Saturn鈥檚 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鈥檚 upper atmosphere.
at the University of Leicester, UK, and his colleagues used the in Hawaii to observe radiation from electrically charged hydrogen molecules in Saturn鈥檚 upper atmosphere, or ionosphere. Sunlight makes this region glow at infrared wavelengths, but Keck II revealed dark bands running parallel to Saturn鈥檚 equator.
鈥淲e don鈥檛 see this with Jupiter, which has a much more uniform ionosphere,鈥� says team member at University College London.
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The unique stripes seemed to implicate Saturn鈥檚 magnificent rings, which are far denser than Jupiter鈥檚 paltry ring system.
Brilliant rings
Sure enough, the team鈥檚 calculations showed that the dark bands are magnetically linked with the densest and most brilliant of Saturn鈥檚 rings, which are made of orbiting chunks of ice and water vapour.
The sun鈥檚 radiation ionises the water molecules, the team say, which then get swept up by the planet鈥檚 magnetic field and channelled down to Saturn. As this drizzle of charged particles hits the ionosphere, it destroys the glowing hydrogen molecules.
The densest 鈥� and brightest 鈥� 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鈥檛 dump water ions on the planet, so they leave bands where the ionosphere naturally glows brightest.
鈥淲e wouldn鈥檛 see these with our own eyes, even if we were on one of Saturn鈥檚 moons,鈥� says Miller. 鈥淭he darker regions are only visible at specific wavelengths in the infrared.鈥�
Mysteries solved
O鈥橠onoghue estimates that the rings dump 1 to 10 Olympic-sized swimming pools of charged molecules on Saturn鈥檚 ionosphere per day.
The newly discovered rain could explain some long-standing mysteries, says O鈥橠onoghue. Saturn鈥檚 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鈥檚 lower atmosphere is much wetter than predicted; the rain from the rings could be drenching the planet.
Journal reference: , DOI: 10.1038/nature12049
Correction: When this article was first published, it did not give the timescale for James O鈥橠onoghue鈥檚 estimate of 1 to 10 Olympic-sized swimming pools of charged particles dumped from the rings.
