Saturn's eccentric orbit might be the cause of the uneven distribution of ethane and methane lakes on the ringed planet's moon, Titan.
Images from NASA's Cassini spacecraft reveal that liquid methane and ethane lakes in Titan's northern high latitudes cover 20 times more area than lakes in the southern high latitudes.
Cassini data also show that there are significantly more partially filled and now-empty lakes in the north.
But why Titan showed these north-to-south differences wasn't known, though scientists have floated various ideas to explain the mismatch. Now a new and potentially more plausible explanation is offered.
Topography vs. seasons
Scientists initially considered the idea that "there is something inherently different about the northern polar region versus the south in terms of topography, such that liquid rains, drains, or infiltrates the ground more in one hemisphere," said Oded Aharonson of Caltech.
But scientists didn't know of any such topographic differences between Titans two poles, Aharonson said.
One alternative to this theory is that the differences are seasonal. One year on Titan lasts 29.5 Earth years. Every 15 Earth years, the seasons of Titan reverse, so that it becomes summer in one hemisphere and winter in the other.
According to the seasonal variation hypothesis, methane rainfall and evaporation vary in different seasons ? recently filling lakes in the north while drying lakes in the south.
The problem with this idea, Aharonson says, is that it accounts for decreases of about 3 feet (1 meter) per year in the depths of lakes in the summer hemisphere. But Titan's lakes are a few hundred meters deep on average, and wouldn't drain (or fill) in just 15 years.
Seasonality also can't account for the variation in the number of lakes between the two hemispheres. The north polar region has roughly three times as many dried-up lake basins as the south and seven times as many partially filled ones.
"The seasonal mechanism may be responsible for part of the global transport of liquid methane, but it's not the whole story," Aharonson said.
A more plausible explanation, say Aharonson and his colleagues, is related to the eccentricity of the orbit of Saturn ? and hence of Titan, its satellite ? around the sun.
"We propose that, in this orbital configuration, the difference between evaporation and precipitation is not equal in opposite seasons, which means there is a net transport of methane from south to north," Aharonson said.
This imbalance would lead to an accumulation of methane ? and hence the formation of many more lakes ? in the northern hemisphere.
This situation is only true right now, however. Over very long time scales of tens of thousands of years, Saturn's orbital parameters vary, at times causing Titan to be closer to the sun during its northern summer and farther away in southern summers. (A similarly long shift in Earth's orbital parameters is though to be a cause of Earth's ice ages.)
This reversal should produce a reverse in the net transport of methane and lead to a buildup of the methane and ethane ? and an abundance of lakes ? in the southern hemisphere.
Aharonson and his colleagues described their theory in the Nov. 29 issue of the journal Nature Geoscience.