P42A-01 10:20h
Origin and Evolution of Titan's Atmosphere: Testing the Models
With early results from Cassini and the imminent deployment of Huygens, it is useful to consider what observations the orbiter and the probe will be able to make to test existing models for the origin and evoution of Titan's atmosphere. This atmosphere must have resulted from some mixture of cometary impacts plus outgassing of the materials making up the bulk of the satellite. The nature of the icy planetestimals that accreted to form Titan is thus a key unknown in attempts to deduce atmospheric origin. Ground-based observations of $^{15}$N/$^{14}$N in HCN and $^{18}$O/$^{16}$O in CO suggest massive escape of the early atmosphere. The rapid photolysis of atmospheric CH$_{4 }$ and the near-normality of $^{13 }$C/$^{12 }$C both require a continuing source for methane, and some form of renewal is also necessary to maintain the abundance of CO. Subterranean oceans, cryovolcanism, continuing Fischer-Tropsch-type reactions and the metabolism of hypothetical sub-surface organisms are all alluring solutions to these problems. Predictions for isotope ratios and abundances flowing from the different models can be tested by the array of instruments carried by the Cassini-Huygens orbiter and probe.
P42A-02 10:35h
Titan: Nature's Laboratory for Organic Synthesis
It has been nearly a third of a century since Strobel's (1973) pioneering study of hydrocarbon chemistry and Axel's (1972) seminal work on the hydrocarbon aerosols. Methane, the parent molecule of hydrocarbons, is abundant on Titan. Photolysis of methane results in the synthesis of more complex hydrocarbons. The hydrocarbon chemistry is unique and rich and the same reactions apply to all reducing atmospheres. The hydrocarbon chemistry inevitably leads to the formation of high molecular weight products, giving rise to aerosols when the ambient atmosphere is cool enough for them to condense. We will review the progress in the hydrocarbon chemistry since the Voyager encounters (Yung et al., 1984). A number of pathways to organic synthesis from simple to complex hydrocarbons have been identified. Neutral schemes involve radicals such as C$_3$H$_3$ and C$_3$H$_5$. Ion schemes involve charged radicals such as c-C$_3$H$_2^+$ and C$_4$H$_3^+$. Both ringed and chained compounds can be synthesized. The organic chemistry is a very sensitive function of the H and H$_2$ concentrations in the atmosphere. In fact, we can "retrieve" the concentration of H atoms in the atmosphere from the recent ground-based observation of the allene to methylacetylene ratio. Preliminary results from Cassini will also be discussed.
P42A-03 10:50h
Cassini observations of ULF waves during the TA Titan flyby
ULF waves are effective in dissipating free energy produced in the plasma during the flow of plasma past bodies with atmospheres and they provide diagnostics of the processes taking place. At Io in the jovian magnetosphere strong ion cyclotron waves were observed by Galileo but during the Voyager encounter with Titan at Saturn no clear ion cyclotron waves were seen. During the TA encounter we will attempt to detect ion cyclotron waves to assess the rate of loss of atmosphere from Titan through ion pickup. If present, we can use the amplitude and spatial extent of these waves to assess the role of fast neutrals in contributing to the loss process, contrasting the interaction with that of Io. Finally, we examine the nature of waves across the full frequency spectrum to determine the power spectral density and nature of all waves generated in the interaction.
P42A-04 11:05h
Cassini Magnetometer Observations of the State of Saturn's Magnetosphere During the Titan Encounter on 26th October 2004
Using Cassini magnetometer observations and modelling results we report on the state of the Kronian magnetosphere during the targeted Cassini Titan encounter (TA). The flyby is in the morning sector at 10h36m SLT and since Titan's orbital radius is close to the nominal magnetopause stand off distance, the encounter may occur in the magnetosphere, magnetosheath or solar wind. We will distinguish between these scenarios and discuss bow shock and magnetopause crossings, and the state of magnetospheric current systems. We will also compare the observed magnetic fields with model fields - essential for extracting satellite-magnetosphere interactions from the background magnetospheric field. For solar wind and magnetosheath encounters we will report on the structure of the IMF and the proximity of the encounter to Saturn's magnetosphere.
P42A-05 11:20h
Modelling Surface Temperatures on Titan by a General Circulation Model: Constraints on the Surface Type
This study investigates the seasonal and spatial variation of the surface temperature and air temperature in the lower troposphere by a 3-dimensional general circulation model for different putative surface types (porous icy regolith, rock-ice mixture, hydrocarbon lakes) in an effort toconstrain the likely surface type and to understand the meteorology near the surface. For any viable surface type the surface temperature is unlikely to be constant through the year and should more or less vary seasonally and even diurnally, most likely by a few K. Unless the surface is liquid the lower troposphere gets convective near the summer pole where patchy tropospheric clouds have recently been observed. The predicted surface temperature is consistent with Voyager 1 data as long as the thermal inertia of the surface is relativelysmall and/or the surface albedo is low, while a dominance of water ice (high thermal inertia and high albedo) at the surface is unfavourable to reproduce the observation. The lake surface undergoes a larger seasonal variation than the solid surface because of the low albedo and low surface drag. During the Cassini mission the surface temperature ought to be asymmetric about the equator for any surface type, with a warmer south, unless there is a strong accumulation of haze near both poles.The air temperature in the lower troposphere exhibits a smaller latitudinal gradient than the surface and a pole-to-pole gradient due to the presence of a pole-to-pole Hadley circulation, indicating that the temperature within the planetary boundary layer may exhibit a vertical profile characteristic of season, location and scenario. There may be a shallow near-surface inversion layer in cold seasons and a shallow convective layer in warm seasons.
P42A-06 11:35h
The Nature of the Plasma Density Enhancements near Titan
At the time of the Voyager 1 close encounter with Titan, enhancements of density and sharp decreases of electron temperature were observed by the PLS instrument. These were interpreted by Eviatar et al.(1982, JGR, 87, 8091) as plumes drawn out of the ionosphere of Titan by the corotation electric field. A few smaller plumes were identified inside and outside Titan orbit and their displacements correlated with variations in solar wind dynamic pressure observed by Voyager PLS one and two corotation periods earlier. The subplumes were understood to be the remnants of plasma drawn out earlier and the rate of decay was associated with the assimilation of the plume into the ambient plasma. This model was disputed by Goertz (GRL, 1983, 10, 455) who viewed them as blobs of plasmas detached by centrifugal force from the central body of Saturn plasma in the inner magnetosphere. The Voyager PLS instrument was unable to make a firm composition determination which would have resolved the question, although Hartle et al. (JGR, 1982, 87, 1383) did identify an ion of mass to charge ratio 28 in the near-Titan plasma. The Cassini CAPS instrument will be able to determine the composition of the plumes/blobs and to resolve the issue during the Titan encounter of October 26, 2004. Results of the composition analysis will be presented at the December AGU.
P42A-07 11:50h
A Global Hybrid Model of Titan's Ion Escape
Titan, the largest moon of Saturn and an important target for the Cassini mission, has a unique plasma interaction with the Kronian corotating plasma. Titan's dense and nitrogen rich atmosphere is the primary source of nitrogen torus around the Titan's orbit in the Kronian system through the subsonic and superAlfvenic interaction between Saturn's corotating plasma and the exosphere and ionosphere of Titan. We have developed a global numerical model to study the magnetic erosion process at Titan. [Kallio et al., Geoph. Res. Lett., 31, L15703, doi:10.1029/2004GL020344, 2004] Our simulation model is a quasi-neutral hybrid model, which can model both subsonic and supersonic plasma flow. In the presentation we will show analysis based on an improved version of the model that contains four ion species ($H^+, N^+, CH_4^+, N_2^+$) and also mimics the electron impact ionization. We will present runs in which we have studied the effect of the Saturn Local Time on the Titan - co-rotating plasma interaction. Finally, implications of the results for the anticipated scientific return from the Cassini plasma and field instruments will be discussed.
P42A-08 12:05h
Maintenance of Equatorial Superrotation in Titan's Atmosphere
The long-standing problem of the existence of equatorial superrotation in a rotating planetary atmosphere has been solved. Various momentum force terms that could drive the equatorial superrotation are examined. In general, it is found that: (i) the pumping by thermal tides is the only dominant momentum source for the equatorial superrotation; (ii) a small planetary rotation rate is not crucial for the existence of an equatorial superrotation; (iii) it is unlikely that the eddy horizontal momentum transfer in a slowly rotating atmosphere such as Venus' is against its gradient, though this remains an open question; (iv) upward momentum flux by Hadley circulation below the jet center of an equatorial superrotation is a momentum sink, which will impede rather than assist the formation of the equatorial superrotation. Each of ten terms in the averaged zonal momentum equation is evaluated analytically. The analytic forms of the approach explicitly show how an equatorial superrotation of a planetary atmosphere is dependent on various external and internal parameters, including: (i) Sun-planet distance, (ii) radius of planet, (iii) rotation rate, (iv) inclination of the equatorial plane, (v) gravity, (vi) atmospheric scale height, (vii) atmospheric buoyancy frequency, (viii) frictional drag, (ix) albedo, and (x) the pressure level at which the deposition of the solar radiation occurs. When the general solution is applied to Titan's atmosphere, the results are: (i) no solution is found for Titan's stratospheric equatorial superrotation at 1-mb level; (ii) however, if the main absorption layer of the solar radiation in Titan's atmosphere is lifted from 1 mb ($\sim$185 km) to above 0.15 mb ($\sim$270 km) level, a stable equatorial superrotation of $ > $86 m/s can be maintained near the solstice. Two momentum sinks for Titan's superrotation are frictional drag force and meridional advection of wind shear by horizontal branches of the Hadley circulation.