The Pole Orientation, Pole Precession, and Moment of Inertia Factor of Jupiter

This paper discusses our determination of the Jupiter’s pole orientation and precession using a combination of Earth-based and spacecraft based observational data. The direction and precession of Jupiter’s pole are needed to orient the Jupiter gravity field for satellite and spacecraft orbit modelling and to determine Jupiter’s polar moment of inertia for studies of Jupiter’s interior (the precession rate depends upon that moment of inertia). The model for the motion of the pole of Jupiter is based on the rotational equations of motion for a rigid body. The equations assume that Jupiter is axially symmetric and that the torques applied are derived from the Sun, Saturn, and the Galilean satellites acting on Jupiter’s figure as represented by its zonal gravitational harmonics. We numerically integrated the equations over the 200~year period from 1900 to 2100 and fit the integrated pole orientation angles with a Fourier series. A consequence of our pole model is that the Jupiter axial moment of inertia effectively scales the pole rates and the coefficients of the trigonometric series. When we fit the satellite orbits and spacecraft trajectories to the observational data, we estimate the leading terms in the series and the fractional improvement in the moment of inertia as the series scale factor. We present the pole orientation, precession rate, and moment of inertia from our latest analysis of our Jovian system observation set: satellite astrometry through 2018, satellite mutual events through 2015, satellite eclipse timings from 1650 to 2016, and data acquired by the Voyager, Galileo, Cassini, and New Horizons.