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W.R. Ward (Southwest Research Institute), D.P. Hamilton (U. of Maryland & Southwest Research Institute)
The obliquities of Jupiter and Saturn are 3.12 degrees and 26.73 degrees, respectively. If both planets formed via hydrodynamic inflow of gas onto pre-existing cores, why are these values so dissimilar? Random obliquities of the cores could not themselves account for this difference. We propose instead that Saturn's obliquity is a post-formation trait due to a secular spin-orbit resonance with Neptune, and that Saturn is still locked into this state today. The resonance involves a 1:1 commensurability between Saturn's spin axis precession rate and the \nu18 secular frequency of the solar system (1.87 x 106 year period) that is the primary term in the precession of Neptune's orbital node. A simple analytic theory is presented that explains the effects of resonance passage and what is required for Saturn capture and evolution to a ~27 degree obliquity.
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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.