Lake Ontario has some key differences compared to her equally-sized sister lake, Ontario Lacus: The Great Lake has water; Ontario Lacus has methane, ethane, and propane. The Great Lake invites sunbathers; Lacus’ beaches, almost ten times further from the sun, are icy cold. The Great Lake is located on Earth; Lacus on Saturn’s largest moon, Titan. Despite all these distinctions, new research points to an important similarity: liquid levels in both lakes change with the seasons.

From June 2005 to July 2009, the Ontario Lacus shoreline has receded by about 6 miles, Alexander G. Hayes and his coauthors report in two papers submitted to Icarus and the Journal of Geophysical Research. Looking at other lakes in Titan’s southern hemisphere, it seems they are dropping in depth by about three feet per year.

Despite its shoreline’s rapid retreat, there is little worry that Ontario Lacus and other Titan lakes will disappear forever. Scientists expect that the evaporation is just part of a cycle of evaporation and condensation, that changes with the seasons. The four years of observation, carried out by NASA’s Cassini spacecraft, represents only the period from about mid-summer to fall, since a Titan year lasts 29.5 Earth years.

The discovery that Titan’s lakes are evaporating. . . suggests that there are active weather and geological cycles on Titan analogous to those on Earth. But on Titan the liquid driving those cycles is not water but methane, explained Oded Aharonson, a planetary scientist at the California Institute of Technology.“This is a wonderful opportunity and rare in the solar system to observe a planet with working liquid on its surface, a volatile agent that is responsible for altering its geology and participating in its weather cycle by evaporating and precipitating,” Dr. Aharonson said.[New York Times]

The Synthetic Aperture Radar (SAR) on the Cassinni spacecraft provided the data to help the researchers determine the lakes’ properties and see into their depths. Hayes explains:

“[The liquid] is fairly clear to radar energy—that is, transparent, like liquid natural gas.” Because of this, radar can see through the liquid in Titan’s lakes to a depth of several meters. “Then the radar hits the floor, and bounces back,” he says. “Or, if the lake is deeper than a few meters, the radar is completely absorbed, producing a ‘black’ signature.”[California Institute of Technology]

By watching how images created from this radar data (see image right) changed over four years, the researchers witnessed the evaporation in detail.

“Cassini continues to take our breath away as it fills in the details on the surfaces of these far-off moons,” said Linda Spilker, Cassini project scientist based at JPL.[NASA/JPL]

Given all that evaporated methane, a visit to the surface might have a similar effect.

The amount of methane gas produced by the changes seen so far exceeds the methane expelled by all the cows on Earth over a year, according to the press release. Yeah, might want to rethink that vacation after all. [DiscoveryNews]

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Image: NASA & Cassini Radar Science Team, NASA/JPL/Caltech