Stratospheric Haze Research Articles

Titan’s 5-μm window: observations with the Very Large Telescope

We report on mid-resolution ( R∼2000) spectroscopic observations of Titan, acquired in November 2000 with the Very Large Telescope and covering the range 4.75–5.07 μm. These observations provide a detailed characterization of the CO (1–0) vibrational band, clearly separating for the first time individual CO lines (P10 to P19 lines of 13CO). They indicate that the CO/N 2 mixing ratio in Titan’s troposphere is 32±10 ppm. Comparison with photochemical models indicates that CO is not in a steady state in Titan’s atmosphere. The observations confirm that Titan’s 5-μm continuum geometric albedo is ∼0.06, and further indicates a ∼20% albedo decrease over 4.98–5.07 μm. Nonzero flux is detected at the 0.01 geometric albedo level in the saturated core of the 12CO (1–0) band, at 4.75–4.85 μm, providing evidence for backscattering on the stratospheric haze. Finally, emission lines are detected at 4.75–4.835 μm, coinciding in position with lines from the CO(1–0) and/or CO(2–1) bands. Matching them by thermal emission would require Titan’s stratosphere to be much warmer (by ∼ 25 K at 0.1 mbar) than indicated by the methane 7.7-μm emission and the Voyager radio-occultation. We show instead that a nonthermal mechanism, namely solar-excited fluorescence, is a more plausible source for these emissions. Improved observations and laboratory measurements on the vibrational–translational relaxation of CO are needed for further interpretation of these emissions in terms of a CO stratospheric mixing ratio.

Spectrophotometry of Jupiter in the Wavelength Range 320–1100 nm: Long-Term Observations of Variations over the Disk

Based on long-term spectrophotometric observations of Jupiter in the wavelength range 320–1100 nm, we investigate the variations of aerosol extinction (at λ 320–600 nm) and methane–ammonia absorption (at λ 600–1100 nm) over Jupiter's disk. We give estimates of the optical parameters for the upper cloud layer of the planet, the overlying stratospheric haze, and a Rayleigh atmosphere.

New Observational Results Concerning Jupiter's Great Red Spot

The changing size and aspect ratio of the Great Red Spot from 1880 to 2000 are reviewed, indicating that the length of the system has decreased significantly over the last 100 years and continues to decrease at present at a rate of 0.19 degrees per year. Voyager IRIS maps of para hydrogen fraction and potential temperature over the system are presented, showing the internal structure and surrounding areas at pressures from 100 to 500 mbars. In these maps, the GRS appears as a cold temperature anomaly and a low para fraction anomaly. Vertical structure analyses and wind velocity measurements are presented from data acquired by the Galileo spacecraft in 1996 and 2000, respectively. The vertical structure data suggest a very thick, white, cloud deck with a top near 800±100 mbars and an extended, blue-absorbing, optically thick haze up to about 200 mbars with a thin stratospheric haze above it. The wind field shows a substantial increase in the maximum tangential velocity from 150 to 190 m s −1 over the Galileo time period. The vertical structure analyses from Galileo data and the wind and temperature fields from Voyager data all show evidence of a tilt to the Great Red Spot at the cloud deck from north to south and more subtly from east to west. The data support the hypothesis of the GRS as a tilted “pancake” with temporal variability to the tilt.

High-Resolution Infrared Imaging of Neptune from the Keck Telescope

We present results of infrared observations of Neptune from the 10-m W. M. Keck I Telescope, using both high-resolution (0.04 arcsecond) broadband speckle imaging and conventional imaging with narrowband filters (0.6 arcsec resolution). The speckle data enable us to track the size and shape of infrared-bright features (“storms”) as they move across the disk and to determine rotation periods for latitudes −30 and −45°. The narrowband data are input to a model that allows us to make estimates of Neptune's stratospheric haze abundance and the size of storm features. We find a haze column density of ∼10 6 cm −2 for a haze layer located in the stratosphere, and a lower limit of 10 7 cm −2 and an upper limit of 10 9 cm −2 for a layer of 0.2 μm particles in the troposphere. We also calculate a lower limit of 7×10 6 km 2 for the size of a “storm” feature observed on 13 October 1997.

Near-IR Spectrophotometry of Saturnian Aerosols—Meridional and Vertical Distribution

Photometrically calibrated grism spectra of Saturn in the H-(1.45–1.80 μm) and K-band (1.95–2.50 μm) are presented. The spectra were obtained with the 200-inch Hale telescope at Palomar mountain three days after the ring plane crossing of August 10, 1995. By inversion of the spectra, the vertical distribution of the scattering density is obtained as a function of latitude along the central meridian, for pressures ranging from about 10 to 600 mbar. At all latitudes, we find a vertical structure consisting of a stratospheric haze layer and a more dense upper tropospheric haze layer, with density minima both above and below the tropospheric layer. At northern midlatitudes, the upper tropospheric haze is located deeper into the atmosphere than at similar southern midlatitudes. This hemispherical asymmetry can be explained by seasonal influences. The largest scattering densities in the upper tropospheric haze layer are found at tropical latitudes, between about −10° and +15°.

Photochemistry of Saturn's Atmosphere: I. Hydrocarbon Chemistry and Comparisons with ISO Observations

To investigate the details of hydrocarbon photochemistry on Saturn, we have developed a one-dimensional diurnally averaged model that couples hydrocarbon and oxygen photochemistry, molecular and eddy diffusion, radiative transfer, and condensation. The model results are compared with observations from the Infrared Space Observatory (ISO) to place tighter constraints on molecular abundances, to better define Saturn's eddy diffusion coefficient profile, and to identify important chemical schemes that control the abundances of the observable hydrocarbons in Saturn's upper atmosphere. From the ISO observations, we determine that the column densities of CH 3, CH 3C 2H, and C 4H 2 above 10 mbar are 4 +2 −1.5×10 13 cm −2, (1.1±0.3)×10 15 cm −2, and (1.2±0.3)×10 14 cm −2, respectively. The observed ISO emission features also indicate C 2H 2 mixing ratios of 1.2 +0.9 −0.6×10 −6 at 0.3 mbar and (2.7±0.8)×10 −7 at 1.4 mbar, and a C 2H 6 mixing ratio of (9±2.5)×10 −6 at 0.5 mbar. Upper limits are provided for C 2H 4, CH 2CCH 2, C 3H 8, and C 6H 2. The sensitivity of the model results to variations in the eddy diffusion coefficient profile, the solar flux, the CH 4 photolysis branching ratios, the atomic hydrogen influx, and key reaction rates are discussed in detail. We find that C 4H 2 and CH 3C 2H are particularly good tracers of important chemical processes and physical conditions in Saturn's upper atmosphere, and C 2H 6 is a good tracer of the eddy diffusion coefficient in Saturn's lower stratosphere. The eddy diffusion coefficient must be smaller than ∼3×10 4 cm 2 s −1 at pressures greater than 1 mbar in order to reproduce the C 2H 6 abundance inferred from ISO observations. The eddy diffusion coefficients in the upper stratosphere could be constrained by observations of CH 3 radicals if the low-temperature chemistry of CH 3 were better understood. We also discuss the implications of our modeling for aerosol formation in Saturn's lower stratosphere—diacetylene, butane, and water condense between ∼1 and 300 mbar in our model and will dominate stratospheric haze formation at nonauroral latitudes. Our photochemical models will be useful for planning observational sequences and for analyzing data from the upcoming Cassini mission.

Jovian Stratospheric Hazes: The High Phase Angle View from Galileo

High phase angle Galileo images of Jupiter's limb reveal the presence of stratospheric hazes in the equatorial region (latitude 9°N) and the transition zone at the southern edge of the north polar region (60°N). During orbit E4, images were taken in both violet and near-IR continuum filters, effectively probing Jupiter's stratosphere at two different altitudes. A discrete layer detached from the limb is present at 60°N, 315°W, but not 20° further east at 60°N, 295°W. Bright streaks running roughly north–south are also present on Jupiter's crescent at 60°N. No such discrete features have been seen previously in high phase angle images of the giant planets. Nor are such features seen at 9°N, where the haze appears to be more uniformly distributed in height and longitude. Radial intensity profiles across the limb are inverted to give vertical extinction profiles over ∼200 km in Jupiter's stratosphere. These show that extinction in the discrete haze layer is enhanced by a factor of ∼2 over its surroundings in the violet filter, with less pronounced variation in the near-IR filter. Haze distribution models were found for both latitudes in which the near-IR images constrain the haze properties near or above the 100-mbar pressure level, where the mean particle radius is about 0.45 μm and the haze number density is near 0.15 cm−3. In these models the violet images constrain the haze properties about 25 km higher in the atmosphere, near or above the 20-mbar pressure level, where the haze particle size is 0.32±0.01 μm at 9°N and 0.27±0.01 μm at 60°N. At this altitude, the haze number density increases by almost an order of magnitude between the equator and the polar transition region—from 0.1 cm−3 at 9°N to 0.7 cm−3 at 60°N. An alternative solution is possible at 60°N, in which the haze is placed near 1 mbar in both filters, with mean particle sizes of 0.6 μm in the violet and 1.3 μm in the near IR.

Thermal Structure of Uranus' Atmosphere

Application of a radiative–convective equilibrium model to the thermal structure of Uranus'atmosphere evaluates the role of hazes in the planet's stratospheric energy budget and places a lower limit on the internal energy flux. The model is constrained by Voyager and post-Voyager observations of the vertical aerosol and radiatively active gas profiles. Our baseline model generally reproduces the observed tropospheric and stratospheric temperature profile. However, as in past studies, the model stratosphere from about 10−3to 10−1bar is too cold. We find that the observed stratospheric hazes do not warm this region appreciably and that any postulated hazes capable of warming the stratosphere sufficiently are inconsistent with Voyager and ground-based constraints. We explore the roles played by the stratospheric methane abundance, the H2pressure-induced opacity, photochemical hazes, and C2H2and C2H6in controlling the temperature structure in this region. Assuming a vertical methane abundance profile consistent with that found by the Voyager UVS occultation observations, the model upper stratosphere, from 10 to 100μbar, is also too cold. Radiation in the 7.8-μm band from a small abundance of hot methane in the lower thermosphere absorbed in this region can warm the atmosphere and bring models into closer agreement with observations. Finally, we find that internal heat fluxes ⋦60 erg cm−2sec−1are inconsistent with the observed tropospheric temperature profile.

Jupiter's Cloud Structure from Galileo Imaging Data

The vertical structure of aerosols on Jupiter is inferred from data obtained by the NASA Galileo Solid State Imaging system during the first six orbits of the spacecraft. Images at 889 nm (a strong methane band), 727 nm (a weaker methane band), and 756 nm (continuum) taken at a variety of lighting and viewing angles are used. The images are displayed and described in the companion paper by Vasavadaet al.(1998,Icarus135, 265–275). Conservative scattering cloud particles with laterally uniform single scattering properties are assumed in the analysis and are shown to be consistent with the data at these wavelengths. Particles are bright, and the darkest locations on Jupiter correspond to the smallest optical thickness of aerosols. Optical depths and vertical positions of aerosol layers vary from place to place and are the retrieved quantities in the analysis. Only mid and low latitudes are sampled in this data set. A stratospheric haze with an optical depth of roughly a tenth and an upper tropospheric haze with an optical depth of 2 to 6 exist over all regions. Both are consistent with previous conclusions based on data of lower spatial resolution (e.g., Westet al.1986,Icarus65, 161–217). The new data show that these layers contain little lateral structure on scales smaller than the planetary jets. On scales of the jets and ovals, the top and bottom of the upper tropospheric haze vary in elevation. Theconcentrationof particles (optical depth per pressure interval) varies less than does the total optical depth. Near the base of the upper tropospheric haze is a third cloud component, usually at pressurep= 750 ± 200 mb, which is less than a scale height in geometric thickness. Its optical depth varies from zero to about 20 on regional scales and often varies by 50% on scales of a few tens of kilometers. Optical depth variations in this cloud are the principal cause of the features in Jupiter's atmosphere seen at red and longer wavelengths. It is probably composed of ammonia. The expected NH4SH cloud has not been identified in this work, perhaps because it exists only at locations where it is concealed beneath higher clouds. Our retrievals also cannot rule out a pervasive deep haze without small-scale structure. Finally, in one region northwest of the Great Red Spot, a deeper cloud is identified. Parts of it lie at a pressure greater than four bars. It is associated with a rapidly changing storm system with optical depth of several tens (or more) and a range of cloud heights betweenp> 4 bars top∼ 400 mb. It is probably composed of water.

Near-IR Spectrophotometry of Jovian Aerosols—Meridional and Vertical Distributions

Photometrically calibrated grism spectra of Jupiter in the H (1.45–1.8 μm) and K (1.95–2.5 μm) bands with a resolution of about 100 were taken with the 5-m Hale telescope at Palomar in August of 1995. The spectra were obtained as meridional cuts at six different longitudes on the planet, with one cut crossing the Great Red Spot.The technique outlined in D. Banfieldet al.(1996,Icarus121, 389–410) for the retrieval of scatterer density with altitude from near-infrared spectra is used and refined. It is expected that this retrieval technique will find use in the interpretation of many atmospheric near-infrared reflection spectra, especially those from Galileo NIMS and Cassini VIMS. For the wavelengths and spectral resolution used in this study, the sensitivity of the inversions extends from pressure levels near 400 mbar up to about 20 mbar. Employing this inversion technique on the spectra yields well-resolved jovian cloud densities for τ ≲ 0.1, as a function of latitude and altitude.The density of scatterers is minimum at a height where the pressure is about 100 mbar and increases both upward and downward from this level. The minimum near 100 mbar can be explained by coagulation of settling particles, leading to an increase in fall speed. The results indicate that stratospheric haze particles are generated at heights wherep≲ 20 mbar.

Near-Infrared Absolute Photometric Imaging of the Uranian System

We report the first multifilter set of absolutely calibrated near-infrared images of Uranus, its rings, and three major satellites—Titania, Ariel, and Miranda. Along with imagery utilizing the canonical K filter bandpass (effective wavelength 2.20 μm), absolutely calibrated images of the uranian system are presented for the first time for three additional filter bandpasses: J (1.27 μm), H (1.62 μm), and in a narrow bandpass (0.1 μm full-width-at-half-maximum) centered at 1.73 μm (hereafter designated H′) particularly diagnostic of C–H stretch vibrational absorptions common in hydrocarbons. Multifilter-derived spectra of the southern ring ansa including the bright apoapse of the dominant ϵ ring show no absorptions due to condensable volatiles, including water, ammonia, and light (high H:C) hydrocarbons. Plausible near-infrared spherical geometric and single-scattering particle albedos consistent with Voyager-derived phase functions range from 0.069 to 0.102 and from 0.030 to 0.037, respectively. These are approximately 50% greater than visible values, consistent with the optical properties of charcoal, carbonaceous chondrite material, and the darkside of Iapetus, and consistent with the hypothesis that a primary component of the ring particles is high stoichiometric ratio C:H organics produced by charged-particle and/or photochemical weathering of methane clathrate and/or hydrocarbon ice material originating from nearby moonlets. Additional components consistent with the ring spectrum include silicates such as pyroxene, but not olivine. Analytical modeling of the ring structure indicates ϵ-ring near-infrared opacities of 0.37 ± 0.05 and 1.8 ± 0.3 at apoapsis and periapsis, respectively. Ariel is more than 25% brighter than Miranda and 15% brighter than Titania at all near-infrared wavelengths. Comparisons with UV–visible spectra by Karkoschka (1997,Icarus125, 348–363) show consistency with the hypothesis that the water-ice surfaces of Titania and Ariel are contaminated with low-reflectance, near-IR-reddened substances similar to the weathered high C:H material postulated for the uranian ring system. The relatively blue, near-infrared dark surface of Miranda (full-disk near-infrared albedo of ∼0.22 at 2.4° phase angle) exhibits the greatest water ice absorption band-depth and the largest H- and K-filter brightness surge measured among the uranian satellites, consistent with (1) a markedly heterogeneous surface punctuated with localized outcroppings of relatively pure water ice and (2), as with Titania and Ariel, a surface composition dominated in the near infrared by weathered high C:H material. The uranian disk shows marked latitudinal variability in tropospheric cloud structure, with a “polar cap” of aerosols lying within the planet's southern polar region. This is in sharp contrast with the stratospheric haze structure, which is nearly globally uniform.

Galileo UVS Results and Cassini Preview

AbstractUVS results from Jupiter show the Io torus colder and brighter than the Voyager observations. Aurora near Jupiter’s poles emit 1-4 MR from altitudes of 300 – 400 km. Ganymede and Callisto have extended hydrogen exospheres. The near-UV reflectance of the Galilean satellites shows SO2, magnetospheric alteration, and O3. Remote sensing of Jupiter’s stratospheric haze determines that it thickens and darkens towards higher latitudes.

Gaseous abundances and methane supersaturation in Titan's troposphere

Various properties of Titan's troposphere are inferred from an analysis of Voyager 1 infrared spectrometer (IRIS) data between 200 and 600 cm −1. Two homogeneous spectral averages acquired at widely separated emission angles are chosen for the analysis. Both data sets are associated with northern low latitudes very close to that of the radio science ingress occultation point. Solutions require simultaneous nonlinear least-squares fits to the two IRIS data sets, coupled with iteration of the radio occultation refractivity data. Values and associated 1-σ uncertainties of several parameters are inferred from our analysis. These include mole fractions for molecular hydrogen (∼0.0011), argon (small), and methane near the surface (∼0.057). Solutions are also obtained for the hydrogen para-fraction (close to equilibrium, with considerable uncertainty), air temperature near the surface (∼93 K), surface temperature discontinuity (∼1 K), and maximum degree of methane supersaturation in the upper troposphere (∼ 1.5). Actual values for the above-mentioned parameters depend on the amount of ethane cloud near the tropopause. There is no evidence for methane clouds in the upper troposphere, nor is their presence compatible with large degrees of supersaturation. A wave number dependence for the stratospheric haze opacity is inferred similar to that found for a polymeric residue created in laboratory discharge experiments. This haze appears to be uniformly distributed with latitude between altitudes of 40 and 160 km, provided those nighttime data at southern high latitudes that are subject to possible systematic calibration errors are discounted. Assuming uniform haze distribution, both the air temperature and methane vapor mole fraction near the surface are symmetrically distributed about the equator, with lower values at higher latitudes. Either the tropopause temperature or the maximum degree of methane supersaturation is asymmetrically distributed about the equator. In either case, the data are consistent with a decrease of methane supersaturation toward the poles, which suggests an increase in mean annual precipitation at high latitudes compared with the equatorial region. If methane vapor is in saturation equilibrium with Titan's surface, the derived latitudinal gradient of the near surface methane vapor mole fraction implies that the liquid content of the surface is ethane-enriched near the poles.

Vertical Structure of Jupiter's Atmosphere at the Galileo Probe Entry Latitude

The vertical structure of Jupiter's atmosphere is examined to better characterize the latitudinal region of the Galileo probe entry site: +5°–10° planetographic latitude. One-hundred Hubble Space Telescope images of Jupiter taken with the 893- and 953-nm filters on 13 and 17 February 1995, 5 October 1995, and 14 May 1996 are analyzed, and a two-cloud model of the jovian atmosphere (D. M. Kuehn and R. F. Beebe 1993,Icarus101,282–292) is used to interpret center-to-limb variations of bright plume heads and adjacent dark areas in the latitude region +2°–10°. For both the plume heads and dark areas, the top of the ammonia cloud deck reaches a pressure level of roughly 200 mbar, and the cloud has an optical depth of between 6 and 7. The optical depth of the stratospheric haze layer is 0.59 for the plumes and 0.34 for the dark regions, while the single scattering albedo of the ammonia cloud deck is 0.996 for the plumes and 0.950 for the dark regions. Sensitivity tests motivated by the preliminary results from the Galileo probe net flux radiometer and nephelometer experiments indicate that the ammonia cloud deck may have a patchy structure.

High-Phase-Angle Observations of Uranus at 2650 Å: Haze Structure and Particle Properties

Spatially resolved photometric observations of Uranus at 2650 Å obtained with the Voyager 2 Photopolarimeter Subsystem (PPS) are presented and analyzed with model atmospheres. A vertically homogenous atmospheric model with Rayleigh scattering and absorption fits Uranus well at 2650 Å at phase angles from 16° to 157°; no strong forward scattering typical of abundant large particles is observed. A microphysical model developed for Uranus to fit Voyager imaging system data (K. Rages, J. B. Pollack, M. G. Tomasko, and L. R. Doose, 1991,Icarus89, 359–376) predicts the vertical distribution and properties of stratospheric haze aerosols at 22.5° and 65°S. This microphysical model also fits the 2650-Å PPS data at low latitudes. Increasing the aerosol sizes in the Rageset al.size distribution by a factor of 1.2 creates an unobserved forward scattering peak and can be excluded. The low-latitude upper stratospheric aerosols on Uranus are smaller (<0.1 μm radius) and/or much less abundant than the 0.2 to 0.25-μm stratospheric aerosol population found by PPS on Neptune; however, to fit the Uranus 2650-Å data at all latitudes requires darker and/or more abundant (1.5–3 times more) aerosols near the sunlit pole.

Near‐ir spectroscopy of Jupiter at the time of comet Shoemaker‐Levy 9 Impacts: Emissions of CH4, H3+ and H2

Near‐infrared emissions of the SL9 impact sites of Jupiter have been recorded on July 16–18, 1994, at ESO (La Silla, Chile). A very strong emission of methane was recorded between 3.50 and 3.56 µm, shortly after impact H, showing evidence for a temporary increase of the Jovian stratospheric temperature. Emissions of H2 (2.12 µm) and H3+ (3.53 µm) were also detected above some of the impact sites, several hours after the impacts. The observed H3+ emissions, however, seem to be at least partly contaminated by the southern aurora. A strong continuum was also detected at 2.12 µm over most of the impact sites, presumably due to intense scattering of reflected sunlight by stratospheric haze.

Four micron infrared observations of the comet Shoemaker‐Levy 9 collision with Jupiter at the Zelenchuk Observatory: Spectral evidence for a stratospheric haze and determination of its physical properties

We present preliminary results derived from low spectral resolution infrared spectra of the comet impact sites onto Jupiter. Fragments W/K were tracked several days after the collision at the SAO 6m‐telescope on July 22 and 27. A systematic increase of the spectral radiance is observed for fragment W/K spectra as compared to undisturbed region spectra. This change is interpreted as the evidence for a stratospheric haze produced by the collision. From the spectral shape of this relative increase, the albedo of the haze can reach values as low as 0.06 but is constrained between 0.13 and 0.15 if the spot size is 3′. No stringent constraint is obtained for the altitude of this haze even if a 5–50 mbar level seems to be possibly the best estimate. The particles in the haze should play a significant role in the thermal balance over impact sites. The fragments W/K spectra between July 22 and July 27 show a high stability of the haze between both dates.

Titan's Atmosphere from Voyager Infrared Observations: IV. Latitudinal Variations of Temperature and Composition

We have analyzed nine Voyager 1 infrared spectral averages covering Titan's disk from 53°S to 70°N. By use of radiative transfer modeling we have determined the thermal profiles and mean molecular abundances in the stratosphere of the species with signatures in the region 200-1500 cm-1. Temperature latitudinal variations were found in accordance with Flasar and Conrath (1990, Icarus 85, 346-354). A maximal temperature decrease of 17 K at the 0.4-mbar level (225 km of altitude) is observed between 5°S (the warmest region) and 70°N, whereas the temperature drops only by ∼3 K from 5° to 53°S. Mean molecular fractions, associated with atmospheric levels between 4 and 9 mbar, were derived from the best fit of the infrared data. The CO2 abundance remains constant from pole to pole within error bars. HCN shows a steady increase from south to north (total enhancement of > 30). For all the other molecules, variations in composition exist mainly between the equator and the north polar region. Ethane, acetylene, and propane show a moderate enrichment by about a factor of two. C4H2, C2H4, C3H4 show significantly higher mole fractions at latitudes > 50°N (by factors of ∼7-15). C2N2 and HC3N, undetected southward of 50°N, show at least an order of magnitude enhancement near the north pole. The stratospheric haze opacity at wavenumbers larger than 600 cm-1 was found to show a north-to-south enhancement of ∼2.5 ± 0.3. Coldest temperatures, found at high northern latitudes, are associated with enhanced gas concentration and haze opacity, and this may be caused by more efficient radiative cooling (Bézard, B., A. Coustenis, and C. P. McKay 1995, Icarus 113,267-276). The observed latitudinal variations in hydrocarbons and nitriles may be related to seasonal and spatial variations of the solar flux (Yung, Y. L. 1987, Icarus 72, 468-472). The present results set constraints for the future development of 2-D seasonal photochemical models.

Tidal effects of disconnected hydrocarbon seas on Titan.

Thermodynamic and photochemical arguments suggest that Titan, the largest satellite of Saturn, has a deep ocean of liquid hydrocarbons. At visible wavelengths, Titan's surface is obscured by a thick stratospheric haze, but radar observations have revealed large regions of high surface reflectivity that are inconsistent with a global hydrocarbon ocean. Titan's surface has also been imaged at infrared wavelengths, and the highest-resolution data (obtained by the Hubble Space Telescope) show clear variations in surface albedo and/or topography. The natural interpretation of these observations is that Titan, like the Earth, has continents and oceans. But Titan's high orbital eccentricity poses a problem for this interpretation, as the effects of oceanic tidal friction would have circularized Titan's orbit for most configurations of oceans and continents. Here we argue that a more realistic topography, in which liquid hydrocarbons are confined to a number of disconnected seas or crater lakes, may satisfy both the dynamical and observational constraints.

Titan's Thermal Emission Spectrum: Reanalysis of the Voyager Infrared Measurements

We have modeled the far-infrared spectrum of Titan between 200 and 600 cm-1, including the fine structure of the H2-N2 and H2-CH4 dimers around 355 and 585 cm-1 respectively. A selection of 373 Voyager IRIS spectra recorded at low and mid-latitudes provides the observational basis for our analysis. The opacity model is significantly improved over previous work by taking into account recent ab initio calculations of the collision-induced absorption by N2-CH4 pairs, as well as laboratory measurements of the H2-N2 dimer transitions. In addition to the collision-induced gaseous absorption, the radiative transfer model includes scattering and absorption by stratospheric haze particles and potential tropospheric condensation clouds. We investigate the possible presence of argon, mainly through its influence on the thermal profile retrieved from the Voyager radiooccultation measurements. We find the following results: (1) The observations are best fit with significant methane supersaturation in the troposphere (up to a factor of two) and no condensation cloud present. (2) If a condensation cloud is present we find that it must have an optical depth less than 0.7 with a mean particle radius of about 50 μm and be located very near the minimum temperature level around 40 km, significantly higher than the expected condensation level. (3) In any case, the CH4 mole fraction at the cold trap is 0.017-0.045, the H2 mole fraction is 1.0 ± 0.4 × 10-3, the argon mole fraction is less than 0.06 (3σ), and the imaginary index of refraction of the haze material at 600 cm-1 is 30-80% that of tholins.