Charged Dust Dynamics in Saturn's Rings

Abstract

Dust is ubiquitous throughout the Solar System and beyond. In the main rings of Saturn, dust
becomes electrically charged through interactions with sunlight and the plasma environment,
leading to complex phenomena for which many questions still remain to be answered. Key issues
are introduced in Chapter 1, through examples of the elusive spokes of the B ring, streaming
particles from the outer rings, and ring rain.
The dynamics of dust in the presence of planetary magnetic and gravitational fields is studied
using numerical simulations of grain trajectories in Chapter 2; electromagnetic forces are added
to an existing N-body code and novel adaptive integration methods are implemented. In Chapter
3, a constant charge-to-mass ratio is assumed, in order to test the numerical method and independently
verify the theoretical and simulation results of older and more recent literature. The
planet is treated as spherical, with an aligned and centred magnetic dipole, so that analytical
boundaries for stability can be derived for the radial, azimuthal and vertical motions of dust.
Depending on the launch location and their positive or negative surface potential, grains can remain
in orbit, collide with the planet at low latitudes, climb up magnetic field lines to collide at
higher latitudes, or escape the ring system. The full extent of radial distances from the innermost
to the outermost parts of the ring system are considered, for both positive and negative grain potentials,
covering a wide range of charge-to-mass ratios that encompass the gravitationally- and
electromagnetically-dominated regimes.
The effects of random discrete charging events on nanodust dynamics are of paramount importance.
In Chapter 4, with the numerical method verified, the assumption of constant grain surface
potential is relaxed and a novel stochastic charging algorithm is developed so that the dynamics
of nanograins can be studied. The periodic modulating effect on the grain’s charge as it transits
the planetary shadow is included as this can have a significant destabilising effect.
In Chapter 5, attention is then directed back to ring rain, the precipitation of dusty material from
the rings onto the planet’s ionosphere. Results are presented that corroborate the hypothesis that
ring rain may not be a cleaning mechanism, whereby the rings preferentially rain out pollutants,
leaving the young age of Saturn’s rings a possibility. The need for a systematic sensitivity analysis
to fully explore the chaotic nature of grain trajectories under the full range of environmental
and grain parameters is described in Chapter 6.

Date of Award	23 Mar 2021
Original language	English
Awarding Institution	University of Bristol
Supervisor	Zoe M Leinhardt (Supervisor) & Mark Birkinshaw (Supervisor)