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Seasons change, and so does Saturn

NASA / JPL-Caltech / SSI

Saturn and its rings provide a backdrop for the planet's largest moon, Titan, in a true-color picture captured by NASA's Cassini orbiter on May 6.

Saturn's shades of blue and butterscotch are changing along with the planet's seasons, as illustrated by a fresh batch of true-color photos from the bus-sized Cassini orbiter.

When Cassini arrived at Saturn, seven years ago, the planet's northern hemisphere had a tint of azure blue. Since then, Saturn has gone through an equinox and a significant shift in seasons. Summer is approaching in the north, and winter is coming to the south.

The seasonal change means ultraviolet radiation is intensifying in the north, resulting in an increasing amount of yellowish haze. Meanwhile, there's a reduction in radiation hitting the southern hemisphere, and the haze is clearing as a result. The presence of the ring shadow enhances the effect in Saturn's south.

"The reduction of haze and the consequent clearing of the atmosphere make for a bluish hue: the increased opportunity for direct scattering of sunlight by the molecules in the air makes the sky blue, as on Earth," Cassini's imaging team reports in today's advisory. "The presence of methane, which generally absorbs in the red part of the spectrum, in a now-clearer atmosphere also enhances the blue."

Although Saturn has seasons like Earth's, the fact that a Saturnian year lasts 29.5 times longer than an Earth year means that the southern hemisphere's winter solstice won't occur until May 2017. And if Cassini's mission managers have their way, the orbiter will be around to see it.

"The Cassini mission was recently given rave reviews by a panel of planetary scientists and NASA program managers for its contributions to our understanding of the solar system, a circumstance that bodes well for a well-funded continuing mission over the next five years," the imaging team's leader, Carolyn Porco of the Colorado-based Space Science Institute, reported in an email today. "Despite the fact that we can't know exactly what the next five years will bring us, we can be certain that whatever it is will be wondrous."  

Saturn's largest moon, Titan, takes center stage in one of Cassini's newly released views. The moon measures 3,200 miles (5,150 kilometers) across and is covered with its own brand of hydrocarbon-rich haze. Titan is the only moon in the solar system to have an opaque atmosphere. Cassini snapped the picture you see above from a distance of about 483,000 miles (778,000 kilometers).

Here are more pictures that show Titan's true colors:

NASA / JPL-Caltech / SSI

Saturn's rings obscure part of Titan's disk in an image from NASA's Cassini orbiter. Parts of the rings appear dark near the center of this view because of the shadow cast by the planet. This image was obtained on May 16 at a distance of about 1.9 million miles (3 million kilometers) from Titan.

NASA / JPL-Caltech / SSI

Titan's recently formed south polar vortex stands out in this natural-color view of Titan from the Cassini spacecraft. The vortex may be related to the approach of southern winter and the development of a polar "hood" of denser, high-altitude haze. This picture was acquired on July 25 at a distance of about 64,000 miles (103,000 kilometers).

NASA / JPL-Caltech / SSI

NASA's Cassini spacecraft looks toward the night side of Titan and sees sunlight scattering through the periphery of the moon's atmosphere, creating a ring of color. The picture was taken on June 6 from a distance of about 134,000 miles (216,000 kilometers).

More colors from Cassini:

Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.