The dark radial features in Saturn’s B ring are called spokes.
Spokes are transient, spoke-like markings that extend radially across the rings, especially the broad B ring. They appear dark when viewed in backscattered light but can look bright in forward-scattered light. Unlike ordinary ring waves, spokes can persist and rotate around Saturn in a way that does not match the orbital motion expected from solid ring particles.
Voyager spacecraft images revealed the features around 1980, overturning the idea that Saturn’s rings were shaped only by gravity. The leading explanation is that microscopic dust particles become electrostatically charged and are temporarily lifted above the main ring plane. Their exact formation mechanism remains uncertain; lightning, micrometeoroid impacts and electric fields near the ring-plane terminator have all been proposed.
Spokes are also seasonal. Cassini did not see them when it arrived in 2004, but they reappeared in images taken in September 2005. Their visibility changes as Saturn moves through its roughly 29.7-year orbit around the Sun.