The population of propellers in Saturn's A Ring

TL;DR

This study analyzes ~150 propeller-shaped features in Saturn's A Ring, interpreting them as small moonlets, and explores their distribution, composition, and possible formation mechanisms within the ring system.

Contribution

It provides the first extensive dataset of propeller features, revealing their size, persistence, and distribution, and discusses potential origins and ring dynamics affecting their placement.

Findings

Propellers are primarily composed of macroscopic particles.

They are concentrated in three narrow bands within the mid-A Ring.

Most propellers are found within regions free of strong density waves.

Abstract

We present an extensive data set of ~150 localized features from Cassini images of Saturn's Ring A, a third of which are demonstrated to be persistent by their appearance in multiple images, and half of which are resolved well enough to reveal a characteristic "propeller" shape. We interpret these features as the signatures of small moonlets embedded within the ring, with diameters between 40 and 500 meters. The lack of significant brightening at high phase angle indicates that they are likely composed primarily of macroscopic particles, rather than dust. With the exception of two features found exterior to the Encke Gap, these objects are concentrated entirely within three narrow (~1000 km) bands in the mid-A Ring that happen to be free from local disturbances from strong density waves. However, other nearby regions are similarly free of major disturbances but contain no propellers. It is unclear whether these bands are due to specific events in which a parent body or bodies broke up into the current moonlets, or whether a larger initial moonlet population has been sculpted into bands by other ring processes.