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F. Namouni, S. Tremaine (Princeton University)
Saturn's satellite Iapetus and Neptune's satellite Triton are the only regular satellites with anomalous inclinations. Iapetus has a free inclination of 7 deg. with respect to the Laplace invariable surface and Triton's orbit is retrograde and inclined by 23 deg. with respect to Neptune's equator plane. We examined the secular evolution of orbits under the combined influence of planetary oblateness and solar tides and found a variety of interesting behavior, particularly in the transition region where the oblateness effect and solar tides balance for planets with large obliquity. Circular orbits in the Laplace surface are unstable if the obliquity exceeds 68.875 deg. The instability arises when a sequence of stable circular orbits of increasing semimajor axis bifurcates into two sequences: one of unstable circular orbits and the other of stable eccentric orbits. Chaotic orbits were also found near the transition region, suggesting that initially circular prograde satellites orbiting near the Laplace plane can attain large inclinations. This kind of orbit offers a natural possibility for exciting satellite inclinations. Subsequent tidal migration of the satellite would displace it away from the transition zone.
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