ScienceDaily (Nov. 30, 2009) — Researchers at the California Institute of Technology (Caltech) suggest that the eccentricity of Saturn's orbit around the sun may be responsible for the unusually uneven distribution of methane and ethane lakes over the northern and southern polar regions of the planet's largest moon, Titan. On Earth, similar "astronomical forcing" of climate drives ice-age cycles.

This image shows the northern and southern hemispheres of Titan, showing the disparity between the abundance of lakes in the north and their paucity in the South. The hypothesis presented favors long-term flux of volatile hydrocarbons, predominantly methane, from hemisphere to hemisphere. Recently the direction of transport has been from south to north, but the effect would have reversed tens of thousands of years ago. (Credit: The mosaic includes Cassini SAR, ISS, and VIS images (NASA/JPL/Caltech/University of Arizona/Cassini Imaging Team).)

A paper describing the theory appears in the November 29th advance online edition of Nature Geoscience.

As revealed by Synthetic Aperture Radar (SAR) imaging data taken by NASA's Cassini spacecraft, which has been surveying Saturn and its moons since 2004, 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. (In the SAR data, smooth features — like the surfaces of lakes — appear as dark areas, while rougher features — such as the bottom of an empty lake — appear bright.)

Assuming that the asymmetry is not a statistical fluke (which is unlikely because of the large amount of data collected by Cassini), 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," says Oded Aharonson, associate professor of planetary science at Caltech and lead author of the Nature Geoscience paper. However, he notes, there are no substantial known differences between the north and south to support this possibility.

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