If you’ve ever looked at Saturn through a backyard telescope, you know it’s true: Yellow is the dominant color of Saturn’s thick clouds. “Sunlight reflected from those clouds is what gives Saturn its golden hue,” explains West.
But Cassini saw something different. Close to Saturn, the spacecraft was able to photograph the clear air above the planet’s clouds. (“Air” on Saturn is mostly hydrogen.) The color there is blue.
“Saturn’s skies are blue, we think, for the same reason Earth’s skies are blue,” says West. Molecules in the atmosphere scatter sunlight. On Earth the molecules are oxygen (O2) and nitrogen (N2). On Saturn the molecules are hydrogen (H2). Different planets, different molecules, but the effect is the same: blue light gets scattered around the sky. Other colors are scattered, too, but not as much as blue. Physicists call this “Rayleigh scattering.”
End of story? Not quite.
“There are some things we don’t understand,” says West. For example, while Saturn’s northern hemisphere has blue skies, Saturn’s southern hemisphere does not. The south looks yellow. It could be that southern skies on Saturn are simply cloudier, yellow clouds making yellow skies.
Intriguing, especially since we’re reading Shelley at the moment.