U.S. scientists explain methane, ethane lakes on Titan

17:05, November 30, 2009      

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U.S. scientists said the eccentricity of Saturn's orbit around the Sun may be responsible for the uneven distribution of methane and ethane lakes over the northern and southern polar regions of Saturn's moon, Titan.

Researchers at the California Institute of Technology (Caltech) described their theory in the Nov. 29 online edition of Nature Geoscience.

Caltech researchers compared the situation with the Earth, where a similar "astronomical forcing" of climate drives ice-age cycles.

Synthetic Aperture Radar (SAR) imaging data taken by NASA's Cassini spacecraft revealed that liquid hydrocarbon-filled lakes in Titan's northern high latitudes cover 20 times more area than lakes in the southern high latitudes.

There are also significantly more partially filled and now-empty lakes in the north.

Scientists initially thought there was 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. But there are no substantial known differences between the north and south to support this possibility.

Some scientists thought the mechanism on Titan may be seasonal, and methane rainfall and evaporation vary in different seasons, recently filling lakes in the north while drying lakes in the south.

However, the theory only explained decreases of about one 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.

In addition, seasonal variation can't account for the disparity between the hemispheres in the number of empty lakes; the northern pole has roughly three times as many dried-up lake basins as the south, and seven times as many partially filled ones.

Researchers at Caltech now explained that like Earth and the other planets, Saturn's orbit is not perfectly circular, but is instead somewhat elliptical, or eccentric, and oblique. Because of this, during its southern summer, Titan is about 12 percent closer to the sun than it is during the northern summer. As a result, northern summers are long and subdued while southern summers are short and intense.

They held that these differences in the characteristics of the seasons could somehow affect the relative amounts of precipitation and evaporation of methane in the hemispheres' respective summers.

According to the researchers, 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. This imbalance would lead to an accumulation of methane, and hence the formation of many more lakes in the northern hemisphere.

"Like Earth, Titan has tens-of-thousands-of-year variations in climate driven by orbital motions," said Oded Aharonson, associate professor of planetary science at Caltech and lead author of the Nature Geoscience paper.

He said on Earth, these variations, known as Milankovitch cycles, are linked to the global redistribution of water in the form of glaciers, and are responsible for ice-age cycles.

"On Titan, there are long-term climate cycles in the global movement of methane that make lakes and carve lake basins. In both cases we find a record of the process embedded in the geology," he added.

Source:Xinhua
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