Crosby Research Plan

For his senior thesis project in physics, Crosby Burdon will be working on using numerical simulations to explore the stability of moonlets embedded in Saturn's rings. The work will build off of two different lines of previous work. One line is the Cassini observations of propellers in Saturn's rings and the moonlets that are inferred to create them (Seiß et al 2005, Tiscareno et al. 2006, Tiscareno et al. 2008, and Lewis and Stewart 2009). These objects had been predicted earlier, but weren't observable until the Cassini mission arrived. Even with the observations, there are interesting questions related to these objects because of their placement and the fact that they are bright when viewed from the lit or dark sides of the rings. This will also build off work by Porco et al. (2007) looking at the stability of small moons including gap moons in the Saturn system.

The moons of Saturn that are in close to the rings have very low densities. From this they are inferred to be rubble piles. Porco et al. (2007) looked at how these could form and the limits of what will hold together. Their simulations focused on moons that were in orbits away from other material. One might initially assume that moonlets are similar to the small moons, only smaller so that they can't open gaps. However, the moonlets are constantly being hit by ring material which can knock off loosely bound particles. As a result, the moonlets likely can't fill their Roche lobes as completely.

Crosby's work will be to investigate this difference and constrain the properties of moonlets as rubble piles. This exploration will look both at moonlets that are true rubble piles as well as those with a solid core and rubble collected on it using various numerical techniques to set up initial particle distributions to produce these different systems. There are four independent variables in the parameter space.

  • a - Semimajor axis of the moonlet (use Hill radius for core?)
  • adhesion - The strength of non-gravitational adhesive forces between particles (Albers and Spahn 2006) (possibly proportional to friction or Kinetic Energy)


  • density - The density of the individual particles making up the core and rubble ( range between ice and silicate density?)
  • r - The radius of the moonlet core if one is used (use cores with Hill radii reasonable from summer work on propellers)

(Note: consider looking at two-phase accretion where the gap creation separates phases.)

Link to research commentary

Albers, N., Spahn, F., 2006. The influence of particle adhesion on the stability of agglomerates in Saturn’s rings. Icarus 181, 292–301.
Lewis, M. C., Stewart, G. R., Feb. 2009. Features around embedded moonlets in Saturn’s rings: The role of self-gravity and particle size distributions. Icarus 199, 387–412.
Porco, C.C., Thomas, P.C., Weiss, J.W., Richardson, D.C., 2007. Saturn’s small inner satellites: Clues to their origins. Science 318, 1602–1607.
Seiß, M., Spahn, F., Sremˇcevi´ c, M., Salo, H., 2005. Structures induced by small moonlets in Saturn’s rings: Implications for the Cassini Mission. Geophys. Res. Lett. 32. CiteID L11205.
Tiscareno, M.S., Burns, J.A., Hedman, M.M., Porco, C.C., Weiss, J.W., Dones, L., Richardson, D.C., Murray, C.D., 2006. 100-metre-diameter moonlets in Saturn’s A ring from observations of ‘propeller’ structures. Nature 440, 648–650.
Tiscareno, M.S., Burns, J.A., Hedman, M.M., Porco, C.C., 2008. The population of propellers in Saturn’s A ring. Astrophys. J. 135, 1083–1091.