This composite image, constructed from data obtained by NASA's Cassini spacecraft, shows Saturn's rings and southern hemisphere. In a bit of cosmic weirdness, Saturn emitted gradually less energy each year from 2005 to 2009, according to observations by NASA's Cassini spacecraft, with Saturn's southern hemisphere consistently emitting more energy than its northern one.
"The fact that Saturn actually emits more than twice the energy it absorbs from the sun has been a puzzle for many decades now," said Kevin Baines, a Cassini team scientist at NASA's Jet Propulsion Laboratory. "What generates that extra energy?"
The research, reported this week in the Journal of Geophysical Research-Planets, was led by Liming Li of Cornell University in Ithaca, N.Y. (now at the University of Houston).
"The Cassini CIRS data are very valuable because they give us a nearly complete picture of Saturn," Li said. "This is the only single data set that provides so much information about this planet, and it's the first time that anybody has been able to study the power emitted by one of the giant planets in such detail."
The planets in our solar system lose energy in the form of heat radiation in wavelengths that are invisible to the human eye. The CIRS instrument picks up wavelengths in the thermal infrared region, far enough beyond red light where the wavelengths correspond to heat emission.
"In planetary science, we tend to think of planets as losing power evenly in all directions and at a steady rate," Li said. "Now we know Saturn is not doing that." (Power is the amount of energy emitted per unit of time.)
The emitted energy for each hemisphere rose and fell along with the effective temperature. Even so, during this five-year period, the planet as a whole seemed to be slowly cooling down and emitting less energy.
To find out if similar changes were happening one Saturn-year ago, the researchers looked at data collected by the Voyager spacecraft in 1980 and 1981 and did not see the imbalance between the southern and northern hemispheres. Instead, the two regions were much more consistent with each other.
Why wouldn't Voyager have seen the same summer-versus-winter difference between the two hemispheres? One explanation is that cloud patterns at depth could have fluctuated, blocking and scattering infrared light differently.
"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."
Better understanding Saturn's internal heat flow "will significantly deepen our understanding of the weather, internal structure and evolution of Saturn and the other giant planets," Li said.