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Capability Development for Modeling the Spin Axis Evolution of the Giant Planets, 15-9310

Principal Investigator
William R. Ward
Co-Investigator
Douglas P. Hamilton, University of Maryland

Inclusive Dates: 04/10/02 - 08/1/02

Background - A fundamentally important and unsolved question in the solar system history can be stated simply as: "What caused the tilts of planetary spin axes?" The study of the origin of planetary tilts (or obliquities) has a long history. Most efforts have focused on the tilts of the terrestrial planets, which are dominated by statistical effects from their late stage accretion of 1,000-kilometer-sized bodies. Perhaps even more interesting, however, are Jupiter and Saturn, both of which gained the vast bulk of their masses by direct accretion of gas from the solar nebula. This mode of accretion should produce planets with almost zero obliquity and, indeed, Jupiter's obliquity at 3.1 degrees is the lowest among all objects that have not been tidally despun. Mysteriously, Saturn is tilted by 26.7 degrees. Ward and Hamilton believe they have found the reason for this tilt.

Approach - The team has developed the software to numerically track the spin axis evolutions of the giant planets over very long time scales [on order of 10⁹ years]. The problem has been approached both analytically and numerically; including the development of a state-of-the-art in-house code that allowed the team to attack a number of important scientific problems concerning the origin and evolution of the spin states of the planets. The code combines synthetic models of the long period changes in planetary orbits with equations that describe the evolution of the planetary spin axis.

Accomplishments - As  first application, the code was used to track the history of Saturn's obliquity, and it was discovered that Saturn is currently trapped into a secular spin-orbit resonance with Neptune. The spin axis precession period of Saturn matches the precession period of Neptune's orbit, that is, 1.87 × 10⁶ years. The pole position of Saturn can be explained as a consequence of passing through this resonance state twice, once in the forward direction and once in reverse. The investigators have identified the contraction of Saturn due to cooling after its formation as responsible for the first passage by slowing down its pole precession. Then the team identified the erosion of the Kuiper belt, a population of debris orbiting beyond Neptune, as responsible for the second passage by slowing down the orbit precession of Neptune. Detailed numerical simulation can now produce the current Saturn spin state with high fidelity.

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