Models of the protosatellite disk of Saturn: Conditions for Titan's formation

Models of the protosatellite accretion disk of Saturn are developed that satisfy cosmochemical constraints on the volatile abundances in the atmospheres of Saturn and Titan with due regard for the data obtained with the Cassini orbiter and the Huygens probe, which landed on Titan in January 2005. All basic sources of heating of the disk and protosatellite bodies are taken into account in the models, namely, dissipation of turbulence in the disk, accretion of gaseous and solid material onto the disk from the feeding zone of Saturn in the solar nebula, and heating by the radiation of young Saturn and thermal radiation of the surrounding region of the solar nebula. Two-dimensional (axisymmetric) temperature, pressure, and density distributions are calculated for the protosatellite disk. The distributions satisfy the cosmochemical constraints on the disk temperature, according to which the temperature at the stage of the satellite formation ranged from 60-65 K to 90-100 K at pressures from 10(-7) to -10(-4) bar in the zone of Titan's formation (according to estimates, r = 20-35R(Sat)). Variations of the basic input parameters (the accretion rate onto the protosatellite disk of Saturn from the feeding zone of the planet (M) over dot; the parameter alpha characterizing turbulent viscosity of the disk; and the mass concentration ratio in the solid/gas system) satisfying the aforementioned temperature constraint are found. The spectrum of models satisfying the cosmochemical constraints covers a considerable range of consistent parameters. A model with a rather small flux of (M) over dot = 10(-8) M-Sat/yr and a tenfold depletion of Saturn's disk in gas due to gas scattering from the solar nebula is at one side of this range. A model with a much higher flux of (M) over dot = 10(-6) M-Sat/yr and a hundredfold decrease in opacity of the disk matter owing to decreased concentration of dust particles and/or their agglomeration into large aggregates and sweeping up by planetesimals is at the other side of the range.