Organic chemistry on Titan

VOL. 17, NO. 8 REVIEWS OF GEOPHYSICS AND SPACE PHYSICS NOVEMBER 1979

Organic Chemistry on Titan

SHERWOOD CHANG, THOMAS SCATTERGOOD, SHELDON ARONOWITZ, AND JOSE FLORES

Extraterrestrial Research Division, NASA Ames Research Center, Moffett Field, California 94035

Observations of nonequilibrium phenomena on the Saturn satellite Titan indicate the occurrence of organic chemical evolution. Features taken from various models of Titan's atmosphere are combined in a working composite model that provides environmental constraints within which different pathways for organic chemical synthesis are assessed. Experimental results and theoretical modeling studies suggest that the organic chemistry of the satellite is dominated by two atmospheric processes: photochemistry and energetic particle bombardment. Photochemical reactions of CH 4 in the upper atmosphere can account for the presence of C2 hydrocarbons. Reactions initiated in various levels of the atmosphere by cosmic ray, Saturn 'wind,' and solar wind particle bombardment of a CHn-N2 atmospheric mixture can account for the ultraviolet-visible absorbing stratospheric haze, the reddish appearance of the satellite, and some of the C2 hydrocarbons. In the lower atmosphere, photochemical processes will be important if surface temperatures are sufficiently high for gaseous NH3 to exist. Hot H atom reactions initiated by photodissociation of NH3 can couple the chemical reactions of NH3 and CH4; if 0.1% of the incident ultraviolet light from 1600 to 2270/• reaches the lower atmosphere, these reactions will be capable of producing organic matter at a rate comparable to or higher than that resulting from particle-initiated reactions. Elec- tric discharges are highly improbable on Titan; if they occurred at' all, they would be restricted to the lower atmosphere and clouds. Their yield of organic matter might approach that of hot H atom reactions if the conversion of solar to electrical discharge energy on Titan was as efficient as that on earth. These assessments indicate that future missions to Titan should include organic chemical analyses of its atmosphere and surface among the prime science objectives.

CONTENTS

Introduction ........................................................................................ 1923

Working model of the atmosphere .................................................... 1923 Upper atmosphere .............................................................................. 1924

Production of dust and chromophores .......................................... 1924 Production of hydrocarbons .......................................................... 1927

Lower atmosphere .............................................................................. 1928 Ammonia photochemistry ............................................................. 1928 Hot hydrogen atom chemistry: model calculations ...................... 1929 Electrical discharges ....................................................................... 1931

Summary ............................................................................................. 1931

1. INTRODUCTION

Two fundamental observations make the satellite Titan an

object of great interest to exobiologists and organic cosmo- chemists: the unique occurrence of abundant methane (CH4) in the atmosphere [Trafion, 1972] unaccompanied by higher amounts of H2 [Miinch et al., 1977] and the presence of a reddish 'surface' [Morrison and Cruikshank, 1974]. Although there is some doubt in interpreting this surface as the top of a cloud layer or as the solid surface of the satellite, its color has significant implications for chemical evolution in the atmosphere. Interpretation of spectroscopic [Caldwell, 1974; Traf- ton, 1975] and polarimettic [Veverka, 1973; Zellner, 1973] observations of Titan indicates the existence of a thick particle layer in the atmosphere that absorbs ultraviolet and visible light, a layer characterized as a 'photochemical haze' [Dan- ielson et al., 1973; Podolak and Danielson, 1977]. Ethane (C2H6), ethylene (C2H4), and acetylene (C2H2) [Gillett, 1975] are attributable to photochemical reactions of CH4 in the stratosphere [Strobel, 1974]. In the context of Lewis' [1971] physical and chemical model of Titan the presence of C2H6, C2H2, and C2H4, the red surface, and a particle layer that ab-

This paper is not subject to U.S. copyright. Published in 1979 by the American Geophysical Union.

sorbs ultraviolet and visible light clearly indicate a chemical nonequilibrium in the atmosphere. To the organic cosmo- chemist this indicates reactions that yield organic molecules in a prebiotic environment perhaps related to those of primitive earth [Sagan, 1974] and the parent bodies of carbonaceous meteorites. In turn, this draws the interest of exobiologists to the possibility that life may have originated or that terrestrial organisms may survive on Titan.

Recently, Margulis et al. [1977] discussed the models that have been proposed for Titan's atmosphere and their implications for life. No basis now exists to alter their conclusion that 'the likelihood of growth of a terrestrial microbe is vanishingly small.' By implication, the probability of the origin of life forms resembling terrestrial organisms on Titan is also 'vanishingly small.' If organisms with biochemistties funda- mentally different from those of terrestrial organisms evolved and populate Titan today, there are no obvious methods for detecting and identifying them except, perhaps, by imaging techniques.

Rather than address a highly speculative issue such as Titan exobiology in this paper, we discuss several more fundamental issues concerning the satellite's organic chemistry. Current models of Titan's atmosphere described in section 2 will be used to help identify the nature and distribution of possible sources of organic matter. Data from the literature and this laboratory will be used to assess the relevance and productiv- ity of those sources as well as to characterize the nature of the products derived from them. The upper and lower atmospheres will be treated separately in sections 3 and 4, respectively. A summary of our conclusions will be presented in section 5.

2. WORKING MODEL OF THE ATMOSPHERE

Observations of Titan have provide•l support for two basic types of thermal atmosphere models. In the 'thin' atmosphere models, characterized by high-altitude temperature inversion [Danielson et al., 1973; Podolak and Danielson, 1977, Caldwell,

Paper number 9R1120. 1923

1924 CHANG ET AL..' ORGANIC CHEMISTRY ON TITAN

UV UV

o • SOLAR AND • • SATURN COSMIC A ¾ WIND RAYS

HAZE AND GAS

T -• 160øK (CH 4, N 2, H 2) .'

T• 115øK • •- --• CLOUD DECK

(HYDROCARBON //• • •-- • ICES AND DROPLETS;

COLORED MATERIAL)

T • 115øK I 'CLEAR GAS' I

(CH4, N 2, H2, NH 3 ß I SOME ORGANIC • PARTICLES SETTLING I FROM ABOVE)

T' • 135øK ••/ SURFACE (NH 3 AND H20 ICES, CH 4 CLATHRATES, ORGANIC MATTER)

Fig. 1. Composite working model for the atmosphere of Titan. The various features of the model are described in the text. The ar- rows indicate approximate levels of penetration of energy sources into Titan's atmosphere.

1977, 1978], the surface temperature (<100øK) and pressure (--<20 mbar) are low, but absorption of ultraviolet and visible sunlight by a high particle layer heats the upper atmosphere to as high as 160øK. The surface temperature in these models is close to the gray body equilibrium value of 86 øK. In the early 'thick' atmosphere models characterized by greenhouse effects [Sagan, 1973; Pollack, 1973; Cess and Owen, 1973] the stratospheric temperature decreases with depth to a ca4 cloud. Be- low this cloud the temperature increases to the surface where high temperatures (> 140øK) a.nd pressures (_>300 mbar) prevail. The more recent models have both temperature in- versions and either weak [Low and Rieke, 1974] or strong [Hunten, 1978] greenhouse effects. Podolak and Giver [1979] point out that with available data it is not possible to distin- guish the surface of the thin atmosphere model from the top of a thick dust-covered cloud layer below which a massive greenhouse atmosphere might prevail. When they are scaled to a base line surface radius of 2700 km [Caldwell, 1978], the measurements by Briggs [1974] at 37 mm yield a surface (radio brightness) temperature of 99 -4-_ 34OK, which with its un- certainty is consistent with the thin model as well as some thick greenhouse models. Similar scaling [Caldwell, 1978] of the data of Conklin et al. [1977] obtained at 3.3 mm results in a surface temperature of 213 ø + 38øK, which favors a massive atmosphere and a surface pressure of several bars. More recent data obtained at centimeter wavelengths (W. Jaffe et al., unpublished manuscript, 1978), cited by Caldwell [1978], indicate that the temperature is 90 ø + 30øK, consistent with the earlier measurements. These observations conflict in support- ing either a simple inversion or a greenhouse model. Nonethe- less, for any greenhouse model, the low H2 concentration observed by Miinch et al. [1977] indicates that a different gas must drive the greenhouse. Molecular nitrogen [Hunten, 1972, 1978; Owen and Cess,.1975], neon [Cess and Owen, 1973], or NH3 [Sagan, 1977] may fulfill this purpose.

To provide a basis for discussion of a variety of processes that may contribute to the organic chemistry of Titan, we

shaft adopt a composite working model of the atmosphere. The following features taken from various sources are illus- trated schematically in Figure 1. (1) A dust layer in the stratosphere absorbs ultraviolet and visible sunlight. (2) The temperature in the stratosphere is about 160øK and decreases to -<115øK at the top of a thick, dust-covered cloud layer consisting primarily of colorless hydrocarbon ices mixed with reddish material whose composition is as yet unknown. Solid NH3 may also occur in this cloud layer if the temperature at Titan's surface and the thermal structure of the lower atmo-

sphere permit the concentration of gaseous NH3 in the cloud region to exceed its saturation vapor pressure. (3) ca4 and N,• comprise the bulk of the atmospheric gases above and below the cloud layer. (4) A 'clear' atmosphere extends from the bottom of the clouds to the surface with temperature and pressure increasing to _> 135 øK and _> 1 bar, respectively. If particulate matter is formed in the upper atmosphere and/or in the vicinity of the cloud layer (see discussion below), then the lower atmosphere should contain a low concentration of material settling out from above and would not be truly clear. (5) NH3 abundances throughout the atmosphere are determined by its saturation vapor pressure. At the surface, NH3 occurs either in the form of NH3-H20 ice or NH3 (aqueous) solution. PH3 and H:S (in the form of NH4SH) may also be present in Titan's atmosphere, although consideration of relative cosmic abundances would suggest amounts lower than the amount of NH3. Conceivably, products resulting from photodecomposition of PH3 [Prinn and Lewis, 1975] or H,S and NH4SH [Lewis and Prinn, 1970; Sagan, 1974] may contribute to Titan's coloration; however, models for the occurrence and photochemistry of these species on Titan have not yet been described.

3. UPPER ATMOSPHERE

Several factors must be reconciled about the chemistry of the upper atmosphere: evidence for C2H6, C2H4, and C:H2 as minor constituents, with a c2a4/c•a6 ratio of about 4 x 10 -3 [Gillett, 1975]; absence of detectable NH3; evidence for dust particles that strongly absorb ultraviolet and visible light; and the red color of the observable surface (cloud top).

a. Production of Dust and Chromophores

Dust and acetylene polymerization. The high-altitude dust particles are thought to be 'photochemical smog' arising from CH4 photolysis [Strobel, 1974]. What is this photochemical smog, and can it be colored? Scattergood and Owen [1977] suggested that a component of the dust particles might be a yellowish acetylene polymer, similar to cuprene (C2H2),, formed by either particle or photoinduced polymerization of C:H:, or both. Although polymers are routinely formed dur-

TABLE 1. Ultraviolet Absorption Properties of Compounds Synthesized by 150-eV Electron Bombardment of C2H2

Films at 55øK

Compound 3, .... A log e

Diacetylene (HC4H) 2340 Benzene (C6H6) 2030 2.5

2540 3.9

Phenylacetylene (C6HsC2H) 2350 4.2 2720 2.5

Naphthalene (CioH8) 2200 5.0 2750 3.8

Indene (C9H8) 2490 4.1

The data are taken from Hirayama [1965].

CHANG ET AL.: ORGANIC CHEMISTRY ON TITAN 1925

[] C2H6 /X, C3H8

10 • C4H10 • C2H4

• 6--

o ..-..-[9""'"

• 4-- -.4

2 -.2

0 .5 1.5 2.5 3.5 4.5 5.5 TIME, hr

Fig. 2. Product distributions obsc•cd at various times by irradiation of 2.6 mbar of CH4 with 1236-• light. Hydrocarbon abundances wcrc determined by •as chromatography and arc •ivcn in nanomolcs per 100-•1 sample. Left scale, CH4; right scale, other •ascs.

ing the photolytic and thermal decomposition of C2H2, the conditions on Titan may not support such processes.

Plasma polymerization of acetylene [Kobayashi et al., 1974], alone and in the presence of saturated (C2H6) and unsaturated (C:•H4) hydrocarbons, readily occurs at pressures as low as 0.7 mbar to yield gaseous oligomers and solid polymers. The presence of H: in the gas mixture, however, partially inhibits polymerization. The solid polymers are insoluble and appear to con,sist of a highly cross-linked, three-dimensio•/al network containing carbon-carbon double bonds and aromatic struc- tures. It is not clear whether or not these polymers are closely related to the polymers identified as (C:H:), that are produced during radiation-induced gas phase polymerizations of C2H: [Jones, 1959], because the infrared spectra are dissimilar in some respects. Ultraviolet irradiation of pure C:H: at 1470/• [McNesby and Okabe, 1964] or 1849/• [Zelikoff and Aschen- brand, 1956] also yields an ill-characterized solid polymer with smaller amounts of diacetylene (HC-=C-C=CH), vinyl- acetylene (CH:--CH-C=CH), ethylene, and benzene. At 1849 /• the quantum yield for C2H: disappearance and the apparent chain length of the polymers decrease with decreasing pressure. While these polymers form readily under many con-

TABLE 2. Mole Fractions and Quantum Yields of Products Obtained by 1236-,• Irradiation of 2.6-mbar CH4 at 300øK

Product Mole Fraction Molecules per Photon

CH4 0.699 -0.56' C:H: 0.0016 0.0019$ C:zH4 0.0048 0.00675 C2H6 0.107 0.15•: C3H 6 0.0003 C3H8 0.0228 0.035 i-C4H!0 0.0019 0.001õ n-Chili0 0.0021 H: 0.162

The reactions were performed in a stirred 2196-cm 3 flask with an Opthos lamp emitting (6 + 2) x 1015 photons s -•. Mole fractions include H: abundance calculated by mass balance and apply to near steady state condition after 30% CH4 decomposition. Errors are estimated at +_30%.

*Molecules of CH4 destroyed per absorbed photon. $Calculated for 30% CH4 decomposition. $Calculated at 10% CH4 decomposition. õCalculated when first observed, after 18% CH4 decomposition.

2

1.8

1.6

1.4

1.2

1

•.8 .6

.4

.2

0 2000

.... CH4 (COLORLESS) • CH4/N 2 (RED-BROWN) ..... CH4/NH 3 (RED-BROWN)

ROWN)

3000 4000 5000 6000 7000

WAVELENGTH, A

Fig. 3. Ultraviolet-visible absorption spectra of liquid phase products synthesized by MeV proton irradiation of gas mixtures [Scattergood et al., 1975; Scattergood, 1975; Scattergood and Owen, 1977].

ditions through many different energy sources [Jones, 1959; Lind, 1961; Kobayashi et al., 1974], their chemical constitu- tions are not uniquely defined. They are described, however, as being near white to yellowish and as lacking color depth.

Recent laboratory studies provide the bases for assessing the possible existence of acetylene polymers on Titan. As a model for interstellar organic synthesis, Floyd et al. [1973] bombarded a solid film of C:H: at 55øK with 150-eV elec- trons. They reported evidence for a variety of unsaturated and aromatic compounds, all of which are ultraviolet absorbers from 2000 to 3000/• (Table 1). No mention was made of a solid cuprenelike product, although such a product would have been interesting in the context of interstellar organic chemistry. If polymers were absent, then the work of Floyd et al. [1973] reveals an important temperature dependence for polymer formation, because previous polymer syntheses were achieved at room temperature or higher. Polymer synthesis on Titan may be inhibited by the low temperature of the stratosphere.

Recent experiments by Scattergood [1975], Bar-Nun [1975, 1979], and Bar-Nun and Podolak [1979] provide stronger additional arguments against the formation of solid acetylene polymers on Titan. Bombardment of gaseous CH4, C2H:, and CH4-C:H: mixtures with MeV protons [Scattergood, 1975] produced a yellowish powdery polymer in pure C:H2 and in mixtures where the C:H:/CH4 ratio was >0.5. Clear, colorless

WAVELENGTH,# 2.5 3 4 5 6 7 8 9 10 12 15

lOO• i i i IllIll 80

60

40

20 --

0

4000 3500 3000 2500 2000 1800 1600 1400 1200 1000 800 600

FREQUENCY, cm -1

100

80

40

20

Fig. 4. Infrared transmission spectra of liquid phase products obtained from MeV proton irradiation of gas mixtures: curve A, CH4- H:; curve B, CHn-N:z and CHn-NH3; and curve C, CHn-NH3-H:zS [from Scattergood et al., 1975; Scattergood, 1975; Scattergood and Owen, 1977]. The plot for mixture C is offset for clarity.

1926 CHANG ET AL.: ORGANIC CHEMISTRY ON TITAN

TABLE 3. Products Resulting from MeV Proton Irradiation of Various Gas Mixtures

Gas Mixture Product

CH4; CH4-H2 CH4-N2; CH4-NH 3

aliphatic hydrocarbons aliphatic hydrocarbons, aliphatic and alicyclic amines, acetonitrile (CH3CN), and red-brown polymer

Data are from Scattergood [ 1975], Scattergood et al. [ 1975], and Scattergood and Owen [1977]. Aliphatic hydrocarbons include saturated and unsaturated compounds up to C22 in length and less than 10 ppm aromatics (e.g., benzene). Cyclic amines include hexa- methylenetetramine (C6H!2N4) and its methyl and dimethyl homo- logues.

liquids consisting primarily of simple saturated hydrocarbons resulted when the ratio was reduced to 0.1. Apparently, under highly favorable total pressure conditions a low mixing ratio of C2H2 in CH4 prohibits formation of solid colored polymer. We conclude that the C2H2/CH4 ratio on Titan, which is estimated at <10 -s [Gillett, 1975], also would be prohibitively low for C,•H,• polymer synthesis by proton bombardment.

When 2.6 mbar of pure CH4 was photolyzed with 1236-A light, we found that the major products were the saturated hydrocarbons C2H6 and C3H8 (Figure 2). After 30% CH4 decomposition, when the system approached a photochemical steady state, no hydrocarbons higher than Cs were observed by gas chromatography, even when the total product was cryogeni- cally concentrated into a small volume prior to sampling (Table 2). Not only did hydrocarbon production appear to terminate at the Cs stage, but synthesis of unsaturated compounds was inhibited. These results agree with experiments performed by Bar-Nun and Podolak [1979]. At the C2H2/H2 ratio of 0.01 produced by CH4 decomposition the secondary process of C,•H,• photopolymerization to a solid polymer is expected to be inhibited by the presence of H,• [Bar-Nun, 1975, 1979]. Note that the planetary data of Gillett [1975] and Miinch et al. [1977] indicate a C•H•/H2 ratio of •10 -4. Bar- Nun [1979] also found during irradiation of C,•H,• in large ex- cesses of H2 that production of photopolymers decreased with reductions of temperature and initial total pressure below 300øK and 260 mbar (•-1019 cm-3), respectively. The region of CH4 photolysis on Titan occurs high in the atmosphere where the number densities are less than 10 •3 cm -3 [Strobel, 1974]. Thus if the experimental data were extrapolated to Titan and if CH4 were the only hydrocarbon in the initial atmosphere, then the combined effects of a low C2H2/CH4 (•C2H2/H2) ratio, substantially lower temperatures, and pressures orders of magnitude lower would make photochemical formation of an acetylene polymer like (C:H:)n highly unlikely in the Titan stratosphere.

Although synthesis of acetylene polymers in Titan's atmosphere is improbable, their presence in the stratospheric dust cannot be ruled out. However, they certainly cannot account for Titan's reddish surface; the combination of their low abundance with their rather weak coloration would make

them relatively ineffective sources of color. On the other hand, ultraviolet absorbance by these polymers is assured by their double bond and aromatic ring content. Interestingly, electron micrographs of polymers produced by alpha particle irradiation of C2H: show them to consist of 0.1- to 0.13-/•m-diameter spheres [Lind, 1961]. As candidates for the photochemical smog, if they form in a low-pressure medium, these particles conveniently remain solid at the relatively high stratospheric

temperatures (160øK) postulated for all Titan models and are of the size to satisfy some but not all recent albedo (haze) models [Podolak and Giver, 1979; Rages and Pollack, 1978].

High-energy particle chemistry. If cuprenelike polymers comprise at most a minor component of the dust, what com- poses the bulk of it? Apparently, the photochemistry of CH4 alone cannot produce colored material. The products found experimentally (Table 2) cannot account for even the observed absorption of ultraviolet light in Titan's stratosphere. Scattergood and Owen [1977] pointed out that production of colored material would require the presence of a nitrogen- containing gas such as N 2 along with the CH 4. Gases containing phosphorus or sulfur would also satisfy this requirement. According to the working models, however, gases like NH3, PH3, and H2S are not likely to be involved in the stratosphere because of their low abundances, although they may contribute to production of colored material lower in the atmosphere.

For purposes of comparison the ultraviolet-visible and infrared spectra of condensable materials produced during MeV proton irradiation of gas mixtures are shown in Figures 3 and 4. Table 3 contains a list of the major types of products. For N-containing mixtures the spectra are similar and resemble those of the colored product obtained by 2537-A irradiation of NH3-H2S-CH4-C2H6-H20 mixtures [Khare and Sagan, 1973]. Because not one of these materials has spectral properties identical to those on Titan, Scattergood and Owen [1977] proposed that mixtures of materials similar to the CH4-H 2 and CH4-N2 products were present. This is an attractive alterna- tive because the components in appropriate proportions, more readily than the Khare-Sagan polymer alone, would permit the rapid increase in absorption toward the ultraviolet that is exhibited by dust in Titan's stratosphere [see Danielson et al., 1973].

Three types of energetic particles could initiate chemical reactions in the upper atmosphere: galactic cosmic ray, solar wind, and Saturn 'wind.' The occurrence of the latter two types of particles depends on the presence or absence of planetary magnetic fields. If Titan has no magnetic field and if the solar wind particle flux and energy are not significantly af- fected by Saturn's proximity, then solar wind bombardment could contribute to production of high-altitude dust and chromophores. However, should Saturn have a magnetosphere that extends as far as the orbit of Titan, a possibility pointed out by Siscoe [1978], then solar wind particles may make no contribution; but the atmosphere would then be sub-

TABLE 4. Estimates of Energetic Particle Fluxes and Associated Production Rates and Column Abundances for Organic Matter on

Titan

Production Column

Flux, Rate, Abundance, Source eV cm -2 s-• st-• gcm -2 s -• gcm -2

Cosmic rays 2.3 X 109* 1.3 x 10 -15 185 Saturn wind 5.1 X 109• ' 2.9 x 10 -•s 412 Solar wind 2.0 X 1095 1.1 x 10 -•s 156

The production rate is equivalent to the mass of CH 4 converted to higher molecular weight carbonaceous matter. The column abundance is the amount produced after 4.5 x 109 years, assuming constant production rate.

*From L. A. Capone (private communication, 1978). •-Calculated from a 3.4 x 107 cm -2 s -• flux of 150-eV protons in a

model Saturn magnetosphere (0.5-G magnetic field) that extends to 20 Saturn radii and includes Titan within its boundaries 90% of the

time [Siscoe, 1978]. $Calculated from a 2 x 106 cm -2 s -• flux of 1-keV protons.

CHANG ET AL..' ORGANIC CHEMISTRY ON TITAN 1927

ject to bombardment by particles trapped by the magnetosphere, that is, a Saturn wind [Siscoe, 1978]. In either case it is likely that energetic particles will reach the upper atmosphere and initiate reactions. The relatively low energies of solar wind (--. 1 keV) and Saturn wind (• 150 eV [Siscoe, 1978]) ions will not permit their penetration deep into the upper atmosphere. Therefore their maximum energy deposition will

mbar produce many different saturated and unsaturated hydrocarbons and nitriles [Toupance et al., 1974] that could sub- sequently undergo photochemistry and condensation-polymerization reactions to readily form reddish substances if they are formed in Titan's atmosphere. To account for the occurrence of stratospheric dust in the absence of strong atmospheric mixing, however, electrical discharges must occur

occur above the stratosphere in approximately the region of above the dust layer at pressures of <1 mbar in a dry atmo- methane photochemistry. The region of maximum energy sphere containing no clouds. Although they could conceivably deposition for the high-energy (>MeV) cosmic ray particles occur, discharges under such conditions are sufficiently ira- will occur lower in the atmosphere at around the 1018-cm -3 plausible as to permit discounting them. level (L. A. Capone, private communication, 1978), as is Photochemistry. In the only report describing the coupling shown in Figure 1. of C and N in the photochemistry of a CHn-N2 mixture,

In order to evaluate the potential contribution from each of Dodonova [1966] found that irradiations at 7-10 mbar with a the particle sources to Titan's atmospheric chemistry an estimate of the efficiency of conversion of CH4 and N•_ to products by energetic particles is required. We assume that the conversion of kinetic energy into chemical energy in Titan's upper atmosphere is similar to that in the laboratory experiments of Scattergood [1975], despite orders of magnitude dif- ferences in flux and total pressure. We also assume that the efficiency of decomposition of CH4 in the laboratory experiments can serve as an approximate measure of the efficiency of product synthesis. From the incident proton energies

hydrogen lamp (10IS--1016 photons s -!) produced HCN after 8-10 hours; hydrocarbons comprised the other products. HCN was detected with a picrate color test sensitive to 10 -6 g. The explanation given for this result was 'the photoactivation of nitrogen and its interaction with CH radicals resulting from the decomposition of me•thane.' The author described the photoactivation of nitrogen as occurring at 1273 •. A crude estimate based on the reported data places the quantum yield for HCN at about 10 -s. Although several attempts were made at NASA Ames Research Center (J. Bragin and G. Nico!l, un-

of 2 MeV (0.75 MeV absorbed by the gas), the particle flux of published report, 1976) to duplicate the Russian work with a 1.7 x 1013 cm -2 s -! used in his experiments, and estimates of 1236-/1, lamp, they failed to produce any HCN detectable by the total amount of CH4 decomposed (based on decreases in high-sensitivity gas chromatographic analyses. If it were infrared absorption), Scattergood [1975] calculated that 1.7 x formed on Titan by Lyman a photons at the estimated yield, 104 CH4 molecules per 'proton were converted to product however, HCN would be produced at the insignificant rate of (equivalent to 44 eV per CH4). This figure, coupled with the about l0 s cm -•- s -!, or 4.5 x 10 -18 g cm -•- s -!. Further attempts estimated energy fluxes associated with the various particle to study the photochemistry of CH4-N•_ at Lyman a and other sources, gives the production rates'for organic matter listed in wavelengths would be deskable because gaseous HCN is Table 4. Using 2 x 10 -is g cm -•- s -! as the nominal production photochemically converted to more complex organic corn- rate from all particle sources and assuming a density of 1 g pounds and reddish-brown solids [Mizutani et al., 1975]. cm -3 for the products, we calculate that a l-/•m-thick layer of Conclusions. The most attractive mechanism for produc- organic matter would form in 1.3 x 103 years. The 'colunto tion of stratospheric dust on Titan that we have discussed abundances obtainable from the individual sources over 4.5 x involves high-energy proton irradiation, which implies a CH4- 109 years suggest that amounts in excess of 300 g cm -•- will ac- N•_ atmosphere. This source could provide the colored con- cumulate if production by cosmic rays is augmented by either densates that are responsible for the reddish surface of the one of the other particle types. For purposes of comparison we satellite and its low ultraviolet visible albedo. Unless vacuum point out that over the age of the solar system the C•_ hydro- ultraviolet photolysis of CH4-H2 yields HCN more efficiently carbon column abundances resulting from CI-!n photolysis than 10 -s molecules per photon, it can be discounted, along alone are predicted in computational models to b e 2 orders of with electric discharges, as a tenable mechanism. magnitude higher [Strobel, 1978].

Clearly, the assumptions used to obtain these estimates require that they be used with caution. Nonetheless, the role of particle radiation in producing condensable and gaseous trace constituents in Titan's atmosphere cannot be dismissed. Mod- els of CH4 rich atmospheres, which included cosmic ray ioni- zation processes [Capone et al., 1976], predicted CH• radical concentrations that were 3 orders of magnitude higher than those predicted by strictly photochemical models [Strobel, 1974]. Because attainment of higher CH• concentrations with- out significantly lowering the CH•/H radical ratios is expected to favor higher hydrocarbon production, the predictions support the view that particle irradiation would enhance organic synthesis. :'

b. Production of Hydrocarbons

If we now return to the data for 1236-/1, photolysis of CH4 (Figure 2 and Table 2), we can make additional observations pertinent to Titan's atmospheric photochemistry. Although a photochemical steady state had not been reached at the time of measurement of the mole fractions listed in Table 2, it was sufficiently close (Figure 2) that the assumption of steady state is justified. The relative abundances of C•_ and C• hydrocarbons produced in experimental and computer simulations of methane photochemistry on Titan are listed for comparison in Table 5. Also included is the ratio of C•_Hn/C•_H6 obtained by spectroscopic observations of Titan. The experimental results predict that the order of C2 and C3 hydrocarbon abun-

Electrical discharges. In addition to synthesis of C•_H•_ dances in Titan's atmosphere should be C•_H6 > C3H8 > C:•H4 polymers by way of CH4 photolysis and synthesis of con- > C:H•_. Moreover, the relative abundance of C3H8 obtained densable organic matter by particle irradiation of CH4-N•_ in the experiments suggests that it may be detectable in the at- mixtures, vacuum ultraviolet photolysis of CH4-N•_ or electric mosphere, an idea proposed earlier by Bar-Nun and Podolak discharges may generate Titan's stratospheric dust and color- [1979]. The model calculations, however, do not explicitly ation. Electric discharges through CH4-N•_ mixtures at 30 take into account the formation of C3H8; but they do predict a


TABLE 5. Ratios of C 2 and C3 Hydrocarbons for a Methane Atmosphere on Titan

C2H2/C2 Ha C2H4/C2H6 C3Hs/C2H6

0.015 45 x 10 -3 0.21 Experimental (this work)

Theoretical

[Strobel, 1974] Observational

[Gi!!ett, 1975]

1.69-1.73' (5.63-1.02) X 10 -3*

4x 10 -3

* Ratio depends on the value of the eddy diffusion coeflicent used, from 107 to 10 5 cm -2 s -I.

different order of C2 hydrocarbon abundances: C2H2 > C2H6 > C2H4. Thus the values in Table 5 show that large discrepancies exist between those hydrocarbon ratios, C2H4/ C2H6 and C2H2/C2H6, predicted by the photochemical experiments and those values predicted by the model calculations. Although spectroscopic observations have not yet provided a useful measure of the C2H2/C2H6 ratio, they do indicate a

_

C2H4/C2H6 ratio of 4 x 10 -3, which is consistent with the model calculations. It is not known, however, what contribution to the observed ratio is actually made by particle-induced reactions, but it could be negligibly small. Clearly, there are difficulties in duplicating Titan's atmospheric environment both in computer and in laboratory simulations of the satellite's CH4 photochemistry. Whether these difficulties may account for the discrepancies between the sets of results remains to be evaluated.

4. LOWER ATMOSPHERE

a. Ammonia Photochemistry

Recently, Woodman et al. [1977] established an upper limit of 30 cm-am for the abundance of ammonia on Titan. This

Within and below the cloud layer in the working model, NH3 photochemistry will be limited by the absorption and scattering of ultraviolet light in the overlying haze and clouds. The amount of light that would be transmitted in the wavelengths absorbed by NH3 (1600-2270 A) is not known, but it could be nil.

Hunten [1973] first proposed NH3 photolysis as a means for generating large amounts of N2 on Titan. Recently, Atreya et al. [1978] described a model for the evolution of a dense N2 atmosphere that is based on photodecomposition of NH3 in Ti- tan's primordial atmosphere. In the latter study, as well as in theoretical treatments of Jovian atmospheric chemistry [Prinn, 1970; $trobel, 1973a, b; Prasad and Capone, 1976], NH3 and CH4 photolysis reactions are decoupled from each other. Ex- cept in the work of Kuhn et al. [1977], production of organic compounds by means other than CH4 photolysis high in the stratosphere is discounted. However, laboratory experiments [Khare and Sagan, 1973; Becker et al., 1974; Ferris and Chen, 1975; Ferris et al., 1977] suggest that hot H atoms generated by photodissociation of NH3 or H2S (or both) can initiate reactions with CH 4 and C2H6 to produce organic compounds by way of hydrocarbon radicals such as CH3 and their recombination with each other and with NH2 radicals. Data in Table 6 summarize results re16orted by Ferris et al. [1977] and include estimated production'rates at Titan obtained by extrapolation of their results. The product yields in the 115øK experiment were based on an experimental quantum yield for NH3 decomposition of 0.12 at 208øK [Nicodem and Ferris, 1973]. In the low-temperature experiments, some NH3 is condensed, and the gas phase abundances of NH3 correspond to the equilibrium vapor pressures. In Table 6, entries for NH3 loss and CH4 loss show that from 115 ø to 347øK the photolysis of ammonia is accompanied by hot atom induced decomposition of

limit does not conflict with the NH3 abundance expected CH4 at levels higher than 0.6 CH4 molecule per NH3 molecule above the cloud tops of the working model, which are consid- dissociated. Furthermore, photolytic losses of CH4 and NH3 ered to be at a temperature of _< 115 øK. For temperatures as cannot be accounted for by summing the observed products. low as 86øK [Atreya et al., 1978] thee saturation vapor p•essure This lack of mass balance undoubtedly results in par• from of NH3 will permit low levels of NH3 photochemistry to take the difficulty of the analyses. Also, other compounds not spe- place just above the clouds. With a cloud top temperature of cifically sought were likely to have evaded detection. 115øK, soli d CH4 is not likely to be a cloud constituent, but Mass balance for nitrogen requires the formation of the 'N other hydrocarbons and NH3 are plausible components. organics' designated in Table 6. Methylamine (CH3NH2),

TABLE 6. Photolysis of a Mixture of H2 (0.623 bar), He (0.080 bar), CH4 (0.125 bar), and NH3 (0.040 bar) with 1849-A Light at Various Temperatures: Product Yields and Estimated Production Rates at

Titan

Product Yield, Molecules per Photon Estimated Production

Product 347øK 200øK 115øK Rate at Titan, g cm -2 S -1

H2 0.67 N2 0.14 0.040 0.007 0.93 X 10 -12 N organics 0.02 0.04 0.103 1.0 X 10 -12 HCN 1.2 x 10 -4 0.94 x 10 -4 0.79 x 10 -4 2.1 x 10 -15 C2H6 2.3 x 10-4 C3H 8 0.59 x 10-4 CnHio+ CnHx 0.59 x 10 -5 CsHy 0.5 X 10 -7 N2H4 1.7 X 10 -6 0.56 X 10 -3 1.09 x 10 -5 1.5 X 10 -14 (NH3 loss) (0.30) (0.12) (0.12) (0.6 X l011 cm -2 s-l) * (CH 4 loss) (0.25) (0.073) (0.083) (0.4 X 1011 cm -2 s-l) *

Product yields were calculated from data of Ferris et al. [1977], using quantum yields for NH3 photolysis obtained at 347øK [Ferris et aL, 1977] and 208øK [Nicodem and Ferris, 1973]. Estimated production rates were calculated from yields at 200øK, using a photon flux of 2 x 1012 cm -2 s -l between 1600 and 2270 J• at Titan divided by 4. N organics are assumed to be CH3NH2; other possibilities may include NH2CN, CH3CN, and NH•NHCH3.

*Column photodecomposition rate (at 200øK).

CHANG ET AL..' ORGANIC CHEMISTRY ON TITAN 1929

TABLE 7. Reactions Pertinent to Ammonia Photolysis and Hot Atom Chemistry on Titan

Reaction Rate Constant Reference

(R1) H(hot) + H2 --• H(thermal) + H2 calculations used hot atom theory (R2) H(hot) + CH4 --• H2 + CH3 calculations used hot atom theory (R3) H(hot) + NH3 --• H2 + NH2 calculations used hot atom theory (R4) 2H(thermal) + M--• H2 + M 2.9 x 1025(300/T) ø-a (R5) H(thermal) q- CH 3 --• CH 4 2.3 X 10 24 (R6) CH 3 q- CH 3 q- M --• C2Ha + M 2.2 X 1019(M) (R7) H(thermal) + NH2--• NH3 1.8 X 10 23 (R8) NH2 + CH3 '• CH3NH2 6.6 X 1013 (R9) NH2 + NH2--• N2H4 1.5 X 10 23

Ham et al. [1970]* Cheng et al. [ 1977] Dodd and Steacie [1954]* Boyd et al. [1971]* Kuhn et aL [1977] Pham Van Khe et al. [1977]

The rate constants are in units of cm 3 mole-2 s-2 for two-body reactions and cm 6 mole -2 s -2 for three-body reactions. *From Strobel [1973b, Table 4].

with a N?C ratio of 1 and molecular weight of 3 l, is taken to represent these compounds. Early on, R. S. Becker and J. Hong (private communication, 1978) detected CH3NH2 in similar experiments, but not after long irradiations. Appar- ently, facile conversion of CH3NH2 to other compounds under the irradiation conditions can account for its absence in most

experiments. Formation of complex and colored organic matter from HCN, CH3NH2, or both by secondary photolytic processes can be readily envisioned. Mass balance for carbon also indicates the formation of more hydrocarbons than are reported in Table 6. Using the data for 200øK and assuming that the missing carbon is in the form of H(CH2)nH, a hydrocarbon production rate of 0.4 x 10 -12 g cm -2 s -1 is required for mass balance. The total production rate of organic matter from NH3 photochemistry and subsequent hot hydrogen atom chemistry amounts to 1.4 x 10 -12 gcm -2 s -2 at 200øK, or 2.6

transport is not. Also, the model incorporates 'soft sphere' characteristics with respect to thermalization of hot H atoms. As a result, thermalization involves more collisions than would be predicted if atoms and molecules behaved as classi- cal hard spheres. A complete discussion of the approach is beyond the scope of this report and will appear elsewhere [Aronowitz et al., 1979].

In these computations the photochemistry is approximated in the following manner. An assemblage of molecules corresponding to an atmospheric composition is exposed to an ar- bitrarily chosen total number of photons in a series of in- cremental steps. A small fraction of the available NH3 is dissociated by each increment of photons; the resulting primary products H (hot) and NH2 are allowed to interact with each other and with other gas components in the system according to the reactions in Table 7. The resulting mole frac-

x 10 -12 g cm -2 s -2 at 115øK. At the former rate, a 1-•m-thick tions of reactants (e.g., NH3) and products (e.g., N2H4) are de- layer of organic matter (with unit density) would take about 8 days to produce on Titan, and 4.5 x 109 years of production would yield a 2-km-thick layer of organic matter and liberate 103 km-am of N2 (or a column abundance of 2.8 x 10 27 molecules cm-2).

The 115 ø and 200øK experiments of Table 6 represent useful laboratory simulations of organic photochemistry on Ti- tan. The presence of H2 and He in the experiments is relevant, not because they exist on Titan at the experimental abundances but because they represent agents for collisional thermalization of the hot H atom. On Titan, N2 (and perhaps Ne) may be the thermalizing agent. However, H2 and He are expected to be more efficient thermalizers [Wolfgang, 1963]. In our working model, 115øK may occur at the top of the cloud layer, within it, or at some level below it. (The high CH4 partial pressure of the experiment, however, disqualifies its relevance to the thin atmosphere model.) The 200øK temper-

termined iteratively. The calculations are allowed to run to the point at which additional iterations have negligible effect on the mole fractions of stable end products. The photodissociation of NH3 was kept low (<--5%) by limiting the number of photons introduced in the calculations. For purposes of sim- plification the quantum yield for NH3 was assigned a value of 1.0 for all wavelengths. Use of an appropriate scaling factor, however, permits correction of the calculated quantum yields for any photodissociation quantum yield less than 1.0. Sec- ondary photolysis of products is neglected, although further photochemical transformations of N2H4 and CH3NH2 are to be expected (see section 4a) [Kuhn et al., 1977]. Quantum yields obtained for various products were calculated from the number of photons and the mole fractions of products. The simulated environmental parameters and derived quantum yields are listed in Table 8. In each experiment the NH3 partial pressure is governed by its equilibrium vapor pressure (es-

ature corresponds to a level somewhere below the clouds of timated from a fitted equation of the Antoine form) at the in- our working model (and thick models), either at or just above the surface. Very little ultraviolet light may penetrate both a haze and a cloud layer to reach •these depths in the atmosphere. However, even if only 0.1% of the photons from 1600 to 2270/• were transmitted, the production of organic matter (and N2) would still be significant.

b. Hot Hydrogen Atom Chemistry: Model Calculations

To explore hot atom chemistry under other possible planetary conditions, Aronowitz et al. [1979] developed a computational approach that couples hot atom chemistry with the gen- eral kinetic problem by incorporating the kinetic theory of Wolfgang [1963] for hot atoms. Temperature, pressure, and incident photon energy are taken into account, but mass

dicated temperature. These estimates are smaller than those determined by Gulkis et al. [1978], who used a different data extrapolation.

Data in Table 8 reveal several interesting aspects of hot atom chemistry over the range of conditions simulated. Under all conditions, C2H6 and CH3NH2 are formed, and CH3NH2 is the dominant product. With temperature and wavelength constant, increases in total pressure (experiments 1, 2, and 3) lead to increased CH4 decomposition, which is also reflected in higher yields of C2H6. Formation of CH3NH2 appears to be favored by lower pressure. Lowering the temperature from 155 ø to 125øK (experiments 2 and 4) causes little change in the product yields. Quantum yields for CH4 decomposition decrease as the irradiating wavelength increases (experiments 5, 2, and 6). This decrease occurs because the hot H atoms


TABLE 8. Calculated Quantum Yields for Consumption of Reactants and Formation of Products by Hot Atom Reactions Under Various Titan Conditions

Quantum Yield, Molecules per Photon CHn/H2/N2/NH3

Experiment Mole Ratios T, øK P, bar X, A -NH3 -CH4 N2H4 C2H6 CH3NH2

I 1/1/1/(6.7(-6)) 125 0.022 1850 0.996 0.913 0.128 0.087 0.740 2 1/1/1/(2.9(-7)) 125 0.526 1850 0.984 0.926 0.266 0.237 0.452 3 1/1/1/(1.1(-7)) 125 1.316 1850 0.977 0.932 0.340 0.217 0.298 4 1/1/1/(1.6(-3)) 155 0.526 1850 0.986 0.925 0.249 0.218 0.489 5 1/1/1/(2.9(-7)) 125 0.526 1686 0.986 0.956 0.279 0.264 0.428 6 1/1/1/(2.9(-7)) 125 0.526 2014 0.980 0.875 0.265 0.213 0.449 7 100/1/1/(2.2(-4)) 125 0.022 1850 0.999 0.998 0.064 0.063 0.872 8 100/1/1/(9.5(-6)) 125 0.526 1850 0.999 0.999 0.305 0.305 0.389 9 1/1/400/(3.8(-5)) 125 0.526 1850 0.984 0.926 0.266 0.237 0.452

10 1/1/400/(3.8(-5)) 125 0.526 1850 0.482 0.141 0.194 0.024 0.094 11 1/100/400/(4.8(-5)) 125 0.526 1850 0.371 0.018 0.180 0.003 0.011

Experiment 10 is the same as experiment 9 except that the calculations provided for inelastic collisions between N 2 and the hot atoms. Read (6.7(-6)) as (6.7 x 106).

produced at longer wavelengths are less energetic ('hot'). The influence of H2 as a thermalizing agent is striking (experiments 9 and 10). When the H2/CH4 ratio is high, the decomposition of CH4 and NH3 is inhibited, and recombination of CH3 and NH2 radicals with thefinalized H atoms takes place. As a result of inefficient production of CH3 radicals by hot H atoms, substantially diminished yields of C2H6 and CH3NH2 occur. Excess N2 exerts no influence on the thermalization of hot H atoms (experiments 2 and 9). However, when the model calculations permit formation of transitory complexes in collisions of the hot H atom with N2 (experiment 10), quantum yields for CH4 decomposition and C2H6 and CH3NH2 synthesis drop by factors of 7, 10, and 5, respectively.

Over the range of conditions tested by the model calculations, changes in temperature, total pressure, and irradiating wavelength cause relatively small variations in the yields of products. The presence of high abundances of the moderating gases N2 and H2 (e.g., CH4/N2 -< 0.003) reduces the yields by up to 1 order of magnitude. Although H2 is a more effective thermalizer than N2, its abundance on Titan is not expected to exceed that of CH4 [M•inch et al., 1977]. Conditions of 125øK and 22 mbar (total pressure) represent the closest computational simulation of the surface environment of the thin atmo-

sphere model. Other simulations correspond to locations below the clouds of the thick atmosphere models.

To estimate production rates on Titan from the data in Table 8, a value of 0.12 for the quantum 3;ield of NH3 photodissociation is assumed. This value was measured at 208øK in

excess H2 (H2/NH3 -- 749) by Nicodern and Ferris [1973], who found that its magnitude had increased from a value of 0.04 at

ß

313øK. This inverse correlation with temperature suggests that the value of 0.12 may be a lower limit for temperatures below 208øK. For conditions that apply to atmospheric levels above the clouds, transmission of 10% of the incident ultraviolet sunlight is assumed; below the clouds, 1% transmission is assumed. Product quantum yields at 1850 ,& are taken to represent average yields from 1600 to 2270 ,&. The resulting total production rates of hot atom products corresponding to various conditions are listed in Table 9. Production rates based on

the experimental data of Table 6 are also included as the bottom two entries and are in reasonable agreement with theoretical results. Note that organic synthesis should occur at 115øK and may occur at even lower temperatures provided that some NH3 exists in the gas phase. The possibility of hot atom medi- ated synthesis in the solid phase at low temperatures remains to be evaluated. If the surface temperature of the thin atmosphere model ever attained 125øK, a 1-•m layer of organic matter would accumulate every 46 days. If the lowest production rate in Table 9 is taken to represent synthesis below the clouds of the thick atmosphere models, then settling of the products to the surface would yield a l-•m-thick layer after 900 years and a 5-m layer after 4.5 x 109 years.

The experimental and theoretical treatments of NH3 photochemistry described thus far suggest that if hot H atom induced reactions accompanied the photolysis of primordial NH3 on Titan, then evolution of a thick N2 atmosphere as suggested by Atreya et al. [1978] could have been associated with production of large amounts of organic matter. Photo- lysis of PH3 or H2S, if they were present, •could also have yielded organic matter by way of hot H atom reactions, in ad-

TABLE 9. Production Rates for Organic Matter Synthesized by Ammonia Photolysis and Hot H Atom Chemistry Under Various Simulated Titan Conditions

Production Rate

Experiment CH4/H2/N2/NH3 ' (From Table 8) M61e Ratio T, øK P, bar mol cm -2 s -• g cm -2 s -•

I 1/1/1/(6.7(-6)) 125 0.022 5.0 x 10 •ø 2.5 X 10 -•3 3 1/1/1/(1.1(-7)) 125 1.316 3.1 x 10 •ø 1.6 x 10 -•4 4 1/1/1/(1.6(-3)) 155 0.526 4.3 x 10 •ø 2.1 x 10 -•4 7 100/1/1/(2.2(-4)) 125 0.022 5.6 x 10 •ø 2.8 x 10 -•3

10 1/1/400/(3.8(-5)) 125 0.526 0.7 x 10 •ø 0.35 X 10 -•4 0.2/170/0.06' 200 0.87 2.9 X 1010 1.0 X 10 -14 0.2/1/0/0.06' 115 0.87 5.2 X 10 •ø 2.6 X 10 -•4

Production rates are calculated for 1% transmission through the cloud layer, except for experiments 1 and 7, which are calculated for 10%.

*Corresponds to initial NH3 abundance at room temperature (data from Table 6).

CHANG ET AL.: ORGANIC CHEMISTRY Olq TITAN 1931

dition to forming colored inorganic products. We note, how- however, produced reddish ultraviolet-visible light absorbing ever, that if N2 was the primordial form of nitrogen, as was material. Thus in Titan's upper atmosphere, processes initi- suggested by Norris [1978], then production of organic matter ated by galactic cosmic ray and solar wind or Saturn wind would have been dominated by the processes described in sec- particles can couple the reactions of N2 and CH4 to produce tion 3. similar particulate matter. It is not clear whether CH4 can be

coupled with N2 photochemically (• > 1000/•), but this proc- c. Electrical Discharges ess is expected to be insignificant. Both particle-induced teac-

Electric discharge is the last mechanism considered for or- tions and photolysis of CH 4 or CH4 and H2 mixtures will lead ganic synthesis in Titan's lower atmosphere. Discharges might to synthesis of simple colorless hydrocarbons, with the latter arise from atmospheric turbulence at or just below the cloud process dominating. Although N2 is not observable in Titan's deck, although there is as yet no evidence for such phenom- atmosphere, its presence is implicated. The N2 could conceiv- ena. Toupance et al. [1974] have shown that discharges ably originate from two sources: an initial endowment of pri-

mordial N2 or the photodecomposition of primordial NH3 through CHn-N2 mixtures in a flow system (at 26 mbar) produce mainly HCN, hydrocarbons, and a variety of saturated over geological time, or both. Production rates of organic and unsaturated nitriles. Starting with a mixture of 90% N2 compounds of about 10 -•5 g cm -2 s -• are estimated for proc- and 10% CH4, about 60% of the CH 4 was converted to HCN. esses initiated by solar wind, Saturn wind, and cosmic ray par- This conversion factor is used to estimate production of or- ticles. Over 4.5 x 109 years, 300 g cm -2 or more of organic ganic matter by electric discharges on Titan. matter could have been produced to supply the coloration for

From the solar constant at Titan and an effective albedo of the apparent surface (or cloud tops). 0.2 the energy flux available is calculated to be 1.1 x 10 -3 J Production of organic compounds (other than simple hy- cm -2 s -•. For lack of data relevant to Titan the conversion of drocarbons) by photochemical processes may occur pre- this energy to electric discharges is assumed to proceed on Ti- dominantly in the vicinity of the clouds of the working model tan as it does on earth with an efficiency of 10-3%. Therefore and below them. Even if the ultraviolet flux in this region were

reduced by 3 orders of magnitude from the incident value, at- the energy flux carried as electric discharges would be 1.1 x 10 -8 J cm -2 s -•. Assuming a 5-kin lightning stroke with a mospheric photochemistry may nevertheless be significant in shock column of 80 cm 2 cross section [Bar-Nun, 1975] and a initiating organic synthesis through hot H atom reactions. The total energy of 10 •ø J [Krider et al., 1968], a CH4 number den- hot H atoms may be produced by photodissociation of NH3 sity of 5 x 10 •8 cm -3 in a 90% N2 atmosphere, and a discharge (or PH3 and H2S) in the atmosphere. Assuming transmission

of 1% of the incident ultraviolet light to the vicinity of the energy flux of 1.1 x 10 -8 J cm -2 s -•, a production rate of 1.3 x 108 molecules cm -2 s -•, or 6 x 10 -•5 g cm -2 s -•, is obtained. clouds, production rates for organic matter are estimated to be This figure is appropriate for the lower atmosphere of the about 10 -•4 g cm -2 s -•, or 10 times the rate derived from ener- greenhouse models and compares favorably with the produc- getic particle reactions. Electric discharges, if they occurred in tion rates derived from hot H atom reactions. However, the the lower atmosphere, could produce organic material at a laboratory simulations of electric discharges on which the cal- comparable rate. If, however, a massive atmosphere domi- culations are based may have little relevance to lightning dis- nated by N2 has been produced by photolysis of primordial charges in a planetary atmosphere. Moreover, there is no clear NH3 over geological time, then the associated hot H atom re- information regarding the conversion efficiency of solar en- actions could have yielded very large amounts of organic mat- ergy to electric discharge energy on any planet other than ter. earth. These considerations make statements about electric We conclude that the surface of Titan should contain an- discharge synthesis of organic matter on Titan very specula- cient or recent organic matter (or both) produced in the atmo- tive. sphere. Some of this material will remain in the atmosphere as

components of the clouds or of the residual gases. These materials should include both hydrocarbons and organic com-

5. SUMMARY pounds containing N (and possibly P and S). Therefore study The atmospheric models that have been proposed for Titan of the satellite's atmosphere and surface in any probe or

must embody a number of common features in order to ac- lander mission should pay particular attention to the chemis- commodate the available astronomical observations. Titan's try of the organogenic elements (H, C, N, O, P, and S) and in- apparent surface (possibly cloud tops) is reddish; and in the clude organic analysis among the prime science objectives. atmosphere with CH4 are the hydrocarbons C2H6, C2H4, and Note added in proof We have been informed by J.P. C2H2 and a high-altitude layer of ultraviolet-visible light ab- Ferris (private communication, 1979) that in recent experi- sorbing particles. These observations are consistent with the ments the photolysis of NH3 at 1849 • in the presence of CH 4 occurrence of nonequilibrium processes in the atmosphere resulted in 5-7 times less CH4 decomposition due to hot H and can be interpreted as manifestations of organic chemical atom reactions than was previously reported [Ferris and Chen, synthesis. Features taken from various models of Titan's at- 1975; Ferris et al., 1977]. Although the reaction conditions in toosphere are incorporated in a composite working model that the original and new sets of experiments differ, it is probable provides environmental constraints within which various that the production rates for N organics in Table 6 and for to- pathways for organic chemical synthesis are assessed. tal organics in the bottom two entries of Table 9 are over-

estimated. Even order of magnitude lower production rates, In laboratory simulations of upper atmospheric processes on Titan, reactions initiated by energetic particle bombard- however, would still be significant and comparable to those ment or photolysis of CH4 and CH4 q- H2 mixtures yield color- estimated to result from high-energy particle bombardment less substances whose light absorption does not extend to (Table 6). wavelengths longer than the ultraviolet. Inclusion of N: (or Acknowledgments. The authors are indebted to T. Owen, L. Giver, NH3) in the gas mixtures subjected to particle bombardment, L.A. Capone, and D. Goorvitch for their helpful discussions on Titan

1932 CHANG •T AL.: ORGANIC CHEMISTRY ON TITAN

observations and models. S. Aronowitz acknowledges support under NASA Ames contract A39942B. T. Scattergood is an NAS-NRC resi- dent research associate.

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(Received September 20, 1978; accepted June 28, 1979.)

Organic chemistry on Titan

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