We Don’t Even Know What We Don’t Know

This view from NASA’s Cassini spacecraft is the sharpest ever taken of belts of the features called propellers in the middle part of Saturn’s A ring.

The propellers are the small, bright features that look like double dashes, visible on both sides of the wave pattern that crosses the image diagonally from top to bottom.

The original discovery of propellers in this region in Saturn’s rings (see Four Propellers, Propeller Motion​, Locating the Propellers​) was made using several images taken from very close to the rings during Cassini’s 2004 arrival at Saturn. Those discovery images were of low resolution and were difficult to interpret, and there were few clues as to how the small propellers seen in those images were related to the larger propellers Cassini observed later in the mission (for example ‘Earhart’ Propeller in Saturn’s A Ring, Cassini Targets a Propeller in Saturn’s A Ring​, and Bleriot Propeller Close-up​).

This image, for the first time, shows swarms of propellers of a wide range of sizes, putting the ones Cassini observed in its Saturn arrival images in context. Scientists will use this information to derive a “particle size distribution” for propeller moons, which is an important clue to their origins.