From the August 2015 issue

Web Extra: Titan’s seasons slowly change

As Saturn’s largest moon shifts its orbit around the Sun, the changing seasons drive liquid from one pole to the other.
By | Published: August 24, 2015

NASA/JPL-Caltech/Univ. of Arizona/Cassini Imaging & Radar Science Teams

Saturn’s moon Titan experiences seasons, like Earth, because it is tilted with respect to its orbit around the Sun. Titan is currently experiencing northern summer, when its northern hemisphere is tilted toward our star. This also is quite close to coinciding with perihelion, when the world is closest to the Sun.

The distance between the Sun and Earth throughout the year varies by a small amount, so northern and southern summers are essentially equal. But due to Saturn’s larger orbit and larger eccentricity, its moon also experiences large swings in the sunlight it receives throughout one trip around the Sun. Thanks to Kepler’s law that says a planet traveling around a star on an elliptical orbit must carve out equal area in equal time, the world whips through its closest approach to the Sun, experiencing short, hot northern summers. Half a Saturn year later, it is southern summer, but with a more gradual, gentle change of season, as with the simultaneous northern winter.

But northern summer hasn’t always coincided with perihelion. Wind the clock forward or backward a few tens of thousands of years, and the cycle flips so that the southern hemisphere experiences the more intense summers.

These changes in Titan’s seasons are probably why the Cassini spacecraft sees large lakes and seas only in Titan’s northern hemisphere. Over thousands of seasons, the consistently more intense northern summers drive the methane and ethane that make up the lakes out of the south and into the north. But 32,000 years ago, the situation was reversed, as seen in the animation below, and liquid would have been driven to the south.

And indeed, astronomers have spotted both empty southern lakebeds and watched the small southern lakes drying up in passes from year to year. Even though Cassini has only been sending astronomers data for a little over a decade, it can share evidence that has been accumulating for thousands.