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[9.02] Prometheus and Pandora : masses and orbital positions during the Cassini tour

S. Renner, B. Sicardy (Observatoire de Paris, LESIA)

HST images of Prometheus and Pandora show longitude differences of about 20 degrees from the Voyager ephemeris, with an abrupt change in mean motion at the end of 2000 (French, R.G. et al., Icarus, 162, 143-170, 2003; French, R.G., and McGhee, C.A., BAAS, 34, No. 06.07, 2003). These discrepancies arise from chaotic interactions between the two moons, occuring at interval of 6.2 years when their apses are anti-aligned (Goldreich, P. and Rappaport, N., Icarus, 162, 391-399, 2003), a behavior attributed to the overlap of four 121:118 apse-type mean motion resonances (Goldreich, P. and Rappaport, N., Icarus, 166, 320-327, 2003). This is confirmed by numerical integrations that include the perturbations of the major satellites of Saturn (Renner, S. and Sicardy, B., BAAS, 35, No. 04.06, 2003; Cooper, N.J. and Murray, C.D., AJ, 127, 1204-1217, 2004).

We study the Prometheus-Pandora system using a radau-type integrator taking into account Saturn's oblateness up to terms in J₆ and the effects of the major satellites. By fitting the numerical integrations to the HST data (French et al., 2003), we derive the satellite masses. Using the nominal shape of the two moons (Thomas, P.C., Icarus, 77, 248-274, 1989), Prometheus and Pandora densities are 0.40 ±^0.04_0.07 and 0.49 ±^0.06_0.09 g.cm^-3, respectively, with a 99,99 % confidence level. Our numerical fits also allow us to constrain better the time of the latest apse anti-alignment in 2000.

Finally, using our fit, we predict the orbital positions of the two satellites during the Cassini tour. We provide a lower limit of the uncertainties due to chaos, amounting to about 0.2 degrees in mean longitude at the arrival of Cassini in July 2004, and about 3 degrees in 2008.

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Bulletin of the American Astronomical Society, 36 #2
© 2004. The American Astronomical Soceity.