Earth isn't the only planet with a sky full of blue.

In this natural color image taken by NASA's Cassini spacecraft, the skies of Saturn's northern hemisphere take on a blueish hue as light filters through its atmosphere.

Saturn's blue look is caused because light rays travel a longer path as they make their way through the relatively cloud-free zone of the planet's upper atmosphere. But while clouds may be scarce, some gases to remain and scatter the shorter-wavelength blue light ray on the way down.

Researchers aren't completely sure why Saturn's upper atmosphere is so free of clouds above the northern hemisphere. However, they theorize that the phenomenon may be due to colder temperatures brought on by the presence of ring shadows (appearing in this image as dark bands across Saturn) which surround the polar region.

The ring shadows at higher latitudes are cast by rings further away from Saturn so that the uppermost ring shadow is actually cast by the outer edge of Saturn's A ring, which extends the farthest from the planet.

North is up in this image, which was taken by Cassini's wide angle camera on Dec. 14, 2004 but was released earlier this month. Cassini was about 446,900 miles (719,000 kilometers) away from the ringed world when it took this image.

--SPACE.com Staff

Credit: NASA/JPL/Space Science Institute.

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