NASA Cassini: Flaring fairy lights of Saturn's F ring

One of Saturn's rings is flaring and fading in a very strange fashion.

When Voyager 1 visited the planet in 1980, the ring was faint, but it sparkled with bright spots.

By the time the Cassini mission arrived in 2004, the spots were gone – but the ring as a whole had grown twice as bright.

Saturn's F ring has been a puzzle since it was discovered by the Pioneer 11 spacecraft in 1979.

While most of the inner rings have clear, well-defined edges, the F ring, which is the outermost of Saturn's main rings, has a thin central core surrounded by a diffuse skirt of smoke-sized ice particles.

That swirl of smoke forms kinks and knots, and some of it winds around the ring's core in a spiral.

Most of these funny features can be blamed on the little moon Prometheus, which orbits just inside the F ring.

As its elliptical orbit brings Prometheus towards and away from the ring on a 17-year cycle, the moon contorts the ring with its gravity.

It even steals material from the ring when it comes too close, as its mythical namesake stole fire from the gods.

"We knew the F ring changed on a day to day and even hour to hour basis because of interactions with Prometheus," says Robert French, a research assistant at the SETI Institute in Mountain View, California.

"But people always assumed it was stable over the longer term, over decades if not millennia."

So French and colleagues got a surprise when they compared images from the Voyager 1 and 2 missions, which zipped past Saturn in 1980 and 1981, with images from the Cassini spacecraft, which has been orbiting the planet since July 2004.

They found that the ring was twice as bright in 2004 as it was in 1980. It had also grown three times as wide and twice as opaque.

"All of that means there's a lot more dust today than there used to be," French says. Those tiny particles scatter sunlight, so when there are more of them the whole ring shines more brightly.

French's first thought was to blame Prometheus. But while Prometheus's orbit brought it closer to the F ring over the first 5 years of Cassini's stay in the Saturn system, the ring didn't change.

"That's very puzzling," French says. "People usually think of Prometheus as the cause of changes in the F ring, and Prometheus was making large changes in how it interacted with the F ring. Yet the F ring wasn't changing."