Any lingering doubts were removed last year when Cassini’s cameras recorded a glint of sunlight bouncing off Kraken Mare, a large lake-shaped basin near Titan’s North Pole.

Lakes appear dark on the radar because if they are deeper than about 25 feet the radar waves are completely absorbed and do not come back to the spacecraft. Mr. Hayes and his colleagues were able to gauge the drop in lake levels by simply noting the shrinkage of dark nonreflecting areas and combining that with measurements of the slope of the lake bottom.

On the edges of the lake, they could see the lake bottom and thus measure its depth from altimetry, performing what Dr. Aharonson said was the first extraterrestrial bathymetry. The depth results matched the lake shrinkage. Where the slope into the methane was slow and gentle, the lakeshore receded the farthest; where there was a quick drop-off, against some mountains along the northern shore, the lake receded little.

Why should Titan be behaving like this? Dr. Aharonson said that a rough rule of thumb for hydrological systems like Earth or Titan was to expect that moisture would migrate seasonally from one hemisphere to the other, evaporating where it was summer and precipitating where it was winter. Indeed summer just ended in the southern hemisphere and the measurements show lakes were evaporating there, although there is no evidence that northern lakes are gaining yet.

But the actual dynamics of the Titanian atmosphere, he said, can be much more complicated, leading to cycles tens or hundreds of thousands of years long in which one hemisphere or the other could be wetter or colder than the other.

Because of the vagaries of its orbit around Saturn and Saturn’s orbit around the sun, for example, Titan is closer to the sun during its southern summer than in its northern summer. As a result, southern summers are currently shorter and more intense than their northern counterparts. Dr. Aharonson and his colleagues argued in a recent paperthat this could cause liquid to collect more in the northern hemisphere than the southern hemisphere, leading to a net migration of methane to the north over many Titan years.