DPS 34th Meeting, October 2002
Session 28. Solar System Origin, Planet and Satellite Formation
Oral, Chair(s): W.R. Ward and G.J. Consolmagno SJ, Thursday, October 10, 2002, 2:00-4:00pm, Room M

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[28.09] Obliquity of Saturn: Numerical Model

D. P. Hamilton (University of Maryland and Southwest Research Institute), Wm. R. Ward (Southwest Research Institute)

We have identified a secular spin-orbit resonance with Neptune that is responsible for Saturn's tilt. To test our hypothesis, we developed a numerical model which couples the secular variations of giant planet orbits with the evolution of their spin axes. Starting with a zero-obliquity Saturn, we find that capture into the Neptune resonance will drive Saturn's obliquity to 27 degrees. The mechanism requires that Neptune's nodal frequency slowly decrease due to an external agent, which we associate with the depletion of the Kuiper belt. Thus, Saturn's resonance capture implies a lower limit on the amount of mass lost from the primordial belt.

The observed azimuthal position of Saturn's spin axis, however, is inconsistent with this simple scenario. Further experimentation shows that both the obliquity and azimuthal constraints can be met if Saturn had an inclination of some 5-10 degrees when it was captured into the secular resonance with Neptune. We find that this initial inclination can result from the same Neptune resonance, if the resonance was first traversed in the direction that precludes capture. We identify the slow contraction of Saturn, which decreases the planet's precession frequency, as a natural driver for this process.

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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.