The clouds are clearing on Titan as spring takes hold in its northern hemisphere, signaling a shift in the weather patterns on Saturn's largest moon, a new study finds.

Titan is poised for a mostly sunny spring, one that will last seven Earth years, researchers have found. Seasons on Titan last so long because it takes the moon and Saturn about 30 years to orbit the sun. [New photo of Titan clouds.]

Scientists analyzed data from the last six years of observations by NASA's Cassini spacecraft to piece together how Titan's weather cycle works. They found that conditions have changed since August 2009 — when the sun was directly over Titan's equator during its latest equinox.

"The clouds at the south pole completely disappeared just before the equinox, and the clouds in the north are thinning out," study leader Sebastien Rodriguez of the Universite Paris Diderot said in a statement. "We are expecting to see cloud activity reverse from one hemisphere to another in the coming decade as southern winter approaches."

Rodriguez and his colleagues said their findings match predictions by computer models. They presented their results at the European Planetary Science Congress 2010 in Rome on Sept. 22.

On Titan, it rains methane

Titan and its weather have intrigued scientists for decades. Some think Earth resembled Titan before life took hold — only not nearly as cold. Titan's surface averages 290 degrees below zero Fahrenheit (minus 179 degrees Celsius).

Titan has a thick, nitrogen-rich atmosphere, and its surface features have been carved by the action of liquid hydrocarbons like methane, which is the chief component of natural gas here on Earth. Methane rain drizzles from Titan's clouds, pooling in frigid liquid lakes.

To better understand Titan's weather, Rodriguez and his team used data gathered by the Cassini spacecraft since July 2004. They studied about 2,000 cloud images Cassini snapped between then and this past April, when Titan's seven-year spring began in the northern hemisphere.

In Cassini's early images, clouds gather at Titan's north and south poles, as well as in a narrow belt in the southern hemisphere. But in recent images, with the northern spring taking hold, cloud cover has decreased substantially at both poles.

Seasons change on Titan

These findings match predictions by computer models developed by other researchers in the past. Rodriguez and his team combined those models with the actual Titan observations to understand Titan's evolving cloud patterns.

Different cloud-formation mechanisms are likely at work in the different hemispheres, the researchers said.

During the northern winter, ethane and aerosols probably stream down from high in Titan's stratosphere, generating clouds near the north pole. These clouds are made of ethane, and they form at altitudes of 19 to 31 miles (30-50 km).

In the southern summer, on the other hand, methane-rich air wells up from Titan's surface. This action forms methane clouds at middle to high latitudes, the scientists said.

As Titan's seasons turn, these cloud-formation actions and patterns may flip from one hemisphere to the other, the researchers said.

They can see for themselves.

In February the Cassini mission was extended to May 2017, meaning Rodriguez and his team can get cloud-cover data all the way from mid-winter to mid-summer in Titan's northern hemisphere.

"We have learned a lot about Titan's climate since Cassini arrived at Saturn, but there is still a great deal to learn," Rodriguez said. "With the new mission extension, we will have the opportunity to answer some of the key questions about the meteorology of this fascinating moon."