Hydrodynamic simulations of moonlet induced propellers in Saturn's rings: Application to Bleriot

One of the biggest successes of the Cassini mission is the detection of small moons (moonlets) embedded in Saturn's rings which cause S-shaped density structures in their close vicinity, called propellers (Spahn and Sremcevic 2000; Tiscareno et al. 2006; Sremcevic et al. 2007). Here, we present isothermal hydrodynamic simulations of moonlet-induced propellers in Saturn's A ring which denote a further development of the original model (Spahn and Sremcevic 2000). We find excellent agreement between these new hydrodynamic and corresponding N-body simulations. Furthermore, the hydrodynamic simulations confirm the predicted scaling laws (Spahn and Sremcevic 2000) and the analytical solution for the density in the propeller gaps (Sremcevic et al. 2002). Finally, this mean field approach allows us to simulate the pattern of the giant propeller Bleriot, which is too large to be modeled by direct N-body simulations. Our results are compared to two stellar occultation observations by the Cassini Ultraviolet Imaging Spectrometer (UVIS), that intersect the propeller Bleriot. Best fits to the UVIS optical depth profiles are achieved for a Hill radius of 590 m, which implies a moonlet diameter of about 860 m. Furthermore, the model favours a kinematic shear viscosity of the surrounding ring material of $\nu_0 = 340$ cm^2/s, a dispersion velocity in the range of 0.3 cm/s $< c_0 <$ 1.5 cm/s, and a fairly high bulk viscosity $7 < \xi_0/\nu_0 < 17$. These large transport values might be overestimated by our isothermal ring model and should be reviewed by an extended model including thermal fluctuations.