Propellers in Saturn's rings - 3D-structure and the grand finale of the Cassini-mission

• Applicant: Dr. Holger Hoffmann
• Subject Area: Astrophysics and Astronomy
• Term: from 2016 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 287191650

The edge-on illumination of Saturn's rings in the months before and after Saturn's vernal equinox in 2009 revealed the otherwise unobservable vertical structure of moonlet-induced propeller features. Propeller heights of a few hundred meters were deduced from the length of cast shadows shown in images taken by the Cassini spacecraft. These propeller heights are the result of strongly increased values of the ring particle velocity dispersion in the vertically excited propeller region, showing clearly the need for non-isothermal models of propellers.In this project we plan to extend our 3D-propeller-model, which describes the giant propellers orbiting between the Encke and Keeler gaps well, to small and medium sized propellers (the majority of known propellers). Therefore, we will use N-body box simulations of propellers to study the functional dependence of the pressure and transport coefficients like viscosity and heat conduction on the granular temperature and the mass density. With the results of these numerical experiments, we will revise our propeller height relaxation model, evaluating the influence of diffusive processes like heat conduction on the azimuthal height decay of smaller propellers.Further, we want to investigate additional processes which influence the height of propellers: Vertical splashing in the propeller wake region and the vertical excursions of debris released in high-speed impacts of large ring particles to which the debris is otherwise adhesively bound. The results will be applicable to propellers but they might also clarify the nature of the putative moonlet S/2009 S1, which orbits in the outer B ring and was only seen because of its vertical structure. Which lessons, learned from studying the vertically excited region of the propeller, affect our knowledge of the lateral features of propeller structures? We will examine the impact of the massively increased values of the ring particle velocity dispersion (and thus the ring viscosity) on the mass diffusion into the propeller gaps. This will allow us to estimate ring viscosities from the azimuthal evolution of the propeller gap.Additionally, we plan to investigate the implications of the liberated debris on the visibility of the propeller gap region, which might shed light on the fact that to date the largest known propeller Bleriot is the only one showing well formed gaps.The grand finale of the Cassini mission, starting in November 2016, promises propeller observations with the highest resolutions since the Saturn insertion manoeuvre. During this phase, we might not only be able to observe gaps for other propellers than Bleriot, but Cassini might show us, for the first time, individual moonlet-wakes. Our revised models will help to analyze the expected very high resolution images (10 times higher resolution than typical propeller images).

• DFG Programme: Research Grants
• Co-Investigator: Professor Dr. Frank Spahn, Ph.D.