Methane Icebergs Could Float in Titan’s Seas

Floating or disappearing islands — such as Atlantis or Saint Brendan’s Island — are among the most captivating legends and myths, capturing the popular imagination with speculations about their origin and fate. This fascination for disappearing bits of land has found its latest incarnation beyond Earth.

Saturn’s largest moon, Titan, is the only other body in the solar system to have seas and rivers on its surface, but they’re filled with what’s essentially liquefied natural gas. Titan sustains an earthlike hydrological cycle fed by liquid hydrocarbons, such as ethane and methane, which remain liquid in the moon’s frigid temperatures. Methane, for instance, boils at -162°C (-259°F) on Earth, while Titan’s average temperature is only -183°C.

When NASA’s Cassini mission started returning imagery of Titan’s surface in 2004, researchers mapped these lakes and seas, finding most of them clustered near the moon’s north pole. Canyons and rivers drain into them, producing strikingly terrestrial-looking coastlines. But as Cassini repeatedly flew by the moon, it also revealed some bits of land that appeared and disappeared with every passing flyby. Scientists called them “magic islands,” and like the ghost islands of yore, their origin has also been subject to speculation.

Scientists have proposed three main theories to explain their origin. The first two point to artifacts in Cassini’s radar images, which could be easily fooled by either surface waves or bubbles forming under the surface. The last theory points to floating debris that could remain on the surface for a while before sinking.

This last theory has now gained a stronger footing thanks to a new study published in Geophysical Research Letters. There, the researchers show that, under certain conditions, solid materials can float on Titan’s seas, potentially creating iceberg-like structures.

“I’m trying to argue that maybe these islands could be real,” says planetary scientist Xinting Yu (University of Texas, San Antonio), who led the new study. What she and her colleagues found is that organic particles of certain sizes could float on Titan’s lakes and seas “for just the right amount of time to explain the magic island phenomenon.”

To determine what, if anything, could float on Titan’s seas, Yu’s team took to the lab. They simulated what would happen to the kind of materials expected to be present on Titan’s surface if they fell onto the methane- and ethane-filled lakes. Observations have revealed what kind of species exist in Titan’s atmosphere, but researchers needed to know more about their properties such as at what temperatures these materials change phase (such as from solid to liquid) as well as materials’ density to determine how well they float.

The team also looked at other properties such as surface tension or capillary effects. Such effects could make materials float, Yu says, “just like some insects can jump on water because of the surface tension effect.”

Yu and colleagues experimentally determined or compiled from the scientific literature the properties of the likely candidates, a process that took three years due to the complex chemistry in Titan’s atmosphere.

They found that while most materials would either dissolve or sink on Titan’s seas, under certain circumstances some can form porous structures that could float briefly on liquid methane. Yu compares them to pumice rock. “The material itself is denser than water but it takes a long time for the pore space to be filled, explaining why they can stay afloat for a long time,” she explains. If enough of these porous materials fell into the lakes from the coast, much like icebergs calving on Earth, they could stay afloat long enough to produce the magic islands.

In that case, it would make sense to have the islands appear closer to land, just as the images have revealed. “If it’s just waves or bubbles, it could just show up in the middle of the lake — not close to another island,” Yu says.

Additionally, other measurements have shown that Titan’s lakebeds contain organic compounds, implying that this could be the resting place of these icebergs after they sink.

“What the current paper does is to show that it’s plausible to have floating solids,” says Alexander Hayes (Cornell University), who wasn’t involved with the new study. “What it doesn’t say is why floating solids would be the plausible mechanism for the magic islands. That looks like future work to me.”

“My personal preference is still that waves, by Occam’s razor, are still the easiest explanation for these features,” Hayes adds. Still, he's not convinced by floating solids as a solution to the magic islands. “If that was true, why we didn’t see floating solids elsewhere all over the shorelines of the seas.”

What could definitively tell these two theories apart is some compositional data, Yu says. “If it’s waves then if you look at the composition of the magic island you should have the same composition as the lake, but if it looks different it means it’s floating solids.”

Yu hopes to find some of these clues buried in Cassini’s data. “It was there for 13 years,” she says. “There’s so much data!”