When it comes to building waves, Saturn's moon Titan has it all – liquid hydrocarbon seas and lakes, plus a dense atmosphere capable of generating winds that can raise 300-foot-tall dunes near the equator.
Waves on seas and lakes, however, are another story. In the nine years since the joint NASA-European Space Agency Cassini-Huygens mission to Saturn and its moons arrived, nary a ripple has been spotted.
Now, an international team of scientists has offered an explanation for Titan's unexpectedly calm "waters," offering a theory that describes what the small waves should look like for a certain wind speed and a certain range of recipes for the liquid hydrocarbon.
If these small features show up, they could yield important clues about the composition of the liquid hydrocarbons filling the seas and lakes, the researchers say.
"We're laying the groundwork for understanding wave generation on Titan, which is something we didn't have until now," says Alexander Hayes, a planetary scientist at Cornell University and the lead author of the formal paper that lays out the explanation.
As for no detection so far? The mission to date was observing the lakes and seas on Titan during the wrong season there, the researchers suggest. Nearly all of Titan's existing lakes and seas appear in the moon's northern polar region, which has been slowly emerging from winter into spring and summer. The analysis, based on what the team says are the most comprehensive calculations and modeling to date related to wave action on Titan, suggests that the winter chill in the north brought winds there to a virtual standstill.
Now the northern hemisphere is warming up. The models suggest that winds will pick up sufficiently to begin the wave-building process – if the liquid hydrocarbons aren't too viscous for the winds to tussle lake or sea surfaces.
The waves Cassini can detect would be on the order of only a few centimeters tall, based on the characteristics of the radar the craft uses to explore the surfaces of objects in the Saturn system. Depending on how thick or thin the liquid hydrocarbons are, wavelettes could start to appear at windspeeds on the order of about 1 mile an hour.
Some 1 to 2 percent of Titan's surface is covered with liquid hydrocarbons, with 87 to 88 percent of the liquid held in three large seas, or mare. The largest, Kraken Mare in the northern hemisphere, is about the size of the Caspian Sea on Earth. No small part of Titan's fascination for planetary scientists and astrobiologists is its status as the only other body in the solar system with liquids stable on its surface. Like Earth, Titan has an active hydrological cycle – even if at far lower temperatures than any seen at Earth's surface. It's average surface temperature is a flash-freezing 323 degrees below zero Fahrenheit – cold enough to liquify methane and ethane, gases at Earth's balmy temperatures.
Most of the liquid bodies currently appear in the northern hemisphere. Based on observations, researchers estimate that the "wettest" hemisphere changes from north to south and back over 10,000-year cycles – Titan's version of Earth's transition into and out of ice ages.
Hunting for waves on Titan's lakes and seas has yielded measurements of just how remarkably calm the moon's winds can get.
Measurements made in 2008 of Ontario Lacus, a Lake Ontario-size body in Titan's south polar region, showed a swell height – if you could call them swells – of about 3 millimeters over the span of 100 meters. A year later, measurements taken from the northern hemisphere's Kraken Mare, the moon's largest sea, and Jingpo Lacus, a northern lake, indicated "swell" slopes of barely more than 1/10th of a degree over distances of 1 kilometer – for all practical purposes, flat. Additional measurements published more recently point to surface height changes on other bodies of only about a millimeter over 100 meters.
These measurements seemed to defy several lines of evidence suggesting that waves should be forming on these bodies of liquid, says Dr. Hayes, a member of the Cassini science team.
First was the presence of the dunes, which Hayes dubbs "piles of plastic" for the acculumated particles of organic material they represent.
"We see these beautiful 100-meter-high, kilometer-scale dunes" that are comparable in size and shape to dunes seen in the deserts of Namibia or Egypt, he explains, adding that it takes significant winds to make such dunes.
In addition, down south at Ontario Lacus, researchers noticed that the lake is shrinking and exposing sections of shoreline that appear to have been shaped by wave erosion, while other lakes appear bounded by the stair-step shape of successive ancient shorelines, also evidence for possible wave erosion.
Hayes cautions that the evidence at Ontario Lacus is controversial. Nevertheless, taken with the other observations, it would seem that Titan is primed for generating small waves, if not genuine hydrocarbon surf.
Still, when Hayes and his colleagues analyze radar data from Titan's lakes, the lakes "are amazingly flat – outside our experience for liquid bodies," he says. "Why on Titan are these lakes smooth to the millimeter scale" when on Earth coming across a lake so calm that it lacks even tiny, centimeter-scale waves "just doesn't happen?"
To understand the conditions that contribute to this phenomenon, the team used the most recent techniques for modeling wave formation. In addition to wind, these approaches consider the interplay of characteristics such as the liquid's viscosity, surface tension, the densities of the atmosphere and liquid, and gravity on wave formation.
Based on some assumptions about the liquid hyrdocarbon's make-up, the team estimated that winds in the range of 1 to 1.5 miles an hour would be sufficient to begin forming the centimeter-scale "capillary" waves they've sought – waves that over time and with distance can grow larger. These capillary waves fall into a size range Cassini likely could detect with radar and with its Visual and Infrared Mapping Spectrometer.
Meanwhile, models of Titan's climate have yielded a range of estimates for when winds in the north should pick up sufficiently to spawn capillary waves. Some predicted that the winds already should have picked up sufficient speed. Others suggest the acceleration should occur within the next year or two. Still others indicate that the winds won't pick up until after 2017, when Cassini is slated to plunge into Saturn's atmosphere and burn up.
Seeing just one set of waves "would be an amazing, fundamental discovery in itself," Hayes says. But with enough data, researchers could see which of the models is most accurately predicting wind conditions at Titan's surface, not just at the moon's cloud tops, which models do today.
Over the long term, and assuming Cassini can pull in enough data, the model that earns "Best in Show" for this test also could ultimately help researchers more accurately pin down winds speeds at the surface and narrow the range of compositions for the liquid hydrocarbons over which the wind passes.
And if waves still fail to appear in Cassini's data?
"That's still telling us something about the winds speeds and the viscosity of the lakes," Hayes says, which can help narrow the range of estimates for the composition of the hydrocarbons.