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C.C. Porco (CICLOPS/SSI), P. Thomas (Cornell), J. Spitale (CICLOPS/SSI), R.A. Jacobson (JPL), T. Denk (Freie Universitat Berlin), S. Charnoz (Universite Paris 7), D.C. Richardson (Univ. Maryland), L. Dones (SwRI), E. Baker, J. W. Weiss (CICLOPS/SSI)
We present Cassini imaging results on the orbits and physical properties for the small ring-region moons Pan, Atlas, and the Cassini-discovered Keeler gap moon, S/2005 S1 (1), as well as the newly discovered/recovered moons orbiting among the major satellites, Methone (S/2004 S1), Pallene (S/2004 S2), and the Dione co-orbital S/2004 S5 Polydeuces (2,3,4). We find that Atlas is undergoing a 700-km amplitude longitudinal perturbation by Prometheus, Methone is undergoing a 30,000-km amplitude longitudinal perturbation by Mimas, and Pallene is undergoing a long-term 75-km amplitude longitudinal perturbation by Enceladus. Orbital integrations involving Atlas return a mass of GMAtlas = (0.43 ±0.18) X 10-3 km3/sec2, three times larger than previously reported (4).
Reasonably high resolution images have also allowed refinement of physical dimensions and spectral properties of these small moons. Results will be presented. At the time of writing, we find that Atlas has polar and equatorial diameters of 19 km, 38 km and 46 km, respectively. Its volume is (1.5 ±0.4) X 104 km3, yielding a density of 0.43 ±0.20 gm/cm3. Pan's polar diameter is 23 km, and differences in its equatorial axes are not well constrained; they both appear to be ~ 35 km. Pan's volume is (1.4 ±0.7) X 104 km3. Using the most currently reliable mass, GMPan = (0.33 ±0.05) \times 10-3 km3/sec2 (4), Pan's density is roughly 0.4 ±0.2 gm/cm3. Both Pan and Atlas appear to be synchronous rotators, but libration cannot be ruled out yet. Given its shape, it is possible that Atlas is in a secondary spin-orbit resonance that could force a libration. Preliminary idealized rubble pile simulations have been performed which show that, at the orbits of Atlas and Pan, a simple self-gravitating ice-particle aggregate, with equal equatorial dimensions, would be stable against tides; a body with sufficiently unequal equatorial dimensions would not.
[1] IAUC 8524. [2] IAUC 8389. [Correction: Pallene (S/2004 S2) is the same body as S/1981 S14 (IAUC 6162)]. [3] IAUC 8432. [4] Porco et al. 2005, Science, 307, 1226.
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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.