The disappearing “magic islands” on Saturn’s largest moon Titan have intrigued scientists since NASA’s Cassini mission spotted them during flybys a decade ago. Now, researchers believe they have unravelled the phenomenon’s secrets.
The ephemeral features were first thought to be made of fizzing gas bubbles, but astronomers now believe they may be honeycomb-like glaciers made of organic material that fall down onto the moon’s surface.
Scientists regard Titan as one of the most fascinating moons in our solar system because it shares some similarities with Earth. In many ways, however, it also presents a baffling alien landscape.
Titan, larger than both our moon and the planet Mercury, is the only moon in our solar system with a thick atmosphere. The atmosphere is largely composed of nitrogen with a bit of methane, which gives Titan its fuzzy orange appearance. Titan’s atmospheric pressure is about 60% greater than Earth’s, so it exerts the kind of pressure humans feel swimming about 50 feet (15 metres) below the ocean surface, according to NASA.
Titan is also the only other world in our solar system that has Earth-like liquid bodies on its surface — but the rivers, lakes and seas are composed of liquid ethane and methane, which form clouds and cause liquid gas to rain down from the sky.
The Cassini mission’s orbiter, which carried the Huygens probe that landed on Titan in 2005, conducted more than 100 flybys of Titan between 2004 and 2017 to reveal much of what scientists know about the moon today.
Among the most puzzling aspects of Titan are its magic islands, observed by scientists as moving bright spots on Titan’s sea surfaces that can last a few hours, several weeks or longer. Cassini’s radar images captured the unexplained bright regions in Ligeia Mare, the second-largest liquid body on Titan’s surface. The sea is 50% larger than Lake Superior and is made up of liquid methane, ethane and nitrogen.
Astronomers thought these regions might be clumping bubbles of nitrogen gas, actual islands made of floating solids or features attributed to waves (although the waves only reach a few millimetres in height).
Planetary scientist Xinting Yu, an assistant professor at the University of Texas at San Antonio, focused on analyzing the connections between Titan’s atmosphere, liquid bodies and solid materials that fall like snow to see if they might be related to the magic islands.
“I wanted to investigate whether the magic islands could actually be organics floating on the surface, like pumice that can float on water here on Earth before finally sinking,” said Yu, lead author of a study published January 4 in the journal Geophysical Research Letters.
Scientists are aiming to understand as much as they can about Titan before sending a dedicated mission to the moon. The Dragonfly mission, led by the Johns Hopkins Applied Physics Laboratory in partnership with NASA, is expected to launch in 2028 and reach Titan in the 2030s.
ANALYZING AN UNUSUAL WORLD
A diverse range of organic molecules exist in Titan’s upper atmosphere, including nitriles, hydrocarbons and benzene. The surface temperature is so cold at minus 290 degrees Fahrenheit (minus 179 degrees Celsius) that the rivers and lakes were carved out by liquid methane — the way rocks and lava helped to form features and channels on Earth.
The organic molecules in Titan’s atmosphere bind together in clumps before freezing and falling onto the moon’s surface. Plains and dark dunes of organic material have been spotted across Titan, and scientists think the features were largely created by Titan’s “snow.”
But what happens when the hydrocarbon snow falls on the eerily smooth surfaces of Titan’s liquid gas lakes and rivers? Yu and her colleagues investigated the different scenarios that might occur.
Yu’s team determined that the solid organic material falling from the upper atmosphere wouldn’t dissolve when it landed on Titan’s liquid bodies because those are already saturated with organic particles.
“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 some time, but not for forever, either.”
But liquid ethane and methane have low surface tension, which means that it’s harder for solids to float on top of them.
Yu’s team simulated different models and determined that the frozen solid material wouldn’t float unless it was porous, like honeycomb or Swiss cheese. The small particles also likely wouldn’t float by themselves unless they were large enough.
The team’s analysis resulted in a scenario in which the frozen hydrocarbon solids clump together near the shore, then break off and float across the surface like glaciers on Earth. Liquid methane slowly seeps into the frozen clumps, eventually causing them to disappear from view.
Additionally, a possible thin layer of frozen solids on Titan’s seas and lakes may explain why the moon’s liquid bodies are so smooth, according to the researchers.
GETTING UP CLOSE WITH TITAN
In the coming decade, Dragonfly is expected largely to investigate the organic material plains in Titan’s equatorial region, rather than its liquid bodies.
The rotorcraft lander will sample materials on Titan’s surface, study the potential habitability of its unique environments and determine which chemical processes are taking place on the moon.
Organic chemicals essential to life on Earth are also found on Titan, such as nitrogen, oxygen and other carbon-based molecules. Beneath Titan’s thick crust, made of ice, is an internal ocean of salty water not unlike other intriguing ocean world moons orbiting Saturn such as Enceladus, or Jupiter’s moon Europa — which are considered some of the best places to search for life beyond Earth.
Titan sounds inhospitable, but it’s possible that conditions there may be conducive to life relying on different chemistry and forms in ways beyond our current understanding, according to NASA.