Chapter 8 High-Altitude Production of Titan s Aerosols

Transcription

1 Chapter 8 High-Altitude Production of Titan s Aerosols J.H. Waite, Jr., D.T. Young, J.H. Westlake, J.I. Lunine, C.P. McKay, and W.S. Lewis Abstract Measurements with the Cassini Ion and Neutral Mass Spectrometer (INMS) and two Cassini Plasma Spectrometer (CAPS) sensors, the Ion beam Spectrometer (IBS) and the Electron Spectrometer (ELS), have revealed the presence of a significant population of heavy hydrocarbon and nitrile species well above the homopause, with masses as large as several thousand Daltons (Da). The INMS ion and neutral spectra cover the mass range Da. The IBS has measured positive ions up to 350 Da, while the ELS has detected concentrations of negative ions as high as 0% of the total negatively charged ionosphere component extending to over 13,000 Da. These measurements have motivated the development of new atmospheric models and have significant implications for our knowledge and understanding of Titan s haze layers. The existence of a thick haze obscuring Titan s surface was inferred from remote-sensing observations at infrared and ultraviolet wavelengths during the mid-1970s (Danielson et al. 1973; Veverka 1973; Zellner 1973; Trafton 1975) and confirmed by Voyager 1 and imaging, which revealed the existence of two principal haze layers, a main layer and a thin detached layer 100 km above it, both merging at high northern latitudes (Smith et al. 1981, 198). It was recognized early on (e.g., Danielson et al. 1973) that photochemistry occurring in the upper atmosphere of Titan was the likely source of the haze-forming aerosols, and in the years J.H. Waite, Jr.,( ), D.T. Young, J.H. Westlake, and W.S. Lewis Southwest Research Institute, P.O. Drawer 8510 San Antonio, TX 788, USA hwaite@swri.edu J.H. Waite, Jr., J.H. Westlake University of Texas at San Antonio, One UTSA Blvd. San Antonio, TX 7849, USA J.I. Lunine Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 8571, USA C.P. Mckay NASA Ames Research Center, Mail Stop 45 3, Moffett Field, CA 94035, USA leading up to the Voyager encounters several laboratory experiments were performed in an attempt to synthesize materials whose properties were similar to those of the postulated hazes (see reviews by Chang et al and Cabane and Chassefière 1995). Substances investigated as possible candidates for the haze-forming aerosols included polymers of acetylene, ethylene, and HCN (Scattergood and Owen 1977; Podolak and Bar-Nun 1979) and tholins, complex organic solids, brownish in color, produced in a simulated reducing planetary atmosphere through UV irradiation and electric discharge (Khare and Sagan 1973; Sagan and Khare 1979). Prior to the Voyager encounters, the only species known with certainty to be present in Titan s atmosphere were C and, although there was evidence for the presence of and as well (Gillett 1975). The presence of N, predicted by Hunten (1977) and Atreya et al. (1978), had not yet been established, although Titan s reddish-brown albedo suggested that nitrogen-bearing species (and/or sulfur-bearing ones) should be present in the haze aerosols (Scattergood and Owen 1977; Chang et al. 1979). The Voyagers revealed that Titan s atmosphere consists predominantly (>90%) of molecular nitrogen (Broadfoot et al. 1981; Tyler et al. 1981) with methane as the next most abundant species and provided positive identifications of several hydrocarbons including,, and H 8 as well as of the nitriles HCN, H N, and N (Hanel et al. 1981, 198; Kunde et al. 1981; Maguire et al. 1981). During the interval between the Voyager encounters and the arrival of Cassini in the Saturn system, several photochemical models were developed to describe the production of hydrocarbons and nitriles resulting from the dissociation of N and C in Titan s upper atmosphere by electron impact (N ) and UV irradiation (C ) (e.g., Yung et al. 1984; Toublanc et al. 1995; Wilson and Atreya 004). More a number of laboratory, modeling, and theoretical studies were undertaken to investigate the formation of the haze layers and the physical, optical, and chemical properties of the aerosols in light of both the Voyager data and new remote-sensing R.H. Brown et al. (eds.), Titan from Cassini-Huygens, Springer Science Business Media B.V

2 0 J.H. Waite et al. observations (see reviews by Cabane and Chassefière 1995 and McKay et al. 001). Post-Voyager experiments to synthesize aerosol analogs in the laboratory involved both the production of tholins in a simulated Titan N -C atmosphere (e.g., Thompson et al. 1994; Coll et al. 1999) and the creation of the photopolymers of,, and HCN (Bar- Nun et al. 1988; Scattergood et al. 199) as well as of H N and H N/ (Clarke and Ferris 1997). The spectral and optical properties of tholins were found to be consistent with Titan s albedo and with the refractive properties of Titan s haze particles, suggesting that tholins are good analogs for Titan s aerosols (Khare et al. 1984). The ultimate sources of Titan s aerosols are the gas-phase dissociation products of C and N. However, as noted by Lebonnois et al. (00), the transition from gas-phase compounds to solid-phase aerosols is poorly understood. They suggested three possible chemical pathways that could polymerize simple molecules to macromolecules, which are the presumed precursors to aerosol particles, producing: (1) polymers of acetylene and cyanoacetylene, () polycyclic aromatics, and (3) polymers of HCN and other nitriles, and polyynes. Their model suggested a total production rate of g cm s 1 and a C/N ratio of 4, in a production zone slightly lower than 00 km altitude. Wilson and Atreya (003) considered similar pathways and concluded that the growth of polycyclic aromatic hydrocarbons (PAH) throughout the lower stratosphere could play an important role in haze formation. They suggested that the peak chemical production of haze would lie near 0 km, with a column integrated production rate of g cm s 1. Wilson and Atreya (003) pointed out that the discovery of benzene in Titan s atmosphere by ISO (Coustenis et al. 003) favored the PAH pathway. Trainer et al. (004) found that for particles produced from a mixture of 10% C in N the results were consistent with a large fraction of aromatics, including specific mass spectral peaks likely due to PAHs. However, at lower concentrations of C (1% and lower), the mass fraction of PAHs greatly diminished, and an aliphatic pathway dominated. Laboratory simulations also indicate a possible key role for PAHs. Khare et al. (00) reported on an analysis of the time-dependent chemical evolution of gas phase products in a Titan simulation. They found an early dominance of aromatic ring structures that led in the later stages of the experiment to the appearance of nitrile and amine compounds. Thompson et al. (1991) reported the yields of gaseous hydrocarbons and nitriles produced at pressures (1,700 Pa and 4 Pa) in a continuous-flow, low-dose, cold plasma discharge excited in an atmosphere consisting of 10% C and 90% N at 95 K. At 1,700 Pa, 59 gaseous species including 7 nitriles were detected while at 4 Pa, 19 species are detected, including six nitriles and three other unidentified N-bearing compounds. The types of molecules formed changed even more markedly, with high degrees of multiple bonding at 4 Pa prevailing over more H-saturated molecules at 1,700 Pa. Imanaka et al. (004) conducted a series of experiments from high (,300 Pa) to low (13 Pa) pressure. They found an increase in the aromatic compounds and a decrease in C/N ratio in tholins formed at low pressures, indicating the presence of the nitrogen-containing polycyclic aromatic compounds in tholins formed at low pressures. They concluded that the haze layers at various altitudes might have different chemical and optical properties, but most importantly they found that there is a fundamental change in the nature of haze production between pressures above and below roughly 100 Pa. 8.1 Cassini Observations of Heavy Hydrocarbons in Titan s Upper Atmosphere Earlier models of the photochemistry responsible for initiating the production of complex acetylene polymers and polyaromatic hydrocarbons (PAHs) suggested that the formation of heavy hydrocarbons such as benzene occurs primarily in the well-mixed portion of Titan s atmosphere below the homopause (pressure 10 3 Pa near 750 km) (e.g., Wilson and Atreya 003, 004). Ion-neutral reactions near the ionospheric peak (pressure Pa at 1,100 km) were thought to be an additional, although much weaker source of complex hydrocarbons. Thus, prior to the arrival of Cassini, it was expected that there would be little benzene or other complex hydrocarbons, and thus they would be only marginally detectable at altitudes above 950 km, the region sampled by the Cassini orbiter during its passes through Titan s upper atmosphere. However, measurements with the Cassini Ion and Neutral Mass Spectrometer (INMS) and two Cassini Plasma Spectrometer (CAPS) sensors, the Ion beam Spectrometer (IBS) and the Electron Spectrometer (ELS), have revealed the presence of a significant population of heavy hydrocarbon and nitrile species well above the homopause, with masses as large as several thousand Daltons (Da). The INMS ion and neutral spectra cover the mass range Da (Fig. 8.1; Tables 8.1 and 8.) (Waite et al. 005; Magee et al. 009). The IBS has measured positive ions up to 350 Da (Fig. 8.; Crary et al. 009), while the ELS has detected a concentration of negative ions as high as 0% of the total negatively charged ionospheric component extending to over 13,000 Da (Fig. 8.3; Coates et al. 009). The INMS is a true (quadrupole) mass spectrometer designed to measure the abundance of ion and neutral species in Titan s upper atmosphere (Waite et al. 004). The IBS and ELS sensors, on the other hand, measure ion flux as a funatic of ion energy/charge from which pseudo-mass spectra can be derived. IBS, designed for the supersonic solar wind and the

3 8 High-Altitude Production of Titan s Aerosols 03 Fig. 8.1 Composite mass spectra for neutrals (top) and ions (bottom) based on Cassini INMS data (black line) acquired during 17 flybys of Titan. Data were taken between 1,000 and 1,100 km. The mass deconvolution used to produce Table 8.1 is indicated above the spectrum and the totals are marked on the spectrum (top panel) with red dots, the top panel (from Waite et al. (007) and is reprinted with permission from AAAS) cold ionosphere of Titan, has relatively high energy resolution (DE/E = 1.7%). ELS, which is designed to measure hot plasma electrons, has lower resolution (DE/E = 17%) but is in addition sensitive to negative ions (Waite et al. 007; Coates et al. 007, 009). As a consequence of the low temperatures of ions in Titan s ionosphere (10 50 K between 950 and 1,600 km respectively), ion thermal velocities are small (100 s of m/s, depending on ion mass). During encounters with Titan Cassini travels at supersonic velocities ( 6 km/s) relative to the cold ionosphere, which allows the use of IBS and ELS energy/charge spectra to infer ion mass/charge regardless of charge state or polarity. Crary et al. (009) used INMS data combined with IBS to extract mass spectra and estimate ion temperatures and flow speeds. Similarly Coates et al. (007, 009) and Waite et al. (007) analyzed ELS data to produce negative ion mass spectra. The peak flux measured by IBS or ELS can be identified with ion mass by the relationship E i = m i V s/c / 8 kt where V s/c is spacecraft velocity and m i is the mass of the ith species (Crary et al. 009). Taking T = 150 K as a characteristic temperature, E i = m i ev. Carbon is the smallest mass of interest here, so m i 1 Da and m i V s/ / >>; 8 kt. Thus E i = m i V s/c / to a very good approximation c

4 04 J.H. Waite et al. Table 8.1 Neutral species mixing ratios measured by INMS in the altitude region between 1100 and 1000 km. Values are globally averaged over 0 flybys as reported by Magee et al. (009) Major species N Mixing ratio ± N 15 N ± C ± C ± Minor species HCN ± Mixing ratios Ar CH 3 CCH H 8 C 4 N C 6 HCN H 3 CN H 5 CN C 7 H Table 8. Ion densities measured by INMS for five passes T16 T17 T18 T1 T3 CH (0.1) (0.3) (0.18) (0.03) (0.5) H (0.) (0.6) (0.1) (0.04) (0.87) HCNH (0.3) (1.3) (0.48) (0.10) (.74) H (0.4) (1.4) (1.10) (0.87) (4.48) C 4 H (0.3) (0.5) (0.7) (0.1) (1.13) C 4 H (0.3) (0.4) (0.30) (0.15) (1.38) C 6 H (0.4) (0.1) (0.3) (0.8) (1.47) C 6 H (0.4) (0.1) (0.7) (0.0) (.6) C 7 H (0.9) (0.) (0.50) (0.7) (5.79) Altitude (km) LST and mass (in Daltons) can be inferred using m i = 5.3E i. Under these circumstances the energy resolution of IBS is equivalent to an effective mass resolution of 30 at 8 Da. Peaks in the IBS energy spectra can be resolved up to 00 Da (Fig. 8.), while the maximum mass observed so far is 350 Da. By assuming maxwellian velocity distributions and comparing IBS with INMS, Crary et al. (009) were able to correct IBS data for spacecraft potential and ion winds along the spacecraft track to obtain pseudo-mass spectra (Fig. 8.4). Figure 8.5 shows an example of ion density calculated by Crary et al. using this technique. The agreement between the two instruments below 1,600 km is very good as is agreement with the Cassini Langmuir Probe measurements of total plasma density (Wahlund et al. 009). The lack of agreement above this altitude is caused by ion heating, which invalidates the cold ion assumption used to interpret IBS data. Heavy ions >100 Da become a major constituent below 1,00 km and tend to increase deeper in the ionosphere, becoming as much as 50% of the total at 950 km, the lowest altitudes visited by Cassini (Crary et al. 009). Altitude profiles strongly suggest that the abundance of ions >100 Da continues to increase rapidly with depth in the atmosphere (Fig. 5; also Wahlund et al. 009). Detailed knowledge of the abundance of heavy ions is important for understanding the chemistry of ion formation. Using data from all 14 Titan encounters studied so far, Crary et al. were able to obtain 130 mass spectra. (The number of spectra is limited by the need to swing the CAPS sensors across the Cassini ram direction every 60 s as is evidenced by the spacing of peaks in Fig. 8.6, which shows the percent occurrence calculated for each mass bin from 100 to 00 Da. On more recent passes such as T55, motion of the sensors has been halted to yield a much larger number of spectra in the ram direction. These have not yet been analyzed.) When taken together and examined statistically the spectra show a consistent mass peak spacing of 1 14 Da as expected for compounds consisting of carbon and nitrogen. Analysis of the abundant groups >100 Da, together with INMS data <100 Da and inferences of possible chemical reactions, suggests that ions in this range are most likely aromatic hydrocarbons (Waite et al. 007). Crary et al. (Fig. 8.7) also infer that acetylene and nitrile polymers, particularly naphthalene, are relatively common, appearing in >70% of the spectra. In addition they conclude that aromatic hydrocarbons, particularly PAHs, are the most likely component of the positive ion spectra. Sittler et al. (009) have advanced arguments to the effect that the negative ions observed by ELs could be fullerenes (C60), a suggestion based on analogies to observations of fullerenes formed under laboratory conditions similar to those in Titan's atmosphere. Since neutrals and ions are cold, energy would come from hot electrons (0.1 ~ few ev) that are abundant in the ionosphere (Coates et al., 009). Sittler et al. go on to suggest that because there are a number of sources of oxygen present in the atmosphere, particularly ~ke V oxygen and water group ions driven in to the atmosphere by corotation, oxygen might in fact be trapped in the fullerenes, a phenomenon also observed in the laboratory. Once formed the fullerenes would settle into haze layers as discussed above, eventually reaching the surface where they would represent a source of oxygen that might contribute to

5 8 High-Altitude Production of Titan s Aerosols 05 Fig. 8. IBS spectrum from flyby T6. The match to the Cassini INMS ion spectrum below 100 Da is marked in red. Note the significant ion densities above 100 Da Fig. 8.3 Inferred mass/charge spectrum of negatively charged ions using ELS energy/ charge data (energy scale is shown at top of figure). Spectra were taken at 953 km during the T16 pass. Upper trace in top panel shows count rate spectrum corrected for photoelectron contribution. Lower panel shows spectrum converted to differential number density (from Coates et al. (007) ) pre-biological chemistry. Although somewhat speculative in nature (there are many steps involved, not all of which are well established) the possibility of fullerene formation is certainly worth further investigation. Figure 8.8 shows ELS energy-time spectrograms for four encounters. The sharp spikes in all four panels indicate the presence of cold negative ions that are seen only when ELS sweeps through the ram direction. Although ELS has much lower energy resolution than IBS, the same principles of analysis can be applied, enabling energy spectra to be converted into pseudo-mass spectra (Fig. 8.3). Mass resolution is limited to 5 at 16 Da, and, as for IBS, drops at higher masses. Because of the low instrument resolution and high ion masses observed by ELS, corrections for spacecraft potential and winds can be neglected. The finding of abundant heavy negative ions using ELS is one of the truly surprising discoveries made with Cassini. Although photoelectron peaks are seen during daylight encounters, it is very clear that the peaks in the spectra identified as negative ions are far too narrow and unidirectional to be misidentified as electrons, which are both isotropic and hot (>>1 ev, equivalent to >> 10,000 K). As Fig. 8.3 shows, the mass of negative ions extends from 17 Da to >10,000 Da. Negative ions are a permanent feature of the ionosphere, having been observed on all 3 encounters thus far during which spacecraft pointing was favorable for observations. Two very clear negative ion peaks can typically be identified in the spectra at ± 4 Da and 44 ± 8 Da with a possible third peak at 8 ± 14 Da (Figs. 8 and of Vuitton et al. 007).

6 06 J.H. Waite et al. Fig. 8.4 Mass spectra from the INMS (upper panel) and the IBS (lower panel) from 1,05 km during the ingress leg of the T6 encounter. The lower panel shows a best fit to the IBS data below 100 Da using the method described in the text. Note that, although poorly resolved, mass peaks are still visible above 100 Da. The bottom panel (from Crary et al. (009) with permission from Elsevier) Fig. 8.5 Comparison of INMS and IBS ion densities during the T6 encounter (Crary et al. 009). INMS total ion density is shown in black. Using data from the IBS, Crary et al. calculated the total ion density (red), density of ions below 100 Da (blue) and density of ions heavier than 100 Da (green) (reprinted with permission from Elsevier) Fig. 8.6 The percent occurrence calculated for each mass bin from 100 to 00 Da (Crary et al. 009). Total percent occurrence for each group is shown in the box above each peak (reprinted with permission from Elsevier)

7 8 High-Altitude Production of Titan s Aerosols 07 Fig. 8.7 Color-coded figure showing the likelihood of chemical groups to be present in the high mass ion population. Probabilities are determined from the percent occurrence spectrum shown in Fig. 8.6 (reprinted from Crary et al. (009) with permission from Elsevier) Fig. 8.8 Energy-time spectrograms of ELS data taken during T16 to T19 and centered on closest approach to Titan (Coates et al. 007). The prominent peaks in each panel are due to cold negative ions rammed into the instrument. Intense fluxes below 10 ev correspond to photoelectrons from the spacecraft which disappear when spacecraft potential becomes negative in the ionosphere. Fluxes between 10 and 30 ev correspond to ionospheric photoelectrons

8 08 J.H. Waite et al. CAPS observations combining IBS and ELS data show that heavy positive and negative ions are present on every pass during the primary mission, 14 in all, where pointing was appropriate. (cf. Table 1 of Crary et al. 009 and Table 1 of Coates et al. 009; Wahlund et al Analysis has not yet been completed for passes in the extended mission, which began in summer 008.) Heavy positive ions become a significant component of the positive ion component of the ionosphere below 1,00 km, while at the same altitude heavy negative ions begin to become a prominent fraction ( 0%) of the total negatively charged ionospheric component, presumably electrons (Coates et al. 007, 009; Waite et al. 007). Total negative ion density increases with decreasing altitude as does the maximum negative ion mass (Fig. 8.9). The latter is strongly dependent on altitude, varying from approximately few 100 Da at 1,400 km to as much as 13,800 Da at 950 km (Coates et al. 007, 009). At that altitude negative ions can reach densities up 00 cm 3 and make up as much as 0% of all the negative charge present (Wahlund et al. 009). Although a particle with mass of 13,800 Da is a very large molecule indeed, it is doubtful that these are true molecules. Rather they are most likely aerosols formed by the clumping of smaller molecules. Since only mass/charge can be inferred from ELS measurements, if ion charge is >1 electron, then ion mass and size would be proportionately larger. Although simple to estimate in principle, calculating particle size depends on an assumption of density. Solid particles might have a characteristic density 1 g/cm 3 whereas fractal particles might conceivably have densities as low as g/cm 3. The estimated radius for maximum observed mass (13,800 Da) is then in the range 3.8 to 38 nm, the size of small aerosols which, it has been suggested, could be precursors of larger aerosols seen at lower altitudes (see Chapter 1 and references therein). Since size and electrical charge are important microphysical properties of the aerosols, estimations of size are critical to developing theories of formation and growth (Tomasko et al. 008; Lavvas et al. 009). At 1,000 km altitude the size of heavy negative ions is much smaller than the local Debye length (approximately few centimeters) leading to an estimated particle potential j.5 kt e /e (Goertz 1989). Since only mass/charge is known, in order to estimate particle size we need to establish ion charge. The electron energy spectrum measured by ELS is variable, at times in sunlight dominated by a photoelectron peak at a few electronvolts (Vuitton et al. 009). In shadow, the characteristic equilibrium electron temperature is 1,000 K. On the other hand, the assumption of thermal equilibrium between electrons and ions would result in a temperature of 150 K. These values lead to a wide range of particle potentials ranging from 5 to 0.01 V depending on conditions and location. With an estimate of potential, particle charge can be estimated from Q = 4pe 0 aj exp( a/l D ) where a is particle radius which we earlier estimated to be in the range of 3.8 to 38 nm depending on density. Since a << l D we neglect the exponent. Substituting values for the constants and units gives the simple equation: Q 0.7 aj (electrons) where a is in nm and j in V. The total particle mass M = km = m/q = 1.4 m/aj in units of amu, where k is a factor for converting pseudo-mass to total mass. For example a 10,000 Da particle with 1 nm radius and a potential of 1 V would have a total mass of 14,000 amu (Da is a measure of mass/charge). Given the resolution of ELS the identity of the ions is necessarily somewhat speculative. The high electron affinity of Fig. 8.9 Heavy negative ion densities as a function of altitude showing the rapid increase in density in the aerosol formation layer (below 1,000 km) (reprinted from Coates et al. (009) with permission from Elsevier)

9 8 High-Altitude Production of Titan s Aerosols N (0.77 ev) and CN (3.86 ev) makes them candidates for the components of the first peak at Da. (Although ion masses are 16 and 6 respectively both could be present in varying amounts.) Similar considerations suggest that the second peak might be NCN, HNCN or H (see Coates et al. 007) or C 4 H (Vuitton et al. 009). Vuitton et al. also suggest C 5 N as a possible candidate for third peak. Candidates for heavier ions involve polyynes, nitriles, PAHs, Fullerenes (Sittler et al., 009) and cyano-nitriles. There is no shortage of possibilities, but at present there is not enough information to narrow the selection. However details of composition matter less than the fact that there is a rich soup of heavy organic compounds conducive to forming aerosols. 8. New Chemical Models Based on the Cassini Results The measurements made by Cassini have motivated a new round of modeling and analysis, which has contributed to our present level of understanding of organic formation in Titan s atmosphere. Waite et al. (007) were the first to highlight the correspondence of the ion and neutral mass spectra in the upper atmosphere and suggested that this demonstrated a strong degree of coupling in the ion-neutral chemistry that resulted in the observed complexity of the organic compounds. Furthermore, they showed that ion neutral chemistry involving C 4 H 3 was the likely formation path of the observed benzene and that benzene was in chemical equilibrium with its protonated ion counterpart, C 6 H 7. Vuitton et al. (006) had earlier postulated that most of the unexpected ion peaks measured by INMS in the T5 flyby were the result of protonated nitrile compounds. They used this basic premise to develop a complex zero-dimensional model of the ion neutral chemistry and to infer much of the trace nitrile composition in the neutral atmosphere. Carrasco et al. (008) examined this same ion neutral data set using two chemical reaction pruning methods coupled with a Bayesian statistical analysis of the reaction rate uncertainty to conclude that only 35 key ion molecule reactions were needed to describe the basic ion-neutral coupling and that they were not dominated by proton transfer as Vuitton et al. had implied. Rather they consisted of 3 growth reactions leading to chemical complexity (see Table 8.3) including condensation (bond rearrangement reactions), 5 protonations, and 5 charge transfer reactions. Using a method that accounts for diurnal variations in the energy input into the upper atmosphere, De La Haye et al. (008) have developed a coupled ion-neutral composition model for Titan s atmosphere over the altitude range from 600 to,000 km. The model demonstrates the important role played by ion-neutral chemistry in the formation of both hydrocarbons Table 8.3 Ion neutral reactions deemed important for the upper atmosphere as assessed by Carrasco et al. (008) 09 Reaction Global rate constant, k 10 9 cm 3 s 1 Branching ratio (br) Dk/k CH C H CH CH C H H CH HCN C N H CH CH C H CH CH CH CH CH C H C H CH CH C H C H CH CH C H C H CH H CH C H H H CH C H H H C H c-c H CH H C H c-c H CH H C H C H CH H C H C H C H H O H 5 3 O H HCN 5 HCNH H C H H C H C H H N( 3 P) C CH 3 NH N( 3 P) C C N N( 3 P) C HCNH N( 3 P) N N( 3 P) CNC N( 3 P) N N H N H H N CH CH H 4 N N CH CH H N N C H C H N N C H C H H N N H C CH 5 N H 3 O HCN HCNH O HCNH CH 3 CN N HCN HCNH NH 3 N HCN CNC H HCN CNC N Source: Adapted from Carrasco et al. (008), with permission from Wiley Interscience. Wiley Interscience, New York, 008. and nitriles, although the relative importance of ion-neutral vs neutral chemistry varies with local time, altitude, and species. More comprehensive one-dimensional models of the atmosphere that extend from the surface to the exobase and include both gases and particulate chemistry have been recently presented by Lavvas et al. (008a,b) and Krasnopolsky (009). Krasnopolsky assumed the temperature structure measured by HASI, whereas Lavvas et al. calculated the thermal structure self consistently, which in fact agrees with the HASI derived profile. Both models generate the haze structure from the gaseous species photochemistry and the authors compare their results to the Cassini INMS and CIRS composition measurements in the thermosphere and stratosphere, respec-

10 10 J.H. Waite et al. tively, and to the DISR measurements of the haze from the Huygens probe. Lavvas et al. identified a new source of haze particles from 500 to 900 km based on nitrile-hydrocarbon copolymer chemistry, but did not include ion-neutral chemistry and thus failed to identify the source of the high latitude macromolecules observed by CAPS and INMS. Haze formation in their model is dominated by nitrile and aromatic chemistry below 300 km. The total rate of precipitation they calculate is g cm s 1 (4 kg cm Gy 1 ), which is in the range estimated by McKay et al. (001), g cm s 1 ( kg cm Gy 1 ). Krasnopolsky (009) included ion-neutral chemistry, ambipolar diffusion, and atmospheric escape as well as both positive and negative ion chemistry that to first order agrees with the CAPS and INMS results. The major haze production in this case is via the reactions C 6 H C 4 (polyyne chemistry), N C 4 (copolymer chemistry), and the condensation of hydrocarbons below 100 km first suggested by Hunten (006). Overall, the estimated precipitation rate is equal to g cm s 1 (4 7 kg cm Gy 1 ). Tables 8.4 and 8.5 compare the models discussed above to INMS measurements at 1,050 km and to CIRS measurements at 300 km, respectively. The modeling community before 008 in general over-predicted the abundance of the C group, possibly due to overproduction of H 5 and H 5 in the case of the primarily ionospheric models of Vuitton et al. (006, 007), or to the lack of ion-neutral chemistry in the case of earlier models. Lavvas et al. (008 a, b); De La Haye et al. (008); Lara et al. (1996); and Yung et al. (1984) all underestimate the abundance of the C3 group. Moreover the estimates of benzene mixing ratios vary significantly. Lavvas et al. (008a,b) and Wilson and Atreya (004) find mixing ratios from a few times to , while Krasnopolsky (009) and De La Haye (008) find values in the 10 7 range. Vuitton et al. (007) calculate a ratio of , which comes closest to observations, possibly because of their inclusion of a currently undefined -body reaction process. Two recent papers have addressed the structure of the haze particles based on Cassini Huygens observations. Liang et al. (007) used UVIS ultraviolet observations of the continuum near 190 nm to infer the altitude distribution and size of the haze scattering the incident solar flux. Their analysis indicated that the upper atmosphere near 1,000 km contained up to 10 4 cm 3 macromolecules with sizes of the order of 10 0 nm. The photochemical calculations of Liang et al. suggest that polyyne polymers play a major role in the haze formation process. Lavvas et al. (009) combined the UVIS ultraviolet observations with the ISS visible scattering observations to address the relationship between the detached haze layer and the haze layers extending to the surface. They found that the detached haze layer is formed as a result of changes in the particle sedimentation rate with altitude. They assert that the observed mass flux.7 to g cm s 1, which is approximately what McKay et al. (001), needed to explain the main haze layer, originates from production mechanisms dominated by upper atmospheric processes first identified by Waite et al. (007). They therefore suggest that the main haze layer in Titan s stratosphere is formed primarily by sedimentation and coagulation of particles in the detached layer. More recently Sittler et al. (009) suggest that sufficient fullerenes are formed to contribute ~7% to the total aerosol infall. Figure 8.10 summarizes the post Cassini Huygens understanding of the conversion of methane and nitrogen to organic macromolecules, then to organic aerosols, and eventually to organic materials on the surface of Titan. The process begins with the dissociation and ionization of methane and molecular nitrogen by solar ultraviolet radiation and energetic particles in the upper atmosphere of Titan. This process sets in motion a rich ion neutral chemistry that produces heavy positive ions and neutrals that eventually form macromolecules, many of which are negatively charged. Macromolecules precipitate rapidly from the formation layer near 1,000 to 550 km where they reach a size of 40 nm. They then slow through atmospheric viscosity to form the detached haze layer (Lavvas et al. 009, 008a,b). Some additional radical chemistry occurs during this descent, but most of the formation chemistry has already taken place in the low-pressure upper atmosphere as a result of ion neutral chemistry. Below 550 km the particles began to coagulate into the main aerosol layer and some additional chemistry likely takes place in this growth process. The cold temperatures of the troposphere lead to condensation of ethane and other organics onto the aerosols (Hunten 006), before they finally precipitate onto the surface. The loss of hydrogen to space guarantees that the process will irreversibly convert methane in the atmosphere into organic residue on times scales from 10 to 70 million years (Mandt et al. 009). 8.3 Conclusions: Laboratory Simulations and the Future of Titan Exploration No laboratory simulations have been published that explicitly try to match the Cassini data. However, it is instructive to look at published results from Titan simulations and compare them to the INMS spectra. For example, comparison of the results of Thompson et al. (1991) to the INMS results of Waite et al. (007) shows that there is a broad similarity in the two results and suggests that properly conducted laboratory simulations will be able to reproduce the INMS spectrum (Fig. 8.11). The laboratory mass spectrometer measurements of Imanaka et al. (004) are compared to the mass peaks identi-

11 8 High-Altitude Production of Titan s Aerosols 11 Table 8.4 Comparison of mixing ratios from the INMS with several models Species Magee et al. (009) Krasnopolsky (009) Carrasco et al. (008) Lavvas (008a) De La Haye et al. (008) Vuitton et al. (007) Vuitton et al. (006) Wilson and Atreya (004) Toublanc et al. (1995) Lara et al. (1996) Yung et al. (1984) Max Min Max Min Max Min HCN Max Min Max Min H 8 Max Min C 4 Max Min N Max Min H N Max Min H 3 CN Max Min H 5 CN Max Min C 6 Max Min C 7 H 8 Max Min Source: From Magee et al. (009), with permission from Elsevier.

12 1 J.H. Waite et al. Table 8.5. Comparison of CIRS data with several models. CIRS data Models Vinatier (007) Teanby (007) Krasnopolsky (009) Lavvas (008) Wilson and Atreya (004) Lara (1996) Toublanc (1995) Yung (1984) Max Min Max Min Max Min HCN Max Min H 8 Max Min C 4 Max Min H N Max Min C Fig Cartoon illustrating tholin formation as result of high-altitude ion-neutral chemistry. Over tens of millions of years methane is, with the loss of hydrogen to space, irreversibly converted to the complex hydrocarbon-nitrile compounds that are the precursors of the aerosols (adapted from Waite et al. (007) with permission from AAAS) Fig Comparison of INMS spectrum and laboratory simulations. Solid line and red symbols are INMS observations of Titan s atmosphere at an altitude of 1,000 km (Waite et al. 005). Blue and gray symbols are laboratory results at 4 and 1,700 Pa respectively. The general comparability of the different results suggests that properly conducted laboratory simulations will be able to assist in understanding the INMS spectrum.

13 8 High-Altitude Production of Titan s Aerosols 13 Fig. 8.1 The correspondence between the peaks observed in the high-mass positive ions from the IBS/INMS cross correlation and a mass spectrum of laboratory tholins. The second peak, which would correspond to the C9 group, is similar in both cases. The top panel (reprinted from Crary et al. (009) ), and the bottom panel (adapted from Imanaka et al. (004), with permission from Elsevier) fied in the IBS ion mass spectrum in Fig While they are suggestive, the results indicate that the very low pressure ion-neutral chemistry present in Titan s upper atmosphere has not been adequately explored in the laboratory and should be the focus of future investigations. Sittler et al. (009) have carefully examined laboratory processes that lead to fullerence formation and their results suggest further lines of inquiry. The richness of Titan s chemical environment discovered by Cassini, and the complexity of chemical processes being explored in the laboratory together point to the need for further exploration in both arenas. In the case of Cassini this will take the form of an additional 60 encounters with the fascina ting atmosphere of Titan during the Cassini Solstice Mission. References Atreya S, Donahue T, Kuhn W (1978) Evolution of a nitrogen atmosphere on Titan. Science 01: Bar-Nun A, Kleinfeld I, Ganor E (1988) Shape and optical properties of aerosols formed by photolysis of acetylene, ethylene, and hydrogen cyanide. J Geophys Res 93:115 1 Broadfoot A, Sandel B, Shemansky D, Holberg J, Smith G et al (1981) Extreme ultraviolet observations from Voyager 1 encounter with Saturn. Science 1:06 11 Cabane M, Chassefière E (1995) Laboratory simulations of Titan s atmosphere: organic gases and aerosols. Planet Space Sci 43:47 65 Carrasco N, Plessis S, Dobrijevic M, Pernot P (008) Toward a reduction of the bimolecular reaction model for titan s ionosphere. Int J Chem Kinet 40: Chang S, Scattergood T, Aronowitz S, Flores J (1979) Organic chemistry on Titan. Rev Geophys Space Phys 17(8): Clarke D, Ferris J (1997) Titan haze: structure and properties of cyanoacetylene and cyanoacetylene acetylene photopolymers. Icarus 17: Coates AJ, Crary FJ, Lewis GR, Young DT, Waite Jr JH, Sittler Jr EC (007) Discovery of heavy negative ions in Titan s ionosphere. Geophys Res Lett 34:L103 Coates AJ, Lewis G, Wellbrock A, Jones G, Young DT et al (009) Heavy negative ions in titan s ionosphere: altitude and latitude dependence. Planet Space Sci doi: /j.pss Coll P, Coscia D, Smith N, Gazeau M, Ramrez S et al (1999) Experimental laboratory simulation of Titans atmosphere: aerosols and gas phase. Planet Space Sci 47: Coustenis A, Salama A, Schulz B, Ott S, Lellouch E et al (003) Titan s atmosphere from ISO mid-infrared spectroscopy. Icarus 161: Crary FJ, Magee BA, Mandt KE, Waite JH, Westlake JH, Young DT (009) Heavy ions, temperatures and winds in titan s ionosphere: Combined cassini caps and inms observations. Planet Space Sci. In press Danielson RE, Caldwell J, Larach DR (1973) An inversion in the atmosphere of Titan. Icarus 0: De La Haye V, Waite JH, Cravens TE, Robertson IP, Lebonnois, S (008) Coupled ion and neutral rotating model of Titan s upper atmosphere. Icarus 197: Gillett F (1975) Further observations of the 8 13 micron spectrum of Titan. Astrophys J 01:L41 Goertz CK (1989) Dusty plasmas in the solar system. Rev Geophys 7:71 9 Hanel R, Conrath B, Flasar F, Kunde V, Maguire W et al (1981) Infrared observations of the Saturnian system from Voyager 1. Science 1:19 00

14 14 J.H. Waite et al. Hanel R, Conrath B, Flasar F, Kunde V, Maguire W et al (198) Infrared observations of the Saturnian system from Voyager. Science 15: Hunten DM (1977) Titan s atmosphere and surface. In: IAU Colloq.#8: Planetary satellites. University of Arizona Press, Tucson, AZ Hunten, DM (006) The sequestration of ethane on Titan in smog particles. Nature 443: Imanaka H, Khare B, Elsila J, Bakes E, McKay C et al (004) Laboratory experiments of Titan tholin formed in cold plasma at various pressures: implications for nitrogen-containing polycyclic aromatic compounds in Titan haze. Icarus 168: Khare BN, Sagan C (1973) Red clouds in reducing atmospheres. Icarus 0: Khare BN, Sagan C, Arakawa ET, Suits F, Callcott TA, Williams MW (1984) Optical constants of organic tholins produced in a simulated Titanian atmosphere: from soft X-ray to microwave frequencies. Icarus 60: Khare B, Bakes ELO, Imanaka H, McKay C, Cruikshank D, Arakawa E (00) Analysis of the time-dependent chemical evolution of Titan haze tholin. Icarus 160: Krasnopolsky V (009) A photochemical model of Titan s atmosphere and ionosphere. Icarus 01:6 56 Kunde VG, Aikin AC, Hanel RA, Jennings DE, Maguire WC, Samuelson RE (1981) C 4, H N and N in Titan s Atmosphere. Nature 9: Lara LM, Lellouch E, Lopez-Moreno JJ, Rodrigo R (1996) Vertical distribution of Titan s atmospheric neutral constituents. J Geophys Res 101: Lavvas PP, Coustenis A, Vardavas IM (008a) Coupling photochemistry with haze formation in Titan s atmosphere, Part I: Model description. Planet Space Sci 56:7 66 Lavvas PP, Coustenis A, Vardavas IM (008b) Coupling photochemistry with haze formation in Titan s atmosphere, Part II: Results and validation with Cassini/Huygens data. Planet Space Sci 56:67 99 Lavvas P, Yelle R, Vuitton, V (009) The detached haze layer in Titan s mesosphere. Icarus 01: Lebonnois S, Bakes ELO, McKay C (00) Transition from gaseous compounds to aerosols in Titan s atmosphere. Icarus 159: Liang M, Yung Y, Shemansky D (007) Photolytically generated aerosols in the mesosphere and thermosphere of Titan. Astrophys J Lett 661:199 0 McKay CP, Coustenis A, Samuelson RE, Lemmon MT, Lorenz RD et al (001) Physical properties of the organic aerosols and clouds on Titan. Planet Space Sci 49:79 99 Magee BA, Waite JH, Mandt KE, Bell J, Westlake J, Gell DA, De La Haye V (009) INMS derived composition of titan s upper atmosphere: Analysis methods and model comparison. Planet Space Sci. In press doi: /j.pss Maguire WC, Hanel RA, Jennings DE, Kunde VG, Samuelson RE (1981) H 8 ad in Titan s Atmosphere. Nature 9: Mandt KE, Waite JH, Lewis W, Lunine J, Magee B (009) Isotopic evolution of titan s main constituents. Planet Space Sci. In press doi: /j.pss Podolak M, Bar-Nun A (1979) A constraint on the distribution of Titan s atmospheric aerosol. Icarus 39:7 76 Sagan C, Khare BN (1979) Tholins organic chemistry of interstellar grains and gas. Nature 77: Scattergood T, Owen T (1977) On the sources of ultraviolet absorption in spectra of Titan and the outer planets. Icarus 30: Scattergood TW, Lau EY, Stone BM (199) Titan s aerosols. I Laboratory investigations of shapes, size distributions, and aggregation of particles produced by UV photolysis of model Titan atmospheres. Icarus 99: Sittler EC, Jr, Ali A, Cooper JF, Hartle RE, Johnson RE, Coates AJ, Young DT (009) Heavy ion formation in Titan's ionosphere: Magnetospheric introduction of free oxygen and a source of Titan's aerosols? Planet. Space Sci. (009) doi: /j.pss Smith B, Soderblom L, Beebe R, Boyce J, Briggs G et al (1981) Encounter with Saturn: Voyager 1 imaging science results. Science 1: Smith G, Strobel D, Broadfoot A, Sandel B, Shemansky D, Holberg J (198) Titan s upper atmosphere: Composition and temperature from the EUV solar occultation results. J Geophys Res 87: Teanby NA, Irwin PG, de Kok R, Vinatier S, Bézard B et al (007) Vertical profiles of HCN, H N, and in Titan s atmosphere derived from Cassini/CIRS data. Icarus 186: Thompson RW, Henry T, Schwartz J, Khare B, Sagan C (1991) Plasma discharge in N C at low pressures: experimental results and applications to Titan. Icarus 90:57 73 Thompson RW, McDonald G, Sagan C (1994) The Titan haze revisited: magnetospheric energy sources and quantitative tholin yields. Icarus 11: Tomasko MG, Doose L, Engel S, Dafoe LE, West R et al (008) A model of Titan s aerosols based on measurements made inside the atmosphere. Planet Space Sci 56: Toublanc D, Parisot JP, Brillet J, Gautier D, Raulin F, McKay CP (1995) Photochemical modeling of Titan s atmosphere. Icarus 113: 6 Trafton L (1975) Near-infrared spectrophotometry of Titan. Icarus 4: Trainer M, Pavlov A, Jimenez J, McKay C, Worsnop D et al (004) Chemical composition of Titan s haze: are PAHs present? Geophys Res Lett 31:17 17 Tyler GL, Eshleman VR, Anderson JD, Levy GS, Lindal GF et al (1981) Radio science investigations of the Saturn system with Voyager 1 Preliminary results. Science 1:01 06 Veverka J (1973) Titan: Polarimetric evidence for an optically thick atmosphere? Icarus 18: Vinatier S, Bezard B, Fouchet T, Teanby N, Kok R et al (007) Vertical abundance profiles of hydrocarbons in Titan s atmosphere at 15 S and 80 N retrieved from Cassini/CIRS spectra. Icarus 188: Vuitton V, Yelle RV, Anicich VG (006) The nitrogen chemistry of Titan s upper atmosphere revealed. Astrophys J Lett 647: Vuitton V, Yelle RV, McEwan MJ (007) Ion chemistry and N-containing molecules in Titan s upper atmosphere. Icarus 191:7 74 Vuitton V, Lavvas P, Yelle R, Galand M, Wellbrock A et al (009) Negative ion chemistry in titan s upper atmosphere. Planet Space Sci. In press doi: /j.pss Wahlund J-E, Galand M, Mueller-Wodarg I, Cui J, Yelle RV et al (009) On the amount of heavy molecular ions in Titan s ionosphere. Planet Space Sci. In press Waite JH, Lewis WS, Kasprzak WT, Anicich VG, Block BP et al (004) The Cassini Ion and Neutral Mass Spectrometer (INMS) investigation. Space Sci Rev 114: Waite JH, Niemann H, Yelle RV, Kasprzak W, Cravens T et al (005) Ion neutral mass spectrometer results from the first flyby of Titan. Science 308: Waite JH, Young DT, Cravens TE, Coates AJ, Crary FJ, Magee B, Westlake, J (007) The process of tholin formation in Titan s upper atmosphere. Science 316:870 Wilson EH, Atreya SK (003) Chemical sources of haze formation in Titan s atmosphere. Planet Space Sci 51: Wilson EH, Atreya SK (004) Current state of modeling the photochemistry of Titan s mutually dependent atmosphere and ionosphere. J Geophys Res 109:E0600 Yung YL, Allen M, Pinto JP (1984) Photochemistry of the atmosphere of Titan comparison between model and observations. Astrophys J Suppl. S. 55: Zellner B (1973) The polarization of Titan. Icarus 18: