Blue Skies on Saturn

NASA's Cassini spacecraft has discovered another world with blue skies: Saturn.

Fast forward 100 years: You're an astronaut piloting an airplane in the upper atmosphere of Saturn. The gas giant has no solid surface to walk on and no seas to put a boat in. Exploring Saturn means flying, dipping in and out of strangely-colored clouds, racing through ring shadows. It's a totally alien world.

It's so alien that you start to feel homesick. So you do what they taught you in astronaut training. Take a deep breath, look up at the sunny blue sky and pretend to be back on Earth. Works every time!

Sunny blue skies ... on Saturn? It's true. NASA's Cassini spacecraft discovered them in 2005.

"We were surprised," recalls JPL's Bob West, a member of the Cassini imaging team. "Saturn is supposed to be yellow."

The blue skies of Saturn, photographed by Cassini in January 2005. In the foreground is Saturn's moon Mimas. The long, dark lines on the atmosphere are sun-shadows cast by the planet's rings.

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."

"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.

Saturn, photographed by Geoff Chester of Alexandria, VA, on Jan. 29, 2005, using an 8-inch telescope. Saturn's blue north is hidden behind the planet's rings.

The mystery, says West, isn't why the south is cloudy--that's normal. It's why the north is clear. "For some [unknown] reason, Saturn's northern clouds have sunk deeper into the planet, leaving clear blue air behind."

Saturn's north is so blue that West believes amateur astronomers could see the hue from Earth. Unfortunately, the north of Saturn is hidden at the moment behind Saturn's rings, a situation that will persist for another year or so.

For now, Cassini is in the best position to investigate. Will Saturn's blue skies fade? Or grow to envelop the whole planet? No one knows. It is an alien world, after all.