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Ring plane crossings and Saturn’s polar precession rate II.

Presentation #112.04 in the session Many Planets, More Rings (Oral Presentation)

Published onOct 23, 2023
Ring plane crossings and Saturn’s polar precession rate II.

An accurate measurement of Saturn’s spin axis precession rate, and thus of its moment of inertia, would contribute substantially to our knowledge of the planet’s interior structure, augmenting information gleaned from studies of its shape, gravitational and magnetic fields and internal modes of oscillation. Present estimates of Saturn’s precession rate are largely based on precise measurements of the orientation of its ring plane, as determined by a combination of radio and stellar occultation observations, as well as on astrometric data for Titan and the mid-sized icy satellites. By combining data from two ring occultations observed by Voyagers 1 and 2 in 1980/81 with Earth-based observations of the occultation of 28 Sgr in 1989, French et al. (Icarus, 1993) obtained a precession rate of (0.63±0.23)”/yr, consistent with the predicted long-term average value of 0.74”/yr (Ward, AJ, 1975). Subsequently, French et al. (Icarus, 2017) used a complete set of Cassini radio and stellar ring occultations to obtain a significantly-lower mean rate of (0.451±0.014)”/yr over the period 2005 - 2017.

Here we present the results of a critical re-analysis of Saturn’s ring plane crossing (RPX) times, as observed from Earth over the period 1714 – 1995, updating preliminary results reported by Nicholson & French (BAAS, 1997; 1999, 2005). By fitting the deviations in the observed RPX times from those predicted using the 1980 pole of French et al. (1993) to a model of uniform precession, we estimate the mean polar precession rate over this period.

Our preliminary result of (0.63±0.05)”/yr, based on a subset of 15 RPX times, is consistent with that of French et al. (1993) but significantly greater than the Cassini value (French et al. 2017). The tightest constraints are provided by the RPXs of Oct 1907, Jan 1908, Feb 1921, Dec 1966 and May & Aug 1995, so this may be regarded in practice as an average over the 20th century. The effect of precession is to shift the RPX time by ~50 min over 15 yr and up to 4-5 hr over 75 yr, depending on the geometry of each crossing, while our post-fit residuals range from <10 min to 3 hr for the best data.

We suggest that the greater rate prior to the year 2000 reflects the varying torques exerted by Titan, and to a lesser extent Rhea, Tethys and Mimas, on Saturn’s oblate figure due to their varying orbital inclinations wrt the planet’s equatorial plane, as modeled by Vienne & Duriez (A&A, 1992), Jacobson (AJ, 2022) and Wisdom et al. (Science, 2022). We will present the results of fitting such a quasi-numerical model to the RPX data, in order to constrain the polar moment of inertia directly.

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