Titan's Haze Research Articles

Physical properties of the organic aerosols and clouds on Titan

Titan's haze is optically thick in the visible, with an optical depth at 0.5 μm of about three. The haze varies with latitude in a seasonal cycle and has a detached upper layer. Microphysical models, photochemical models, and laboratory simulations all imply that the production rate of the haze is in the range of 0.5– 2×10 −14 g cm −2 s −1 . Given the rate of sedimentation, the total mass loading is about 250 mg m −2 . The transparency of the haze is high for wavelengths above 1 μm because the haze material becomes almost purely scattering and the optical depth decreases with increasing wavelength. The particles in the main haze deck are probably fractal in structure with an equivalent volume radius of 0.2 μm . The haze material is organic and, if similar to laboratory tholin, has a C/N ratio in the range of 2–4 and a C/H ratio of about unity. The haze significantly affects the thermal balance of Titan, causing an antigreenhouse effect that cools the surface by 9 K. Titan's faintly banded appearance suggests strong zonal winds in the lower stratosphere. Condensate clouds of ethane or methane, if present, are thin, patchy, or transient. Stratospheric clouds of condensed nitriles and (possibly) hydrocarbons appear to be associated with, though not contained entirely in, the polar shadow, suggesting abundances may vary with the season. Precipitating condensate particles from the stratosphere probably act as nucleating centers for the formation and rapid growth of methane ice particles in the troposphere, where the gas phase appears to be highly supersaturated. Once formed, fallout times for these hailstones are ∼2 h or less. Melting, and possible subsequent fragmentation of methane raindrops should occur at ∼12 km and below. Almost complete evaporation should occur just above the surface. A thin residue of ethane-enriched fog particles would then slowly settle to the surface, steadily modifying an existing surface or subsurface residue of liquid hydrocarbons. The optical properties of the haze in the 1 to 3 μm spectral region and the implications for the visibility of the surface are probably the most pressing current research questions. Other key questions include the nature of the high altitude detached haze layer, altitude and seasonal changes in composition of the haze, the role of haze particles as condensation nuclei for clouds, and the nature of any condensate clouds.

Titan: High-Resolution Speckle Images from the Keck Telescope

Saturn's large moon Titan is unique among planetary satellites in that it possesses a thick atmosphere and a haze layer that is opaque to visible light. This haze is believed to be composed of organic compounds produced by the photolysis of methane. It has been suggested that the photochemical products of methane photolysis, primarily ethane, would “rain out” over time and may produce reservoirs of liquid hydrocarbons on Titan's surface. Such material would appear very dark, with an albedo ≤0.02 (Khare et al. 1990, Bull. Am. Astron. Soc.22, 1033). Such low-albedo regions have not been previously detected on Titan's surface. Here we report observations of Titan at a resolution of 0.04 arcsec (0.02 arcsec/pixel) using the technique of speckle imaging from the 10-m Keck I Telescope. By observing Titan at specific infrared wavelengths which are windows through its atmosphere, we have made both an albedo map of Titan's surface at 1.6 and 2.1 μm and an estimate of Titan's haze optical depth at these wavelengths. We clearly distinguish low-albedo features (reflectance <0.05) on Titan's surface.

The Ultraviolet Albedo of Titan

Ultraviolet spectra of Titan obtained with the Faint Object Spectrograph of the Hubble Space Telescope in October 1991 and August 1992 have yielded a disk-average geometric albedo of 0.02–0.044 from 1800–3300 Å. These results are in excellent agreement with previous UV measurements from 2200–3300 Å spanning two decades, but have a significantly higher signal to noise ratio and spectral resolution. We add ∼400 Å of new spectral information from 1800–2200 Å, a wavelength region in which Titan has not been detected previously. The albedo above 2200 Å is a factor of 1.18 higher in August 1992 than October 1991, which is most likely due to the 4.3° difference in Titan's phase angle between the two dates. Below 2200 Å the albedo decreases only modestly with decreasing wavelength and does not show unambiguous evidence for discrete spectral features characteristic of several of the known hydrocarbons, particularly acetylene (C2H2). Comparison of the albedo with best-fit models provides constraints on the optical properties and distribution of the small particle component of Titan's haze and on the C2H2mole fraction. The small particle haze seems to be darker in the UV than the “Titan” tholins analog produced in the laboratory, and the vertical extent limited to altitudes above ∼120–150 km. The acetylene distribution is consistent with the Voyager IRIS determination in the 130- to 180-km altitude range, where the mole fraction is of order 2 × 10−6.

Titan Haze: Structure and Properties of Cyanoacetylene and Cyanoacetylene–Acetylene Photopolymers

The structure and morphological properties of polymers produced photochemically from the UV irradiation of cyanoacetylene and cyanoacetylene mixtures have been examined to evaluate their possible contribution to the haze layers found on Titan. A structural analysis of these polymers may contribute to our understanding of the data returned from the Huygens probe of the Cassini mission that will pass through the atmosphere of Titan in the year 2004. Infrared analysis, elemental analysis, and thermal methods (thermogravimetric analysis, thermolysis, pyrolysis) were used to examine structures of polycyanoacetylenes produced by irradiation of the gas phase HC 3N at 185 and 254 nm. The resulting brown to black polymer, which exists as small particles, is believed to be a branched chain of conjugated carbon–carbon double bonds, which, on exposure to heat, cyclizes to form a graphitic structure. Similar methods of analysis were used to show that when HC 3N is photolyzed in the presence of Titan's other atmospheric constituents (CH 4, C 2H 6, C 2H 2, and CO), a copolymer is formed in which the added gases are incorporated as substituents on the polymer chain. Of special significance is the copolymer of HC 3N and acetylene (C 2H 2). Even in experiments where C 2H 2was absorbing nearly all of the incident photons, the ratio of C 2H 2to HC 3N found in the resulting polymer was only 2:1. Scanning electron microscopy was used to visually examine the polymer particles. While pure polyacetylene particles are amorphous spheres roughly 1 μm in diameter, polycyanoacetylenes appear to be strands of rough, solid particles slightly smaller in size. The copolymer of HC 3N and C 2H 2exhibits characteristics of both pure polymers. This is particularly important as pure polyacetylenes do not match the optical constants measured for Titan's atmospheric hazes. The copolymers produced by the incorporation of other minor atmospheric constituents, like HC 3N, into the polyacetylenes are expected to have optical constants more comparable to those of the Titan haze.

Organic chemistry in Titan's atmosphere: New data from laboratory simulations at low temperature

Many experiments have already been carried out to simulate organic chemistry on Titan, the largest satellite of Saturn. They can provide fruitful information on the nature of minor organic constituents likely to be present in Titan's atmosphere, both in gas and aerosol phases. Indeed, all the organic compounds but one already detected in Titan's atmosphere have been identified in simulation experiments. The exception, C 4N 2, as well as other compounds expected in Titan from theoretical modeling, such as other N-organics, and polyynes, first of all C 6H 2, have never been detected in experimental simulation thus far. All these compounds are thermally unstable, and the temperature conditions used during the simulation experiments were not appropriate. We have recently started a new program of simulation experiments with temperature conditions close to that of Titan's environment. It also uses dedicated analytical techniques and procedures compatible with the analysis of organics only stable at low temperatures, as well solid products of low stability in the presence of O 2 and H 2O. Spark discharge of N 2CH 4 gas mixtures was carried out at low temperature in the range 100–150 K. Products were analysed by FTIR, GC and GC-MS techniques. GC-peaks were identified by their mass spectrum, and, in most cases, by comparison of the retention time and mass spectrum with standard ones. We report here the first detection in Titan simulation experiments of C 6H 2 and HC 5N. Their abundance is a few percent relative to C 4H 2 and HC 3N, respectively. Preliminary data on the solid products indicate an elemental composition corresponding to (H 11C 11N) n. These results open new prospects in the modeling of Titan's haze making.

The Titan Haze Revisited: Magnetospheric Energy Sources and Quantitative Tholin Yields

We present laboratory measurements of the radiation yields of complex organic solids produced from N 2/CH 4 gas mixtures containing 10 or 0.1% CH 4. These tholins are thought to resemble organic aerosols produced in the atmospheres of Titan, Pluto, and Triton. The tholin yields are large compared to the total yield of gaseous products: nominally, 13 (C + N)/100 eV for Titan tholin and 2.1 (C + N)/100 eV for Triton tholin. High-energy magnetospheric electrons responsible for tholin production represent a class distinct from the plasma electrons considered in models of Titan's airglow. Electrons with E > 20 keV provide an energy flux ∼1 × 10 -2 erg cm -2 sec -1, implying from our measured tholin yields a mass flux of 0.5 to 4.0 × 10 -14 g cm -2 sec -1 of tholin. (The corresponding thickness of the tholin sedimentary column accumulated over 4 Gyr on Titan's surface is 4 to 30 m.) This figure is in agreement with required mass fluxes computed from recent radiative transfer and sedimentation models. If, however, these results, derived from experiments at ∼2 mb, are applied to lower pressure levels toward peak auroral electron energy deposition and scaled with pressure as the gas-phase organic yields, the derived tholin mass flux is at least an order of magnitude less. We attribute this difference to the fact that tholin synthesis occurs well below the level of maximum electron energy deposition and to possible contributions to tholins from UV-derived C 2-hydrocarbons. We conclude that Titan tholin, produced by magnetospheric electrons, is alone sufficient to supply at least a significant fraction of Titan's haze—a result consistent with the fact that the optical properties of Titan tholin, among all proposed materials, are best at reproducing Titan's geometric albedo spectrum from near UV to mid-IR in light-scattering models.

Effects of Time-Varying Haze Production on Titan's Geometric Albedo

Two versions of the Toon et al. (1992, Icarus 95, 24-53) Titan haze model were used to simulate the observed blue and yellow geometric albedo variations using a time-dependent haze production. Steady-state calculations were used to suggest that the range of production rate changes necessary to explain the observations was a factor of between 2 and 4. However, the blue and yellow geometric albedo variations were about 10 times smaller than the observations, when computed with a time-varying production rate with this same range. Long haze residence times were the cause of this decreased response. This result was insensitive to the details of the functional representation of the production. In addition to the amplitude of the geometric albedo, we found that the phase between albedo response and the production forcing is a critical discriminate when comparing model results to the observations. Long haze time scales also produced a phase delay in the albedo response to seasonal solar forcing which can agree with the observations (Sromovsky et al. 1981, Nature 292, 698-702). Unfortunately, the size of the phase delay was dependent on model parameters. We found that raising the level of rainout well into the stratosphere (90 km) decreased the residence time of the haze and allowed us to fit the magnitude of the albedo variation. However, the resulting phase delay (-60°) was not in agreement with observations (+90°) and such a high altitude for rainout is probably un-physical. Our results suggest that a time-varying mass production cannot explain Titan's geometric albedo variations.

Formation and growth of photochemical aerosols in Titan's atmosphere

Recent developments in our understanding of the morphology (shape and size) of haze aerosols in Titan's atmosphere, aggregate particles and their associated optical properties (West 1990, West and Smith 1991), have been considered in light of a microphysical modeling of aerosols. Using an Eulerian model rather similar to the one of Toon et al. (1980), it is shown that the growth of particles may be divided in two stages. The first one corresponds to the initial growth near the formation altitude by accretion of very small elementary particles (synthetized oligomers). Due to the large number of accreted small particles, this stage leads to the formation of nearly spherical particles (called “monomers” by West and Smith). In the second stage, these particles (“monomers”) settle in the atmosphere and stick together forming aggregates, of the same kind as those obtained experimentally by Bar-Nun et al. (1988). The classical assumption of spherical particles, which is made in all existing models, is shown to be valid during the first growth stage. The mean ratio between the masses of two particles colliding together is quantified as a function of altitude using a specific index (mass sharing index, MSI). From this criterion, the two previously defined regions (the formation region inside which monomers are generated, the settling region where aggregates build up) may be clearly separated and the boundary between them precisely defined and located. Identifying the monomer radius with the mass-averaged radius at the altitude of the boundary, it is shown that this radius is extremely sensitive to the altitude where aerosols are created. The reason is that the residence time of particles near the formation altitude, which determines to a large extent the size of monomers, is an increasing function of the pressure. The strong dependence of the monomer radius on the pressure (thus the altitude) is calculated using different sets of parameters (eddy diffusion coefficient, electrical charge, mass production rate, …) in order to cover the widest range of possibilities. Using parameters favored by West in his analysis of the polarizing properties of Titan's haze, the formation altitude of aerosols is found to lie in the range from 350 to 400 km.

Titan's surface and troposphere, investigated with ground-based, near-infrared observations

New observations of Titan's near-infrared spectrum (4000–5000 cm −1) combined with points taken from Fink and Larson's (1979) spectrum (4000–12500 cm −1) provide information on Titan's haze, possible clouds, surface albedo, and atmospheric abundance of H 2. In the near-infrared, the main features in Titan's spectrum result from absorption of solar radiation by CH 4. The strength of this absorption varies considerably with wavelength, allowing us to probe various atmospheric levels down to the surface itself by choosing specific wavelengths for analysis. At 4715 cm −1, the pressure-induced S(1) fundamental band of H 2 lies in the wings of CH 4 bands. Based on current values for the CH 4 line parameters, Titan's spectrum can be best interpreted with a volume mixing ratio of H 2 between 0.5 and 1.0%. Our observations suggest the existence of an optically thin CH 4 cloud layer. The optical depths that we derive for Titan's haze and clouds are small enough to allow us to sense the surface of Titan at 4900, 6250, and 7700 cm −1. The most plausible interpretation of the albedos determined at these wavenumbers suggests a surface dominated by “dirty” water ice. A global ethane ocean is not compatible with these albedos.

UV spectroscopy of Titan's atmosphere, planetary organic chemistry and prebiological synthesis: II. Interpretation of new IUE observations in the 220–335 nm Range

We report on new observations of Titan with the International Ultraviolet Explorer in the mid-UV range ( λ ≈ 220−335 nm). We use these data to determine upper limits for the abundances of simple organic compounds in the gas phase and to further constrain the properties of the high altitude haze on Titan. As a baseline, we adopted the parametrized microphysical model of McKay et al. (1989) which is successful at reproducing Titan's thermal structure while satisfying several other observational constraints in the visible and IR regions. However, we find that such a model—in which all particles at a given altitude are assumed to have the same size—cannot match simultaneously the IUE observations and the visible/IR data, even when allowance is made for a wide range of values in the adjustable parameters. On the other hand, a good overall agreement is obtained when considering a bimodal size distribution, with small haze particles or “polymers” ( r <0.02 μm) acting as strong Rayleigh absorbers below 300 nm and larger haze particles ( r ≈ 0.1−0.5 μm) being responsible for the characteristics of the albedo spectrum in the near-UV, visible, and IR regions. This approach is consistent with the results of several previous investigations of the properties of Titan's haze, although our preferred vertical structure for the haze + polymer material somewhat differs from earlier solutions. On the basis of simple dynamical considerations, we adopted a uniformly mixed layer between 150 and 600 km. The IUE data allow us to place fairly stringent constraints on the abundance of the Rayleigh absorbers, if we assume that their optical properties are similar to those of laboratory-synthesized “tholins”: The column-mass density of this material—the essential observable that can be determined from our study—is of the order of 5 μg cm −2. This would correspond to number-densities between 10 3 and 10 7 cm −3 in the 150–600 km altitude range, if the average particle radius is between 0.001 and 0.02 μm. Such high number densities are a priori at odds with the estimated coagulation lifetime for particles of that size. Thus, our proposed bimodal size distribution is plausible only if inhibiting processes act to slow down considerably the coagulation of polymers in Titan's stratosphere.