Allyson Ducey

Introduction

I'm an undergraduate Physics/Art major Astronomy minor at Trinity University, entering my senior year at the end of the summer. During the last ten weeks I have worked with Dr. Mark Lewis on analyzing Saturn's rings simulations to study the Keeler gap. This page documents my research.

Lewis' work on the Keeler gap prior to this can be found here.

The Keeler gap is a narrow division in the outer part of the A ring.

When it was initially discovered by Voyager, the Keeler gap was assumed to be a miniature version of the Encke gap; maintained by a small embedded moon, with far-reaching radial features and sinusoidal edge wakes which last as many as 500 orbital periods all the way back until they meet up with the moon again before damping out.

What Cassini found turned out to be strikingly different. While there is indeed an embedded moon, Daphnis, the moon's significant eccentricity relative to the ring particles causes a particulaly non-sinusoidal edge shape. Additionally, the edge wakes damp out in only a few orbital periods and the radial features that are so prominent in the ring material near the Encke gap are hardly visible near the Keeler gap. These unexpected differences show that the Keeler gap is a valuable region of study for ring dynamics, and can contribute to our knowledge of ring structure and formation.

This research did not attempt to construct an exact model of the Keeler gap system; our interest was in examining the dynamics of systems similar to the Keeler gap and analyzing in detail how the structures are influenced by parameters such as inclination and longitude of the ascending node.

Abstract

We have seen that gap moons within Saturn’s rings perturb the ring material on either side and create wakes on the edges of the rings. A parameter that has not been heavily investigated is the inclination of the gap moon; our research show how this orbital element contributes to the wake structure. We have also studied features in the z direction of the edge structure. This is accomplished with the use of N-body spherical particle simulations with collisions and self-gravity that model the Keeler gap region of Saturn’s rings.  The simulations are input into a visualization program called SwiftVis, from which we can easily analyze different aspects of the resulting wake structure. In the simulations, the known characteristics of the Keeler gap and its moon Daphnis, such as eccentricity, mass, and width of the gap, remain at the observed values of e = 3.3E-5, m = 8.4E13 kg, and d = 42 km, respectively. Characteristics which greatly affect the efficiency of the simulations, such as particle size, density, and optical depth of the ring material, are kept as accurate as the available computing power allows. Our simulations use r = 7.56 m, ρ = 0.5 g/cm^3, and τ = 0.2; these values lead to particle simulations with N = 30 million. Additionally, the simulations show that the location of the longitude of the ascending node affects the size of the minor extrema in z, located between the major extrema. In the future it may be possible to ascertain the location of the ascending node and compare its precession rate to expected values.

Cassini Keeler Gap Images

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