Tidal Despinning of Iapetus, the Eighth Satellite of Saturn

Details Tidal Despinning of Iapetus, the Eighth Satellite of Saturn Tidal Despinning of Iapetus, the Eighth Satellite of Saturn

May 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009dda....40.0804e&link_type=abstract

American Astronomical Society, DDA meeting #40, #8.04; Bulletin of the American Astronomical Society, Vol. 41, p.903

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Scientific paper

While currently Iapetus is locked into a 1:1 spin-orbit resonance with Saturn, its original state was likely one of rapid rotation. Simulations (Aleshkina 2009) have demonstrated that: (1) with a high arbitrariness in the initial conditions, Iapetus had passed without delay through the 5:2, 2:1, and 3:2 spin-orbit commensurabilities, and (2) its tidal-despinning time was about hundred billion years, an unphysical result for a moon in a spin-orbit lock. (For the other large satellites of Saturn these times are shorter by orders of magnitude.) Hence the necessity to seek efficient tidal-despinning mechanisms.
One such mechanism was introduced by Castillo-Rogez et al. (2007), who derived accelerated tidal dissipation from the presence of short-lived Al and Fe isotopes. These could have created a warm and low-viscosity interior permitting an initial despinning phase, with the rotation period of 7 hr decreasing to 15-16 hr, while preserving the non-hydrostatic,
fossil oblateness.
Since the amount of 26Al used in models implies a range of times of formation for Iapetus and thus the Saturnian system, it is crucial to constrain the processes and parameters involved. In Castillo-Rogez et al. (2007), the dynamical equations were simplistic, while the dissipation calculation relied on the Maxwell model, not on laboratory data. Recent developments increase the level of realism in coupled geophysical and dynamical modeling. Efroimsky and Lainey (2008) and Efroimsky and Williams (2009) have improved the theories of tidal lagging and despinning, while the measurements tackled at the JPL (Castillo-Rogez et al. 2009) yield a realistic dissipation model for ices in the conditions relevant to Iapetus. We are exploring if the realistic frequency-dependence of the attenuation rate can lead to efficient tidal despinning. Our purpose is to determine whether or not 26Al is required to achieve synchronous rotation. This will enable us to constrain the time of Iapetus’ formation.

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