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S. Renner, B. Sicardy (Observatoire de Paris, LESIA)
Recent HST images of Prometheus and Pandora show that their longitudes differ by about 20 degrees from the predictions based on Voyager images. Moreover abrupt changes in mean motion occurred around the end of 2000 (French, R.G. et al., Icarus, 162, 143-170, 2003). These discrepancies are anticorrelated and arise from chaotic interactions between the two moons, the strongest jumps of orbital elements happening at interval of 6.2 years when their apses are anti-aligned (Goldreich, P. and Rappaport, N., Icarus, 162, 391-399, 2003).
We study the system Prometheus-Pandora using a radau-type integrator taking into account Saturn's oblateness and the perturbations by the major saturnian satellites. We first confirm the chaotic behaviour of the system due to the overlap of four 121:118 apse-type mean motion resonances. Then we show how the available observations suggest values as low as 0.5 g.cm-3 for Prometheus and Pandora. These values compare well the densities derived from the density waves excited in Saturn's rings by the two satellites.
Finally we discuss the long-term evolution of the system : using a wide range of initial conditions compatible with observations, we do not detect in general any systematic transfer of angular momentum from Pandora to Mimas via the nearby 3:2 resonances.
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.