Like Jupiter, Saturn is orbited by a large extended family of moons - 62, at last count - ranging in size from the gigantic 3,200-mile-wide Titan, wrapped in thick clouds, to the barely 2-mile-wide Methone, smooth as a river rock. But there are even more moons in the ringed planet's retinue, tiny worlds embedded inside the icy rings themselves. Even with the Cassini spacecraft they are nearly impossible to see... until they give themselves away with their shining "propellers."
In the image above we get a view across 9,000 miles of Saturn's A ring, the outermost of the main ring structures, with Saturn itself well off frame to the left. Inside one of the darker segments of the rings, at lower left center, are two short, bright streaks - one pointing up, one pointing down. This is what the Cassini science team calls a "propeller," a clumping of ring particles in front of and behind a tiny moonlet located between the two "blades."
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The moonlet is too small to be resolved here directly - it's less than half a mile across - but its gravity is still strong enough to affect the tiny particles that comprise Saturn's rings. Made mostly of water ice, the more the particles gather together the more they tend to reflect sunlight - highlighting the moonlet's location for Cassini.
Depending on the angle of sunlight, propellers can also appear darker than the surrounding rings.