atmosphere. The experimental device, currently under development, consists of a reactor where the initial gas mixture, representative of 5. Conclusion the main composition of the atmosphere: CH4/N2 (2/ 100), will be submitted simultaneously, for the first time, Despite the fact that methane photolysis has been the to both major energy sources that are responsible for the subject of many theoretical and experimental studies, a chemical evolution of TitanÕs atmosphere: electrons and complete understanding of the process has not been photons. Cold plasma and UV irradiation (248 nm) will achieved yet. As we have shown, the available literature be respectively used to dissociate N2 and CH4 (by a two- provides contradictory values for the branching ratios photon mechanism). Time-resolved analysis of the of the various possible CH4 photolysis pathways at resulting gas sample will be done by means of spectro- Lyman-a. scopic methods. Photochemical models of TitanÕs atmosphere have This work should provide adequate kinetic data and been built using alternatively the most recent methane appropriate reaction pathways in order to improve photodissociation scheme proposed by experimental- models and therefore to enhance our knowledge of ists or theoreticians, but none of those models have Titan complex atmospheric chemistry. given satisfactory fits for the whole set of observed compounds. Indeed, a comprehensive description of the hydrocarbon photochemistry is still a great chal- Acknowledgements lenge. Sensitivity studies on the choice of methane photolysis scheme done by Wilson and Atreya (2000) The LISAÕs group is grateful for the CNES grants have shown that no observational constraints are able that allowed us to attend COSPAR meeting and to Prof. to discriminate between those schemes. The reason is R. Navarro-Gonzalez for his invitation at session: F3.2/ ARTICLE IN PRESS

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