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We present results of an axisymmetric global circulation model of Titan with a simplified suite of atmospheric physics forced by seasonally varying insolation. The recent discovery of midlatitude tropospheric clouds on Titan has caused much excitement about the roles of surface sources of methane and the global circulation in forming clouds. Although localized surface sources, such as methane geysers or 'cryovolcanoes,' have been invoked to explain these clouds, we find in this work that clouds appear in regions of convergence by the mean meridional circulation and over the poles during solstices, where the solar forcing reaches its seasonal maximum. Other regions are inhibited from forming clouds because of dynamical transports of methane and strong subsidence. We find that for a variety of moist regimes, i.e., with the effect of methane thermodynamics included, the observed cloud features can be explained by the large-scale dynamics of the atmosphere. Clouds at the solsticial pole are found to be a robust feature of Titan's dynamics, whereas isolated midlatitude clouds are present exclusively in a variety of moist dynamical regimes. In all cases, even without including methane thermodynamics, our model ceases to produce polar clouds [approximately equal to] 4-6 terrestrial years after solstices. atmospheres | climate | planetary science

Recent exploration by the Cassini/Huygens mission has stimulated a great deal of interest in Saturn's moon, Titan. One of Titan's most captivating features is the thick organic haze layer surrounding the moon, believed to be formed from photochemistry high in the C[H.sub.4]/[N.sub.2] atmosphere. It has been suggested that a similar haze layer may have formed on the early Earth. Here we report laboratory experiments that demonstrate the properties of haze likely to form through photochemistry on Titan and early Earth. We have used a deuterium lamp to initiate particle production in these simulated atmospheres from UV photolysis. Using a unique analysis technique, the aerosol mass spectrometer, we have studied the chemical composition, size, and shape of the particles produced as a function of initial trace gas composition. Our results show that the aerosols produced in the laboratory can serve as analogs for the observed haze in Titan's atmosphere. Experiments performed under possible conditions for early Earth suggest a significant optical depth of haze may have dominated the early Earth's atmosphere. Aerosol size measurements are presented, and implications for the haze layer properties are discussed. We estimate that aerosol production on the early Earth may have been on the order of [10.sup.14] g x [year.sup.-1] and thus could have served as a primary source of organic material to the surface. planetary atmospheres | tholins | atmospheric aerosol | Archaen | astrobiology

Fractal behavior of one Titan profile acquired by the Cassini altimeter during its first flyby is demonstrated and quantitatively analyzed. The inadequacy of popular nonfractal models to represent scale-dependent behavior is also discussed. Our results lead to the conjecture that a proper geometrical model of the Titan surface is provided by the fractional Brownian motion stochastic process and that the particular acquired profile has a fractal dimension of about 2.4. In addition, use of the developed statistical model allows the design of model-based filters for a significant improvement of the measurement accuracy, at the expense of the horizontal resolution. Index Terms--Extraterrestrial exploration, filtering, fractals, radar altimetry.

Though Titan is in synchronous rotation around Saturn, it experiences gravitational tides as a consequence of its eccentric orbit. It is proposed that the vertical transport of aerosols by these tides produces the haze layers in Titan's upper atmosphere. Analysis shows that the zonal winds in Titan's superrotating atmosphere have a profound influence on which tidal components are effective in establishing the multiple detached-haze layers. If the Huygens Doppler winds are representative of the equatorial global superrotation, then the westward propagating s = 2 mode is the responsible tidal component even though its forcing is significantly weaker than that of the s = 0 and eastward s = 2 components. The eastward s = 2 tidal mode is eliminated by critical levels while the s = 0 mode is viscously damped in the strong high altitude winds. At polar latitudes, however, the gravest s = 0 mode is the one most likely to produce layering. It is also suggested that the atmospheric gravitational tides could be responsible for decelerating the superrotating atmosphere as seen in the Huygens Doppler wind velocity profile at about 80 km altitude. Keywords: Titan; Tides, atmospheric; Atmospheres, dynamics; Satellites of Saturn

Tidal waves driven by Titan's orbital eccentricity through the time-dependent component of Saturn's gravitational potential attain nonlinear, saturation amplitudes ([absolute value of T'] > 10 K, [absolute value of u'] > 20 m [s.sup.-1], and [absolute value of w'] > 5 cm [s.sup.-1]) in the upper atmosphere ([greater than or equal to] 500 km) due to the approximate exponential growth as the inverse square root of pressure. The gravitational tides, with vertical wavelengths of ~100-150 km above 500 km altitude, carry energy fluxes sufficient in magnitude to affect the energy balance of the upper atmosphere with heating rates [greater than or equal to] [10.sup.-9] erg [cm.sup.-3] [s.sup.-1] in range of 500-900 km. Keywords: Titan; Tides, atmospheric; Atmospheres, dynamics; Atmospheres, structure; Satellites, atmospheres

Sediment transport by surficial flow likely occurs on Titan. Titan is thought to have a volatile cycle, such as on Earth and likely in the past on Mars, which would entail surficial liquid flow. And surficial flow is implied in interpretations of Cassini-Hyugens data as showing fluvial channels, which would require sediment transport by surficial flow to form the observable features. We present calculations from basic hydraulic formulae of sediment entrainment and transport by surficial flow. First, we describe the conditions for (non-cohesive) sediment entrainment by grain size through use of the Shields' threshold curve. We then calculate settling velocities by grain size to describe the type of sediment transport--washload, suspended load, or bedload--that would follow entrainment. These calculations allow derivation of required flow depths for sediment transport by grain size over a given slope. A technique to estimate required flow velocities and unit discharges is also presented. We show the results of these calculations for organic and water ice sediment movement by liquid methane flow under Titan gravity. For comparative purposes, plots for movement of quartz sediment by water on Earth and basalt sediment by water on Mars are also included. These results indicate that (non-cohesive) material would move more easily on Titan than on Earth or Mars. Terrestrial field observations suggest that coarse grain transport is enhanced by hyperconcentration of fine-grained sediment; and the apparent availability of organic (fine grained) sediment on Titan, in conjunction with the possibility of convection-driven rainstorms, may lead to hyperconcentrated flows. Thus, significant sediment transport may occur on Titan during individual overland flow events. Keywords: Titan; Mars, surface; Surfaces, planets; Satellites of Saturn; Surfaces, satellite

Mid- and far-infrared spectra from the Composite InfraRed Spectrometer (CIRS) have been used to determine volume mixing ratios of nitriles in Titan's atmosphere. HCN, H[C.sub.3]N, [C.sub.2][H.sub.2], and temperature were derived from 2.5 [cm.sup.-1] spectral resolution mid-IR mapping sequences taken during three flybys, which provide almost complete global coverage of Titan for latitudes south of 60[degrees] N. Three 0.5 [cm.sup.-1] spectral resolution far-IR observations were used to retrieve [C.sub.2][N.sub.2] and act as a check on the mid-IR results for HCN. Contribution functions peak at around 0.5-5 mbar for temperature and 0.1-10 mbar for the chemical species, well into the stratosphere. The retrieved mixing ratios of HCN, H[C.sub.3]N, and [C.sub.2][N.sub.2] show a marked increase in abundance towards the north, whereas [C.sub.2][N.sub.2] remains relatively constant. Variations with longitude were much smaller and are consistent with high zonal wind speeds. For 90[degrees]-20[degrees] S the retrieved HCN abundance is fairly constant with a volume mixing ratio of around 1 x [10.sup.-7] at 3 mbar. More northerly latitudes indicate a steady increase, reaching around 4 x [10.sup.-7] at 60[degrees] N, where the data coverage stops. This variation is consistent with previous measurements and suggests subsidence over the northern (winter) pole at approximately 2 x [10.sup.-4] m [s.sup.-1]. H[C.sub.3]N displays a very sharp increase towards the north pole, where it has a mixing ratio of around 4 x [10.sup.-8] at 60[degrees] N at the 0.1-mbar level. The difference in gradient for the HCN and H[C.sub.3]N latitude variations can be explained by H[C.sub.3]N's much shorter photochemical lifetime, which prevents it from mixing with air at lower latitude. It is also consistent with a polar vortex which inhibits mixing of volatile rich air inside the vortex with that at lower latitudes. Only one observation was far enough north to detect significant amounts of [C.sub.2][N.sub.2], giving a value of around 9 x [10.sup.-10] at 50[degrees] N at the 3-mbar level. Keywords: Titan; Atmospheres, composition

The partitioning of atmospheric hydrocarbons into vapor and condensed phases when the species count is large is considered using the formalism of continuous thermodynamics. The vapor saturation pressures and condensate species distribution are parameterized using the species normal boiling temperatures. Qualitative trends in activity coefficient values and phase equilibrium behavior that are relevant to the outer planets and Titan are discussed in terms of a much simplified perspective on these aspects of partitioning. The trends found are generally consistent with those from other published atmospheric model results.

Titan's atmospheric winds, like those on Venus, exhibit superrotation at high altitudes. Titan general circulation models have yielded conflicting results on whether prograde winds in excess of 100 [ms.sup.-1] at the 1 mbar level are possible based on known physical processes that drive wind systems. A comprehensive two-dimensional (2D) model for Titan's stratosphere was constructed to systematically explore the physical mechanisms that produce and maintain stratospheric wind systems. To ensure conservation of angular momentum in the limit of no net exchange of atmospheric angular momentum with the solid satellite and no external sources and sinks, the zonal momentum equation was solved in flux form for total angular momentum. The relationships among thermal wind balance, meridional circulation, and zonal wind were examined with numerical experiments over a range of values for fundamental input parameters, including planetary rotation rate, radius, internal friction due to wave stresses, and net radiative drive. The magnitude of mid-latitude jets is most sensitive to a single parameter, the planetary rotation rate and results from the conversion of planetary angular momentum to relative angular momentum by the meridional circulation, whereas the strength of meridional circulation is mainly determined by the magnitude of the radiative drive. For Titan's slowly rotating atmosphere, the meridional temperature gradient is vanishingly small, even when the radiative drive is enhanced beyond reasonable magnitudes, and can be inferred from zonal winds in gradient/thermal wind balance. In our 2D model large equatorial superrotation in Titan's stratosphere can be only produced through internal drag forcing by eddy momentum fluxes, which redistribute angular momentum within the atmosphere, while still conserving the total angular momentum of the atmosphere with time. We cannot identify any waves, such as gravitational or thermal tides, that are sufficiently capable of generating the required eddy forcing of >50 [ms.sup.-1] [Titan-day.sup.-1] to maintain peak prograde winds in excess of 100 [ms.sup.-1] at the 1 mbar level. Keywords: Titan; Atmospheric dynamics; Meteorology

Analysis of the 250-560 [cm.sup.-1] spectral continuum of Titan's north polar hood just after spring equinox reveals, in addition to the ubiquitous aerosol, a tenuous but relatively uniform cloud of small particles permeating the lower stratosphere at altitudes between 58 and 90 km. Voyager 1 IRIS data suggest the particles are highly scattering, almost certainly condensed organics, with radii between 1 and 5 [micro]m. Mole fractions for the condensed material range between 4 x [10.sup.-8] and 4 x [10.sup.-6], depending upon particle size. Vapor pressure arguments imply condensed nitriles near 90 km, the most likely being HCN, with condensed hydrocarbons such as [C.sub.2][H.sub.6] restricted to regions considerably nearer the tropopause. No direct chemical identification is possible. Negligible methane supersaturation in the troposphere at 67.4[degrees] N latitude, when compared with degrees of supersaturation at other latitudes, hints at precipitation fluxes of north polar stratospheric condensates during the previous northern winter that were perhaps three orders of magnitude greater than those at low latitudes during that time. A scale height of 1.5 times the density scale height above 160 km is reaffirmed for the photochemical aerosol of the north polar hood. There appears to be a depletion of aerosol somewhere below 160 km. An aerosol mole fraction ~8 x [10.sup.-8] at 160 km is inferred, about 33% greater than the value derived in a previous study. The Cassini CIRS instrument, with its expanded spectral range and higher spectral resolution, should be able to provide highly complementary information for the time period coveting most of the northern winter season. Keywords: Titan; Condensates; Stratosphere; Clouds

The European Space Agency (ESA) and NASA worked together for the Titan probe mission, called Cassini-Huygens mission, to probe the largest moon of Saturn and the most fascinating object in the solar system. The Huygens probe successfully landed on Titan in January 2005 and found dried riverbeds and channels that likely once held liquid methane.

We propose an analytic model that allows rapid computation of the secondary ion production due to electron impact from the primary photo-production in the ionosphere of Titan. The model parameters are given for each of the 5 major ion productions ([N.sup.+.sub.2], [CH.sup.+.sub.4], [N.sup.+], [CH.sup.+.sub.3], [N.sup.++.sub.2]) as well as for the electron production. Keywords: Titan, Ionospheres; Solar radiation

Using synthetic spectra derived from an updated model atmosphere together with a continuum model that includes contributions from haze, cloud and ground, we have re-analyzed the recently published (Geballe et al., 2003, Astrophys. J. 583, L39-L42) high-resolution 3/[micro]m spectrum of Titan which contains newly-detected bands of HCN (in emission) and [C.sub.2][H.sub.2] and C[H.sub.3]D (in absorption), in addition to previously detected bands of C[H.sub.4]. In the 3.10-3.54 [micro]m interval the analysis yields strong evidence for the existence of a cloud deck or optically thick haze layer at about the 10 mbar (~ 100 km) level. The haze must extend well above this altitude in order to mask the strong C[H.sub.4] lines at 3.203.50/[micro]m. These cloud and haze components must be transparent at 2.87-2.92/[micro]m, where analysis of the C[H.sub.3]D spectrum demonstrates that Titan's surface is glimpsed through a second cloud deck at about the 100 mbar (~ 50 km) level. Through a combination of areal distribution and optical depth this cloud deck has an effective transmittance of ~ 20%. The spectral shape of Titan's continuum indicates that the higher altitude cloud and haze particles responsible for suppressing the C[H.sub.4] absorptions have a largely organic make-up. The rotational temperature of the HCN ranges from 140 to 180 K, indicating that the HCN emission occurs over a wide range of altitudes. This emission, remodeled using an improved collisional deactivation rate, implies mesospheric mixing ratio curves that are consistent with previously predictions. The stratospheric and mesospheric [C.sub.2][H.sub.2] mixing ratios are ~ [10.sup.-5], considerably less than previous model predictions (Yung et al., 1984), but approximately consistent with recent observational results. Upper limits to mixing ratios of H[C.sub.3]N and [C.sub.4][H.sub.2] are derived from non-detections of those species near 3.0/[micro]m. Keywords: Titan; Atmosphere; Molecules; Clouds; Infrared

We report the three-dimensional fluorescence spectra of Titan tholins in water and acetonitrile, and separate some of the fluorescent components of tholins using two-dimensional thin layer chromatography. In acetonitrile, tholins exhibit a broad fluorescence with a maximum at 471 nm, and an excitation maximum at 410 nm. The fluorescence spectrum of a water extraction displays a more complicated spectrum with multiple peaks. TLC results indicate the presence of at least three distinct fluorescent species. In addition, we obtained the two-dimensional fluorescence spectrum of tholins in water ice at 77 K, close to the surface temperature of Titan. The fluorescence of tholins in a 77 K ice matrix is red-shifted in comparison to a water solution, and undergoes a further red-shift when the water solution is heated prior to freezing. These results suggest that a simple fluorescence probe could be used on the surface of Titan to identify sites where tholins have been mixed with water, and possibly reveal information about the extent of heating the tholin-water mixture has undergone. This would be useful for a future mission with the goal of examining the organic chemistry of Titan. Keywords: Titan; Organic chemistry; Prebiotic chemistry; Prebiotic environments

We have developed a coupled general circulation model of Titan's atmosphere in which the aerosol haze is treated with a microphysical model and is advected by the winds. The radiative transfer accounts for the non uniform haze distribution and, in turn, drives the dynamics. We analyze the GCM results, especially focusing on the difference between a uniform haze layer and a haze layer coupled to the dynamics. In the coupled simulation the aerosols tend to accumulate at the poles, at latitudes higher than [+ or -] 60[degrees]. During winter, aerosols strongly radiate at thermal infrared wavelengths enhancing the cooling rate near the pole. Since this tends to increase the latitudinal gradients of temperature the direct effect of this cooling excess, in contrast to the uncoupled haze case, is to increase the strength of the meridional cells as well as the strength of the zonal winds and profile. This is a positive feedback of the haze on dynamics. The coupled model reproduces observations about the state of the atmosphere better than the uniform haze model, and in addition, the northern polar hood and the detached haze are qualitatively reproduced. Keywords: Titan; Haze; Dynamics; Photometry

We have reanalyzed the Voyager 1 UVS solar occultations by Titan to expand upon previous analyses and to resolve inconsistencies that have been noted in the scientific literature. To do so, we have developed a detailed model of the UVS detector and improved both the data reduction methods and retrieval techniques. In comparison to the values previously determined by Smith et al. (1982, J. Geophys. Res. 87, 1351-1359) we find [N.sub.2] densities that are 25-60% higher, C[H.sub.4] densities that are smaller by a factor of 3-7, and [C.sub.2] [H.sub.2] densities that are roughly two orders of magnitude smaller. Our values for the thermospheric temperature are 153-158 K, which are approximately 20-40 K colder than previous estimates. We also report the first-ever determination from Voyager UVS data of density profile information [C.sub.2][H.sub.4], HCN, and H[C.sub.3]N Finally, we present a simple engineering model that is consistent with our new results in the upper atmosphere and merges smoothly with the model of Yelle et al. (1997, in: HUYGENS Science, Payload and Mission, in: ESA SP, vol. 1177, pp. 243-256) in the lower atmosphere. Our results provide improved constraints for photochemical models and offer scientists a better understanding of Titan's upper atmosphere as we head into the Cassini era in the exploration of the saturnian system. Keywords: Titan; Occultations; Aeronomy; Atmospheres, structure

We present an analysis of images of Saturn's moon Titan, obtained by the Voyager 1 spacecraft on November 8-12, 1980. Orange filter (590-640 nm) images were photometrically corrected and a longitudinal average removed from them, leaving residual images with up to 5% contrast, and dominated by surface reflectivity. The resultant map shows the same regions observed at 673 nm by the Hubble Space Telescope (HST). Many of the same albedo features are present in both datasets, despite the short wavelength (600 nm) of the Voyager 1 images. A very small apparent longitudinal offset over the 14 year observation interval places tight constraints on Titan's rotation, which appears essentially synchronous at 15.9458 [+ or -] 0.0016 days (orbital period = 15.945421 [+ or -] 0.000005 days). The detectability of the surface at such short wavelengths puts constraints on the optical depth, which may be overestimated by some fractal models. Keywords: Titan, surface, atmosphere; Satellites; Image processing

All previous observations of seasonal change on Titan have been of physical phenomena such as clouds and haze. We present here the first observational evidence of chemical change in Titan's atmosphere. Images taken during 1999-2002 (late southern spring on Titan) with the W.M. Keck I 10-meter telescope at 8-13 lain show a significant accumulation of ethylene ([C.sub.2][H.sub.4]) in the south polar stratosphere as well as north-south stratospheric temperature variation (colder at poles). Our observations restrict this newly discovered south polar ethylene accumulation to latitudes south of 60[degrees] S. The only other observations of the spatial distribution of [C.sub.2][H.sub.4] were those of Voyager I, which found a significant north polar accumulation in early northern spring. We see no build-up in the north, although the highest northern latitudes are obstructed from view in the current season. Our observations constrain any unobserved north polar accumulation of [C.sub.2][H.sub.4] to north of 50[degrees] N latitude. Comparison of the Voyager I results with our new results show seasonal chemical change has occurred in Titan's atmosphere. Keywords: Titan, satellites; Atmospheres, infrared observations

We present results from 14 nights of observations of Titan in 1996-1998 using near-infrared (centered at 2.1 microns) speckle imaging at the 10-meter W.M. Keck Telescope. The observations have a spatial resolution of 0.06 arcseconds. We detect bright clouds on three days in October 1998, with a brightness about 0.5% of the brightness of Titan. Using a 16-stream radiative transfer model (DISORT) to model the central equatorial longitude of each image, we construct a suite of surface albedo models parameterized by the optical depth of Titan's hydrocarbon haze layer. From this we conclude that Titan's equatorial surface albedo has plausible values in the range of 0-0.20. Titan's minimum haze optical depth cannot be constrained from this modeling, but an upper limit of 0.3 at this wavelength range is found. More accurate determination of Titan's surface albedo and haze optical depth, especially at higher latitudes, will require a model that fully considers the 3-dimensional nature of Titan's atmosphere. Keywords: Titan; Infrared observations; Surfaces, satellite

We calculate the amount of methane that may form via reactions catalyzed by metal-rich dust that condenses in the wake of large cometary impacts. Previous models of the gas-phase chemistry of impacts predicted that the terrestrial planets' atmospheres should be initially dominated by CO/C[O.sub.2], [N.sub.2], and [H.sub.2]O. C[H.sub.4] was not predicted to form in impacts because gas-phase reactions in the explosion quench at temperatures ~ 2000 K, at which point all of the carbon is locked in CO. We argue that the dust that condenses out in the wake of a large comet impact is likely to have very effective catalytic properties, opening up reaction pathways to convert CO and [H.sub.2] to C[H.sub.4] and C[O.sub.2], at temperatures of a few hundred K. Together with C[O.sub.2], C[H.sub.4] is an important greenhouse gas that has been invoked to compensate for the lower luminosity of the Sun ~ 4 Gyr ago. Here, we show that heterogeneous (gas-solid) reactions on freshly-recondensed dust in the impact cloud may provide a plausible nonbiological mechanism for reducing CO to C[H.sub.4] before and during the emergence of life on Earth, and perhaps Mars as well. These encouraging results emphasize the importance of future research into the kinetics and catalytic properties of astrophysical condensates or 'smokes' and also more detailed models to determine the conditions in impact-generated dust clouds. Keywords: Atmospheres, evolution; Impact processes; Prebiotic environments; Planetary formation; Terrestrial planets; Titan

Titan, the largest satellite of Saturn, has a thick nitrogen/methane atmosphere with a thick global organic haze. A laboratory analogue of Titan's haze, called tholin, was formed in an inductively coupled plasma from nitrogen/methane = 90/10 gas mixture at various pressures ranging from 13 to 2300 Pa. Chemical and optical properties of the resulting tholin depend on the deposition pressure in cold plasma. Structural analyses by IR and UV/VIS spectroscopy, microprobe laser desorption/ionization mass spectrometry, and Raman spectroscopy suggest that larger amounts of aromatic ring structures with larger cluster size are formed at lower pressures (13 and 26 Pa) than at higher pressures (160 and 2300 Pa). Nitrogen is more likely to incorporate into carbon networks in tholins formed at lower pressures, while nitrogen is bonded as terminal groups at higher pressures. Elemental analysis reveals that the carbon/nitrogen ratio in tholins increases from 1.5-2 at lower pressures to 3 at 2300 Pa. The increase in the aromatic compounds and the decrease in C/N ratio in tholin formed at low pressures indicate the presence of the nitrogen-containing polycyclic aromatic compounds in tholin formed at low pressures. Tholin formed at high pressure (2300 Pa) consists of a polymer-like branched chain structure terminated with -C[H.sub.3], -N[H.sub.2], and -C[equivalent to]N with few aromatic compounds. Reddish-brown tholin films formed at low pressures (13-26 Pa) shows stronger absorptions (almost 10 times larger k-value) in the UV/VIS range than the yellowish tholin films formed at high pressures (160 and 2300 Pa). The tholins formed at low pressures may be better representations of Titan's haze than those formed at high pressures, because the optical properties of tholin formed at low pressures agree well with that of Khare et al. (1984a, Icarus 60, 127-137), which have been shown to account for Titan's observed geometric albedo. Thus, the nitrogen-containing polycyclic aromatic compounds we find in tholin formed at low pressure may be present in Titan's haze. These aromatic compounds may have a significant influence on the thermal structure and complex organic chemistry in Titan's atmosphere, because they are efficient absorbers of UV radiation and efficient charge exchange intermediaries. Our results also indicate that the haze layers at various altitudes might have different chemical and optical properties. Keywords: Titan; Haze; Tholin; Spectroscopy; Organic chemistry; Nitrogen-containing polycyclic aromatic compounds

Titan's haze consists of long chain polymers of pure and N-mixed hydrocarbons (Coustenis et al., 1989, Icarus 80, 54-76, 1991, Icarus 89, 152-167). These polymers have regularly alternating (i.e., conjugated) double/single and triple/single bonds, which open either spontaneously (free aging) or under the action of some external factors (forced aging), the latter being very diverse, e.g., charging, photolysis, radiolysis, thermolysis, chemical effect of environment, etc. An essential of free aging was examined previously (Dimitrov and Bar-Nun, 2002, Icarus 156, 530-538). The main distinction between free and any forced aging is that both of them possess the same thermodynamics while different kinetics, the forced aging in any case being faster, proceeding in different pathways than the free aging. The more extensive is the list of the external effects and the more intensive they are, the faster and more variably the forced aging proceeds. In this paper we quantified the kinetics of forced aging, considering charging of Titan's aerosol population. It was found that forced aging proceeds approximately hundred times faster as compared to the free aging. Various physico-chemical properties of Titan's aerosol material, including coagulation coefficients, depending on particle size and medium conditions, were defined. The comparison of the aging rate, rate of sedimentation and rate of the particle increase proves that Titan's aerosol domain can be subdivided conditionally into two big subdomains. The upper one contains minor portion (< 5%) of the total aerosol bulk, unannealed aerosol particles being fine and sticky. The lower subdomain contains the major portion (> 95%) of aerosol bulk, which is completely aged, coarsely dispersed particles. We established the border between these subdomains at the altitude Z ~620 km. Keywords: Titan's aerosols; Forced aging; Charging

We present a numerical study of barotropic waves in Titan's stratosphere based on a shallow-water model. The forcing of the zonal flow by the mean meridional circulation is represented by a relaxation towards a barotropically unstable wind profile. The relaxation profile is consistent with observations and with previous results from a 3D general circulation model. The time constant of the forcing that best matches the northward eddy-transport of zonal momentum from the 3D model is [tau] ~5 Titan days. The eddy wind field is a zonal wavenumber-2 wave with a peak amplitude about 10% of the mean wind speed. The latitudinal transport of angular momentum by the wave tends to keep the flow close to marginal stability by carrying momentum upgradient, from the core of the jets into the low latitudes. Although the strongest eddy motions occur at the latitudes of the wind maxima, the strongest mixing takes place at the barotropically unstable regions, close to [+ or -] 30[degrees] and spanning about 30[degrees] in latitude. An eddy-mixing time constant of the order of 1 Titan day is inferred within these regions, and of a few tens of days within regions of stable flow. Horizontal gradients in transient tracer fields are less than 10% of the latitudinal gradient of the meridional tracer profile. Cassini's detection of such waves could provide a direct observation of wind speeds at stratospheric levels. Keywords: Titan; Atmospheres, dynamics; Atmospheres, composition

We present a 2D general circulation model of Titan's atmosphere, coupling axisymmetric dynamics with haze microphysics, a simplified photochemistry and eddy mixing. We develop a parameterization of latitudinal eddy mixing by barotropic waves based on a shallow-water, longitude-latitude model. The parameterization acts locally and in real time both on passive tracers and momentum. The mixing coefficient varies exponentially with a measure of the barotropic instability of the mean zonal flow. The coupled GCM approximately reproduces the Voyager temperature measurements and the latitudinal contrasts in the distributions of HCN and [C.sub.2][H.sub.2], as well as the main features of the zonal wind retrieved from the 1989 stellar occultation. Wind velocities are consistent with the observed reversal time of the North-South albedo asymmetry of 5 terrestrial years. Model results support the hypothesis of a non-uniform distribution of infrared opacity as the cause of the Voyager temperature asymmetry. Transport by the mean meridional circulation, combined with polar vortex isolation may be at the origin of the latitudinal contrasts of trace species, with eddy mixing remaining restricted to low latitudes most of the Titan year. We interpret the contrasts as a signature of non-axisymmetric motions. Keywords: Titan; Atmospheres, dynamics; Atmospheres, composition

Solar UV is the principal energy source impinging the atmosphere of Titan while the energy from the electrons in Saturn's magnetosphere is less than 0.5% of the UV light. Titan haze analogs were prepared by the photolysis of a mixture of gases that simulate the composition of its atmosphere (nitrogen, methane, hydrogen, acetylene, ethylene, and cyanoacetylene). The real (n) and imaginary (k) parts of the complex refractive index of haze analogs formed from four different gas mixtures were calculated from the spectral properties of the solid polymer in UV-visible, near infrared and infrared wavelength spectral regions. The value of n was constant at 1.6 [+ or -] 0.1 throughout the 0.2-2.5[micro]m region. The variation of k with wavelength for the values derived for Titan has a lower error than the absolute values of k so the more significant comparisons are with the slopes of the k([lambda]) plots in the UV-VIS region. Three of the photochemical Titan haze analogs had slopes comparable to those derived for Titan from the Voyager data (Rages and Pollack, 1980, Icarus 41, 119-130; McKay and Toon, 1992, in: Proceedings of the Symposium on Titan, in: ESA SP, Vol. 338, pp. 185-190). The slopes of the k([lambda]) plots for haze analogs prepared by spark discharge (Khare et al., 1984, Icarus 60, 127-137) and plasma discharge (Ramirez et al., 2002, Icarus 156, 515-529) were also comparable to Titan's. These finding show that the k([lambda]) plots do not differentiate between different laboratory simulations of atmospheric chemistry on Titan in the UV-VIS near IR region (0.2-2.5 microns). There is a large difference between the k([lambda]) in the infrared between the haze analogs prepared photochemically and analogs prepared using a plasma discharges (Khare et al., 1984, Icarus 60, 127-137; Coll et al., 1999, Planet. Space Sci. 47, 1331-1340; Khare et al., 2002, Icarus 160, 172-182). The C/N ratio in the haze analog prepared by discharges is in the 2-11 range while that of the photochemical analogs is in the 18-24 range. The use of discharges and UV light for initiating the chemistry in Titan's atmosphere is discussed. Keywords: Photochemistry; Titan; Refractive index; Atmospheres; Haze; UV spectroscopy; IR spectroscopy; Electric discharges; Tholin

We conduct three-dimensional hydrodynamical simulations of hypervelocity impacts into the crust of Titan to determine the fraction of liquid water generated, under the reasonable assumption that the crest is largely water ice, and to track the fate of the organic-rich layer that is thought to overlie the ice over much of the surface. Impactors larger than a kilometer in diameter are only slightly affected by the atmosphere, while those well under that size are strongly decelerated and broken up before reaching the surface. Impact of a 2 km diameter icy projectile into the crust at velocities of 7 km per second or higher, and angles of impact between 30[degrees] and 45[degrees], generate about 2-5% melt by volume within the crater. Our results for the amount of aqueous melt generated in impacts on Titan are broadly consistent with the analytic model developed by Thompson and Sagan (1992) although our numerical model allows us to more precisely quantify the fraction of melt, and fate of the organics, as a function of the impact parameters. While much of the organic surface layer is heavily shocked and ejected from the immediate region of the crater, a significant fraction located behind the oblique impact trajectory is only lightly shocked and is deposited in the liquid water at the crater base. Simple calculations suggest that the resulting aqueous organic phase may remain liquid for hundreds of years or longer, enough time for the synthesis of simple precursor molecules to the origin of life.

Simulations of Titan's atmospheric transmission and surface reflectivity have been developed in order to estimate how Titan's atmosphere and surface properties could affect performances of the Cassini radar experiment. In this paper we present a selection of models for Titan's haze, vertical rain distribution, and surface composition implemented in our simulations. We collected dielectric constant values for the Cassini radar wavelength (~ 2.2 cm) for materials of interest for Titan: liquid methane, liquid mixture of methane-ethane, water ice, and light hydrocarbon ices. Due to the lack of permittivity values for Titan's haze particles in the microwave range, we performed dielectric constant ([[epsilon].sub.r]) measurements around 2.2 cm on tholins synthesized in laboratory. We obtained a real part of [[epsilon].sub.r] in the range of 2-2.5 and a loss tangent between [10.sup.-3] and 5 x [10.sup.-2]. By combining aerosol distribution models (with hypothetical condensation at low altitudes) to surface models, we find the following results: (1) Aerosol-only atmospheres should cause no loss and are essentially transparent for Cassini radar, as expected by former analysis. (2) However, if clouds are present, some atmospheric models generate significant attenuation that can reach -50 dB, well below the sensitivity threshold of the receiver. In such cases, a 13.78 GHz radar would not be able to measure echoes coming from the surface. We thus warn about possible risks of misinterpretation if a 'wet atmosphere' is not taken into account. (3) Rough surface scattering leads to a typical response of ~ -17 dB. These results will have important implications on future Cassini radar data analysis. Keywords: Titan; Satellites, atmospheres; Surfaces, satellite; Radar

We investigate the role of seasonal variations of Titan's stratospheric composition on the temperature. We use a general circulation model coupled with idealized chemical tracers that reproduce variations of ethane ([C.sub.2] [H.sub.6]), acetylene ([C.sub.2] [H.sub.2]), and hydrogen cyanide (HCN). Enhancement of the mole fractions of these compounds, at high latitudes in the winter hemisphere relative to their equatorial values, induces a relative decrease in temperature above approximately 0.2 mbar, with a peak amplitude around -20 [KAPPA], and a relative increase in temperature below, around 1 mbar, with a peak amplitude around +7 [KAPPA]. These thermal effects are mainly due to the variations of the cooling to space induced by the varying distributions. The ethane, acetylene, and hydrogen cyanide variations affect the cooling rates in a similar way, with the dominant effect being due to ethane, though its latitudinal variations are small. Keywords: Titan; Atmospheres, composition; Radiative transfer

Narrow-band images of Titan were obtained in November 1999 with the NASA/GSFC-built acousto-optic imaging spectrometer (AImS) camera. This instrument utilizes a tunable filter element that was used within the 500-to 1050-nm range, coupled to a CCD camera system. The images were taken with the Mount Wilson 2.54-m (100 in.) Hooker telescope, which is equipped with a natural guide star adaptive optics system. We observed Titan at 830 and 890 nm and at a series of wavelengths across the 940-nm window in Titan's atmosphere where the methane opacity is relatively low. We determined the absolute reflectivity (I/F) of Titan and fit the values at 940 nm to a Minnaert function at Titan's equator and at -30[degrees] latitude (closer to the subsolar point) and obtained average values for the Minnaert limb-darkening slope,[kappa], of 0.661 [+ or -] 0.007 and 0.775 [+ or -] 0.018, respectively. Comparison with models suggests that the equatorial value of [kappa] is consistent with rain removal of haze in the lower atmosphere. The higher value of [kappa] at -30[degrees] is consistent with the southern hemisphere being brighter than the equator. However, the fits are not unique. The data and models at 890 are consistent with no limb brightening or darkening at this wavelength either at the equator or at -30[degrees]. Keywords: Titan; Atmospheres, Structure; Radiative transfer; Instrumentation

We propose a new interpretation of the D/H ratio in C[H.sub.4] observed in the atmosphere of Titan. Using a turbulent evolutionary model of the subnebula of Saturn (O. Mousis et al. 2002, Icarus 156, 162-175), we show that in contrast to the current scenario, the deuterium enrichment with respect to the solar value observed in Titan cannot have occurred in the subnebula. Instead, we argue that values of the D/H ratio measured in Titan were obtained in the cooling solar nebula by isotopic thermal exchange of hydrogen with C[H.sub.3]D originating from interstellar methane D-enriched ices that vaporized in the nebula. The rate of the isotopic exchange decreased with temperature and became fully inhibited around 200 K. Methane was subsequently trapped in crystalline ices around 10 AU in the form of clathrate hydrates formed at 60 K, and incorporated into planetesimals that formed the core of Titan. The nitrogen-methane atmosphere was subsequently outgassed from the decomposition of the hydrates (Mousis et al. 2002). By use of a turbulent evolutionary model of the solar nebula (O. Mousis et al. 2000, Icarus 148, 513-525), we have reconstructed the entire story of D/H in C[H.sub.4], from its high value in the early solar nebula (acquired in the presolar cloud) down to the value measured in Titan's atmosphere today. Considering the two last determinations of the D/H ratio in Titan--D/H = (7.75 [+ or -] 2.25) x [10.sup.-5] obtained from ground-based observations (Orton 1992, In: Symposium on Titan, ESA SP-338, pp. 81-85), and D/H = ([8.75.sup.+3.25.sub.-2.25]) x [l0.sup.-5], obtained from ISO observations (Coustenis et al. 2002, submitted for publication)--we inferred an upper limit of the D/H ratio in methane in the early outer solar nebula of about 3 x [10.sup.-4]. Our approach is consistent with the scenario advocated by several authors in which the atmospheric methane of Titan is continuously replenished from a reservoir of clathrate hydrates of C[H.sub.4] at high pressures, located in the interior of Titan. If this scenario is correct, observations of the satellite to be performed by the radar, the imaging system, and other remote sensing instruments aboard the spacecraft of the Cassini-Huygens mission from 2004 to 2008 should reveal local disruptions of the surface and other signatures of the predicted outgassing. Key Words: Titan; solar nebula; deuterium; satellites, atmospheres.

Previous modeling by Banaszkiewicz et al. (2000a,b) showed that the C[H.sub.4] thermospheric mixing ratio on Titan could vary as much as 35-40% due to ion-neutral chemical reactions. A new vertical methane profile has been computed by simultaneously modifying the stratospheric methane mixing ratio and the K(z) previously considered by Lara et al. (1996) and Banaszkiewicz et al. (2000a,b). A satisfactory fit of the methane thermospheric abundance and stratospheric mixing ratio of other minor constituents is achieved by placing the homopause at ~1000 km and increasing the methane stratospheric mixing ratio ([q.sub.C[H.sub.4]]) up to 3.8%. The new proposed eddy diffusion coefficient steadily rises from 1 x [10.sup.7] [cm.sup.2] [s.sup.-1] at 700 km to 1 x [10.sup.10] [cm.sup.2] [s.sup.-1] at 1500 km, whereas the stratospheric values are in the range (4-20) x [10.sup.3] [cm.sup.2] [s.sup.-1]. Other likely ionization sources that can influence the methane distribution are (i) a metallic ion layer produced by micrometeoroid infall and (ii) frequent X-rays solar flares. Analysis of the effects of these ionization sources on the methane distribution indicates that, unlike previously assumed, C[H.sub.4] can suffer considerable variations. These variations, although proved in this work, must be cautiously regarded since several assumptions have to be made on the rate of [N.sub.2] and C[H.sub.4] ionization by the processes previously mentioned. Hence, these results are only indicative of methane sensitivity to ionospheric chemistry. Key Words: Titan; C[H.sub.4] thermospheric abundance; atmospheres; ionospheres.

We quantify the charge states of submicrometer aerosols and aromatic macromolecules in Titan's organic haze. The aerosol charge is balanced between the recombination of positive ions with the aerosol plus the ejection of electrons from the aerosol via the UV-driven photoelectric effect and the recombination of electrons with the aerosol. During the day, the dominant charge state for submicrometer aerosols is positive. Macromolecules composed of fewer than 32 carbon atoms with low electron affinities ( Key Words: Titan; stratospheric chemistry; aromatics; aerosol charging.

Ethane spectral lines were observed in emission from Titan in August 1993, October 1995, and September 1996, at a spectral resolution of [lambda]/[DELTA][lambda] [approximately equal to] [10.sup.6], at wavelength 11.7-11.9 [micro]m using the Goddard Infrared Heterodyne Spectrometer at the NASA Infrared Telescope Facility on Mauna Kea, Hawaii. The ethane mole fraction is determined to be (8.8 [+ or -] 2.2) x [10.sup.-6] (68.3% confidence limits, '1[sigma]'), averaging the retrievals from each observing run obtained using the 'recommended' thermal profile of R. V. Yelle, D. Strobel, E. Lellouch, and D. Gautier (1997, in Huygens: Science, Payload, and Mission (J.-P. Lebreton, Ed.), pp. 243-256, European Space Agency SP-1177). Key Words: satellites of Saturn; Titan; atmospheres, composition; infrared observations.

Using adaptive optics on the W. M. Keck II telescope, we imaged Titan several times during 1999 to 2001 in narrowband near-infrared filters selected to probe Titan's stratosphere and upper troposphere. We observed a bright feature around the south pole, possibly a collar of haze or clouds. Further, we find that solar phase angle explains most of the observed east-west brightness asymmetry of Titan's atmosphere, although the data do not preclude the presence of a 'morning fog' effect at small solar phase angle. Key Words: Titan; satellites; atmospheres; infrared observations.

General constraints on geyser phenomena are developed and applied to speculative methane geysers on Titan. Variation of boiling point with depth of ethane-methane-nitrogen fluids is found to be of order 0.3 K [m.sup.-1], in contrast with around 2 K [m.sup.-1] for water on Earth. It is found that geysers are possible on Titan but require enhancements of ~100 above global-average geothermal heat flux, a factor similar to that required on Earth. Eruption velocities of order 25 m [s.sup.-1] appear to be typical for 10-m-deep geyser vents on both Earth and Titan. While eruption velocities on Earth are usually limited by the low sound speed in water-steam mixtures, sound speeds in Titan fluids are higher and the Titan limit is imposed by the available energy in the boiling fluid. Eruption intervals should be nearly the same for geysers with equivalent plumbing. There is an interesting symmetry between geysers on Titan and those on Earth: The volatility of the relevant fluid scales with the available heat flow. Key Words: Titan; geological processes; volcanism.

We have elaborated an evolutionary turbulent model of the subnebula of Saturn derived from that of Dubrulle (1993, Icarus 106, 59-76) for the solar nebula, which is valid for a geometrically thin disk. We demonstrate that if carbon and nitrogen were in the form of CO and [N.sub.2], respectively, in the early subnebula, these molecules were not subsequently converted into C[H.sub.4] and N[H.sub.3] during the evolution of the disk, contrary to the current scenario initially proposed by Prinn and Fegley (1981, Astrophys. J., 249, 308-317). However, if the early subnebula contained some C[H.sub.4] and N[H.sub.3], these gases were not subsequently converted into CO and [N.sub.2]. We argue that Titan must have been formed from planetesimals migrating from the outer part of the subnebula to the present orbit of the satellite. These planetesimals were relics of those embedded in the feeding zone of Saturn prior to the completion of the planet and contained hydrates of N[H.sub.3] and clathrate hydrates of C[H.sub.4]. It is shown that, for plausible abundances of C[H.sub.4] and N[H.sub.3] in the solar nebula at 10 AU, the masses of methane and nitrogen trapped in Titan were higher than the estimate of masses of these components in the primitive atmosphere of the satellite. If our scenario is valid and if our turbulent model properly describes the structure and the evolution of the actual subnebula of Saturn, the Xe/C ratio should be six times higher in Titan's atmosphere today than in the Sun, while the current scenario would probably result in a quasi solar Xe/C ratio. The mass spectrometer and gas chromatograph instrument aboard the Huygens Titan probe of the Cassini mission has the capability of measuring this ratio in 2004, thus permitting us to discriminate between the current scenario and the one proposed in this report. Key Words: Titan; solar nebula; satellites; atmospheres.

On 1995 August 21, Saturn's moon Titan occulted the 13th magnitude star GSC5254-00997. The predawn event was observed at 2.3 [micro]m with the NASA 3.0-m Infrared Telescope Facility. We measured the average atmospheric scale height between latitudes 50 [degrees] S and 67 [degrees] S by fitting isothermal models to the immersion portion of the occultation lightcurve. Within the altitude range of 300 to 500 km, we find an isothermal scale height of Titan's sunset atmosphere of 55 [+ or -] 9 km. Our derived temperature of 180 [+ or -] 30 K is consistent with modeled temperature profiles derived from Voyager 1 IRIS observations (A. Coustenis and B. B6zard 1995, Icarus 115, 126-140) and the 28 Sagittarii occultation event (W. B. Hubbard et al. 1993, Astron. Astrophys. 269, 541-563). The atmospheric albedo asymmetry due to low level haze (Caldwell et al. 1992, Icarus 96,1-9) is inferred from the difference in Titan's center-of-light (determined from precise astrometric measurements of images of Titan and the star before and after the occultation) and the actual center of Titan (determined from the occultation analysis). We find the southern hemisphere to be brighter than the northern hemisphere, a result consistent with Combes et al. (1997, Icarus 129, 482-497), Gibbard et al. (1999, Icarus 139, 189-201), and Smith et al. (1996 Icarus, 119, 336-349). Key Words: Titan, atmospheres; structure, occultations.

I report preliminary measurements of the complex permittivity of frozen aqueous ammonia solutions at liquid nitrogen temperatures, representative of those in the saturnian system. The real part of the dielectric constant of 30% ammonia ice is around 4.5 at near-DC frequencies and at [approximately]1 MHz, compared with around 3.1 for pure water ice. The loss tangents of ammonia-rich ices seem somewhat ([approximately]50%) higher than those for water ice, for which the few low-temperature experiments to date indicate values comparable with predictions by Thompson and Squyres (1990, Icarus 86, 336-354) and Maetzler (1998, in Solar System Ices (B. Schmitt, C. DeBergh, and M. Festou, Eds.), pp. 241-257, Kluwer Academic, Dordrecht), but considerably higher than models by Chyba et al. (1998, Icarus, in press). Ammonia-rich ice may reconcile the radar and optical appearance of Titan's surface: the detectability of water-ammonia ice on Titan by the Cassini mission and the implications for Titan's origin and evolution are discussed.

Measurements of the Doppler frequency change in a microwave beam sent to and transponded back from an interplanetary spacecraft are very sensitive to the quadrupole gravitational field of a nearby natural satellite. This method will be used in Cassini's mission to the saturnian system in relation to Titan during some of the numerous flybys. The use of very stable coherent signals at X and Ka bands will allow an excellent determination of the quadrupole dimensionless coefficients, with absolute accuracy between [10.sup.-8] and [10.sup.-9]. Titan's quadrupole deformation is due to its rotation and the saturnian tidal forces; the determination of the corresponding two parameters will greatly constrain the interior models. The measurement accuracy should be sufficient to determine also the variation of the tidal parameter with the orbital period, as a consequence of Titan's eccentricity; this will provide information on its rigidity and indicate whether it has an internal ocean of water and ammonia. The analysis of the complicated dependence on the geometry of Doppler measurements of the quadrupole can be done by means of an analytical expression of the covariance matrix.

We have measured the spectrum of Titan near 5 [[micro]meter] and have found it to be dominated by absorption from the carbon monoxide 1-0 vibration-rotation band. The position of the band edge allows us to constrain the abundance of CO in the atmosphere and/or the location of the reflecting layer in the atmosphere. In the most likely case, 5 [[micro]meter] radiation is reflected from the surface and the mole fraction of CO in the atmosphere is [Mathematical Expression Omitted], significantly lower than previous estimates for tropospheric CO. The albedo of the reflecting layer is approximately [Mathematical Expression Omitted] in the 5 [[micro]meter] continuum outside the CO band. The 5 [[micro]meter] albedo is consistent with a surface of mixed ice and silicates similar to the icy Galilean satellites. Organic solids formed in simulated Titan conditions can also produce similar albedos at 5 [[micro]meter].

We examine various mechanisms of erosion on Titan and their likely effect on Titan's geomorphology. Specifically we evaluate the solubility of likely crustal ices (water and ammonia) in liquid methane and ethane, and compare these with the solubility of rocks on Earth. Aeolian and diurnal weathering can be neglected, and glacial erosion is unlikely unless the atmosphere has frozen out in the past. Erosion due to rainfall is at most comparable to periglacial environments on Earth. We find that additional laboratory experiments are required if Cassini observations are to be adequately interpreted.

Studies of various physical models of Titan's interior and oceans reveal that the eccentricity of the satellite's orbit cannot be adequately explained by the hypothesized tidal dissipation in its interior or in oceans in its surface. This finding indicates that the expected global ocean does not exist and that Titan's eccentric orbit originated only more recently. Alternatively, it is also possible that rapid interior cooling characterized Titan's thermal history.