Theoretical Modelings

Titan's orbit is slightly eccentric (e = 0.029): this suggests the absence of a shallow ocean on Titan's surface which would tend to reduce the eccentricity value, transforming the orbit into a circular one. Now, the mean density of Titan is 1.88 g cm-3. This low value (even lower than Europa) indicates an internal structure made of low density materials—ices, like water ice, of a mean density of approximately 1 g cm-3 mixed with high density materials, such as rocks and silicates, of a mean density of about 3 g cm-3.

Titan originated about 4.6 billion years ago from the accretion of small planetesimals present in Saturn's sub-nebula. The heat generated by gravity processes and radioactive decay melted the icy components making the rocks sink toward the centre of the forming Titan. The resulting initial internal structure was a central core made of undifferentiated silicate, mixing rocks and ices, covered by a silicate outer core and a liquid mantle of water-ammonia mixture, in contact with a dense atmosphere (Fig. 3.6). The origin and composition of this primordial atmosphere are still under debate. Recent models of the evolution of the Saturn sub-nebula indicate that the planetesimals which formed Titan included CH4 and NH3 in the form of hydrates (and not CO or N2 which are not efficiently trapped as hydrates). Then CH4 and NH3 were progressively released to the atmosphere. NH3 was converted into N2 by photochemical processes in the upper regions of the atmosphere or by impact shock catalyzed dissociations. At that time Titan's surface was warm enough to maintain an aqueous environment for some (up to 100) millions of years. As its energies became less abundant, it cooled down and liquid water started freezing out of the H2O-NH3 solution. An ice I crust (Ice I is the form of all natural snow and ice on Earth) formed, the thickness of which rapidly increased to 30 km in about 70 million years, covering an internal ocean initially in contact with a rocky bottom. The high pressure at this bottom induced the crystallization of water ice VI (high pressure water ice) and ammonia hydrates, enriching the ocean in

Figure 3.6. Internal structure of Titan; left: primordial Titan; right present Titan (adapted from Icarus, Vol. 146, Fortes24; Copyright 2000 with permission from Elsevier).

Figure 3.6. Internal structure of Titan; left: primordial Titan; right present Titan (adapted from Icarus, Vol. 146, Fortes24; Copyright 2000 with permission from Elsevier).

ammonia. The thickness and composition of this ocean depend on the heat flow and convective transport; it could be about 200 km thick, containing up to 15 wt % of NH3.

Concerning the evolution and present state of the atmosphere, several kinetic models, mainly photochemical models, have been published. Their main output is the vertical concentration profile of major and trace species. Because ofthe lack of detailed observations and data on the eddy diffusion coefficient prior to Cassini-Huygens, these profiles were relatively poorly constrained. However, these models provided a preliminary understanding of the chemical processes involved in this complex atmospheric chemistry. They assumed that the chemical scheme starts with the dissociation of CH4 and N2 by the solar UV photons and the electrons from Saturn's magnetosphere. The resulting primary species, formed in the high atmosphere, yield simple hydrocarbons and nitrogen-containing molecules, particularly C2H2 and HCN. Once they are formed, theses two compounds diffuse to the lower atmospheric regions where they can still absorb the available UV photons (in the near UV) because their UV spectra extend to that region. Although less energetic than the Mid-UV, these photons can produce photo-dissociation. The resulting radicals produce more complex hydrocarbons and nitriles. In the chemical processes polyynes, such as C4H2 and C6H2 and cyanopolyynes, like HC5N, may play an important role in the formation of high molecular weight products. Additional CH4 dissociation probably occurs in the low stratosphere through photo-catalytic processes involving C2H2 and polyynes. The end pro duct ofthis chemistry would be a macromolecular organic compound mainly made of C, H and N. However, as described below, the measurements reformed by the Cassini-Huygens mission15,16 on Titan's ionosphere suggest that organic processes in this high atmospheric zone must also contribute to the formation of this macromolecular matter. But detailed models of this ion chemistry coupled to photochemical models still need to be developed.

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