NASA Astrobiology Institute (NAI)

1. Origin of Irregular Satellites

   Project Investigators: Nader Haghighipour, David Jewitt

   Summary

   An interesting feature of the giant planets of our solar system is the existence of regions around these objects where no irregular satellites are observed. Surveys have shown that, around Jupiter, such a region extends from the outermost regular satellite Callisto to the vicinity of Themisto, the innermost irregular satellite. This project aims to understand the reason for the existence of such a satellite–void region by numerically integrating the orbits of several hundred small objects, distributed in a region between 30 and 80 Jupiter-radii.

   Astrobiology Roadmap Objectives:

   • Objective 1.1: Models of formation and evolution of habitable planets

   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.

   
   

   Lifetimes of test particles with initial eccentricities of ep = 0.4 and orbital inclinations of ip = 40&#9702; (black) and ip = 140&#9702; (red). As shown here, particlesin retrograde orbits (red) are more stable. The locations of mean-motion resonances with Ganymede and Callisto are also shown.

   
   

   Graph of the region of the stability of Jovian irregular satellites. Prograde satellites are in purple and retrograde ones are in orange. The inner boundary of this region, shown in light blue, corresponds to the curve of constant-periastron of Themisto. Its outer boundary, shown in purple for prograde irregulars and in orange for retrograde ones, is a curve for constant-apastron equal to the largest distance a Jovian irregular satellite can travel before its orbit becomes unstable by solar perturbation. The curves of constant-periastron corresponding to the influence zones of Ganymede and Callisto and the stable (black) and unstable (red) test particles, within the region of 30RJ to 80RJ , are also shown.

Publications

Haghighipour, N. & Jewitt, D.  (2007).  Irregular Satellites of the Planets: Products of Capture in the Early Solar System.  Annual Review of Astronomy and Astrophysics, 45:261-295.

Haghighipour, N. & Jewitt, D.  (2008).  A Region Void of Irregular Satellites Around Jupiter.  Astronomical Journal, 136:909-918.