Titan ’ s spin , gravity , and moments of inertia

نویسندگان

  • Bruce G. Bills
  • Francis Nimmo
چکیده

Introduction Analysis of Doppler tracking data and radar images from the Cassini spacecraft have recently provided estimates of the low degree gravity field [1], and spin pole direction [2] of Titan. We examine implications of these measurements for the internal structure and rotational dynamics of that body. We derive separate estimates of the polar moment of inertia of Titan from the degree two gravity field, under the assumption of hydrostatic equilibrium, and from the spin pole direction, under the assumption of a fully damped spin-orbit configuration, or multi-frequency Cassini state. These estimates are quite different. We interpret the gravity-derived value as the actual moment of inertia of Titan, and the larger spin-derived value as an effective moment of inertia of a mechanically decoupled ice shell. This implies a sub-surface ocean, as the decoupling agent. Gravity constraints: For a body in hydrostatic equilibrium and synchronous rotation, the imposed tidal and rotational potentials together induce changes in the mass distribution which are mainly manifest as degree two spherical harmonic coefficients in the gravitational potential [3]:

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تاریخ انتشار 2009