- Email: [email protected]
The physical nature of Titan's aerosols: Laboratory simulations
Adv. Space Res. Vol. 15, No. 3, pp. (3)313-(3)316, 1995 1994 COSPAR Printed in Great Britain. 0273-l 177/95 $7.00 + 0.00
THE PHYSICAL NATURE OF TITAN’S AEROSOLS: LABORATORY SIMULATIONS T. W. Scattergood S.U.N.Y.at Stony Brook and NASA Ames Reseurch Center
ABSTRACT The atmosphere of Titan is known to contain aerosols, as evidenced by the Voyager observations of at least three haze layers. Such aerosols can have significant effects on the reflection spectrum of Titan and on the chemistry and thermal structure of its atmosphere. To investigate some of these effects, laboratory simulations of the chemistry of Titan’s atmosphere have been done. The results of these studies show that photolysis of acetylene, ethylene, and hydrogen cyanide, known constituents of Titan’s atmosphere, yields sub-micron sized spheres, with mean diameters ranging from 0.4 to 0.8 urn, depending on the pressures of the reactant gases. Most of the spheres are contained in nearlinear aggregates. The formation of the aggregates is consistent with models of Titan’s reflection spectrum and polarization, which are best fit with non-spherical particles. At room temperature, the particles are very sticky, but their properties at low temperatures on Titan are presently not known. INTRODUCTION Titan’s atmosphere is known to contain aerosols, as evidenced by the observation of at least three haze layers by the Voyager spacecraft. One of these layers is the ubiquitous yellow-orange cloud deck that envelops the satellite. As particles can be effective absorbers and scatterers of light, the importance of aerosols in affecting the reflection spectrum (albedo) of Titan has long been recognized /l/. These aerosols must have a significant influence on the propagation of solar radiation through the atmosphere and are probably responsible for the high temperature of the stratosphere /2/. They also have a significant effect on the atmospheric thermal profile and surface temperature /2/, and may play important roles in local chemistry. At present, little is known about the physical and chemical properties of Titan’s atmospheric aerosols, Some constraints have been placed on the sizes and optical properties of the particles through analysis and computational modeling of Titan’s geometric albedo /3/ and phase-angle observations /l/. Analyses of the polarization of the light reflected from Titan back to Earth suggests that the particles near and below the visible limb are smaller than about 0.1 pm in radius /4/. Examination of high-phase-angle spectra indicates that larger particles between 0.2 and 0.5 urn in radius must be present near and above the limb /l/. The results of West and Smith /5/ and new analyses by Courtin et al. /6/ of International Ultraviolet Explorer (IUE) observations indicate that a population of very small particles, about 0.02 pm in radius, must also be present in Titan’s atmosphere. Non-spherical particles have been suggested to explain the discrepancy between the polarization and phase-angle data /5/. Modeling of the scattering properties of aggregate particles, using the optical properties of a laboratory synthesized material, called tholin, made by sparking mixtures of methane (CH4) and nitrogen (N2) /7/, support this hypothesis /5/. However, the results of recent modeling by Toon et al. /3/ show that the discrepancy could be resolved by invoking a layer of large (r > 0.15 pm) particles overlying the main (colored) haze layer, a scenario suggested earlier by Tomasko and Smith /4/. Also, the modeling results suggest that the mean radius of the particles in the bulk of Titan’s haze
(3)314
T. W. Scattergood
layer is =: 0.15 pm, which is consistent with the particle size determined from analyses of polarization observations 141. However, this size is too small to explain the phase-angle observations /I/. Different particles at different altitudes in the atmosphere must be responsible for the two sets of observations. How are these hazes made? To date there is no information that directly answers this question. However, many laboratory simulations and computational studies have been done that place limits on the answer. As sunlight is most certainly the dominant source of energy in Titan’s atmosphere, photochemistry of simple atmospheric species, primarily CH4, and their decomposition products is most likely the process responsible for the production of aerosols, and a detailed but limited model of the photochemistry in Titan’s atmosphere has been developed /8/. Photolysis of CH4 leads to the production of species, such as acetylene (C2H2) and ethylene (C2H4), that can readily polymerize to form particles. However, a careful determination of the optical properties that best reproduce the behavior of Titan’s geometric albedo in the range from 0.3 to 1.1 pm indicates that the tholin made by electrical discharge /7/ is a better analog of Titan’s aerosols than are either pure polyacetylene or polyethylene /9/. Also, the means by which the nitriles (CN compounds) are made is still not known. Photodissociation of N2 to form species that can directly react with CH4 and other hydrocarbons is a minor process at best, due to the low solar flux at h < 1lOOA where N2 absorbs. However, cosmic rays and energetic particles from Saturn’s magnetosphere can dissociate N2 to make HCN and the other CN compounds, which can then be further processed to make aerosols containing materials that may be more like the spark tholin. Perhaps in this way aerosols with the proper optical properties can be made, but this issue is certainly not yet resolved (Bar-Nun, personal communication). Following the lead of Bar-Nun et al, who in 1988 reported the production of sub-micron sized spheres from the photolysis of C2H2, C2H4, or HCN /lo/, I report here some of the results of an experimental program to investigate the production of aerosols from photolysis of Titan-like atmospheres. Although there is some question about using polyacetylenes as analogs for Titan’s aerosols, these compounds have been proposed as possible aerosol constituents /9,10/ and, in terms of physical formation and behavior, production of these compounds via photolysis of C2H2, etc. may provide useful insights about the physical nature of Titan’s aerosols. EXPERIMENTAL PROCEDURE Various mixtures of C2H2. C2H4, and HCN in N2 or Helium (He) were prepared in either a cylindrical cell (4.5 cm o.d. by 25.4 cm long) or 5 liter glass bulb. Both vessels were fitted with Spectrosil quartz windows to allow admission of 1849A (and 2537A) light from a microwave-driven low pressure mercury lamp. The output of the lamp at 1849A was monitored by ammonia (NH3) actinometry and was found to be typically 7 x 10 5 photons set-’ into the gas. The exposure time was typically 1 hour. Onset of particle production was determined by the appearance of light scattered from the beam of a He/Ne laser (h = 6328A). The particles were allowed to settle onto clean glass disks placed at the bottom of the cylinder or the bulb. The disks were carefully removed from the reaction vessels and quickly coated under vacuum with a few Angstrom thick layer of gold to preserve the particles and to enable imaging by scanning electron microscopy (SEM). Polaroid photographs were taken of the SEM images at magnifications up to 30,000x. The diameters of the individual particles and the numbers of particles in the aggregates were determined by direct measurement from the photographs. These studies were carried out with the help of Dr. Bradley Stone and Samuel Clegg (SJSU) and Edmond Lau (UCSB). RESULTS The individual particles produced from the photolysis of C2H2 at pressures ranging from 10 torr down to 0.01 torr in 55 torr N2 were completely spherical, apparently amorphous, and quite sticky. A typical example of the particles is shown in Figure 1. As can be seen from the figure, both single particles and numerous aggregates with up to at least 10 units were made. The formation of the particles was observed within a few minutes of the onset of irradiation for all pressures of C2H2
Physical Nature of Titan’s Aerosols
except 0.01 torr for which no scattering from the laser beam was observed. indeed produced, however, and a few were found on the glass SEM disk.
(3)315
Spherical particles were
Fig. 1. Polyacetylene particles produced by 1849A photolysis of 10 torr C2H2 in 55 torr N2 The diameters of the unit spheres (particles) range from 0.2 to 1.8 pm with a mean (lognormal) of 0.77 f 0.3 pm (1 KS). Magnification in this figure is 9000x. The mean diameters of the polyacetylene particles were found to decrease with decreasing initial pressure of C2H2 For our experiments, the mean diameters were about 0.8 pm at 10 torr, 0.6 pm at 1 torr, and 0.4 pm at 0.1 torr. The particle sizes were found to be best fit with Gaussian (normal) distributions except for the 10 torr case, for which a lognormal distribution gave the best fit. This latter behavior was also seen in the data of Bar-Nun et al. /lo/. The reason for this change in shape of the size distribution is not presently known. One experiment with an initial pressure of C2H2 of 0.01 torr has also been done. 17 particles were found, with a mean diameter of 1.2 pm. The reason for this much larger size than were found for higher pressures of C2H2 is not known but, hopefully, should be by the time this paper is published. All of the experiments described above were done with C2H2 as the only reactant gas. Since the atmosphere of Titan is known to contain other compounds, such as C2H4 and HCN, some experiments were done in which these compounds were included with the C2H2. The particles produced in these experiments were very similar in appearance and size to those produced from C2H2 alone. However, the distribution of particle sizes was much broader, that is, the particles sizes were more evenly distributed over the observed size range (0.1 - 1.2 urn). An important property of the particles is that they appear to be quite sticky, at least at room temperature. To visually verify this, the SEM disk was tilted 70’ from the direction of the electron beam and detectors in one experiment, in essence providing a side view. The aggregates were observed to be mostly standing on end rather than lying flat on the disk, as they would be if they did not stick to each other. This stickiness may also account for the tendency for the particles to form aggregates rather than to remain as single particles. Also, the aggregates were mostly near-linear chains and not spheroidal clumps, suggesting that their formation was effected by motions in the background gas (an effect that needs to be investigated further). DISCUSSION AND FUTURE STUDIES Because of the growing recognition of the importance of aerosols to both the energetic and chemistry
(3)316
T. W. Scattergood
of planetary atmospheres, there has been increasing experimental and theoretical efforts to model the formation, growth, and properties of aerosols. Microphysical models of these processes and analyses of appropriate observations have placed some constraints on aerosols properties, such as sizes and optical properties, as noted in the Introduction. Based upon the laboratory studies of Bar-Nun et al. /lo/ and of this author, it is clear that photolysis of reactive constituents of Titan’s atmosphere will produce sub-micron sized spheres and aggregates of these spheres, at room temperature. The formation of non-spherical aggregates is consistent with current models of Titan’s polarization and geometric albedo. Although very small particles (r < 0.1 urn) have yet to be observed in the experiments, they must certainly be formed as intermediates between molecules (r = l-2 A) and the observed particles (r - 0.3 pm). The absence of such small particles may be due to their continued growth during settling and collection on the SEM plates. The properties of aerosols formed at low temperatures appropriate for Titan and the outer planets are Most organic (polymeric) materials become hard and brittle at low not presently known. temperatures, e.g., 77 K, thus aerosols present in outer planet atmospheres may not be spherical or may not aggregate very well. An experimental program to investigate this is being developed at NASA Ames and some results should be available by the time of publication of this article. Finally, there remains the apparent dilemma of chemical nature of Titan’s aerosols. While photolysis of the simple organic constituents to produce polymeric compounds seems the most likely source for the aerosols, optical properties of materials made from electrical discharge seem to give a better tit to Titan’s visible albedo than do optical properties of, e.g., polyacetylene. Perhaps Titan’s aerosols are some mixture of compounds made by photolysis and irradiation by energetic particles. REFERENCES 1.
K. Rages and J.B. Pollack, Vertical,distribution of scattering hazes in Titan’s upper atmosphere, Icarus 55, 50 (1983).
2.
C.P. McKay, J.B. Pollack, and R. Courtin, The thermal structure of Titan’s atmosphere, Icarus 80, 23 (1989).
3.
O.B. Toon, C.P. McKay, C.A. Griffith, and R.P. Turco, A physical model of Titan’s aerosols, Icarus 95, 24 (1992).
4.
M.G. Tomasko and P.H. Smith, Photometry and polarimetry of Titan: Pioneer 11 observations and their implications for aerosol properties, Icarus 51, 65 (1982).
5.
R.A. West and P.H. Smith, Evidence for aggregate particles in the atmospheres of Titan and Jupiter, Icarus 90, 330 (1991).
6.
R. Courtin, R. Wagener, C.P. McKay, J. Caldwell, K.-H. Fricke, F. Raulin, and P. Bruston, UV spectroscopy of Titan’s atmosphere, planetary organic chemistry, and prebiological synthesis. II. Interpretation of new IUE observations in the 220-335 nm range, Icarus 90, 43 (1991).
7.
B.N. Khare, C. Sagan, E.T. Arakawa, F. Suits, T.A. Callcott, and B. Nagy, Optical constants of From x-ray to microwave organic tholins produced in a simulated Titan atmosphere: frequencies, Icarus 60, 127 (1984).
8.
Y.L. Yung, M.A. Allen, and J.P. Pinto, Photochemistry of the atmosphere of Titan: Comparison between models and observations, Astrophys. J. Suppl. 55, 465 (1984).
9.
C.P. McKay, O.B. Toon, and R. Courtin, Titan’s Albedo, 0.2 to 1 urn: Laboratory Analogs and Power-Laws, B. A. A. S. 24, 949 (1992).
10. A. Bar-Nun, I. Kleinfeld, and E. Ganor, Shape and optical properties of aerosols formed by photolysis of acetylene, ethylene, and hydrogen cyanide, J. Geophys. Res. 93, 8383 (1988).
THE PHYSICAL NATURE OF TITAN’S AEROSOLS: LABORATORY SIMULATIONS T. W. Scattergood S.U.N.Y.at Stony Brook and NASA Ames Reseurch Center
ABSTRACT The atmosphere of Titan is known to contain aerosols, as evidenced by the Voyager observations of at least three haze layers. Such aerosols can have significant effects on the reflection spectrum of Titan and on the chemistry and thermal structure of its atmosphere. To investigate some of these effects, laboratory simulations of the chemistry of Titan’s atmosphere have been done. The results of these studies show that photolysis of acetylene, ethylene, and hydrogen cyanide, known constituents of Titan’s atmosphere, yields sub-micron sized spheres, with mean diameters ranging from 0.4 to 0.8 urn, depending on the pressures of the reactant gases. Most of the spheres are contained in nearlinear aggregates. The formation of the aggregates is consistent with models of Titan’s reflection spectrum and polarization, which are best fit with non-spherical particles. At room temperature, the particles are very sticky, but their properties at low temperatures on Titan are presently not known. INTRODUCTION Titan’s atmosphere is known to contain aerosols, as evidenced by the observation of at least three haze layers by the Voyager spacecraft. One of these layers is the ubiquitous yellow-orange cloud deck that envelops the satellite. As particles can be effective absorbers and scatterers of light, the importance of aerosols in affecting the reflection spectrum (albedo) of Titan has long been recognized /l/. These aerosols must have a significant influence on the propagation of solar radiation through the atmosphere and are probably responsible for the high temperature of the stratosphere /2/. They also have a significant effect on the atmospheric thermal profile and surface temperature /2/, and may play important roles in local chemistry. At present, little is known about the physical and chemical properties of Titan’s atmospheric aerosols, Some constraints have been placed on the sizes and optical properties of the particles through analysis and computational modeling of Titan’s geometric albedo /3/ and phase-angle observations /l/. Analyses of the polarization of the light reflected from Titan back to Earth suggests that the particles near and below the visible limb are smaller than about 0.1 pm in radius /4/. Examination of high-phase-angle spectra indicates that larger particles between 0.2 and 0.5 urn in radius must be present near and above the limb /l/. The results of West and Smith /5/ and new analyses by Courtin et al. /6/ of International Ultraviolet Explorer (IUE) observations indicate that a population of very small particles, about 0.02 pm in radius, must also be present in Titan’s atmosphere. Non-spherical particles have been suggested to explain the discrepancy between the polarization and phase-angle data /5/. Modeling of the scattering properties of aggregate particles, using the optical properties of a laboratory synthesized material, called tholin, made by sparking mixtures of methane (CH4) and nitrogen (N2) /7/, support this hypothesis /5/. However, the results of recent modeling by Toon et al. /3/ show that the discrepancy could be resolved by invoking a layer of large (r > 0.15 pm) particles overlying the main (colored) haze layer, a scenario suggested earlier by Tomasko and Smith /4/. Also, the modeling results suggest that the mean radius of the particles in the bulk of Titan’s haze
(3)314
T. W. Scattergood
layer is =: 0.15 pm, which is consistent with the particle size determined from analyses of polarization observations 141. However, this size is too small to explain the phase-angle observations /I/. Different particles at different altitudes in the atmosphere must be responsible for the two sets of observations. How are these hazes made? To date there is no information that directly answers this question. However, many laboratory simulations and computational studies have been done that place limits on the answer. As sunlight is most certainly the dominant source of energy in Titan’s atmosphere, photochemistry of simple atmospheric species, primarily CH4, and their decomposition products is most likely the process responsible for the production of aerosols, and a detailed but limited model of the photochemistry in Titan’s atmosphere has been developed /8/. Photolysis of CH4 leads to the production of species, such as acetylene (C2H2) and ethylene (C2H4), that can readily polymerize to form particles. However, a careful determination of the optical properties that best reproduce the behavior of Titan’s geometric albedo in the range from 0.3 to 1.1 pm indicates that the tholin made by electrical discharge /7/ is a better analog of Titan’s aerosols than are either pure polyacetylene or polyethylene /9/. Also, the means by which the nitriles (CN compounds) are made is still not known. Photodissociation of N2 to form species that can directly react with CH4 and other hydrocarbons is a minor process at best, due to the low solar flux at h < 1lOOA where N2 absorbs. However, cosmic rays and energetic particles from Saturn’s magnetosphere can dissociate N2 to make HCN and the other CN compounds, which can then be further processed to make aerosols containing materials that may be more like the spark tholin. Perhaps in this way aerosols with the proper optical properties can be made, but this issue is certainly not yet resolved (Bar-Nun, personal communication). Following the lead of Bar-Nun et al, who in 1988 reported the production of sub-micron sized spheres from the photolysis of C2H2, C2H4, or HCN /lo/, I report here some of the results of an experimental program to investigate the production of aerosols from photolysis of Titan-like atmospheres. Although there is some question about using polyacetylenes as analogs for Titan’s aerosols, these compounds have been proposed as possible aerosol constituents /9,10/ and, in terms of physical formation and behavior, production of these compounds via photolysis of C2H2, etc. may provide useful insights about the physical nature of Titan’s aerosols. EXPERIMENTAL PROCEDURE Various mixtures of C2H2. C2H4, and HCN in N2 or Helium (He) were prepared in either a cylindrical cell (4.5 cm o.d. by 25.4 cm long) or 5 liter glass bulb. Both vessels were fitted with Spectrosil quartz windows to allow admission of 1849A (and 2537A) light from a microwave-driven low pressure mercury lamp. The output of the lamp at 1849A was monitored by ammonia (NH3) actinometry and was found to be typically 7 x 10 5 photons set-’ into the gas. The exposure time was typically 1 hour. Onset of particle production was determined by the appearance of light scattered from the beam of a He/Ne laser (h = 6328A). The particles were allowed to settle onto clean glass disks placed at the bottom of the cylinder or the bulb. The disks were carefully removed from the reaction vessels and quickly coated under vacuum with a few Angstrom thick layer of gold to preserve the particles and to enable imaging by scanning electron microscopy (SEM). Polaroid photographs were taken of the SEM images at magnifications up to 30,000x. The diameters of the individual particles and the numbers of particles in the aggregates were determined by direct measurement from the photographs. These studies were carried out with the help of Dr. Bradley Stone and Samuel Clegg (SJSU) and Edmond Lau (UCSB). RESULTS The individual particles produced from the photolysis of C2H2 at pressures ranging from 10 torr down to 0.01 torr in 55 torr N2 were completely spherical, apparently amorphous, and quite sticky. A typical example of the particles is shown in Figure 1. As can be seen from the figure, both single particles and numerous aggregates with up to at least 10 units were made. The formation of the particles was observed within a few minutes of the onset of irradiation for all pressures of C2H2
Physical Nature of Titan’s Aerosols
except 0.01 torr for which no scattering from the laser beam was observed. indeed produced, however, and a few were found on the glass SEM disk.
(3)315
Spherical particles were
Fig. 1. Polyacetylene particles produced by 1849A photolysis of 10 torr C2H2 in 55 torr N2 The diameters of the unit spheres (particles) range from 0.2 to 1.8 pm with a mean (lognormal) of 0.77 f 0.3 pm (1 KS). Magnification in this figure is 9000x. The mean diameters of the polyacetylene particles were found to decrease with decreasing initial pressure of C2H2 For our experiments, the mean diameters were about 0.8 pm at 10 torr, 0.6 pm at 1 torr, and 0.4 pm at 0.1 torr. The particle sizes were found to be best fit with Gaussian (normal) distributions except for the 10 torr case, for which a lognormal distribution gave the best fit. This latter behavior was also seen in the data of Bar-Nun et al. /lo/. The reason for this change in shape of the size distribution is not presently known. One experiment with an initial pressure of C2H2 of 0.01 torr has also been done. 17 particles were found, with a mean diameter of 1.2 pm. The reason for this much larger size than were found for higher pressures of C2H2 is not known but, hopefully, should be by the time this paper is published. All of the experiments described above were done with C2H2 as the only reactant gas. Since the atmosphere of Titan is known to contain other compounds, such as C2H4 and HCN, some experiments were done in which these compounds were included with the C2H2. The particles produced in these experiments were very similar in appearance and size to those produced from C2H2 alone. However, the distribution of particle sizes was much broader, that is, the particles sizes were more evenly distributed over the observed size range (0.1 - 1.2 urn). An important property of the particles is that they appear to be quite sticky, at least at room temperature. To visually verify this, the SEM disk was tilted 70’ from the direction of the electron beam and detectors in one experiment, in essence providing a side view. The aggregates were observed to be mostly standing on end rather than lying flat on the disk, as they would be if they did not stick to each other. This stickiness may also account for the tendency for the particles to form aggregates rather than to remain as single particles. Also, the aggregates were mostly near-linear chains and not spheroidal clumps, suggesting that their formation was effected by motions in the background gas (an effect that needs to be investigated further). DISCUSSION AND FUTURE STUDIES Because of the growing recognition of the importance of aerosols to both the energetic and chemistry
(3)316
T. W. Scattergood
of planetary atmospheres, there has been increasing experimental and theoretical efforts to model the formation, growth, and properties of aerosols. Microphysical models of these processes and analyses of appropriate observations have placed some constraints on aerosols properties, such as sizes and optical properties, as noted in the Introduction. Based upon the laboratory studies of Bar-Nun et al. /lo/ and of this author, it is clear that photolysis of reactive constituents of Titan’s atmosphere will produce sub-micron sized spheres and aggregates of these spheres, at room temperature. The formation of non-spherical aggregates is consistent with current models of Titan’s polarization and geometric albedo. Although very small particles (r < 0.1 urn) have yet to be observed in the experiments, they must certainly be formed as intermediates between molecules (r = l-2 A) and the observed particles (r - 0.3 pm). The absence of such small particles may be due to their continued growth during settling and collection on the SEM plates. The properties of aerosols formed at low temperatures appropriate for Titan and the outer planets are Most organic (polymeric) materials become hard and brittle at low not presently known. temperatures, e.g., 77 K, thus aerosols present in outer planet atmospheres may not be spherical or may not aggregate very well. An experimental program to investigate this is being developed at NASA Ames and some results should be available by the time of publication of this article. Finally, there remains the apparent dilemma of chemical nature of Titan’s aerosols. While photolysis of the simple organic constituents to produce polymeric compounds seems the most likely source for the aerosols, optical properties of materials made from electrical discharge seem to give a better tit to Titan’s visible albedo than do optical properties of, e.g., polyacetylene. Perhaps Titan’s aerosols are some mixture of compounds made by photolysis and irradiation by energetic particles. REFERENCES 1.
K. Rages and J.B. Pollack, Vertical,distribution of scattering hazes in Titan’s upper atmosphere, Icarus 55, 50 (1983).
2.
C.P. McKay, J.B. Pollack, and R. Courtin, The thermal structure of Titan’s atmosphere, Icarus 80, 23 (1989).
3.
O.B. Toon, C.P. McKay, C.A. Griffith, and R.P. Turco, A physical model of Titan’s aerosols, Icarus 95, 24 (1992).
4.
M.G. Tomasko and P.H. Smith, Photometry and polarimetry of Titan: Pioneer 11 observations and their implications for aerosol properties, Icarus 51, 65 (1982).
5.
R.A. West and P.H. Smith, Evidence for aggregate particles in the atmospheres of Titan and Jupiter, Icarus 90, 330 (1991).
6.
R. Courtin, R. Wagener, C.P. McKay, J. Caldwell, K.-H. Fricke, F. Raulin, and P. Bruston, UV spectroscopy of Titan’s atmosphere, planetary organic chemistry, and prebiological synthesis. II. Interpretation of new IUE observations in the 220-335 nm range, Icarus 90, 43 (1991).
7.
B.N. Khare, C. Sagan, E.T. Arakawa, F. Suits, T.A. Callcott, and B. Nagy, Optical constants of From x-ray to microwave organic tholins produced in a simulated Titan atmosphere: frequencies, Icarus 60, 127 (1984).
8.
Y.L. Yung, M.A. Allen, and J.P. Pinto, Photochemistry of the atmosphere of Titan: Comparison between models and observations, Astrophys. J. Suppl. 55, 465 (1984).
9.
C.P. McKay, O.B. Toon, and R. Courtin, Titan’s Albedo, 0.2 to 1 urn: Laboratory Analogs and Power-Laws, B. A. A. S. 24, 949 (1992).
10. A. Bar-Nun, I. Kleinfeld, and E. Ganor, Shape and optical properties of aerosols formed by photolysis of acetylene, ethylene, and hydrogen cyanide, J. Geophys. Res. 93, 8383 (1988).