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Project Progress

Due to their proximity, the irregular satellites of Jupiter have been subjects of extensive observational and theoretical research. As shown by Sheppard & Jewitt (2003), the region extending from the orbit of Callisto at 26 Jupiter-radii (RJ), to the periastron of Themisto (?76RJ), is void of irregulars. The focus of this project is on understanding the reason for the lack of irregular satellites in the boundary between regulars and irregulars. We numerically integrated the orbits of several hundred test particles in a region interior to the orbit of Themisto. We assumed that the regular satellites of Jupiter were fully formed and studied the perturbative effects of the Galilean satellites on the dynamics of small objects. Our simulations showed that, except at large distances from Ganymede and Callisto, the lack of irregular satellites can be attributed to the instability of these objects caused by their interactions with the two outermost Galilean satellites (Figure 1). At larger distances (e.g., between <40RJ and 80RJ for particles in circular orbits, and between <60RJ and 80RJ for particles with initial orbital eccentricities of 0.4), the perturbations of Galilean satellites do not seem to be able to account for the instability of small bodies. A possible explanation is that their instability is the result of interactions with Jovian satellitesimals and protosatellites during the formation of Jupiter’s regular moons. Figure 2 shows the region of stability of Jovian irregular satellites obtained from our numerical simulations. As shown here, this region coincides with the region where irregular satellites actually exist.

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