Coupling photochemistry with haze formation in Titan's atmosphere, Part II: Results and validation with Cassini/Huygens data

The new one-dimensional radiative-convective/photochemical/microphysical model described in Part I is applied to the study of Titan's atmospheric processes that lead to haze formation. Our model generates the haze structure from the gaseous species photochemistry. Model results are presented for the species vertical concentration profiles, haze formation and its radiative properties, vertical temperature/density profiles and geometric albedo. These are validated against Cassini/Huygens observations and other ground-based and space-borne measurements. The model reproduces well most of the latest measurements from the Cassini/Huygens instruments for the chemical composition of Titan's atmosphere and the vertical profiles of the observed species. For the haze production we have included pathways that are based on pure hydrocarbons, pure nitriles and hydrocarbon/nitrile copolymers. From these, the nitrile and copolymer pathways provide the stronger contribution, in agreement with the results from the ACP instrument, which support the incorporation of nitrogen in the pyrolized haze structures. Our haze model reveals a new second major peak in the vertical profile of haze production rate between 500 and 900 km. This peak is produced by the copolymer family used and has important ramifications for the vertical atmospheric temperature profile and geometric albedo. In particular, the existence of this second peak determines the vertical profile of haze extinction. Our model results have been compared with the DISR retrieved haze extinction profiles and are found to be in very good agreement. We have also incorporated in our model heterogeneous chemistry on the haze particles that converts atomic hydrogen to molecular hydrogen. The resultant H₂ profile is closer to the INMS measurements, while the vertical profile of the diacetylene formed is found to be closer to that of the CIRS profile when this heterogenous chemistry is included.     

Latitudinal variations in Titan’s methane and haze from Cassini VIMS observations

We analyze observations taken with Cassini’s Visual and Infrared Mapping Spectrometer (VIMS), to determine the current methane and haze latitudinal distribution between 60°S and 40°N. The methane variation was measured primarily from its absorption band at 0.61 μm, which is optically thin enough to be sensitive to the methane abundance at 20-50 km altitude. Haze characteristics were determined from Titan’s 0.4-1.6 μm spectra, which sample Titan’s atmosphere from the surface to 200 km altitude. Radiative transfer models based on the haze properties and methane absorption profiles at the Huygens site reproduced the observed VIMS spectra and allowed us to retrieve latitude variations in the methane abundance and haze. We find the haze variations can be reproduced by varying only the density and single scattering albedo above 80 km altitude. There is an ambiguity between methane abundance and haze optical depth, because higher haze optical depth causes shallower methane bands; thus a family of solutions is allowed by the data. We find that haze variations alone, with a constant methane abundance, can reproduce the spatial variation in the methane bands if the haze density increases by 60% between 20°S and 10°S (roughly the sub-solar latitude) and single scattering absorption increases by 20% between 60°S and 40°N. On the other hand, a higher abundance of methane between 20 and 50 km in the summer hemisphere, as much as two times that of the winter hemisphere, is also possible, if the haze variations are minimized. The range of possible methane variations between 27°S and 19°N is consistent with condensation as a result of temperature variations of 0-1.5 K at 20-30 km. Our analysis indicates that the latitudinal variations in Titan’s visible to near-IR albedo, the north/south asymmetry (NSA), result primarily from variations in the thickness of the darker haze layer, detected by Huygens DISR, above 80 km altitude. If we assume little to no latitudinal methane variations we can reproduce the NSA wavelength signatures with the derived haze characteristics. We calculate the solar heating rate as a function of latitude and derive variations of ∼10-15% near the sub-solar latitude resulting from the NSA. Most of the latitudinal variations in the heating rate stem from changes in solar zenith angle rather than compositional variations.     

Diagnostics of Titan's stratospheric dynamics using Cassini/CIRS data and the 2-dimensional IPSL circulation model

The dynamics of Titan's stratosphere is discussed in this study, based on a comparison between observations by the CIRS instrument on board the Cassini spacecraft, and results of the 2-dimensional circulation model developed at the Institute Pierre-Simon Laplace, available at http://www.lmd.jussieu.fr/titanDbase [Rannou, P., Lebonnois, S., Hourdin, F., Luz, D., 2005. Adv. Space Res. 36, 2194-2198]. The comparison aims at both evaluating the model's capabilities and interpreting the observations concerning: (1) dynamical and thermal structure using temperature retrievals from Cassini/CIRS and the vertical profile of zonal wind at the Huygens landing site obtained by Huygens/DWE; and (2) vertical and latitudinal profiles of stratospheric gases deduced from Cassini/CIRS data. The modeled thermal structure is similar to that inferred from observations (Cassini/CIRS and Earth-based observations). However, the upper stratosphere (above 0.05 mbar) is systematically too hot in the 2D-CM, and therefore the stratopause region is not well represented. This bias may be related to the haze structure and to misrepresented radiative effects in this region, such as the cooling effect of hydrogen cyanide (HCN). The 2D-CM produces a strong atmospheric superrotation, with zonal winds reaching 200 m s⁻¹ at high winter latitudes between 200 and 300 km altitude (0.1-1 mbar). The modeled zonal winds are in good agreement with retrieved wind fields from occultation observations, Cassini/CIRS and Huygens/DWE. Changes to the thermal structure are coupled to changes in the meridional circulation and polar vortex extension, and therefore affect chemical distributions, especially in winter polar regions. When a higher altitude haze production source is used, the resulting modeled meridional circulation is weaker and the vertical and horizontal mixing due to the polar vortex is less extended in latitude. There is an overall good agreement between modeled chemical distributions and observations in equatorial regions. The difference in observed vertical gradients of C₂H₂ and HCN may be an indicator of the relative strength of circulation and chemical loss of HCN. The negative vertical gradient of ethylene in the low stratosphere at 15° S, cannot be modeled with simple 1-dimensional models, where a strong photochemical sink in the middle stratosphere would be necessary. It is explained here by dynamical advection from the winter pole towards the equator in the low stratosphere and by the fact that ethylene does not condense. Near the winter pole (80° N), some compounds (C₄H₂, C₃H₄) exhibit an (interior) minimum in the observed abundance vertical profiles, whereas 2D-CM profiles are well mixed all along the atmospheric column. This minimum can be a diagnostic of the strength of the meridional circulation, and of the spatial extension of the winter polar vortex where strong descending motions are present. In the summer hemisphere, observed stratospheric abundances are uniform in latitude, whereas the model maintains a residual enrichment over the summer pole from the spring cell due to a secondary meridional overturning between 1 and 50 mbar, at latitudes south of 40-50° S. The strength, as well as spatial and temporal extensions of this structure are a difficulty, that may be linked to possible misrepresentation of horizontally mixing processes, due to the restricted 2-dimensional nature of the model. This restriction should also be kept in mind as a possible source of other discrepancies.     

HST spectral imaging of Titan's haze and methane profile between 0.6 and 1 μm during the 2000 opposition

We report on the analysis of high spatial resolution visible to near-infrared spectral images of Titan at Ls=240° in November 2000, obtained with the Space Telescope Imaging Spectrograph instrument on board the Hubble Space Telescope as part of program GO-8580. We employ a radiative transfer fractal particle aerosol model with a Bayesian parameter estimation routine that computes Titan's absolute reflectivity per pixel for 122 wavelengths by modeling the vertical distribution of the lower atmosphere haze and tropospheric methane. Analysis of these data suggests that Titan's haze concentration in the lower atmosphere varies in strength with latitude. We find Titan's tropospheric methane profile to be fairly consistent with latitude and longitude, and we find evidence for local areas of a CH₄-N₂ binary saturation in Titan's troposphere. Our results suggest that a methane and haze profile at one location on Titan would not be representative of global conditions.     

Cassini/VIMS hyperspectral observations of the HUYGENS landing site on Titan

Titan is one of the primary scientific objectives of the NASA–ESA–ASI Cassini–Huygens mission. Scattering by haze particles in Titan's atmosphere and numerous methane absorptions dramatically veil Titan's surface in the visible range, though it can be studied more easily in some narrow infrared windows. The Visual and Infrared Mapping Spectrometer (VIMS) instrument onboard the Cassini spacecraft successfully imaged its surface in the atmospheric windows, taking hyperspectral images in the range 0.4–5.2 μm. On 26 October (TA flyby) and 13 December 2004 (TB flyby), the Cassini–Huygens mission flew over Titan at an altitude lower than 1200 km at closest approach. We report here on the analysis of VIMS images of the Huygens landing site acquired at TA and TB, with a spatial resolution ranging from 16 to14.4 km/pixel. The pure atmospheric backscattering component is corrected by using both an empirical method and a first-order theoretical model. Both approaches provide consistent results. After the removal of scattering, ratio images reveal subtle surface heterogeneities. A particularly contrasted structure appears in ratio images involving the 1.59 and 2.03 μm images north of the Huygens landing site. Although pure water ice cannot be the only component exposed at Titan's surface, this area is consistent with a local enrichment in exposed water ice and seems to be consistent with DISR/Huygens images and spectra interpretations. The images show also a morphological structure that can be interpreted as a 150 km diameter impact crater with a central peak.     

Titan global climate model: A new 3-dimensional version of the IPSL Titan GCM

We have developed a new 3-dimensional climate model for Titan’s atmosphere, using the physics of the IPSL Titan 2-dimensional climate model with the current version of the LMDZ General Circulation Model dynamical core. Microphysics and photochemistry are still computed as zonal averages. This GCM covers altitudes from surface to 500 km altitude, with barotropic waves now being resolved and the diurnal cycle included. The boundary layer scheme has been changed, yielding a strong improvement in the tropospheric zonal wind profile modeled at Huygens descent position and season. The potential temperature profile is fairly consistent with Huygens observations in the lowest 10 km. The latitudinal profile of the near-surface temperature is close to observed values. The minimum of zonal wind observed by the Huygens probe just above the tropopause is also present in these simulations, and its origin is discussed by comparing solar heating and dynamical transport of energy. The stratospheric temperature and wind fields are consistent with our previous works. Compared to observations, the zonal wind peak is too weak (around 120 m/s) and too low (around 200 km). The temperature structures appear to be compressed in altitude, and depart strongly from observations in the upper stratosphere. These discrepancies are correlated, and most probably related to the altitude of the haze production. The model produces a detached haze layer located more than 150 km lower than observed by the Cassini instruments. This low production altitude is due to the current position of the GCM upper boundary. However, the temporal behaviour of the detached haze layer in the model may explain the seasonal differences observed between Cassini and Voyager 1. The waves present in the GCM are analyzed, together with their respective roles in the angular momentum budget. Though the role of the mean meridional circulation in momentum transport is similar to previous work, and the transport by barotropic waves is clearly seen in the stratosphere, a significant part of the transport at high latitudes is done all year long through low-frequency tropospheric waves that may be baroclinic waves.     

Titan: Preliminary results on surface properties and photometry from VIMS observations of the early flybys

Cassini observations of the surface of Titan offer unprecedented views of its surface through atmospheric windows in the 1-5 μm region. Images obtained in windows for which the haze opacity is low can be used to derive quantitative photometric parameters such as albedo and albedo distribution, and physical properties such as roughness and particle characteristics. Images from the early Titan flybys, particularly T0, Ta, and T5 have been analyzed to create albedo maps in the 2.01 and 2.73 μm windows. We find the average normal reflectance at these two wavelengths to be 0.15±0.02 and 0.035±0.003, respectively. Titan's surface is bifurcated into two albedo regimes, particularly at 2.01 μm. Analysis of these two regimes to understand the physical character of the surface was accomplished with a macroscopic roughness model. We find that the two types of surface have substantially different roughness, with the low-albedo surface exhibiting mean slope angles of ∼18°, and the high-albedo terrain having a much more substantial roughness with a mean slope angle of ∼34°. A single-scattering phase function approximated by a one-term Henyey-Greenstein equation was also fit to each unit. Titan's surface is back-scattering (g∼0.3-0.4), and does not exhibit substantially different backscattering behavior between the two terrains. Our results suggest that two distinct geophysical domains exist on Titan: a bright region cut by deep drainage channels and a relatively smooth surface. The two terrains are covered by a film or a coating of particles perhaps precipitated from the satellite's haze layer and transported by eolian processes. Our results are preliminary: more accurate values for the surface albedo and physical parameters will be derived as more data is gathered by the Cassini spacecraft and as a more complete radiative transfer model is developed from both Cassini orbiter and Huygens Lander measurements.     

Investigation of energetic proton penetration in Titan's atmosphere using the Cassini INCA instrument

Saturn's largest moon, Titan, provides an interesting opportunity to study how dense atmospheres interact with the surrounding plasma environment. Without an intrinsic magnetic field, this satellite's nitrogen-rich atmosphere is relatively unprotected from plasma interactions. Therefore, the energy-deposition rate is important for understanding chemistry and dynamics in Titan's atmosphere. Since the plasma environment can vary significantly we focus here on the T18 Titan encounter using in-situ data from instruments on board the Cassini spacecraft. These instruments cannot provide in-situ information below the spacecraft closest approach altitude (∼>960 km) so we use the Cassini magnetospheric imaging instrument (MIMI) ion-neutral camera (INCA) to remotely image energetic hydrogen particle fluxes (20-80 keV) at altitudes below Titan closest approach. We also use the MIMI low-energy magnetosphere measurements system (LEMMS) to measure the incident ion fluxes as the spacecraft approaches Titan and combine these data sets with an atmospheric model to first reproduce INCA images. We then use this model to calculate the energy-deposition profiles for the observed incident proton flux. Our model is able to reproduce the INCA observations and give the energy density deposited vs. altitude in Titan's atmosphere; however, we find that the incident fluxes and energy-deposition profiles vary significantly during the encounter.     

Compositional evidence for Titan's stratospheric tilt

Five years of Cassini CIRS infrared spectra have been used to determine the tilt of Titan's stratospheric symmetry axis with respect to the solid body rotation axis. Measurements of HCN abundance centred around 5 mbar (125 km altitude) at equatorial latitudes show the symmetry axis is tilted by 4.0±1.5^° in a direction 70±40^°W of the sub-solar point. This value is consistent with tilts determined from temperature and haze measurements by Achterberg et al. (2008a) and Roman et al. (2009). The consistency of results from three independent methods suggests that Titan's entire stratosphere is tilted and provides a powerful constraint on the underlying atmospheric dynamics.     

The structure of Titan’s atmosphere from Cassini radio occultations

We present results from the two radio occultations of the Cassini spacecraft by Titan in 2006, which probed mid-southern latitudes. Three of the ingress and egress soundings occurred within a narrow latitude range, 31-34°S near the surface, and the fourth at 52.8°S. Temperature-altitude profiles for all four occultation soundings are presented, and compared with the results of the Voyager 1 radio occultation (Lindal, G.F., Wood, G.E., Hotz, H.B., Sweetnam, D.N., Eshleman, V.R., Tyler, G.L. [1983]. Icarus 53, 348-363), the HASI instrument on the Huygens descent probe (Fulchignoni, M. et al. [2005]. Nature 438, 785-791), and Cassini CIRS results (Flasar, F.M. et al. [2005]. Science 308, 975-978; Achterberg, R.K., Conrath, B.J., Gierasch, P.J., Flasar, F.M., Nixon, C.A. [2008b]. Icarus 194, 263-277). Sources of error in the retrieved temperature-altitude profiles are also discussed, and a major contribution is from spacecraft velocity errors in the reconstructed ephemeris. These can be reduced by using CIRS data at 300 km to make along-track adjustments of the spacecraft timing. The occultation soundings indicate that the temperatures just above the surface at 31-34°S are about 93 K, while that at 53°S is about 1 K colder. At the tropopause, the temperatures at the lower latitudes are all about 70 K, while the 53°S profile is again 1 K colder. The temperature lapse rate in the lowest 2 km for the two ingress (dawn) profiles at 31 and 33°S lie along a dry adiabat except within ∼200 m of the surface, where a small stable inversion occurs. This could be explained by turbulent mixing with low viscosity near the surface. The egress profile near 34°S shows a more complex structure in the lowest 2 km, while the egress profile at 53°S is more stable.     

Influence of high abundances of aerosols on the electrical conductivity of the Titan atmosphere

Observations of optical depth and scattering by instrumentation onboard the Huygens probe have been used by Tomasko et al. [Tomasko et al., 2005. Rain, winds and haze during Huygens probe's descent to Titan's surface. Nature 438 (8), 765-778] to deduce that the size and abundance of Titan aerosols could be nearly independent of altitude. Here we show that by assuming a constant mass flux with altitude and using the measured optical depth as a constraint, we obtain more realistic size and abundance distributions. In particular, the calculated abundance decreases from 3.5×10⁷ m⁻³ at 100 km to 8×10⁶ m⁻³ near the surface while the particle radius varies from 0.25 μm at 150 km to 1.1 μm at the surface. These distributions are consistent with the reported measurements for these quantities. Our results are then employed to compute electron and ion densities and conductivities for various solar UV photoelectron emission thresholds. Our model shows that to get agreement with the published (preliminary) conductivity measurements, photoemission cannot be an important source of electrons and ions. To get agreement with the electron and ion conductivity observations, both an additional population of aerosol embryos above 50 km and a trace amount of an electrophillic molecular species below 50 km are needed.     

Far-infrared opacity sources in Titan’s troposphere reconsidered

We use Cassini far-infrared limb and nadir spectra, together with recent Huygens results, to shed new light on the controversial far-infrared opacity sources in Titan’s troposphere. Although a global cloud of large CH₄ ice particles around an altitude of 30 km, together with an increase in tropospheric haze opacity with respect to the stratosphere, can fit nadir and limb spectra well, this cloud does not seem consistent with shortwave measurements of Titan. Instead, the N₂-CH₄ collision-induced absorption coefficients are probably underestimated by at least 50% for low temperatures.     

Measurements of methane absorption by the descent imager/spectral radiometer (DISR) during its descent through Titan's atmosphere

New low-temperature methane absorption coefficients pertinent to the Titan environment are presented as derived from the Huygens DISR spectral measurements combined with the in-situ measurements of the methane gas abundance profile measured by the Huygens Gas Chromatograph/Mass Spectrometer (GCMS). The visible and near-infrared spectrometers of the descent imager/spectral radiometer (DISR) instrument on the Huygens probe looked upward and downward covering wavelengths from 480 to 1620 nm at altitudes from 150 km to the surface during the descent to Titan's surface. The measurements at continuum wavelengths were used to determine the vertical distribution, single-scattering albedos, and phase functions of the aerosols. The gas chromatograph/mass spectrometer (GCMS) instrument on the probe measured the methane mixing ratio throughout the descent. The DISR measurements are the first direct measurements of the absorbing properties of methane gas made in the atmosphere of Titan at the pathlengths, pressures, and temperatures that occur there. Here we use the DISR spectral measurements to determine the relative methane absorptions at different wavelengths along the path from the probe to the sun throughout the descent. These transmissions as functions of methane path length are fit by exponential sums and used in a haze radiative transfer model to compare the results to the spectra measured by DISR. We also compare the recent laboratory measurements of methane absorption at low temperatures [Irwin et al., 2006. Improved near-infrared methane band models and k-distribution parameters from 2000 to 9500 cm⁻¹ and implications for interpretation of outer planet spectra. Icarus 181, 309-319] with the DISR measurements. We find that the strong bands formed at low pressures on Titan act as if they have roughly half the absorption predicted by the laboratory measurements, while the weak absorption regions absorb considerably more than suggested by some extrapolations of warm measurements to the cold Titan temperatures. We give factors as a function of wavelength that can be used with the published methane coefficients between 830 and 1620 nm to give agreement with the DISR measurements. We also give exponential sum coefficients for methane absorptions that fit the DISR observations. We find the DISR observations of the weaker methane bands shortward of 830 nm agree with the methane coefficients given by Karkoschka [1994. Spectrophotometry of the jovian planets and Titan at 300- to 1000-nm wavelength: the methane spectrum. Icarus 111, 174-192]. Finally, we discuss the implications of our results for computations of methane absorption in the atmospheres of the outer planets.     

Retrieval and tentative indentification of the 3 μm spectral feature in Titan's haze

Recently, an unidentified 3.3-3.4 μm feature found in the solar occultation spectra of the atmosphere of Titan observed by Cassini/VIMS was tentatively attributed to the C-H stretching mode of aliphatic hydrocarbon chains attached to large organic molecules, but without properly extracting the feature from adjacent influences of strong CH₄ and weak C₂H₆ absorptions (Bellucci et al., 2009). In this work, we retrieve the detailed spectral feature using a radiative transfer program including absorption and fluorescent emission of both molecules, as well as absorption and scattering by haze particles. The spectral features of the haze retrieved from the VIMS data at various altitudes are similar to each other, indicating relatively uniform spectral properties of the haze with altitude. However, slight deviations observed near 127 km and above 300 km suggest inhomogeneity at these altitudes. We find that the positions of the major spectral peaks occur at 3.33-3.37 μm, which are somewhat different from the typical 3.3 μm aromatic or 3.4 μm aliphatic C-H stretches usually seen in the spectra of dust particles of the interstellar medium and comets. The peaks, however, coincide with those of the solid state spectra of C₂H₆, CH₄, and CH₃CN; and a broad shoulder from 3.37 to 3.50 μm coincides with those of C₅H₁₂ and C₆H₁₂ as well as those of typical aliphatic C-H stretches. This result combined with high-altitude (∼1000 km) haze formation process recently reported by Waite et al. (2007) opens a new question on the chemical composition of the haze particles. We discuss the possibility that the 3 μm feature may be due to the solid state absorption bands of these molecules (or some other molecules) and we advocate additional laboratory measurements for the ices of hydrocarbon and nitrogen-bearing molecules present in Titan's atmosphere for the identification of this 3 μm feature.     

Huygens probe entry dynamic model and accelerometer data analysis

During the first phase of Huygens arrival into Titan's atmosphere the probe is subjected to gravitational and aerodynamic forces in aerodynamic hypersonic regime. Atmospheric drag exerts a strong deceleration on the capsule measured by Huygens atmospheric structure instrument (HASI) servo accelerometer. A 6 DOF (Degree of Freedom) model of the Huygens probe entry dynamics has been developed and used for data analysis. The accelerometer data are analysed and the model allows the retrieval of dynamics information of Huygens probe from 1545 km altitude down to end of the entry phase. Probe's initial conditions (velocity and position) were refined to match the measured deceleration profile resulting in a different altitude at interface epoch with respect to those of the Cassini Navigation Team. Velocity and position of probe at interface epoch are compatible with those used by Descent Trajectory Working Group (DTWG).Measurements acquired before atmosphere detection are used to estimate probe's angular rate, bound attitude and characterise the angle of attack profile which results to be lower than 4^° during the whole entry. Probe's spin calculated (6.98 RPM) is slightly different with respect to DTWG of 7.28 RPM but considering a 2% error in the Inertia matrix these results are inside the 1-σ error band.     

On the discovery of CO nighttime emissions on Titan by Cassini/VIMS: Derived stratospheric abundances and geological implications

We present a quantitative analysis of CO thermal emissions discovered on the nightside of Titan by Baines et al. [2005. The atmospheres of Saturn and Titan in the near-infrared: First results of Cassini/VIMS. Earth, Moon, and Planets, 96, 119–147]. in Cassini/VIMS spectral imagery. We identify these emission features as the P and R branches of the 1-0 vibrational band of carbon monoxide (CO) near 4.65 μm. For CH₃D, the prominent Q branch of the ν₂ fundamental band of CH₃D near 4.55 μm is apparent. CO₂ emissions from the strong v₃ vibrational band are virtually absent, indicating a CO₂ abundance several orders of magnitude less than CO, in agreement with previous investigations. Analysis of CO emission spectra obtained over a variety of altitudes on Titan's nightside limb indicates that the stratospheric abundance of CO is 32±15 ppm, and together with other recent determinations, suggests a vertical distribution of CO nearly constant at this value from the surface throughout the troposphere to at least the stratopause near 300 km altitude. The corresponding total atmospheric content of CO in Titan is 2.9±1.5×10¹⁴ kg. Given the long lifetime of CO in the oxygen-poor Titan atmosphere (0.5–1.0 Gyr), we find a mean CO atmospheric production rate of 6±3×10⁵ kg yr⁻¹. Given the lack of primordial heavy noble gases observed by Huygens [Niemann et al., 2005. The abundances of constituents of Titan's atmosphere from the GCMS on the Huygens probe. Nature, 438, 779–784], the primary source of atmospheric CO is likely surface emissions. The implied CO/CH₄ mixing ratio of near-surface material is 1.8±0.9×10⁻⁴, based on an average methane surface emission rate over the past 0.5 Gyr of 1.3×10⁻¹³ gm cm⁻² s⁻¹ as required to balance hydrocarbon haze production via methane photolysis [Wilson and Atreya, 2004. Current state of modeling the photochemistry of Titan's mutually dependent atmosphere and ionosphere. J. Geophys. Res. 109, E06002 Doi:10.1029/2003JE002181]. This low CO/CH₄ ratio is much lower than expected for the sub-nebular formation region of Titan and supports the hypothesis [e.g., Atreya et al., 2005. Methane on Titan: photochemical-meteorological-hydrogeochemical cycle. Bull. Am. Astron. Soc. 37, 735] that the conversion of primordial CO and other carbon-bearing materials into CH₄-enriched clathrate-hydrates occurs within the deep interior of Titan via the release of hydrogen through the serpentinization process followed by Fischer–Tropsch catalysis. The time-averaged predicted emission rate of methane-rich surface materials is 0.02 km³ yr⁻¹, a value significantly lower than the rate of silicate lava production for the Earth and Venus, but nonetheless indicative of significant active geological processes reshaping the surface of Titan.     

A photochemical model of Titan's atmosphere and ionosphere

A global-mean model of coupled neutral and ion chemistry on Titan has been developed. Unlike the previous coupled models, the model involves ambipolar diffusion and escape of ions, hydrodynamic escape of light species, and calculates the H₂ and CO densities near the surface that were assigned in some previous models. We tried to reduce the numbers of species and reactions in the model and remove all species and reactions that weakly affect the observed species. Hydrocarbon chemistry is extended to C₁₂H₁₀ for neutrals and C₁₀H⁺₁₁ for ions but does not include PAHs. The model involves 415 reactions of 83 neutrals and 33 ions, effects of magnetospheric electrons, protons, and cosmic rays. UV absorption by Titan's haze was calculated using the Huygens observations and a code for the aggregate particles. Hydrocarbon, nitrile, and ion chemistries are strongly coupled on Titan, and attempt to calculate them separately (e.g., in models of ionospheric composition) may result in significant error. The model densities of various species are typically in good agreement with the observations except vertical profiles in the stratosphere that are steeper than the CIRS limb data. (A model with eddy diffusion that facilitates fitting to the CIRS limb data is considered as well.) The CO densities are supported by the O⁺ flux from Saturn's magnetosphere. The ionosphere includes a peak at 80 km formed by the cosmic rays, steplike layers at 500-700 and 700-900 km and a peak at 1060 km (SZA = 60°). Nighttime densities of major ions agree with the INMS data. Ion chemistry dominates in the production of bicyclic aromatic hydrocarbons above 600 km. The model estimates of heavy positive and negative ions are in reasonable agreement with the Cassini results. The major haze production is in the reactions C₆H + C₄H₂, C₃N + C₄H₂, and condensation of hydrocarbons below 100 km. Overall, precipitation rate of the photochemical products is equal to 4-7 kg cm⁻² Byr⁻¹ (50-90 m Byr⁻¹ while the global-mean depth of the organic sediments is ∼3 m). Escape rates of methane and hydrogen are 2.9 and 1.4 kg cm⁻² Byr⁻¹, respectively. The model does not support the low C/N ratio observed by the Huygens ACP in Titan's haze.     

Photoelectric Charging of Submicron Aerosols and Macromolecules in the Titan Haze

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 submicro-meter aerosols is positive. Macromolecules composed of fewer than 32 carbon atoms with low electron affinities (<1.0 eV) are neutral, while the rest are mainly neutral and negatively charged with a small fraction (∼10%) becoming positively charged at higher (≥300 km) altitudes. At night, Titan's aerosol population becomes uniformly neutral and negatively charged. The time taken for a nighttime aerosol to change from being negatively charged to its most probable daytime positive charge is on the order of a few seconds for the largest submicrometer aerosols, while macromolecules tend to persist in an anionic charge state for one to several Earth days. Charging strongly influences aerosol agglomeration via Coulomb attraction and may account for the seasonal variations in the albedo of the Titan haze at midrange (∼200-250 km) altitudes. Enhanced agglomeration may also efficiently produce a source of condensation nuclei for the daily rainout of methane. In addition, the difference in aerosol charge between Titan's day and night (or summer and winter) phases will produce dramatically different chemistries which must be accounted for in future photochemical models. Finally, if there are PAH-like macromolecules in the Titan haze, Cassini Huygens should be able to observe these charge differences, with neutral macromolecules emitting strongly at 3.3 and 11.2 μm, cationic macromolecules emitting between 6.2 and 8.6 μm, and anionic macromolecules emitting in both infrared spectral regions.     

Heavy ion formation in Titan's ionosphere: Magnetospheric introduction of free oxygen and a source of Titan's aerosols?

Discovery by Cassini's plasma instrument of heavy positive and negative ions within Titan's upper atmosphere and ionosphere has advanced our understanding of ion neutral chemistry within Titan's upper atmosphere, primarily composed of molecular nitrogen, with ~2.5% methane. The external energy flux transforms Titan's upper atmosphere and ionosphere into a medium rich in complex hydrocarbons, nitriles and haze particles extending from the surface to 1200 km altitudes. The energy sources are solar UV, solar X-rays, Saturn's magnetospheric ions and electrons, solar wind and shocked magnetosheath ions and electrons, galactic cosmic rays (GCR) and the ablation of incident meteoritic dust from Enceladus’ E-ring and interplanetary medium. Here it is proposed that the heavy atmospheric ions detected in situ by Cassini for heights >950 km, are the likely seed particles for aerosols detected by the Huygens probe for altitudes <100 km. These seed particles may be in the form of polycyclic aromatic hydrocarbons (PAH) containing both carbon and hydrogen atoms C_nH_x. There could also be hollow shells of carbon atoms, such as C₆₀, called fullerenes which contain no hydrogen. The fullerenes may compose a significant fraction of the seed particles with PAHs contributing the rest. As shown by Cassini, the upper atmosphere is bombarded by magnetospheric plasma composed of protons, H₂⁺ and water group ions. The latter provide keV oxygen, hydroxyl and water ions to Titan's upper atmosphere and can become trapped within the fullerene molecules and ions. Pickup keV N₂⁺, N⁺ and CH₄⁺ can also be implanted inside of fullerenes. Attachment of oxygen ions to PAH molecules is uncertain, but following thermalization O+ can interact with abundant CH₄ contributing to the CO and CO₂ observed in Titan's atmosphere. If an exogenic keV O+ ion is implanted into the haze particles, it could become free oxygen within those aerosols that eventually fall onto Titan's surface. The process of freeing oxygen within aerosols could be driven by cosmic ray interactions with aerosols at all heights. This process could drive pre-biotic chemistry within the descending aerosols. Cosmic ray interactions with grains at the surface, including water frost depositing on grains from cryovolcanism, would further add to abundance of trapped free oxygen. Pre-biotic chemistry could arise within surface microcosms of the composite organic-ice grains, in part driven by free oxygen in the presence of organics and any heat sources, thereby raising the astrobiological potential for microscopic equivalents of Darwin's “warm ponds” on Titan.     

High altitude haze: Influence of monomer particles on Titan’s temperature profile

The present study investigates the role of high altitude monomer particles in the energy balance of Titan’s upper atmosphere above an assumed low and high aggregate formation altitude of 385 km and 535 km. A ‘single particle approach’ was applied, where the starting point is the energy balance of an individual aerosol. In our analysis 0.01–0.06 μm radius aerosol particles were chosen for the proposed monomer formation regions. These particles absorb solar radiation, emit in the infrared, and are energetically linked to the surrounding gas by thermal conduction. To compute the monomer particle heating effect, the aerosols are assumed to radiate directly to space. We found that high altitude monomers may affect the profile of Titan’s thermosphere from 2 to 20 K depending on the formation altitude of fluffy non-spherical aggregates, the monomer size and distribution. The actual Titan temperature profile in this altitude range including all heating effects will be measured by the HASI instrument during the descent of the Huygens probe.