Imagine if all of the water in the Great Lakes evaporated, moved to the Southern Hemisphere, and rained down to form new lakes in Argentina. Then thousands of years later, the process repeated and the water returned north. That's what researchers say could be happening on Saturn's largest moon, Titan. Understanding the process could shed light on how long-term climate cycles operate on other worlds.
Titan's lakes aren't anything like those on Earth. Although some are as large and deep as our own Great Lakes--one, called Ontario Lacus, is about the size of Lake Ontario--they contain mostly methane, which becomes liquid at temperatures below -180°C. Even stranger, of the hundreds of lakes spotted so far, almost all are in Titan's far northern latitudes, and there seem to be no lakes at all near the moon's equator.
Titan's "lake asymmetry," as researchers call it, isn't due to differences in topography. Radar mapping of the moon by NASA's Cassini spacecraft reveals that the northern and southern hemispheres contain plenty of areas where liquid methane could accumulate. Nor does the difference seem to be the result of seasonal temperature variations. Titan's summers last about 7 years, but that's not nearly long enough to evaporate such deep lakes.
Instead, much longer climate processes may be at work, suggests a team of researchers online this week in Nature Geoscience. Just as long-term variations in Earth's orbit and spin axis contribute to our planet's 100,000-year ice age cycle, similar processes cause a 45,000-year cycle on Titan, the team has calculated. But instead of glaciation, Titan's climate oscillations seem to produce a wholesale migration of its hydrocarbon lakes. The researchers hypothesize that, at one end of the cycle, higher temperatures caused by the prolonged tilting of one of Titan's hemispheres toward the sun evaporates the methane in the lakes. Then atmospheric circulation transports the gaseous hydrocarbons toward the opposite pole, where they condense, fall as rain, and form new lakes. About halfway through the cycle, Titan's climate process reverses: The southern lakes evaporate, the methane gas migrates, and the northern basins begin refilling.
Although the researchers have hypothesized most of this scenario, some observational evidence exists, says planetary scientist and co-author Jonathan Lunine of the University of Arizona, Tucson. "Ontario Lacus has shrunk over 5 years of observation," he says, "so we do seem to see this evaporation process in action."
The paper "presents a plausible explanation" for the discrepancy in the distribution of Titan's polar lakes, says planetary scientist Ellen Stofan of Proxemy Research in Gaithersburg, Maryland, a small firm that performs planetary analysis for NASA. But "to really get a handle" on Titan's methane cycle, they'll need to monitor the exchange of methane between the lakes and the atmosphere. And that, she says, will require "a future lake-lander mission."