Titan’s ‘magic islands’ are likely honeycombed hydrocarbon icebergs, study finds

Titan’s “magic islands” are likely floating chunks of porous, frozen organic solids, a new study suggests, departing from previous work suggesting they were gas bubbles. The study was published in Geophysical research letters.

A hazy orange atmosphere, 50 percent thicker than Earth’s and rich in methane and other carbon-based or organic molecules, covers Saturn’s largest moon, Titan. Its surface is covered in dark dunes of organic matter and seas of liquid methane and ethane. What’s even stranger is what appears in radar images as changing bright spots on the sea surface that can last from a few hours to several weeks, or even longer.

Scientists first spotted these ephemeral “magic islands” in 2014 with the Cassini-Huygens mission and have been trying to understand what they are ever since. Previous studies suggested they could be ghost islands caused by waves or real islands made of suspended solids, floating solids or bubbles of nitrogen gas.

Xinting Yu, a planetary scientist and lead author of the new study, wondered whether a closer look at the relationship between Titan’s atmosphere, liquid lakes and solid materials deposited on the moon’s surface could reveal the cause of these mysterious islands.

“I wanted to check if the magical islands could actually be organic matter floating on the surface, like pumice that can float on water here on Earth before eventually sinking,” Yu said.

A strange world of organic products

Titan’s upper atmosphere is dense with various organic molecules. Molecules can clump together, freeze and fall onto the Moon’s surface, including its eerily smooth rivers and lakes of liquid methane and ethane, with waves only a few millimeters high.

Yu and his team were interested in the fate of these organic clumps when they reached Titan’s hydrocarbon lakes. Will they sink or float?

To find the answer, the team first investigated whether Titan’s organic solids would simply dissolve into the Moon’s methane lakes. Because the lakes are already saturated with organic particles, the team determined that falling solids would not dissolve when they reached the liquid.

“For us to see the magic islands, they can’t float for a second and then sink,” Yu said. “They have to float for a while, but not forever either.”

Titan’s lakes and seas are mostly made of methane and ethane, both of which have low surface tension, making it more difficult for solids to float. The models suggested that most of the frozen solids were too dense and the surface tension too low to create Titan’s magical islands, unless the clumps were porous like Swiss cheese.

If the ice clumps were large enough and had the right ratio of holes and narrow tubes, liquid methane could seep in slowly enough that the clumps could persist on the surface, the researchers found.

Yu’s modeling suggests that individual clusters are likely too small to float on their own. But if enough clumps cluster together near the shore, larger pieces could break off and float away, the same way glaciers calve on Earth. With a combination of larger size and good porosity, these organic glaciers could explain the phenomenon of magical islands.

In addition to the magical islands, a thin layer of frozen solids covering Titan’s seas and lakes could explain the unusual softness of liquid bodies. Thus, the results of this study could explain two of Titan’s mysteries.