Axel dust on Saturn and Titan

The scattering and absorption properties of Axel dust were investigated by means of Mie theory. We find that a flat distribution of particle radii between 0 and 0.1 μm, and an imaginary part of the index of refraction which varies as λ^?2.5 produce a good fit to the variation of Titan's geometric albedo with wavelength (λ) provided that τ_ext, the extinction optical depth of Titan's atmosphere at 5000 Å, is about 10. The real part of the complex index is taken to be 2.0. The model assumes that the mixing ratio of Axel dust to gas is uniform above the surface of Titan. The same set of physical properties for Axel dust also produces a good fit to Saturn's albedo if τ_ext = 0.7 at 5000 Å. To match the increase in albedo shortward of 3500 Å, a clear layer (containing about 7 km-am H₂) is required above the Axel dust. Such a layer is also required to explain the limb brightening in the ultraviolet. These models can be used to analyze the observed equivalent widths of the visible methane bands. The analysis yields an abundance of the order of 1000 m-am CH₄ in Titan's atmosphere. The derived CH₄/H₂ mixing ratio for Saturn is about 3.5 × 10^?3 or an enhancement of about 5 over the solar ratio.     

Ultraviolet reflectivity and geometrical albedo of Titan

Intermediate resolution (6Å) photoelectric spectral scans of Titan, Saturn, Saturn's Rings and the Moon appear in two forms: ratio spectra of Titan vs the Rings and of Saturn vs the Rings, and relative reflectivities, which are compared to previously published results. Titan's geometrical albedo of 0.094 ± 0.012 was measured at 4255Å with a 50Å bandpass. From this and the spectral measurements, we derived the geometrical albedo as a function of wavelength. We find that the wavelength dependences of Titan's uv spectrum and the spectrum of Saturn's Rings are remarkably similar. No trace of any absorption bands is apparent. These results imply that uv gaseous absorption and Rayleigh scattering play a strongly subdued role in Titan's atmosphere. Any homogeneous atmospheric model implies that the absorber responsible for Titan's uv spectral albedo varies strongly with wavelength. On the other hand, we find that the uv observations can be satisfied by an absorber having a relatively weak dependence upon wavelength if an inhomogeneous atmospheric model is employed. In particular, a fine dust, which absorbs as 1/λ, can explain the uv observations provided that it is preferentially distributed high up in Titan's atmosphere where the optical depth from Rayleigh scattering is low. The likely presence of such a dust in Jupiter's atmosphere and the difficulty in explaining the nature of a continuous uv absorber which varies rapidly with wavelength suggest that the gas and aerosol in Titan's atmosphere are inhomogeneously distributed.     

The composition and origin of Titan's atmosphere

The discovery that Titan had an atmosphere was made by the identification of methane in the satellite's spectrum in 1944. But the abundance of this gas and the identification of other major constituents required the 1980 encounter by the Voyager 1 spacecraft. In the intervening years, traces of C₂H₂, C₂H₄, C₂H₆ and CH₃D had been posited to interpret emission bands in Titan's i.r. spectrum. The Voyager Infra-red Spectrometer confirmed that these gases were present and added seven more. The atmosphere is now known to be composed primarily of molecular nitrogen. But the derived mean molecular weight suggests the presence of a significant amount of some heavier gas, most probably argon. It is shown that this argon must be primordial, and that one can understand the evolution of Titan's atmosphere in terms of degassing of a mixed hydrate dominated by CH₄, N₂ and ³⁶Ar. This model satisfactorily explains the absence of neon and makes no special requirements on the satellite's surface temperature.     

Near-infrared spectrophotometry of Titan

Detailed analysis of the R(5) manifold of Titan's 3ν₃ CH₄ band confirms that the column abundance of Titan's spectroscopically visible atmosphere is greater than 1.6 kmamagats. This agrees with the value estimated from the strength of Titan's 3ν₃ CH₄Q branch and is at least 25 times the value for the column abundance of Mars' atmosphere. Moreover, the enhanced strength of the weaker CH₄ lines in Titan's spectrum relative to Saturn's spectrum suggests that CH₄ constitutes a significant fraction of this bulk.Recently discovered strong, unidentified absorptions in Titan's spectrum at 1.05–1.06 μm have been compared with laboratory spectra of a number of gases including CH₄, C₂H₄, C₂H₆, and C₃H₈ with negative results. These comparisons, however, have not excluded the possibility that these features arise from a very large quantity of CH₄ or from an isotope of CH₄. The fundamental transition of the responsible molecule may affect the interpretation of Titan's 8–14 μm spectrum since its wavelength may lie in this window. Comparison with Uranus' spectrum suggests that the visible abundance of this molecule in Titan's atmosphere may be much greater than in Uranus' relatively clear, deep atmosphere.Spectra of features at λ8150.7 and λ8272.7 attributed possibly to H₂ have been obtained at high resolution also during the apparitions of 1971, 1972, and 1973. These are presented for comparison with the results of the 1970 apparition. The existence of the λ8150.7 feature is established definitively but further observations are needed to establish whether the λ8272.7 feature exists beyond doubt.     

The abundance of CH3D in the atmosphere of Titan,derived from 8- to 14-μm thermal emission

The 8.6-μm emission feature of Titan's infrared spectrum was analyzed using the Voyager temperature-pressure profile. Although both C₃H₈ and CH₃D have bands at that wavelength, we show that CH₃D dominates the observed emission on Titan. We derived a CH₃D/CH₄ mixing ratio using this band and the strong CH₄ band at 7.7 μm. The corresponding D/H ratio is 4.2_?1.5⁺² × 10^?4, neglecting deuterium fractionation with other molecules. The main uncertainty in this value comes from the continuum emission characteristics. The D/H ratio is apparently significantly enhanced on Titan with respect to published values for Saturn.     

Chemistry and evolution of Titan's atmosphere

The chemistry and evolution of Titan's atmosphere is reviewed in the light of the scientific findings from the Voyager mission. It is argued that the present N₂ atmosphere may be Titan's initial atmosphere rather than photochemically derived from an original NH₃ atmosphere. The escape rate of hydrogen from Titan is controlled by photochemical production from hydrocarbons. CH₄ is irreversibly converted to less hydrogen rich hydrocarbons, which over geologic time accumulate on the surface to a layer thickness of ～0.5 km. Magnetospheric electrons interacting with Titan's exosphere may dissociate enough N₂ into hot, escaping N atoms to remove ～0.2 of Titan's present atmosphere over geologic time. The energy dissipation of magnetospheric electrons exceeds solar e.u.v. energy deposition in Titan's atmosphere by an order of magnitude and is the principal driver of nitrogen photochemistry. The environmental conditions in Titan's upper atmosphere are favorable to building up complex molecules, particularly in the north polar cap region.     

Detection of a CH4 atmosphere on Pluto

Using a low-resolution spectrograph and a CCD array, a spectrum of Pluto from 0.58 to 1.06 μm was obtained. The spectrum had a resolution of $\text{～25}\text{A}\text{?}$ and a signal-to-noise ratio of ～300. It showed CH₄ absorption bands at 6200, 7200, 7900, 8400, 8600, 8900 and 10,000 Å. The strongest of these bands was at 8900 Å with an absorption depth of 0.23. This band was heavily saturated, compared to the weaker bands, providing proof for the gaseous origin of the observed absorptions. By applying CH₄ band model parameters to our data, a total CH₄ abundance of 80 ± 20 m-am was derived. This translates into a one-way abundance of 27 ± 7 m-am and a CH₄ surface pressure of 1.5 × 10^?4 atm. An upper limit to the total pressure of ～0.05 atm could be set. First-order calculations on atmospheric escape showed that this methane atmosphere would be stable if the mass of Pluto is increased 50% over its current value and its radius is 1400 km. Alternatively a heavier gas mixed with the CH₄ atmosphere would aid its stability. The relatively large amount of gaseous CH₄ observed implies that the absorption bands recently reported at 1.7 and 2.3 μm are likely due to atmospheric CH₄ absorptions rather than surface frost as interpreted earlier.     

The spectrum of titan in the far-infrared and microwave regions

The pressure-induced absorptions of gaseous nitrogen (N₂) and methane (CH₄) are computed on the basis of the collisional lineshape theory of G. Birnhaum and E.R. Cohen [Canad. J. Phys.54, 593–602 (1976)]. Laboratory data at 300 and 124°K for N₂ and at 296 and 195°K for CH₄ are used to determine the collisional time constant and their temperature dependence. The spectrum of Titan from the microwave to the far-infrared region (0.1–600 cm^?1) is then modeled using these opacities and a temperature profile of Titan's atmosphere derived from the Voyager 1 radio occultation experiment. The model atmosphere is composed of N₂ and CH₄, their relative proportions being determined by the vapor pressure law of CH₄. A model with gaseous opacity alone is ruled out by the far-infrared observations. An additional opacity, thought to be associated with a methane cloud, is confirmed. The effective temperature of Titan is estimated at T_e = 83.2 ± 1.4°K.     

Detection of the Kuiper bands in the spectrum of Titan

3ew spectra of Titan centered at 7500 Å, at resolutions of 4 and 1 Å are presented. Weak absorptions coincident with features observed in the spectra of Uranus and Neptune are found. This observation suggests methane abundances in excess of 1 km-am, thereby emphasizing the complexity of line formation in Titan's atmosphere. The question of the total atmospheric pressure of Titan must be reexamined.     

The absence of endogenic methane on Titan and its implications for the origin of atmospheric nitrogen

We calculate the D/H ratio of CH₄ from serpentinization on Titan to determine whether Titan’s atmospheric CH₄ was originally produced inside the giant satellite. This is done by performing equilibrium isotopic fractionation calculations in the CH₄-H₂O-H₂ system, with the assumption that the bulk D/H ratio of the system is equivalent to that of the H₂O in the plume of Enceladus. These calculations show that the D/H ratio of hydrothermally produced CH₄ would be markedly higher than that of atmospheric CH₄ on Titan. The implication is that Titan’s CH₄ is a primordial chemical species that was accreted by the moon during its formation. There are two evolutionary scenarios that are consistent with the apparent absence of endogenic CH₄ in Titan’s atmosphere. The first is that hydrothermal systems capable of making CH₄ never existed on Titan because Titan’s interior has always been too cold. The second is that hydrothermal systems on Titan were sufficiently oxidized so that C existed in them predominately in the form of CO₂. The latter scenario naturally predicts the formation of endogenic N₂, providing a new hypothesis for the origin of Titan’s atmospheric N₂: the hydrothermal oxidation of ¹⁵N-enriched NH₃. A primordial origin for CH₄ and an endogenic origin for N₂ are self-consistent, but both hypotheses need to be tested further by acquiring isotopic data, especially the D/H ratio of CH₄ in comets, and the ¹⁵N/¹⁴N ratio of NH₃ in comets and that of N₂ in one of Enceladus’ plumes.     

Estimates of methane and ammonia abundance in the Jovian atmosphere with allowance for scattering in the clouds

Results of photoelectric measurements of the intensity in CH₄ 5430, 6190, and 7250 Å absorption bands, CH₄ absorption lines in the 3ν₃ band, and the NH₃ 6457.1 Å line are examined from the point of view of a model which takes into account the role of multiple scattering inside a homogeneous semi-infinite cloud layer in the formation of absorption components in the Jovian spectrum. Introduced are a number of simple ratios between depths of lines and bands and the parameters which characterize the properties of the cloud layer and the atmosphere above the clouds for occurrence of the Henyey-Greenstein scattering phase function at various degrees of asymmetry in g. The CH₄ content inside the cloud layer is determined as an equivalent thickness on the mean free path between scattering events. The latter was found to be equal to A_L ? 10 ± 2 m-amagat at g = 0.75 or A_L ? 20 ± 3 m-amagat at g = 0.5 along all the above-mentioned CH₄ absorption bands. For NH₃ it is A_L ? 31 ± 4 cm-amagat at g = 0.75 and A_L ? 62 ± 8 cm-amagat at g = 0.5.The weakening of the CH₄ absorption bands toward the edges of the Jovian disc requires a volume scattering coefficient in the cloud layer of σ_a ～ 10^?6 cm^?1. The mean specific abundance of NH₃ obtained within the cloud layer does not contradict the calculated abundance of saturated gaseous ammonia.     

Ground-based measurements of the methane distribution on Titan

Using spectra taken with NIRSPEC (Near Infrared Spectrometer) and adaptive optics on the Keck II telescope, we resolved the latitudinal variation of the 3ν₂ band of CH₃D at 1.56 μm. As CH₃D is less abundant than CH₄ by a factor of 50±10×10^-5, these CH₃D lines do not saturate in Titan’s atmosphere, and are well characterized by laboratory measurements. Thus they do not suffer from the large uncertainties of the CH₄ lines that are weak enough to be unsaturated in Titan. Our measurements of the methane abundance are confined to the latitude range of 32°S-18°N and longitudes sampled by a 0.04″ slit centered at ∼195°W. The methane abundance below 10 km is constant to within 20% in the tropical atmosphere sampled by our observations, consistent with the low surface insolation and lack of surface methane [Griffith, C.A., McKay, C.P., Ferri, F., 2008. Astrophys. J. 687, L41-L44].     

Titan’s atmosphere from ISO mid-infrared spectroscopy

We have analyzed Titan observations performed by the Infrared Space Observatory (ISO) in the range 7-30 μm. The spectra obtained by three of the instruments on board the mission (the short wavelength spectrometer, the photometer, and the camera) were combined to provide new and more precise thermal and compositional knowledge of Titan’s stratosphere. With the high spectral resolution achieved by the SWS (much higher than that of the Voyager 1 IRIS spectrometer), we were able to detect and separate the contributions of most of the atmospheric gases present on Titan and to determine disk-averaged mole fractions. We have also tested existing vertical distributions for C₂H₂, HCN, C₂H₆, and CO₂ and inferred some information on the abundance of the first species as a function of altitude. From the CH₃D band at 1161 cm⁻¹ and for a CH₄ mole fraction assumed to be 1.9% in Titan’s stratosphere, we have obtained the monodeuterated methane-averaged abundance and retrieved a D/H isotopic ratio of 8.7−1.9+3.2 × 10⁻⁵. We discuss the implications of this value with respect to current evolutionary scenarios for Titan. The ν₅ band of HC₃N at 663 cm⁻¹ was observed for the first time in a disk-averaged spectrum. We have also obtained a first tentative detection of benzene at 674 cm⁻¹, where the fit of the ISO/SWS spectrum at R = 1980 is significantly improved when a constant mean mole fraction of 4 × 10⁻¹⁰ of C₆H₆ is incorporated into the atmospheric model. This corresponds to a column density of ∼ 2 × 10¹⁵ molecules cm⁻² above the 30-mbar level. We have also tested available vertical profiles for HC₃N and C₆H₆ and adjusted them to fit the data. Finally, we have inferred upper limits of a few 10⁻¹⁰ for a number of molecules proposed as likely candidates on Titan (such as allene, acetonitrile, propionitrile, and other more complex gases).     

Methane absorption variations in the spectrum of Pluto

Absolute spectrophotometry of Pluto in the wavelength range of 5600 to 10,500 Å was obtained on 4 nights covering lightcurve phases of 0.18, 0.35, 0.49, and 0.98. The four phases included minimum light (0.98) and one near maximum light (0.49). The spectra reveal significant variations in the absorption depths of the methane bands at 6200, 7200, 7900, 8400, 8600, 8900, and 10,000 Å. The minimum amount of absorption was found to occur at minimum light. This variation would imply a 30° change in the column abundance of methane within 3 days. A model employing an anisotropic surface distribution of methane frost and a clear layer of CH₄ gas was developed to explain the variation in absorption strength with rotational phase. The fit to the overall spectrum requires the presence of a frost with particle sizes on the order of a few millimeters. An upper limit of 5.5 m-am is derived for the one-way column abundance of CH₄ gas. An equally good fit to the variation of the 7200-Å band is obtained if the atmosphere is removed from the model entirely.     

Application of methane band-model parameters to the visible and near-infrared spectrum of Uranus

Laboratory band-model absorption coefficients of CH₄ are used to calculate the Uranus spectrum from 5400 to 10,400 Å. A good fit of both strong and weak bands for the Uranus spectrum over the entire wavelength interval is achieved for the first time. Three different atmospheric models are employed: a reflecting layer model, a homogeneous scattering layer model, and a clear atmosphere sandwiched between two scattering layers. The spectrum for the reflecting layer model exhibits serious discrepancies but shows that large amounts of CH₄ (5–10 km-am) are necessary to reproduce the Uranus spectrum. Both scattering models give reasonably good fits. The homogeneous model requires a particle scattering albedo $\text{(}\text{g}\text{?}\text{w}{}_{\mathrm{<mtext>p</mtext>}}\text{) ? 0.998}$ and an abundance per scattering mean free path $\text{(}\text{a}\text{?}\text{)}\text{of}\text{a}\text{?}\text{1}\text{km-am}$. The parameters derived from the sandwich layer model are: forsb the upper scattering layer a continuum single scattering albedo ($\text{g}\text{?}\text{w}{}_0$) of 0.995 and a scattering optical depth variable with wavelength consistent with Rayleigh scattering; for the clear layer they are a CH₄ abundance (a) of 2.2 km-am and an effective pressure (p) ? 0.1 atm; for the lower cloud deck a Lambert reflectivity (L) of 0.9 resulted. A severe depletion of CH₄ in the upper scattering layer is required. An enrichment of CH₄/H₂ over the solar ratio by a factor of 4–14 in the lower atmosphere is, however, indicated.     

The abundances of ammonia in the atmospheres of Jupiter,Saturn, and Titan

An investigation of low-resolution ratio spectra of Jupiter, Saturn, and Titan in the region 5400–6500 Å has permitted new evaluations of ammonia absorption bands. The distribution of ammonia over the disk of Jupiter is very inhomogeneous. The carbon-to-nitrogen ratio is distinctly different from the solar value, but this is probably a result of uneven mixing of methane and ammonia, as suggested previously by Kuiper, rather than a compositional anomaly. The abundance of ammonia on Saturn also shows spatial variations, but appears constant in time over a 3-yr period. Two weak, unidentified absorptions were discovered in the red region of Titan's spectrum, in the absence of any detectable ammonia. The new upper limit is ηN < 120 cm-am.     

The millimeter spectrum of Titan: Detectability of HCN,HC3N,and CH3CN and the CO abundance

The flux emitted by Titan's disk in millimeter lines of HCN, HC₃N, CH₃CN, and CO is calculated by means of a radiative transfer formulation which takes into account the sphericity of the atmosphere. It is demonstrated that the plane-parallel approximation for radiative transfer is no longer valid, especially in the core of emission lines, when Titan is not spatially resolved. The antenna temperatures which would be measured by large radiotelescopes observing Titan at frequencies of (1?0) and (2?1) transitions of CO, of (1?0), (2?1), and (3?2) transitions of HCN, and of selected transitions of HC₃N and CH₃CN in the range 80–300 GHz are calculated. The observability of these transitions is investigated. It is concluded that there is the possibility of inferring the vertical stratospheric distribution of these species from line shape measurements to be achieved with existing or forthcoming radioastronomical instrumentation. The determination of the CO abundance by D. O. Muhleman, G. L. Berge, and R. T. Clancy (1984, (Science (Washington, D.C.), 223, 393–396) from measurements at 115.3 GHz in two 200 MHz bands, is reinterpreted by means of this radiative transfer formulation. A CO mixing ratio between 3 × 10^?5 and 18 × 10^?5, with a most plausible value of 7.5 × 10^?5, is found.     

The composition of liquid methane-nitrogen aerosols in Titan’s lower atmosphere from Monte Carlo simulations

Molecular level Monte Carlo simulations have been performed with various model potentials for the CH₄-N₂ vapor-liquid equilibrium at conditions prevalent in the atmosphere of Saturn’s moon Titan. With a single potential parameter adjustment to reproduce the vapor-liquid equilibrium at a higher temperature, Monte Carlo simulations are in excellent agreement with available laboratory measurements. The results demonstrate the ability of simple pair potential models to describe phase equilibria with the requisite accuracy for atmospheric modeling, while keeping the number of adjustable parameters at a minimum. This allows for stable extrapolation beyond the range of available laboratory measurements into the supercooled region of the phase diagram, so that Monte Carlo simulations can serve as a reference to validate phenomenological models commonly used in atmospheric modeling. This is most important when the relevant region of the phase diagram lies outside the range of laboratory measurements as in the case of Titan. The present Monte Carlo simulations confirm the validity of phenomenological thermodynamic equations of state specifically designed for application to Titan. The validity extends well into the supercooled region of the phase diagram. The possible range of saturation levels of Titan’s troposphere above altitudes of 7 km is found to be completely determined by the remaining uncertainty of the most recent revision of the Cassini-Huygens data, yielding a saturation of 100 ± 6% with respect to CH₄-N₂ condensation up to an altitude of about 20 km.     

The role of photochemical processes in evolution of the isotopic composition of the atmosphere of Titan

Molecular nitrogen, the main component of the modern atmosphere of Titan, may have formed without significant changes in the nitrogen and hydrogen isotopic composition from the clathrate hydrate of ammonia NH₃ · H₂O_SLD, which is the main accreted form of nitrogen. The most preferable transformation mechanism of NH₃ · H₂O_SLD into atmospheric N₂ is its thermal decomposition in the interior of Titan rather than the photochemical decomposition of ammonia in the upper atmosphere of early Titan. The photolysis of ammonia does not lead to a change in the isotopic composition of nitrogen, as all the nitrogen remains in Titan’s atmosphere. The photolysis of NH does not lead to a change in the isotopic composition of nitrogen in Titan’s atmosphere. Fractionation of hydrogen and nitrogen isotopes during the impacts of comets with Titan does not seem to be significant either. It will be possible to determine the dissociative fractionation factor, the original ratio ¹⁴N/¹⁵N, and the mass of Titan’s original atmosphere when fractionation of nitrogen isotopes in Titan’s atmosphere is examined in additional theoretical and experimental studies that take into account processes occurring during the formation of a system of Saturn’s satellites.     

Titan: Far-infrared and microwave remote sensing of methane clouds and organic haze

It is shown that Titan's surface and plausible atmospheric thermal opacity sources—gaseous N₂, CH₄, and H₂, CH₄ cloud, and organic haze—are sufficient to match available Earth-based and Voyager observations of Titan's thermal emission spectrum. Dominant sources of thermal emission are the surface for wavelenghts λ ? 1 cm, atmospheric N₂ for 1 cm ? λ ? 200 μm,, condensed and gaseous CH₄ for 200 μm ? λ ? 20 μm, and molecular bands and organic haze for λ ? 20 μm. Matching computed spectra to the observed Voyager IRIS spectra at 7.3 and 52.7° emission angles yields the following abundances and locations of opacity sources: CH₄ clouds: 0.1 g cm^? at a planetocentric radius of 2610–2625 km, 0.3 g cm^?2 at 2590–2610 km, total 0.4 ± 0.1 g cm^–2 above 2590 km; organic haze: $\text{4 ± 2 × 10}{}^{\mathrm{?6}}\text{,}\text{g cm}\text{,}{}^{\mathrm{?2}}$ above 2750 km; tropospheric H₂: 0.3 ± 0.1 mol%. This is the first quantitative estimate of the column density of condensed methane (or CH₄/C₂H₆) on Titan. Maximum transparency in the middle to far IR occurs at 19 μm where the atmospheric vertical absorption optical depth is $\text{?0.6}$ A particle radius $\text{r}\text{? 2 μ}\text{m}$ in the upper portion of the CH₄ cloud is indicated by the apparent absence of scattering effects.