Saturn's largest moon rains down snow and ice of ethane, methane and higher organic compounds. On the ground, accumulated chunks of material could act like glaciers to form short-lived floating "magic islands" at the edges of lunar methane lakes. A new study finds that Titan's "magic islands" are likely porous, frozen floating masses of organic solids, whereas previous research thought they were bubbles.
The research was published in Geophysical Research Letters, a journal of the American Geophysical Union (AGU) that publishes high-impact, short-form reports that have a direct impact on Earth and space science.
Titan's unique atmosphere and surface
Saturn's largest moon, Titan, is shrouded in a hazy orange atmosphere 50 percent thicker than Earth's and rich in methane and other carbon-based or organic molecules. Titan's surface is covered with dark sand dunes made of organic matter and oceans of liquid methane and ethane. What's even more peculiar is that in radar images, these bright spots on the ocean surface constantly change, lasting from hours to weeks or even longer.
The Mystery of the Magic Islands
Scientists first discovered these ephemeral "magic islands" during the Cassini-Huygens mission in 2014 and have been trying to figure out what they are ever since. Previous research has suggested they could be island-like phantoms created by ocean waves, or they could be real islands made of suspended solids, floating solids or nitrogen bubbles. Planetary scientist Xinting Yu, lead author of the new study, wondered whether a closer look at the relationship between Titan's atmosphere, liquid lakes and solid material deposited on the moon's surface could shed light on the origins of these mysterious islands.
"I want to study whether these magical islands are really organic matter floating on the sea, just like pumice stones on Earth, which can float on the water and eventually sink to the bottom of the sea," Yu said.
Strange organic world
Titan's upper atmosphere is densely populated with a variety of organic molecules. The molecules would condense together, form ice, and then fall to the lunar surface—including into rivers and lakes of eerily slick liquid methane and ethane in waves just a few millimeters high.
Yu and her team are interested in the fate of these organic clumps once they reach Titan's hydrocarbon lakes. Will they sink or float?
Research on floating theory
To find out, the team first looked at whether Titan's organic solids could simply dissolve in the moon's methane lakes. Because the lake is already saturated with organic particles, the team determined that the falling solids do not dissolve when they reach the liquid.
"In order for us to see the magic islands, they can't just float for a second and then sink," Yu said. "They have to float for a while, but they can't float forever."
Titan's lakes and oceans are dominated by methane and ethane, both of which have low surface tension, making it difficult for solids to float. Models suggest that most frozen solids are too dense and have too low surface tension to form Titan's magical islands, unless the solid clumps are as porous as Swiss cheese.
The researchers found that if the ice mass is large enough and has the right proportion of holes and narrow tubes, liquid methane can slowly seep in, allowing the ice mass to linger on the surface.
The formation of the Magic Islands
Modeling suggests that individual clumps may be too small to float on their own. But if enough clumps clump together near a coast, larger clumps break off and float away, similar to how glaciers form on Earth. These organic glaciers combine greater volume and appropriate porosity to explain the magic island phenomenon.
In addition to its magical islands, Titan's oceans and lakes are covered in a thin layer of frozen solids, which could explain the liquid body's unusual slickness. The study's findings could therefore explain two of Titan's mysteries.
Compiled source: ScitechDaily