Summer on Titan may make its lakes ripple with waves

CATCH a wave. Fleeting features on the face of Titan are probably waves rippling across the moon’s seas.

As far as we know, Saturn’s largest moon is the only world apart from Earth to host liquids on its surface, in the form of methane and ethane lakes and seas. The lakes are usually flat and calm, but NASA’s Cassini orbiter has spotted mysterious intermittent glints. Their reappearance seems to confirm something is afoot.

“Now we have confirmation that these seas are not stagnant ponds just sitting there, but there is activity in them,” says Jason Hofgartner of NASA’s Jet Propulsion Laboratory.

Cassini first spotted a glint in 2013 in Ligeia Mare, a 26,000-square-kilometre sea in Titan’s north polar region about 50 per cent bigger than Lake Superior. In later observations, the glint disappeared. Another view in 2014 found the bright spot again, and it was gone again by January 2015. The glints could be icebergs, but not much floats in methane, Hofgartner says. Or they could be bubbles.

On Earth, waves ruffled by a breeze are the most common cause of a shimmering sea. That’s probably true on Titan, too. That is surprising, because Titan’s lakes appear glassy smooth. But a faint breeze would be enough to ripple the surface according to Hofgartner’s calculations (Icarus, doi.org/bcwn).

“If it is waves, it could be because of the change of seasons,” Hofgartner says. “Titan’s year is 30 years long, and it is going toward summer in the northern hemisphere. We would expect that winds would be picking up and it could get more common.”

Cassini is making one last visit in April 2017, in the middle of Titan’s summer, and may pick up more evidence of the ephemeral shimmers.

“There are many reasons why we would want to go back to Titan,” Hofgartner says. “Hopefully, one day we would get to observe these from a lander in the sea.”

This article appeared in print under the headline “Waves ripple surface of Saturn’s moon”