Scientists at Cal Tech say they have cracked the puzzling conundrum of the polar patio-gas lakes of Titan, moon of Saturn. The reason why such bodies of fluid are found at the moon's north pole but not at its antarctic is apparently eccentricity in Saturn's orbit, of the same type as that governing ice ages on Earth.
Regular Reg readers will be familiar with the bodies of chilly -180°C liquefied methane, ethane and propane which adorn the Titanian arctic, among them the Sea of Krakens and the island-speckled Ligeian. They are formed by hydrocarbon drizzle condensing out of the moon's thick atmosphere.
What has had boffins stumped, however, is the fashion in which the seas are clustered at the north pole but not at the south, where similar conditions prevail. There are also many more dried-up lake beds in the north than in the south, indicating that the clustering is not simply a matter of liquid evaporating and being redeposited at the other pole owing to seasonal variations (Titan's seasons change on a thirty Earth-year cycle).
"How do you move the hole in the ground?" asks Cal Tech prof Oded Aharonson. "The seasonal mechanism may be responsible for part of the global transport of liquid methane, but it's not the whole story."
According to Aharonson, the missing factor is orbital eccentricity, the fact that Saturn doesn't circle the Sun but rather moves around it on an elliptical path such that at some times of year it is further away. At present, the entire Saturnian system - moons and all - is closer to the sun during Titan's southern summer. This means hotter summers at the south pole, and cooler ones up north, so permitting much more hydrocarbon "rain" and lake formation in the arctic.
A similar effect, known as Milankovitch cycles, controls the spread and retreat of glaciers on Earth and the associated ice ages. Aharonson says that after a few tens of thousands of years the seasonal imbalance on Titan will reverse, and the moon's antarctic will be covered in bodies of fluid - while today's Ligeian and Sea of Krakens will have disappeared so permanently that even their dried-out bottoms will have smoothed out.
"Like Earth, Titan has tens-of-thousands-of-year variations in climate driven by orbital motions," Aharonson says. "On Titan, there are long-term climate cycles in the global movement of methane that make lakes and carve lake basins."
"We may have found an example of present-day climate change, analogous to Milankovitch climate cycles on Earth, on another object in the solar system," he adds.
The new paper, An Asymmetric Distribution of Lakes on Titan as a Possible Consequence of Orbital Forcing, can be read here by subscribers to Nature Geoscience; or there's a less brain-melting website from Aharonson here. ®