Among all the strange things about Saturn's rings one of the strangest are the mysterious radial "spokes" in its B Ring.
At first thought to be caused by the same sort of gravitational interactions between Saturn, its moons and the ring material that create the myriad divisions in the rings, information gathered by the Cassini spacecraft in 2005 established that the spokes are caused by the planet's magnetic field.
The spokes are generally thought to have been first observed by the Voyager spacecraft during their flybys of Saturn in the early 1980s (top image) That they are not a transient feature was proven by recent images returned by the Cassini spacecraft, where they appear as bright streaks on the B ring (below).
There is no question but that these features in the B ring are quite real and not any sort of optical illusion.
Although the spokes were a surprise to scientists when they saw the first Voyager photos of them, they may have actually been observed for more than a century. But these early observations only add to the mystery of the spokes. The mystery is that although the radial spokes observed in the 19th and early 20th centuries resemble the Voyager spokes in every way—for instance, they are radial and usually tapered, sometimes light and sometimes dark—they were in the wrong ring.
Instead of appearing in the broad B ring as in the Voyager and Cassini images, the earlier visual astronomers sketched them in the outer A ring. They show up in drawings created as late as 1960. Many of these are by such expert and knowledgeable observers as E.M. Antoniadi and Lucien Rudaux.
Carolyn Porco, leader of the imaging science team on the Cassini mission, believes that the "spokes" seen by the visual observers were only "an optical effect of observing the azimuthal asymmetry and the gradient of brightness around the A ring. The human eye tends to inject straight lines into a gradient in visual brightness, and there is an azimuthal one in the A ring, so it makes sense. We've had more than 25 years between Voyager and Cassini, with Hubble Space Telescope Observations in between, and in all instances, the spokes are in the B ring. It's hard to imagine why they would change to the A ring on rare occasions."
In other words, while the spokes photographed by Voyager and Cassini are unquestionably real physical features the spokes observed by earth-based astronomers are actually an optical illusion.
While this certainly makes sense, the optical illusion explanation has never, to the best of my knowledge, been put to the test...for instance, in the way that Edward Maunder successfully tested the optical illusion theory of Mars' canals in 1903. Nor does it seem to answer why the early observers saw "spokes" only in the A Ring when there are gradients of brightness in the other rings as well. Perhaps the B Ring is too uniformly bright and the inner C Ring too dim.
It is true that since optical illusions can subjective some people may see spokes during a test and others may not. However, if in such a test even a few individuals saw markings similar to those observed by the visual astronomers that would certainly suggest that the A Ring "spokes" are in fact as illusory as were the Martian canals.