According to Cassini observations, Saturn emitted less and less energy in the years from 2005 to 2009, but the planet's southern hemisphere emitted more than the northern hemisphere, changing with the seasons.

These trends were identified thanks to Cassini's Composite Infrared Spectrometer (CIRS) as well as archival Voyager data, and offers the first chance to study the energy emitted by a gas giant in so much detail. The data could eventually lead to scientists uncovering the nature of Saturn's internal heat source.

“In planetary science, we tend to think of planets as losing power evenly in all directions and at a steady rate,” says Liming Li of Cornell University. “Now we know Saturn is not doing that.”

It turns out that the planet's southern hemisphere emitted one-sixth more energy than the northern hemisphere, correlating with summer in the southern hemisphere and winter in the northern hemisphere. As a change of season approached, the effective temperature, which characterises its thermal emission to space, started to warm up or cool down, but because Saturn's atmosphere retains heat, the temperature changes were complicated.

“The effective temperature provides us a simple way to track the response of Saturn’s atmosphere, as a system, to the seasonal changes,” says Li.

Cassini’s observations in the northern hemisphere revealed that the effective temperature gradually dropped from 2005 to 2008 and then started to warm up again by 2009. In the southern hemisphere, the effective temperature cooled from 2005 through to 2009, as the equinox started to approach. The energy for each hemisphere also rose and fell with the temperate, but over the five year period, the planet as a whole seemed to be slowly cooling.

The fact that Voyager did not see these changes could mean that the amount of energy from the Sun, which is the ultimate driving force of atmospheric temperatures and phenomena, could have fluctuated from one Saturn year to the next, or, Saturn's cloud cover patterns could have varied.

“It’s reasonable to think that the changes in Saturn’s emitted power are related to cloud cover,” says Amy Simon-Miller, who heads the Planetary Systems Laboratory at Goddard. “As the amount of cloud cover changes, the amount of radiation escaping into space also changes. This might vary during a single season and from one Saturn year to another. But to fully understand what is happening on Saturn, we will need the other half of the picture: the amount of power being absorbed by the planet.”

Li and colleagues are now working on precisely that. By comparing the changes in the incoming solar radiation with that being emitted, scientists will get one step closer to finding out the nature of Saturn's internal power engine, and whether this too, changes over time.

“The answer will significantly deepen our understanding of the weather, internal structure and evolution of Saturn and the other giant planets,” adds Li.

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