High-Altitude Production of Titan's Aerosols
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 1–100 Da. The IBS has measured positive ions up to 350 Da, while the ELS has detected concentrations of negative ions as high as 20% 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 2 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, 1982). 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 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. 1979 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 CH4 and C2H6, although there was evidence for the presence of C2H2 and C2H4 as well (Gillett 1975). The presence of N2, 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 C2H2, C2H4, and C3H8 as well as of the nitriles HCN, HC3N, and C2N2 (Hanel et al. 1981, 1982; 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 N2 and CH4 in Titan's upper atmosphere by electron impact (N2) and UV irradiation (CH4) (e.g., Yung et al. 1984; Toublanc et al. 1995; Wilson and Atreya 2004). 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 observations (see reviews by Cabane and Chassefière 1995 and McKay et al. 2001). Post-Voyager experiments to synthesize aerosol analogs in the laboratory involved both the production of tholins in a simulated Titan N2-CH4 atmosphere (e.g., Thompson et al. 1994; Coll et al. 1999) and the creation of the photopolymers of C2H2, C2H4, and HCN (Bar-Nun et al. 1988; Scattergood et al. 1992) as well as of HC3N and HC3N/C2H2 (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 CH4 and N2. However, as noted by Lebonnois et al. (2002), 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, (2) polycyclic aromatics, and (3) polymers of HCN and other nitriles, and polyynes. Their model suggested a total production rate of 4 × 10−14 g cm−2 s−1 and a C/N ratio of 4, in a production zone slightly lower than 200 km altitude. Wilson and Atreya (2003) 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 220 km, with a column integrated production rate of 3.2 × 10−14 g cm−2 s−1. Wilson and Atreya (2003) pointed out that the discovery of benzene in Titan's atmosphere by ISO (Coustenis et al. 2003) favored the PAH pathway. Trainer et al. (2004) found that for particles produced from a mixture of 10% CH4 in N2 the results were consistent with a large fraction of aromatics, including specific mass spectral peaks likely due to PAHs. However, at lower concentrations of CH4 (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. (2002) 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 24 Pa) in a continuous-flow, low-dose, cold plasma discharge excited in an atmosphere consisting of 10% CH4 and 90% N2 at 295 K. At 1,700 Pa, 59 gaseous species including 27 nitriles were detected while at 24 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 24 Pa prevailing over more H-saturated molecules at 1,700 Pa. Imanaka et al. (2004) conducted a series of experiments from high (2,300 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.
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