Saturn's rings leave ghostly imprint on atmosphere

Saturn's iconic rings are already a stunning sight. Now they have a ghostly shadow. New measurements indicate that charged particles are raining down from the icy rings, painting dark stripes in Saturn's upper atmosphere.

James O'Donoghue at the University of Leicester, UK, and his colleagues used the Keck II Telescope in Hawaii to observe radiation from electrically charged hydrogen molecules in Saturn's upper atmosphere, or ionosphere. Sunlight makes this region glow at infrared wavelengths, but Keck II revealed dark bands running parallel to Saturn's equator.

"We don't see this with Jupiter, which has a much more uniform ionosphere," says team member Steve Miller at University College London.

The unique stripes seemed to implicate Saturn's magnificent rings, which are far denser than Jupiter's paltry ring system.

Brilliant rings

Sure enough, the team's calculations showed that the dark bands are magnetically linked with the densest and most brilliant of Saturn's rings, which are made of orbiting chunks of ice and water vapour.

The sun's radiation ionises the water molecules, the team say, which then get swept up by the planet's magnetic field and channelled down to Saturn. As this drizzle of charged particles hits the ionosphere, it destroys the glowing hydrogen molecules.

The densest ? and brightest ? parts of the rings produce the heaviest rainfall, and so they paint the darkest stripes around the glowing ionosphere. The gaps between the rings don't dump water ions on the planet, so they leave bands where the ionosphere naturally glows brightest.

"We wouldn't see these with our own eyes, even if we were on one of Saturn's moons," says Miller. "The darker regions are only visible at specific wavelengths in the infrared."

Mysteries solved

O'Donoghue estimates that the rings dump 1 to 10 Olympic-sized swimming pools of charged molecules on Saturn's ionosphere.

The newly discovered rain could explain some long-standing mysteries, says O'Donoghue. Saturn's ionosphere is hundreds of degrees hotter than expected, and the infall of charged water molecules could be providing the extra energy needed to heat it.

In addition, Saturn's lower atmosphere is much wetter than predicted; the rain from the rings could be drenching the planet.

Journal reference: Nature, DOI: 10.1038/nature12049

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