14 DAY TRIAL // Scientists are celebrating the first Cassini spacecraft sighting of spokes, the ghostly radial markings discovered in Saturn's rings by NASA's Voyager spacecraft 25 years ago.
A sequence of images taken on the side of the rings not illuminated by the sun has captured a few faint, narrow spokes in the outer B ring, about 3,500 kilometers long and about 100 kilometers wide (2,200 miles by 60 miles).
Previously, scientists believed the visibility of spokes depended on the elevation of the sun above the rings. The less sunlight, the more visible the spokes. For this reason, they weren't expecting to see spokes until later in the mission when the sun angle will be low.
In Voyager images from 25 years ago, the spokes appeared dark when seen at low sun angles and bright when seen at high sun angles. This behavior indicated that they were comprised of extremely small icy particles. Since Voyager days, spokes had been seen in images taken by NASA's Hubble Space Telescope. The new Cassini images were taken at very high sun angles, where small particles can brighten substantially, making them more visible.
Some ideas suggest that spokes result from meteoroid impacts onto the rings; others suggest that they are created by instability in Saturn's magnetic field, which surrounds the planet, near the rings. Whatever the cause, imaging team members will study the new spoke images and maintain their vigil for additional spoke sightings.
This was Cassini's eighth flyby out of 45 Titan flybys planned in the nominal four-year tour.