Titan Not Windy Enough For Waves? Cassini Will See About That.
Saturn’s largest moon Titan has often been likened to a primordial Earth, with its thick atmosphere, changing weather patterns, and — most intriguing of all — vast amounts of liquid on its surface in the form of lakes, streams, and rivers. One big difference though: nearly ten times farther from the Sun than we are, temperatures on Titan are a chilly 290 degrees below zero (F) and so the liquid isn’t water, it’s methane… what’s called natural gas on Earth.
Still, methane does a good job replacing water in Titan’s own version of a hydrologic cycle. Methane rain falls to fill streams, carving gullies and canyons through the frozen landscape (where water is harder than rock) and eventually filling methane lakes and seas — some as large as our Great Lakes! Cassini has found evidence of Titan’s lakes in both repeated radar scans as well as visible-light images… but what hasn’t been found yet, strangely enough, are waves on the surfaces of these lakes. If they are indeed liquid methane, and Titan has weather capable of creating rain and sculpting dunes, then why are these enormous lakes so incredibly flat?
As seasons slowly change, Cassini will find out the answer.
Here on Earth, bodies of water are rarely still. Breezes blowing across the surface cause waves to ripple and break; raindrops striking sea surfaces also provide some roughness. Yet on Titan, the lakes are eerily smooth, with no discernable wave action down to the millimeter scale, according to radar data from Cassini.
“We know there is wind on Titan,” says Alex Hayes, a planetary scientist on the Cassini radar team at Cornell University. “The moon’s magnificent sand dunes prove it.”
Add to that the low gravity of Titan—only 1/7th that of Earth—which offers so little resistance to wave motion, and you have a real puzzle.
Researchers have toyed with several explanations. Perhaps the lakes are frozen. Hayes thinks that is unlikely, however, “because we see evidence of rainfall and surface temperatures well above the melting point of methane.” Or maybe the lakes are covered with a tar-like substance that damps wave motion. “We can’t yet rule that out,” he adds.
The answer might be found in the results of a study Hayes and colleagues published in the July 2013 online edition of the journal Icarus. Taking into account the gravity of Titan, the low viscosity of liquid hydrocarbons, the density of Titan’s atmosphere, and other factors, they calculated how fast wind on Titan would have to blow to stir up waves: A walking-pace breeze of only 1 to 2 mph should do the trick.
This suggests a third possibility: the winds just haven’t been blowing hard enough. Since Cassini reached Saturn in 2004, Titan’s northern hemisphere (where most of the lakes are located) has been locked in the grip of winter. Cold heavy air barely stirs, and seldom reaches the threshold for wave-making.
But now the seasons are changing. In August 2009 the sun crossed Titan’s equator heading north. Summer is coming, bringing light, heat and wind to Titan’s lake country.
“According to [climate models], winds will pick up as we approach the solstice in 2017 and should be strong enough for waves,” he says. And if and when they appear, Cassini will be able to detect them.
(Source article by Dr. Tony Phillips. Read more here on Science @ NASA.)
P.S.: On July 26, Cassini will perform yet another flyby of Titan to monitor seasonal changes in lakes Ligeia Mare and Punga Mare, the second and third largest known bodies of liquid on the moon. Read more here.
*Thanks to space artist Ron Miller, who gave permission to use his illustration of a Titanic lake! To see more of Ron’s beautiful planetary art, click here.