Chemical Sources of Haze Formation in Titan's Atmosphere

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Planetary and Space Science 51 (2003) 1017–1033 www.elsevier.com/locate/pss

Chemical sources of haze formation in Titan’s atmosphere

E.H. Wilsona;∗, S.K. Atreyab

aNASA/Jet Propulsion Laboratory, 4800 Oak Grove Drive M/S 169-237, Pasadena, CA 91109-8099, USA bDepartment of Atmospheric, Oceanic, and Space Sciences, University of Michigan, Ann Arbor, 2455 Hayward Street, Ann Arbor, MI 48109-2143, USA Received 16 January 2003; received in revised form 20 June 2003; accepted 23 June 2003

Abstract

A prominent feature of Titan’s atmosphere is a thick haze region that acts as the end product of hydrocarbon and nitrile chemistry. Using a one-dimensional photochemical model, an investigation into the chemical mechanisms responsible for the formation of this haze region is conducted. The model derives proÿles for Titan’s atmospheric constituents that are consistent with observations. Included is an updated benzene proÿle that matches more closely with—recent ISO observations (Icarus 161 (2003) 383), replacing the proÿle given in the benzene study of Wilson et al. (J. Geophys. Res. 108 (2003) 5014). Using these proÿles, pathways from polyynes, aromatics, and nitriles are considered, as well as possible copolymerization among the pathways. The model demonstrates that the growth of polycyclic aromatic hydrocarbons throughout the lower stratosphere plays an important role in furnishing the main haze layer, with nitriles playing a secondary role. The peak chemical production of haze layer ranges from 140 to 300 km peaking at an altitude of 220 km, with a production rate of 3:2 × 10−14 gcm−2 s−1. Possible mechanisms for polymerization and copolymerization and suggestions for further kinetic study are discussed, along with the implications for the distribution of haze in Titan’s atmosphere. ? 2003 Elsevier Ltd. All rights reserved.

Keywords: Titan; Atmospheric composition; Photochemistry; Organic chemistry

1. Introduction However, the precise sources and mechanisms for the formation of hazes in Titan’s atmosphere are quite uncertain. The surface of Titan is shrouded by a thick haze re- Laboratory simulations of Titan’s atmospheric conditions gion (Smith et al., 1981; Rages et al., 1983), a prominent have sought to answer this question through the analysis of feature in Titan’s opaque atmosphere. This region is split tholins (Sagan and Khare, 1979), brown complex solid or- into a main haze layer that resides below ∼ 220 km and a ganics produced through the irradiation of organic molecules detached layer at 300–350 km (Rages and Pollack, 1983). by UV light or electrical discharge. These simulations have Microphysical models place the altitude of peak aerosol provided some insight into the nature of molecules and the production anywhere from 250 to 600 km (Rannouet al., mechanisms that accompany the formation of sub-micron 1997, and all references within), while ChasseÿÂere and particles. For instance, laboratory simulations have shown Cabane (1995) suggest two formation regions for the main that formation of aerosol particles result from photolysis of haze layer (350–400 km) and detached haze layer (500 acetylene (C2H2), ethylene (C2H4), and hydrogen cyanide –800 km), through di@erent chemical mechanisms. These (HCN) (Scattergood, 1995; Bar-Nun et al., 1988). However, models, using high-phase-angle images (Rages et al., 1983) limitations in experimental conditions restrict the ability of and Infrared Spectrometer (IRIS) data from Voyager and laboratory simulations to fully represent the energy sources geometric albedos (Ne@ et al., 1985; Fink and Larson, and transport that a@ect Titan’s atmospheric constituents. 1979), have derived the haze production rate in the range Photochemical models have attempted to take these con- of 0.5–2 × 10−14 gcm−2 s−1 (McKay et al., 2001). siderations into account in analyzing the pathways that form haze particles. Unfortunately, laboratory measurements of the kinetics of reactions directly leading to the formation ∗ Corresponding author. Tel.: +1-818-354-9971; fax: +1-818-393- 4619. of haze particles under Titanian atmospheric conditions E-mail address: [email protected] (E.H. Wilson). are not available. Nevertheless, an examination of the

0032-0633/$ - see front matter ? 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.pss.2003.06.003 1018 E.H. Wilson, S.K. Atreya / Planetary and Space Science 51 (2003) 1017–1033 precursory processes to haze formation can provide clues the chain, eventually causing nucleation of the soot particle into the composition and formation region of Titan haze. Ni- (Krestinin, 2000). trogen (N2) and methane (CH4) predominate Titan’s com- The low activation energy of the ethynyl-insertion into position and, through dissociation, provide the foundation acetylene followed by H-abstraction mechanism allows (R1) of Titan’s organic-producing laboratory. The possible for- to proceed ePciently at low temperatures, as well (Chastaing mation of haze particles through polyyne formation (Yung et al., 1998). However, kinetic data of polyyne formation et al., 1984; Toublanc et al., 1995; Lara et al., 1996) and above the C4 variety are not available, necessitating assump- through polymerization of the nitrile HCN (Banaszkiewicz tions for rate coePcients. Yung et al. (1984) assumed that et al., 2000) in Titan photochemistry has been examined. these polyacetylenes were precursors of soot and followed However, previous Titan photochemical models have given the fate of polyacetylenes in Titan’s atmosphere up to C8H2. little attention to the aromatic pathway, anchored by the Acknowledging that (R2) will likely proceed in a manner formation of benzene (C6H6), which has been tentatively similar to (R1), Yung et al. assumed that larger polyacety- identiÿed in Titan’s atmosphere (Coustenis et al., 2003). lene radicals will be less reactive than C2H, using a formu- Wilson and Atreya (2001) and Wilson (2002), examining lation polyyne, aromatic, and nitrile mechanisms, suggested that k(C H)=31−nk(C H);n=1; 2; 3: this aromatic pathway may be the primary route to Titan 2n 2 haze formation. Lebonnois et al. (2002) have also stud- At higher temperatures, these radicals are assumed to pro- ied possible chemical routes to Titan haze through nitrile ceed at similar rates to C2H(Frank and Just, 1980; Kiefer and aromatic polymers as well as cyanoacetylene/acetylene and von Drasek, 1990). C2nH2-elimination to produce copolymers. In the following sections, we update this in- C2n+2H2 is not the primary loss process for these molecules. vestigation into the formation of haze via the polyyne, Thus, to determine the upper limit of polyacetylene-polymer aromatic, and nitrile pathways as well as examine possible (PA-polymer) production, the following assumption is mechanisms for copolymerization among these pathways adopted in order to gain insight into the mechanisms that are responsible for the presence of the Titan haze region and its k(C2nH+X )=k(C2H+X );n=2; 3;:::; composition. A list of important reaction rates used in this study is provided in Table 1. X =H2; CH4; C2H2;::: : The basis for the lengthening of polyacetylenic chains is set by (R1). This reaction has been recently measured 2. Pure polyacetylene polymers by Chastaing et al. (1998), using a CRESU (CinÃetique de RÃeaction en Ecoulement Supersonique Uniforme) apparatus,

The polymerization of acetylene (C2H2) through polyyne which measures rate coePcients down to 15 K. Fitting their formation has been postulated as a possible vehicle for the data points measured from 295 to 39 K, we obtained a rate creation of haze particles in the atmosphere of Titan (Allen expression of 9:53 × 10−11 e30:8=T cm3 s−1. This expression et al., 1980). This polyyne process is initiated with the pho- gives rate coePcients for (R1) twice as high as those used in todissociation of acetylene, creating ethynyl radical (C2H) the Toublanc et al. (1995) and Lara et al. (1996) models and through hydrogen abstraction. C2H proceeds to react with 4 times as high as that used in Yung et al. (1984) model in another acetylene molecule to produce diacetylene (C4H2) the temperature region where chemistry plays an important role on Titan. This chain-lengthening process is continued C2H2 +C2H → C4H2 +H; (R1) through the photolysis of C4H2 and the process continues, forming ever-larger successive C4H2 + hv → C4H+H (J1a) polyacetylenes C6H2,C8H2, etc. → C2H+C2H (J1b) C2nH2 +C2H → C2n+2H2 +H;n=1; 2; 3;::: : (R2)

→ C2H2 +C2 (J1c) Such polyynes have been identiÿed as major hydrocarbon in- termediates in pyrolysis processes at high temperatures and → C H : (J1d) soot production (Kiefer and von Drasek, 1990; Krestinin, 4 2 2000). These long chain polyynes also photodissociate and Glicker and Okabe (1987) measured C4H2 photolytic quan- produce polyacetylene radicals (e.g. C4H; C6H), which play tum yields to be the same role as C H in creating longer chains. Under py- 2 R rolytic conditions, H-elimination causes the formation of ¡1650 A:qJ1a =0:2;qJ1b =0:03; radical sites, upon which further polyyne building can con- qJ1c =0:1;qJ1d =0:67; 1650 AR 6 ¡2050A:R qJ1b =0:01; tinue. This “breeding” process can result in cyclization of the complex, shown in Fig. 1, rather than a lengthening of qJ1c =0:06;qJ1d =0:93; ¿ 2050 A:R qJ1d =1:0: E.H. Wilson, S.K. Atreya / Planetary and Space Science 51 (2003) 1017–1033 1019

Table 1 Rate coePcients for important reactions in this study

Reactions Rate coePcientsa;b References

Polyyne chemistry −11 30:8=T C2H + C2H2 → C4H2 + H 9:5 × 10 e Chastaing et al. (1998) −11 30:8=T C2H + C8H2 → PA-polymer 9:5 × 10 e Estimated, k(C2H + C2H2); see text −11 30:8=T C4H + C6H2 → PA-polymer 9:5 × 10 e Estimated, k(C2H + C2H2); see text −11 30:8=T C4H + C8H2 → PA-polymer 9:5 × 10 e Estimated, k(C2H + C2H2); see text −11 30:8=T C6H + C4H2 → PA-polymer 9:5 × 10 e Estimated, k(C2H + C2H2); see text −11 30:8=T C6H + C6H2 → PA-polymer 9:5 × 10 e Estimated, k(C2H + C2H2); see text −11 30:8=T C6H + C8H2 → PA-polymer 9:5 × 10 e Estimated, k(C2H + C2H2); see text

Aromatic chemistry

−6 −7:03 −1390=T C3H3 + C3H3 + M → n-C6H6 + M k0 =8:76 × 10 T e k0 estimated; Wang and Frenklach (1997) −11 k∞ =4:0 × 10 −7 −1:34 −1762=T (a) n-C6H6 + H → C6H6 + H ka =1:44 × 10 T e Wang and Frenklach (1994) −31 −0:52 −504=T (b) +M → n-C6H7 + M kb0 =8:0 × 10 T e −14 0:86 −554=T kb∞ =1:06 × 10 T e 10 −12:77 −5888=T (a) C4H3 +C2H2 +M → C6H5 +M ka0 =1:3 × 10 T e Wang and Frenklach (1994) −17 0:47 −3020=T (b) →n-C6H5+M ka∞ =2:8 × 10 T e −24 −2:66 −1711=T kb0 =2:2 × 10 T e −17 1:40 −1158=T kb∞ =4:8 × 10 T e −17 1:47 −2471=T C4H5 + C2H2 → C6H6 + H k =3:16 × 10 T e Westmoreland et al. (1989) 28 −16:3 −3526=T C6H5 + H + M → C6H6 + M k0 =1:82 × 10 T e Wang and Frenklach (1997) −10 k∞ =1:66 × 10 −10 −8052=T (a) C6H6 + H → C6H5 + H2 ka =4:15 × 10 e −28 Wang and Frenklach (1997); kb0 estimate; (b) +M → C6H7 + M kb0 =1:0 × 10 −11 −1605=T Mebel et al. (1997) kb∞ =5:27 × 10 e −13 −1560=T (a) C6H5 + C2H2 → PAH-polymer ka =3:72 × 10 e Yuet al. (1994) ; −19 −4:08 −403=T (b) +M → PAH-polymer kb0 =4:97 × 10 T e Wang and Frenklach (1994) 15 −8:94 −6039=T kb∞ =6:64 × 10 T e −12 −2168=T C6H6 + C6H5 →PAH-polymer k =1:59 × 10 e Park et al. (1999)

Nitrile chemistry

HCN + C H → C H CN + H 1:1 × 10−12 e−900=T 2 3 2 3 Monks et al. (1993); see text −21 2:7 −770=T CN + C2N2 → NT-polymer 2:19 × 10 T e Yang et al. (1992) −11 CN + C2H3CN → NT-polymer 4:23 × 10 Butterÿeld et al. (1993) −13 CN + C4N2 → NT-polymer 5:4 × 10 Seki et al. (1996) −3 −12 −900=T H2CN + HCN → NT-polymer 10 × 1:1 × 10 e Estimated, k(HCN + C2H3); see text

Copolymer reactions

−12 −2516=T HC3N + C2H → products 3:32 × 10 e Estimated, k(C2H2 + C2H) from Chastaing et al. (1998) −19 1:9 −1058=T HC3N + C2H3 → products 4:17 × 10 T e Estimated, k(C2H2 + C2H3) from Weissmann and Benson (1988) −14 −3520=T HC3N + C2H5 → products 8:32 × 10 e Estimated, k(C2H2 + C2H5) from Kerr and Parsonage (1972) −13 −1560=T HC3N + C6H5 → Co-polymer 3:72 × 10 e Estimated, k(C6H5 + C2H2) ∗ −12 0:5 ∗ HC3N + C2H2 → Co-polymer 1:28 × 10 T Estimated, k(C4H2 + C2H2) from Frost et al. (1996)

a 3 −1 6 −1 All rate coePcients are in units of cm s , except those denoted by k0, which are in units of cm s . b Three-body reactions are calculated according to the expression k = k0k∞M=(k0M + k∞).

∗ Along with polyacetylenic radicals, metastable diacetylene C4H2 , with acetylene and diacetylene (Frost et al., 1995; also contributes in forming higher-order polyynes. This pro- Bandy et al., 1993). Such reactions comprise 40% of the cess occurs through the reaction of metastable diacetylene, total C6H2 formation and 22% of the total C8H2 formation. 1020 E.H. Wilson, S.K. Atreya / Planetary and Space Science 51 (2003) 1017–1033

Fig. 1. Formation of a polyyne complex on a surface radical site, and its transformation into a more stable structure, followed by “breeding” of radical sites. (after Krestinin, 2000).

The main obstacle to any possible acetylene polymerization is acetylenic recycling, which proceeds through the reaction of C2H with CH4,C2H4,C2H6, and C3H8

C2H+H2 → C2H2 +H; (R3)

C2H+CH4 → C2H2 +CH3; (R4)

C2H+H2H4 → C2H2 +C2H3; (R5)

C2H+C2H6 → C2H2 +C2H5; (R6)

C2H+C3H8 → C2H2 +C3H7: (R7) Fig. 2. Abundance proÿles for C4H2,C6H2, and C8H2. The rectangles represent the IRIS observations (Coustenis et al., 1989). Of these reactions, (R4) and (R6) are the most important in Titan’s atmosphere. Opansky and Leone (1996a, b) conducted low temperature measurements of rate coePcients C6H2 +C6H → PA-polymer; (R9) for (R4) and (R6), obtaining a rate coePcient an order of magnitude larger than that used by Lara et al. (1996). This C8H2 +C2H → PA-polymer; (R10) would result in a more ePcient recycling mechanism, although there is agreement among all Titan models that (R4) C8H2 +C4H → PA-polymer; (R11) is the most important recycler.

C6H2 has not yet been identiÿed in the atmosphere of Ti- C8H2 +C6H → PA-polymer; (R12) tan, but laboratory simulations at Titan-like conditions (100 and eventually to Titan haze, the resulting haze precursor –150 K) (de Vanssay et al., 1995) have detected C H at 6 2 column output from this pathway is 8:7 × 102 cm−2 s−1, an abundance of 2–7 × 10−2 relative to C H . In the re- 4 2 peaking at the millibar region in the lower stratosphere, with gion where this temperature range applies and polyyne dis- a larger peak at 10−4 mbar. (R10) is responsible for 39% tribution is governed by chemistry (400–700 km), we ÿnd of PA-polymer formation, with (R8) accounting for 29%. good agreement, with a C6H2 column abundance of 2:3 × −2 10 relative to C4H2 in this region, evidence that polyyne chain-lengthening does indeed occur at low temperatures. 3. Pure nitrile polymers Fig. 2 shows the degree at which the density of successive polyacetylenes falls o@, rendering any possible stratospheric 3.1. Nitrile abundances haze formed from this mechanism minimal. The peak mole fraction of these compounds occur near the 10−5 mbar re- The existence of N2 and CH4 as the main constituents of gion, making the polyacetylene pathway a possible candi- Titan’s atmosphere suggests the status of nitriles as impor- date for the precursors of the detached haze layer, located tant contributors to the formation of haze particles. The most at higher altitudes, although the low mole fractions of C6H2 abundant nitrile, HCN, is formed in the upper atmosphere and C8H2, even at high altitudes, argue against this scenario. as a result of N and CH photolysis Assuming all of the production of polyynes containing 10 C 2 4 4s 4s atoms and more to result in the formation of PA-polymer, 0:5(N2 + hv → N +N );

C6H2 +C4H → PA-polymer; (R8) CH4 + hv → CH3 +H; E.H. Wilson, S.K. Atreya / Planetary and Space Science 51 (2003) 1017–1033 1021

4s N +CH3 → H2CN + H;

H2CN + H → HCN + H2

0:5N2 +CH4 → HCN + H2 +H: (S1) HCN polymers have been proposed to exist readily in the solar system, accounting for phenomena such as the dark surface of comet Halley and the orange–brown clouds of Jupiter and Saturn (Matthews, 1995). Polymerization can be initiated through the reaction of HCN with an excited HCN molecule, producing an HCN dimer, (HCN)2; (Rettig et al., 1992) eventually resulting in poly-HCN, (HCN)n. However, this process is not well understood, and laboratory studies from Khare et al. (1994) suggest that the optical constants of Titan haze do not match those of pure HCN polymers. Lara et al. (1999) analyzed the incorporation of N atoms into Titan haze through the possible polymerization of HCN. Arguing that the neglect of consideration of HCN loss to haze is responsible for their model’s overprediction of HCN abundance from 100 to 300 km, Lara et al. found a production rate for haze from HCN suPcient to reduce the HCN density to match observations. Lebonnois et al. (2001) include 2D dynamics as a supplement to vertical eddy di@usion to account for constituent transport and derive a HCN mole fraction proÿle at the equator that is consistent with observations, arguing that seasonal variations signiÿcantly impact HCN distribution. The attempt to derive a one-dimensional eddy di@usion proÿle (Wilson and Fig. 3. (a) Eddy di@usion coePcients from the nominal model (solid line), Yung et al. (1984) (dotted line), Toublanc et al. (1995) (short-dashed Atreya, 2002) that results in constituent proÿles consistent line), Lara et al. (1996) (long-dashed line), and Lara et al. (2002) with observations yields the proÿle shown in Fig. 3a. The (dot–dashed line). (b) HCN proÿles with (solid line) and without corresponding HCN proÿle (Fig. 3b) shows that we do not (long-dashed line) nitrile pathway to haze. Solid bars represent the Hi- ÿnd the incorporation of N to the haze necessary to ÿt ob- dayat et al. (1997) HCN observations, while the dotted bars represent the servations. Tanguy et al. (1990) observations, and the short-dashed lines represent the proÿle retrieved from the Marten et al. (2002) observations. Nevertheless, photolysis of HCN does result in the formation of long-chain nitriles, analogous to polyacetylenes, through the action of CN. CN acts in a similar fashion as Yung (1987) proposed another mechanism for the formation C2H, reacting with hydrocarbons and other nitriles to form of C N as well as C N through the action of the CHCN larger nitrile molecules, such as cyanoacetylene (HC3N), 4 2 2 2 radical in the reactions acrylonitrile (C2H3CN), and dicyanoacetylene (C4N2). Thompson and Sagan (1989) postulated the formation of CHCN + CHCN → C N +H ; (R14) heteropolymers through the attachment of —CN and — 4 2 2 RCN chains onto nitrile compounds, as shown in Fig. 4. 4s The gas-phase buildup of some of those nitrile compounds CHCN + N → C2N2 +H: (R15) through CN addition has been kinetically studied. HC3Nis produced primarily through the addition of the photolytic Considering estimations by Yung for (R15), the main source product CN and acetylene in the upper atmosphere of dicyanoacetylene is the scheme 2d HCN + hv → CN + H; N2 + hv → 2N ; 2d CN + C2H2 → HC3N+H : (S2) 2[N +C2H2 → CHCN + H]; (S3) HCN + C2H2 → HC3N+H CHCN + CHCN → C4N2 +H2;

The path to ever-larger nitrile molecules can continue in the N2 +2C2H2 → C4N2 +2H+H2 same manner to form C4N2 responsible for 98% of C4N2 production throughout Titan’s CN + HC N → C N +H: (R13) 3 4 2 atmosphere. Likewise, C2N2 is produced through the 1022 E.H. Wilson, S.K. Atreya / Planetary and Space Science 51 (2003) 1017–1033

Fig. 4. Nitrile polymerization process involving the formation of chains by the attachment of —CN and —RCN fragments (after Thompson and Sagan, 1989). scheme

4s 2d N2 + hv → N +N ; 2d N +C2H2 → CHCN + H; (S4) 4s N + CHCN → C2N2 +H2

N2 +C2H2 → C2N2 +2H responsible for virtually all of C2N2 production. Above 400 km, C2H3CN production follows along the same lines as (S2),

HCN + hv → CN + H;

CN + C2H4 → C2H3CN + H; : (S5)

HCN + C2H4 → C2H3CN + 2H while in the lower atmosphere, C2H3CN is produced through

M C2H2 +H→ C2H3;

HCN + C2H3 → C2H3CN + H

HCN + C2H2 → C2H3CN + H (S6)

Abundance proÿles for these nitriles can be found in Fig. 5a. The most abundant of these nitriles in the lower stratosphere, according to model results in C2H3CN. However, C2H3CN has not been identiÿed in Titan’s atmosphere by IRIS or the Infrared Space Observatory (ISO). In addition, recent millimeter observations from the IRAM telescope derive an upper limit abundance of 2:0 × 10−9 for acrylonitrile (Hidayat, 1997; Marten et al., 2002). Furthermore, Coll et al. (1999) in their laboratory study found

−3 [C2H3CN]upper limit=[HCN]upper limit =1:3 × 10 ; much smaller than the order of magnitude factor found in the lower stratosphere by comparing Figs. 3 and 5a. A large C2H3CN density results in C2H3CN photolysis as the main Fig. 5. (a) Abundance proÿles for key nitrile species, assuming fractal haze source of HC3N, and thus, the cause of this apparent overpre- opacities and the Monks et al. (1993) rate coePcient for HCN + C2H3. diction needs to be analyzed. Considering that the synthetic (b) Nitrile abundance proÿles calculated neglecting HCN + C2H3. (c) Nitrile abundance proÿles assuming Mie haze opacities. proÿles of C2H2 and HCN match observations in the stratosphere, the most likely causes of this large prediction are: (1) underconsideration of acrylonitrile loss, (2) incorrect chem- Scenario (1) not enough C2H3CN loss is being consid- ical mechanism regarding the formation of C2H3CN or (3) ered: The photodissociation of C2H3CN is modeled using other considerations a@ecting C2H3CN loss. the unpublished data of F. Raulin (pers. comm.), with the E.H. Wilson, S.K. Atreya / Planetary and Space Science 51 (2003) 1017–1033 1023

Scenario (3) other considerations a:ecting C2H3CN loss: The acrylonitrile cross section extends into the 1800 –2200 AR region, where haze particles provide the dominant opacity (McGrath et al., 1998). Titan haze opacity, however, is still a pressing issue. Microphysical models have attempted to describe the nature of Titan haze and its opacity. Models such as Toon et al. (1992) and McKay et al. (1989) have used Mie scatterers as analogues for Titan haze particles while fractal models (e.g. Cabane et al., 1993), Rannouet al., 1995, 1997 ) have simulated Titan haze using Vu@y aggregates of monomers. Spherical Mie models have been able to reproduce the observed geometric albedo, while until recently (Rannouand McKay, 2003 ), fractal models have had diPculty matching the methane feature at 0:64 m. However, photometric and polarimetric data have suggested that Titan haze does take the form of Vu@y frac- Fig. 6. Absorption cross section for C2H3CN (F. Raulin, pers. comm.). tal aggregates. Fractal particles are more opaque than Mie particles at short wavelengths (Rannouet al., 1995 ), a@ecting the radiation available to photolyze constituents in the cross section shown in Fig. 6. This C2H3CN cross section, stratosphere. Most a@ected are constituents that primarily larger than assumptions made previously (Toublanc et al., absorb at wavelengths larger than 1800 A,R like C2H3CN. 1995) allows photolysis to be easily the dominant loss mech- However, as Titan aerosol densities and optical constants anism for stratospheric C2H3CN, through which cyanoacety- are still not well understood, aerosol opacities provide a lene is the primary benefactor (Derecskei-Kovacs and North, signiÿcant uncertainty for photodissociation coePcients of 1999). If underconsideration of acrylonitrile photolysis were a@ecting constituents. the main cause of C2H3CN overprediction, this cross section To test scenarios (2) and (3), sensitivity studies are con- would have to be signiÿcantly larger. However, considering ducted, considering a case where the reaction HCN+C2H3 is its broad continuum nature and its extent into the NUV, par- omitted and a case using Mie opacities derived from Rannou ticularly in the context of other nitrile spectra, it is diPcult et al. (1995) as opposed to the nominal case which uses frac- to image a larger cross section to the extent of signiÿcantly tal opacities from Rannouet al. (1995) and Lebonnois et al. reducing the C2H3CN stratospheric mole fraction by orders (2001). Fig. 5b shows that the omission of the HCN +C2H3 of magnitude. Furthermore, such a cross section would re- source decreases the acrylonitrile abundance in the lower sult in a larger HC3N modeled abundance, which is already stratosphere by more than four orders of magnitude. The too high compared to observations. Other mechanisms in- corresponding change in HC3N results in a reduction of a volving C2H3CN destruction which are not considered in the factor of three at 105 km. In the Mie example (Fig. 5c), the model may be taking place, but such a mechanism ePcient mole fractions of all of the large nitrile molecules are signif- enough to overwhelm photolysis in the stratosphere is un- icantly reduced in the lower stratosphere, with the exception likely, especially considering photolysis dominates the loss of CH3CN, which does not signiÿcantly absorb longward of all known large stable nitrile molecules in Titan’s strato- of 1800 A(R Suto and Lee, 1985). The value of 5:3 × 10−10 sphere. For these reasons, this scenario is removed from for the HC3N mole fraction in the Mie case agrees with the consideration. ISO disk-averaged value obtained with a HC3N contribution Scenario (2) faulty chemical mechanisms regarding peak at ∼ 105 km (Coustenis et al., 2003). Marten et al. acrylonitrile production: The large densities of HCN and (2002) millimeter observations retrieved much lower abun- C2H2 along with the pressure-induced hydrogen attach- dances for HC3N in the region of the atmosphere, although ment onto acetylene allow (S6) to be very ePcient in the shape of their HC3N proÿle in the lower stratosphere producing C2H3CN, despite the small rate coePcient of sharply di@ers from photochemical models. the producing reaction (k =2:7 × 10−15 cm3 s−1@150 K). It is unclear which of these causes is responsible for the This reaction, measured by Monks et al. (1993), did not large HC3N and C2H3CN densities found in Fig. 5aorto produce a deÿnitive yield in the laboratory as only trace what degree both are responsible. Thus, in order to obtain amounts of C2H3CN were identiÿed in the experiment. more likely abundances for the purpose of estimating yields Monks et al. assumed addition–decomposition as the fa- of various haze-forming mechanisms, k(HCN + C2H3)is −3 vored mechanism of C2H3 + HCN through analogy with reduced by a factor of 10 , with the results shown in HCN + C2H and C2H2 +C2H. However, the hypoth- Fig. 7a. Note that the densities for HC3N and C4N2 in this esis that C2H3CN production through this mechanism nominal case are most likely still too high in the lower −9 may be signiÿcantly smaller than assumed remains a real stratosphere, with a HC3N mole fraction of 4:4 × 10 at possibility. 105 km. 1024 E.H. Wilson, S.K. Atreya / Planetary and Space Science 51 (2003) 1017–1033

In accordance with Thompson and Sagan (1989), the reaction of H2CN with HCN is proposed to contribute to the formation of long-chain nitriles in the path towards polymerization:

H2CN + HCN → NT-polymer: (R19)

(R19) has not been kinetically studied so we assume a rate coePcient equal to that of k(HCN + C2H3), a rate coePcient also considered in Lebonnois et al. (2002) for this reaction. The evidence for kinetically similar behavior for H2CN and C2H3 is that rate co- ePcients for H2CN + H and H2CN + N (Nesbitt et al., 1990) are within a factor of two of the rate coeP- cients for C2H3 +H(Monks et al., 1995) and C2H3 +N (Payne et al., 1996), respectively. However, this assumption must be taken under advisement as HCN does not proceed very quickly through reaction with other nitriles. Stable–RCN molecules (e.g. CH3CN) will most likely proceed very slowly, if at all, with HCN. Fig. 7b shows the polymer production from (R16)–(R19), assuming the Monks et al. (1993) rate for HCN + C2H3 and (R19), while Fig. 7c shows the production rates from the di@erent nitrile mechanisms assuming the reduced Monks et al. rate. It is clear that nitrile-produced haze is triggered by (R19) in the lower stratosphere in both cases. At higher altitudes, polymer is generated through the CN-insertion mechanism.

4. PAH polymers

Polymerization of acetylene can also result in the formation of Polycyclic Aromatic Hydrocarbons (PAHs), which can be found in circumstellar nebulae and in- terplanetary dust particles (Clemett et al., 1994; Buss et al., 1993), and are believed to exist prominently in the interstellar medium (Allamandola et al., 1989). Fig. 7. (a) Nominal abundance proÿles for key nitrile species as- Although most studies regarding PAHs involve high −3 suming k(HCN + C2H3)=10 × k (Monks et al., 1993). (b) Pro- temperature combustion conditions, PAHs have been iden- duction rate proÿles for nitrile-pathway haze production mechanisms, tiÿed in laboratory simulations of Jupiter and Titan haze assuming k(HCN + C2H3)=k (Monks et al., 1993). (c) Production rate proÿles for nitrile-pathway haze production mechanisms, assuming (Sagan et al., 1993). Furthermore, the detection of ben- −3 k(HCN + C2H3)=10 × k (Monks et al., 1993). zene (C6H6), the simplest of aromatic hydrocarbons, on Jupiter and Saturn (BÃezard et al., 2001), the tentative detection of benzene on Titan (Coustenis et al., 2003), 3.2. Haze pathways and identiÿcation of benzene in laboratory simulations of Titan’s atmosphere (Coll et al., 1999) together suggest To account for polymerization through CN-insertion, the that PAH synthesis proceeds in planetary atmospheres, as pure nitrile pathway (NT-polymer) to haze formation is as- well. sumed to follow via the addition of CN to high-order nitriles: Benzene is formed in the atmosphere of Titan primarily through the pressure-induced recombination of CN + C N → NT-polymer; (R16) 2 2 propargyl radicals (Wilson et al., 2003). Once the clo- sure of that ÿrst aromatic ring has occurred, leading CN + C2H3CN → NT-polymer; (R17) to the establishment of the benzene molecule or its hydrogen-abstracted radical equivalent, phenyl (C6H5), the CN + C4N2 → NT-polymer: (R18) propagation of PAH formation is promoted through the E.H. Wilson, S.K. Atreya / Planetary and Space Science 51 (2003) 1017–1033 1025 attachment of ethynyl radical onto benzene or acetylene Wang and Frenklach (1994, 1997) measured rates onto phenyl: of reaction for (R20) and (R21) for minimum temperatures varying from 300 to 500 K, with the tem- C6H5 +C2H2 → PAH-polymer; (R20) perature range extending up to 2500 K. This presents some diPculties when extrapolating down to Titan-like temperatures. Yuet al. (1994) obtained a rate ex- C H +C H → PAH-polymer: (R21) 6 5 2 2 pression for (R21) for the temperature range of 297– This mechanism, known as the HACA (H-abstraction= 523 K. This rate is compared with the Wang and Frenklach rate, assuming high-pressure conditions, in C2H2-addition) sequence, may proceed through continued H-abstraction followed by acetylene addition as proposed Fig. 10a. In Titan’s temperature regime, Yu et al. rate by Wang and Frenklach (1997) or through continued exceeds the Wang and Frenklach rate by as much as acetylene-addition to close the second ring as proposed two orders of magnitude. Yu et al. rate is adopted by Bittner and Howard (1981). These mechanisms, form- as the study is more focused on the low temperature ing the two-ring naphthalene are illustrated in Fig. 8. The regime. However, this discrepancy highlights the need for Bittner-Howard version appears to be more important at low more low temperature kinetic measurements of aromatic temperatures (Bauschlicher Jr. and Ricca, 2000), providing chemistry. the basis for the formation of the PAH-polymer. The impact of the two rates for (R21) on the ben- Wilson et al. (2001) and Wilson (2002) consider the zene proÿle is shown in Fig. 10b. The pathway to HACA mechanism as the primary source of polymerization polymerization is not the main loss mechanism for ben- through aromatic chemistry. Assuming that PAH production zene, but rather photolysis. Thus, the enhanced loss beyond the ÿrst aromatic ring goes into Titan haze, the re- from the Yuet al. rate resultsin only a 20% de- actions to consider for PAH-polymer formation are (R20) pletion in benzene density over what is found using and (R21), which were also considered by Lebonnois et al. the Wang and Frenklach rate. However, as Fig. 10c (2002), along with highlights, the increased rate results in an order of magnitude increase in polymer production in the lower

C6H6 +C6H5 → PAH-polymer (R22) stratosphere, with the peak of production rising from 180 to 220 km. In this proÿle, (R21) is clearly the domi- with k(R20) and k(R21) taken from the Wang and nating PAH-polymer mechanism at the peak of produc- Frenklach (1997) measurements and k(R22) from Park tion, due to the involvement of the abundant acetylene et al. (1999). molecule. At this altitude, (R20) and (R22) are four and Wilson et al. (2003) suggest the possibility of ben- ÿve orders of magnitude less ePcient, respectively. Above zene formation in Titan’s atmosphere, corroborated by 400 km, (R20) is the dominant PAH-polymer producing the tentative detection of benzene by ISO (Coustenis mechanism. et al., 2003). The observation ÿt by a uniform mole A possible pathway to haze is polymerization through −10 fraction proÿle of 4 ± 3 × 10 was matched by a the aliphatic version of C6H5. Lebonnois et al. (2002) vertical proÿle of benzene from Wilson et al. (2003), considered a path to haze formation through the reaction multiplied by 3:0 ± 0:5, an upper-limit proÿle calcu- of the linear n-C6H5 with C2H2. The reaction kinetics lated assuming reaction rates for aromatic compounds involving the n-C6H5 molecule, however, have not been that correspond to a temperature of 300 K. However, adequately studied in the laboratory to provide much aerosol opacity was not considered in that calculation. guidance as to its kinetic behavior. Wang and Frenklach Haze particles act to shield benzene from photodisso- (1997), examining n-C6H5 + H, assume a rate coePcient ciation, which extends as far as 2700 A(R Pantos et al., for the addition mechanism to be equal to C4H3 +H 1978). Fig. 9a shows the benzene photodissociation co- while the hydrogen abstraction pathway is assumed at ePcient calculated for the Wilson et al. (2003) proÿle 0:5 × k(C2H3 + H), which in the case of C2H2 would compared with that under fractal and Mie opacities, and be an order of magnitude smaller than reaction with the corresponding benzene proÿles are shown in Fig. 9b. C4H3, assuming the Wang and Frenklach rates. In ad- The updated benzene proÿles, including NUV shield- dition to the ambiguity of what is the proper proxy for ing by haze particles and calculated with the nominal n-C6H5 +C2H2, there is large disagreement regarding the Titan temperature proÿle (Yelle et al., 1997), are found reaction rate of C4H3 +C2H2 at low temperatures. Ex- to be consistent with the ISO tentative detection in trapolation of the highly pressure-dependent C4H3 +C2H2 Titan’s stratosphere, with the fractal proÿle at the upper rate coePcient using the Wang and Frenklach (1994) ex- end of the constraint and the Mie proÿle tapering o@ be- pressions yields a rate coePcient of 4:8 × 10−18 cm3 s−1 low 200 km. The larger abundance of C6H6 in the upper for T = 170 K, p = 1 mbar, for the n-C6H5 pathway. atmosphere for the Wilson et al. proÿle is due to the larger However, an extrapolation of the Westmoreland et al. rates corresponding with the 300 K assumption described rate (1989) yields a C4H3 +C2H2 → n-C6H5 rate coeP- above. cient of 1:7 × 10−20 cm3 s−1 for T = 170 K, p = 1 mbar. 1026 E.H. Wilson, S.K. Atreya / Planetary and Space Science 51 (2003) 1017–1033

+ H + H + H2 + H2

+ HCCH

+ H2 + H

+ H H

+ H (b)

+ H + H 2

+ HCCH

(a)

Fig. 8. Mechanisms of PAH formation proposed by (a) Frenklach et al. and (b) Bittner and Howard. (after Bauschlicher and Ricca, 2000).

Such a reduction in rate coePcient translates to a haze Wang and Frenklach rates. The linear version of this precursor rate of 1:4 × 105 cm−2 s−1 from this path- molecule may also collisionally isomerize to phenyl, which way as opposed to 7:0 × 106 cm−2 s−1 assuming the has a much lower ground-state energy (Westmoreland E.H. Wilson, S.K. Atreya / Planetary and Space Science 51 (2003) 1017–1033 1027

Fig. 9. (a) Benzene photodissociation coePcient assuming fractal haze particles (solid line), Mie particles (dashed line), and no haze (dotted line) used in Wilson et al. (2003). (b) Benzene proÿles assuming fractal haze particles (solid line) and Mie particles (dashed line). The benzene proÿle from Wilson et al. (2003) (dotted line) is included. The rectangle represents the ISO detection of benzene (Coustenis et al., 2003). et al., 1989), enhancing the aromatic contribution to haze.

Fig. 10. (a) Rate coePcient for C6H5 +C2H from Yuet al. (1994) (solid 5. Copolymers line) and Wang and Frenklach (1994) (long-dashed line). Short-dashed line represents the boundaries of the temperature region in which As hydrocarbons and nitriles react with each other in Ti- signiÿcant Titan chemistry takes place. (b) Benzene abundance pro- ÿle, assuming for the C6H5 +C2H rate Yuet al. (1994) (solid tan’s atmosphere, there is a likelihood that Titan’s haze par- line) and Wang and Frenklach (1994) (long-dashed line). (c) Produc- ticles are, at least in part, the products of such chemical tion rate proÿles for aromatic-pathway haze production mechanisms, mixing. Copolymerization of hydrocarbon and nitrile mix- assuming k(C6H5 +C2H) = k (Yuet al., 1994 ) (solid line) and tures has been identiÿed in laboratory simulations as ex- k(C6H5 +C2H) = k(Wang and Frenklach, 1994) (long-dashed line). hibiting optical properties that are comparable to Titan haze (Bar-Nun et al., 1988; Clarke and Ferris, 1996).

formation than C2H2 (Clarke and Ferris, 1996). Laboratory 5.1. Aliphatic copolymers studies indicate that gas-phase aromatic compounds 1,2,4- and 1.3.5-tricyanobenzene result from photolysis of HC3N With HC3N much more reactive in the gas phase than at low temperatures, with polymer as the major photoprod- HCN, cyanoacetylene presents a good case as a candidate uct (Ferris and Guillemin, 1990). for polymerization. Laboratory studies have shown that Clarke and Ferris (1997) have studied polymers aris- HC3N polymerizes much faster than HCN (Scattergood ing from the irradiation of mixtures of HC3N and vari- et al., 1992) and is 2–5 times more reactive in polymer ous hydrocarbons (C2H2,C2H6, n-C4H10). They suggest 1028 E.H. Wilson, S.K. Atreya / Planetary and Space Science 51 (2003) 1017–1033

in an attempt to derive an upper limit for possible mechanisms responsible for copolymerization, rate coePcients for (R23)–(R25) are taken from C2H2 +C2H(Chastaing et al., 1998), C2H2+C2H3 (Weissmann and Benson, 1988), and C2H2+C2H5 (Kerr and Parsonage, 1972), respectively. Assuming these rates, (R23) is the dominant producer at all altitudes among these reactions, with (R24) an order of magnitude smaller in the lower stratosphere. However, the question remains whether (R23) represents the initiation of a process that will result in suP- cient polymerization. Lebonnois et al. (2002) considered this reaction in a polymer-producing scheme, assuming cyanodiacetylene (HC5N) to be the product of a C2H-insertion-H-abstraction mechanism. Such a mecha- Fig. 11. Possible structures for HC3N polymers incorporating (a) C2H2 nism, along with CN-insertion-H-abstraction (Seki et al., and (b) C2H6. (After Clarke and Ferris, 1997, Icarus, 127, 158–172). 1996), may be responsible for the formation of such unsatu- rated cyanopolyynes as HC5N and HC7N, which have been identiÿed in interstellar molecular clouds (Winnewisser and incorporation of C2H5 radicals in the HC3N=C2H6 polymer Walmsley, 1978). Lebonnois et al. (2002) also considered and the formation of a copolymer resulting from the irradia- the involvement of the cyanoethynyl radical (C3N) in the tion of the HC3N=C2H2 mixture, with the possible structures formation of HC5N. Assuming C3N to proceed kinetically shown in Fig. 11. at a rate similar to CN and HC N to photodissociate in a In the lower atmosphere, acetylene and ethane undergo 5 similar manner as HC3N yields a HC5N mole fraction of photolysis and pressure-induced hydrogen addition to form −10 8:5 × 10 at 105 km, with C3N+ C2H2 acting as the the more highly reactive C2H, C2H3, and C2H5. To examine primary source of cyanodiacetylene. Little attention has the possibilities of haze formation from copolymerization been given to kinetic processes involving the cyanoethynyl of HC3N with acetylene and ethane, the following reactions radical, other than estimates from Yung (1987) regarding are considered for the initiation of copolymerization: recycling processes back to HC3N. Furthermore, the photodissociation dynamics of HC N has not been studied. HC3N+C2H → products; (R23) 5 Nevertheless, laboratory detection of HC5N suggest much smaller abundances, with de Vanssay et al. (1995) estimat- HC3N+C2H3 → products; (R24) −2 ing a value for HC5Nof10 times that for HC3N, while Coll et al. (1999) ÿnd the HC5N mole fraction within a HC3N+C2H5 → products: (R25) range of 1.3–6:4 × 10−13. Moreover, Marten et al. (2002) place an observational upper limit of 4×10−10 above 80 km. HC3N has many similarities with C2H2, structurally and Considering the unlikelihood of major di@erences between photolytically (Clarke and Ferris, 1996). HC3N absorbs UV light in the wavelength region of 1060–2600 AR and yields HC3N and HC5N photolysis, these results suggest that the photoproducts through three pathways: larger C3N radical might react with hydrocarbons at a sig- niÿcantly lower rate than CN. The omission of C3N+C2H2 −12 HC3N+hv → CN + C2H; (R26) yields a HC5N mole fraction of 9:6 × 10 at 105 km. The laboratory and observational non-detection of HC5N HC3N+hv → C3N+H; (R27) also suggest a pattern that is illustrated in the analysis of polyacetylenes, shown in Section 2. Largely due to the ∗ small yields for radicals responsible for the propagation HC3N+hv → HC3N : (R28) of chemistry (e.g. CN, C2H), the abundances of HC2n+1N The quantum yields for these three pathways are 0.05, cyanopolyynes appear to drop o@ on a similar scale as C2nH2 0.09, and 0.86, respectively (Clarke and Ferris, 1995; polyynes as n increases, a drop-o@ on the order of 10−2– −3 Halpern et al., 1988). The evidence that HC3N kinetically 10 at each level. As demonstrated for polyacetylenes in behaves in a similar fashion is, admittedly, rather sparse, Section 2, such a drop o@ does not bode well for the produc- largely due to the few studies exploring the kinetics of tion of haze from such a mechanism. Any other chemical reactions involving HC3N. Yung (1987) estimated the hy- mechanisms involving HC5N loss, such as hydrogen addi- drogen addition to HC3N to proceed with a rate coePcient tion considered by Lebonnois et al. (2002), will be minor equal to C2H2 + H. In CN-insertion reactions, Halpern compared to photolysis and will produce a compound with et al. (1989) measured a rate coePcient for CN + HC3N an abundance too small to signiÿcantly contribute to haze that is an order of magnitude lower than that for CN +C2H2 formation. On the other hand, the diminution of the C3N ∗ (Sims et al., 1993) at the same temperature. Nevertheless, radical as a consideration and the large yield for HC3N E.H. Wilson, S.K. Atreya / Planetary and Space Science 51 (2003) 1017–1033 1029 from cyanoacetylene photolysis raise the possibility of the in a similar manner as (R21). Assuming the rate coePcient metastable state as a viable pathway towards substantial to be equal to k(R21), this mechanism produces haze pre- polymerization. cursors at a similar rate as the aliphatic copolymer mecha- Both acetylene and diacetylene have dissociation path- nisms. ways that primarily yield the metastable state (Seki and Okabe, 1993; Glicker and Okabe, 1987). At 193 nm, ∗ metastable acetylene (C2H2 ) reacts with the ground state 6. Discussion to produce diacetylene at a faster rate than quenching (Seki et al., 1986). Furthermore, as stated earlier, the reaction of Haze particles are the product of the growth of gas-phase ∗ C4H2 with C4H2 (Bandy et al., 1993) is responsible for molecules which reach a stage of particle inception. At much of the C8H2 formed in Titan’s atmosphere, as well this stage, these molecular precursors of the haze begin to as reaction with acetylene (Frost et al., 1996) acting as a nucleate, which facilitates the conversion from gas-phase primary contributor to the formation of C6H2. to particulate form. In the case of PAHs, this nucleation Clarke and Ferris (1996) measured signiÿcant con- begins around 2000 amu (Richter and Howard, 2000), sumption of HC3N compared to C2H2 in the irradiation which would require an aggregation on the order of about of C2H2=HC3N mixture at 185 nm in the presence of N2. 20 molecules in the haze precursor, assuming each of the Considering the signiÿcant loss of HC3N of polymerization molecules to have the total mass of the reactants in the ∗ and the large photolytic yield of HC3N , underlining the primary reaction (Table 2). As this process is still poorly role the metastable state might play in polymer formation, understood, regardless of temperature or chemical compo- ∗ a reaction involving HC3N and C2H2 is considered as a sition, it is assumed that all precursor pathways nucleate pathway to polymerization at the 20 molecule level. Assuming that as the precursor grows, any loss due to chemistry is negligible, Wilson and ∗ HC3N +C2H2 → Co-Polymer; (R29) Atreya (2001) and Wilson (2002) obtain a haze column production rate by multiplying the precursor production resulting in a copolymer similar to the structure shown in rate by the nucleation mass. This methodology di@ers Fig. 11a. The rate coePcient for this reaction is taken from from that of Lebonnois et al. (2002), which propose that ∗ ∗ the aggregation of 10–100(∼ 20) parent molecules pre- C4H2 +C2H2 (Frost et al., 1996). The deexcitation of HC3N ∗ ceed the nucleation of the macromolecule, wich is on is taken to be equal to the C4H2 deexcitation rate (Zwier and Allen, 1996). the order of 600 amu. Using our methodology, the model calculates haze column production rates for the various mechanisms shown in Table 2. The nominal case yields 5.2. Aromatic copolymers a haze production rate of 3:2 × 10−14 gcm−2 s−1,a little larger than what is suggested by the microphysi- Ricca et al. (2001) analyzed mechanisms involving cal models. This result is considered an upper limit for the incorporation of nitrogen into an aromatic molecule the mechanisms considered, as loss of precursors due to as a source of Titan haze. A possible mechanism in- chemistry, dynamics, and condensation will most cer- cluded a Bittner-Howard-like scheme, commencing with tainly reduce the haze production rate (Wilson, 2002). H-abstraction from pyridine (C5H5N). However, the mech- This haze production rate derives primarily from the anism by which pyridine is formed on Titan is unclear. PAH pathway. There is a signiÿcant nitrile compo- The reaction C5H6 +N → C5H5N + NH is exothermic nent in the lower stratosphere (Fig. 12), which makes by 95 kJ=mol, but C5H6 formation is largely inhibited up about 40% of the haze at 1 mbar and approxi- on Titan, due to the large activation energy of C3H5 + mates the aromatic contribution below 180 km. However, C2H2 → C5H6 +H (Burcat and Dvinyaninov, 1997). the PAH pathway dominates haze production through- C4H5-insertion into HCN is thermodynamically feasible, out much of the lower atmosphere. Above 800 km, nitriles take over haze formation, peaking at 850 km, near C4H5 + HCN → C5H5N+H; the source region suggested by ChasseÿÂere and Cabane (1995). This upper atmosphere source may contribute to XH0 = −122 kJ=mol: (R30) f the detached haze layer that presides at higher altitudes, although there may be an additional source from below, However, assuming (R30) to proceed in a similar fashion which becomes detached as a result of dynamical pro- as HCN + C2H3 renders (R30) a relatively minor pathway cesses (Rannouet al., 2002 ). Copolymers, for the most to polymer formation. part, provide a minor source for Titan haze, according to Another mechanism suggested by Ricca et al. (2001) in- this calculation, while polyynes demonstrate a negligible volves the adding of HC3N to the phenyl radical, e@ect. This result for the total haze production rate agrees with C6H5 +HC3N → Co-polymer; (R31) the value obtained by Lebonnois et al. (2002) of 4:1 × 1030 E.H. Wilson, S.K. Atreya / Planetary and Space Science 51 (2003) 1017–1033

Table 2 Column production rates for the various haze sources

Primary reaction Precursor production Haze production rate (cm−2 s−1) rate (gcm−2 s−1)

2 −18 Pure polyyne PA-Polymer C6H2 + C4H 8:7 × 10 3.6×10 6 −15 Pure nitrile NT-Polymer H2CN + HCN 2.4×10 4:4 × 10 6 −14 PAH PAH C6H5 + C2H2 9:5 × 10 3:2 × 10 5 −15 Aromatic copolymer Co-Polymer C6H5 + HC3N 2:4 × 10 1.0×10 ∗ 5 −16 Aliphatic copolymer HC3N + C2H2 1:2 × 10 3.0×10

et al. (2002) is also enhanced by their HC3N proÿle, which is at least an order of magnitude too high in the lower stratosphere, based on observations (Coustenis et al., 1989; Marten et al., 2002) and laboratory simulations (Coll et al., 1999), although the low C4H2CN density that is hinted at in Lebonnois et al. suggests that HC3N does not play a sig- niÿcant role in copolymerization with acetylene. The nitrile component to the haze in the nominal model agrees with Lebonnois et al. in the lower stratosphere, with the di@erence in the haze production rate for this pathway due more to the way the haze production rate is calculated, rather the chemical output. Of course, there are a number of uncertainties inher- ent in this study. For instance, assuming the Monks et al. Fig. 12. Total haze production rate proÿles for the pure polyyne, pure (1993) rate is appropriate for both HCN+C2H3 and HCN+ nitrile, pure aromatic, aromatic copolymer, and aliphatic copolymer path- H2CN, providing large stratospheric densities for HC3N and ways. C2H3CN, nitriles can be responsible for as much as 9:2 × 10−14 gcm−2 s−1 of haze production, due to the larger rate coePcient, an aspect illustrated in the comparison of Figs. 10−14 gcm−2 s−1, which peaks at a slightly lower level 7b and c. The aliphatic copolymer mechanism provides a ∗ around 170 km. However, the sources responsible for the minor source for the haze from the HC3N +C2H2 reaction, ∗ ∗ haze di@er between the two models. While the nominal assuming HC3N quenches at a similar rate as C4H2 . How- model attributes haze production largely a result of PAH ever, the high rate of HC3N polymerization (Scattergood growth, Lebonnois et al. derive a PAH source approxi- et al., 1992; Clarke and Ferris, 1996) as compared to acety- mately four orders of magnitude smaller. The main cause lene may signal that the metastable variety does not quench ∗ of this di@erence is the small abundance of benzene in their as quickly as acetylene or diacetylene. 10% of the HC3N study (Lebonnois et al., 2002, Fig. 3), 3–4 orders of magni- quenching rate going toward polymerization would result tude smaller than the nominal stratospheric abundance and in a precursor production rate of 1:0 × 107 cm−2 s−1. Fur- ISO observation (Coustenis et al., 2003). Lebonnois et al. thermore, more study into the kinetic behavior of C4H3 and ÿnd acetylene polymers and HC3N=C2H2 copolymers to be n-C6H5 molecules would provide better insight into the con- mainly responsible for haze production, based largely on a tribution of copolymers and aliphatic hydrocarbons to Titan value for the rate coePcient of C4H3 +C2H2 → n-C6H5 haze. This highlights the necessity for more kinetic study of 2 × 10−16 cm3 s−1, attributed to Wang and Frenklach on these reactions at low temperature, as well as the pho- ∗ (1994). However, considering the temperature range in the tochemical trail of molecules like HC3N , which may play region of their maximum haze formation and the Wang and a signiÿcant role in haze formation. In addition, this study Frenklach high-pressure temperature-dependent expression, underlines the potential importance of aromatic molecules, this rate coePcient can be reduced by as much as a factor of whose mechanisms, in particular C6H5 +C2H2, need to be 3.5. Furthermore, it is clear by Wang and Frenklach, 1994, analyzed at temperatures applicable to outer planetary at- 1998 and Westmoreland et al. (1989) studies that this re- mospheres. action has a signiÿcant pressure-dependence, which is not Laboratory simulations of Titan haze production have at- considered in the Lebonnois et al. study. There are also dif- tempted to identify the composition of Titan haze through ferences between the Wang and Frenklach and Westmore- tholin analysis. Electrical discharge experiments have mea- land et al. rate coePcients, which are discussed in Section 2. sured simulated tholin to have a C/N ratio of 1.9–11 and The larger rate from HC3N=C2H2 copolymers in Lebonnois a H/C ratio near unity (McKay et al. 2001, and all refer- E.H. Wilson, S.K. Atreya / Planetary and Space Science 51 (2003) 1017–1033 1031 ences within), suggesting a larger contribution from nitrogen for the detached haze layer, nitrile polymerization in the compounds than what is found in this study. These experi- upper atmosphere may also contribute to the detached layer, ments simulate the inVuence that precipitating electrons and although it is unclear whether this contribution would be UV radiation below 1000 AR have on Titan’s atmospheric large enough to account solely for the detached layer. chemistry, but do not well describe the impact of UV radiation longer than 1000 A(R Tran et al., 2003). The former is responsible for dissociation of nitrogen molecules, which are 7. Conclusions essential in the formation of nitriles, while radiation longer than 1000 AR help propagate hydrocarbon chemistry. This The chemical pathways to haze formation in Titan’s atmo- characteristic of discharge experiments is illustrated in the sphere have been analyzed. These pathways include mech- plasma discharge experiments of Coll et al. (1999). In these anisms involving polyyne, aromatic, and nitrile chemistry, experiments, a C/N ratio of 2.82 was found for the simu- as well as possible mechanisms involving copolymeriza- lated tholin, while 70 organic compounds were detected. In tion, which have been supported by theoretical and labora- comparison with observations, the HCN and HC N output 3 tory studies. Analysis has involved the inclusion of possible found good agreement. However, the hydrocarbons, with the haze pathways in a photochemical model, yielding proÿles exception of methylacetylene, were consistently underrep- of haze production sensitive to Titan’s atmospheric temper- resented. Tran et al. (2003) conducted laboratory simula- ature and pressure. tions using a photochemical Vow reactor and a gas mixture This analysis has found that the production of haze in containing Titan atmospheric constituents that may be im- Titan’s atmosphere is focused in a region around 140– portant in polymerization. In these experiments, they found 300 km, peaking at an altitude of 220 km. The growth of a polymeric product with a C/N ratio of 17.6, much higher aromatic molecules is expected to play an important role than the discharge experiments. It should be noted that these in the polymerization process, although the nitrile pathway experiments were conducted assuming a HC N=C H ratio 3 2 2 in the form of HCN polymerization, possibly responsible of 5:0 × 10−2, consistent with the IRIS north pole observa- for the orange-brown color of the hazes (Matthews, 1995), tions (Coustenis et al., 1991) while much larger than the ISO may also be signiÿcant. It is evident that the spectra of pure disk-averaged observations that derive a HC N=C H ratio 3 2 2 poly-HCN do not characterize Titan spectra (Khare et al., of 9:0 × 10−5 (Coustenis et al., 2003). The experimental 1994), but the combination of PAH and nitrile polymers, as HC N=C H ratio is consistent with ISO disk-averaged ratio 3 2 2 well as possible copolymerization between these two types, of HCN=C H (5:4 × 10−2), and HC N polymerizes much 2 2 3 may yield di@erent results. faster than HCN (Scattergood et al., 1992), which suggests Copolymerization involving HC N through aliphatic that the experimental C/N ratio found by Tran et al. may be 3 and aromatic channels are not shown to be signiÿcant in a lower limit. haze formation, although the potential provided through Microphysical models, assuming a more simplistic haze the metastable state argues for more kinetic study of this production rate proÿle, have used laboratory-generated molecule. Furthermore, more attention on the large hydro- tholin to ÿt Titan’s geometric albedo, obtaining produc- carbon radicals like C H and n-C H and their reactions tion zones from 250 to 600 km, although the bulk of 4 3 6 5 with stable chemical species would elucidate their role in the studies have found zones above 350 km. Rannou the polymerization process. et al. (2002) argue for a dynamical origin to the detached This study highlights the need for more study of reaction haze layer, with a production zone at the level of the de- kinetics among the PAH, nitrile, and copolymer pathways, tached haze layer around 400 km. Although the dynamical in order to identify both rates of reactions and products mechanism postulated for the existence of the detached that might be found by the Cassini-Huygens Gas Chromato- haze layer and polar hood seems plausible, the production graph Mass Spectrometer (GCMS) and Aerosol Collector zone is placed much higher than where chemistry takes Pyrolyzer (ACP). Identiÿcation of species like naphthalene place (Fig. 12). It is unlikely that vertical winds might and nitrile polymer precursors, like n-cyanomethanimine transport haze precursors from a production zone of 220– will yield insight into the processes responsible for poly- 400 km before conversion to haze particles, as the chemical merization. timescales are much smaller. The distribution of haze particles in the vertical plane is evidently controlled by eddy di@usion in regions where eddy di@usion is large (Rannou and McKay, 2003). In such a scenario, the high intensity Acknowledgements of atmospheric eddies, predicted by this model (Fig. 3a), would act to distribute haze particles both above and below EHW acknowledges support received from the National the production zone, which may explain the distribution Research Council Research Associateship Program. SKA of the main haze layer. Of course, the particle distribution acknowledges support received from NASA’s Planetary At- would also be inVuenced by coagulation, coalescence, and mospheres Program, and the Cassini-Huygens GCMS and settling processes. Along with the dynamical explanation ACP projects. 1032 E.H. Wilson, S.K. Atreya / Planetary and Space Science 51 (2003) 1017–1033

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