The disappearing 鈥渕agic islands鈥� on Saturn鈥檚 largest moon Titan have intrigued scientists since NASA鈥檚 Cassini mission spotted them during flybys a decade ago. Now, researchers believe they have unravelled the phenomenon鈥檚 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鈥檚 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鈥檚 atmospheric pressure is about 60% greater than Earth鈥檚, 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 鈥� 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鈥檚 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鈥檚 sea surfaces that can last a few hours, several weeks or longer. Cassini鈥檚 radar images captured the unexplained bright regions in Ligeia Mare, the second-largest liquid body on Titan鈥檚 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鈥檚 atmosphere, liquid bodies and solid materials that fall like snow to see if they might be related to the magic islands.
鈥淚 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,鈥� 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鈥檚 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 鈥� the way rocks and lava helped to form features and channels on Earth.
The organic molecules in Titan鈥檚 atmosphere bind together in clumps before freezing and falling onto the moon鈥檚 surface. Plains and dark dunes of organic material have been spotted across Titan, and scientists think the features were largely created by Titan鈥檚 鈥渟now.鈥�
But what happens when the hydrocarbon snow falls on the eerily smooth surfaces of Titan鈥檚 liquid gas lakes and rivers? Yu and her colleagues investigated the different scenarios that might occur.
Yu鈥檚 team determined that the solid organic material falling from the upper atmosphere wouldn鈥檛 dissolve when it landed on Titan鈥檚 liquid bodies because those are already saturated with organic particles.
鈥淔or us to see the magic islands, they can鈥檛 just float for a second and then sink,鈥� Yu said. 鈥淭hey have to float for some time, but not for forever, either.鈥�
But liquid ethane and methane have low surface tension, which means that it鈥檚 harder for solids to float on top of them.
Yu鈥檚 team simulated different models and determined that the frozen solid material wouldn鈥檛 float unless it was porous, like honeycomb or Swiss cheese. The small particles also likely wouldn鈥檛 float by themselves unless they were large enough.
The team鈥檚 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鈥檚 seas and lakes may explain why the moon鈥檚 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鈥檚 equatorial region, rather than its liquid bodies.
The rotorcraft lander will sample materials on Titan鈥檚 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鈥檚 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鈥檚 moon Europa 鈥� which are considered some of the best places to search for life beyond Earth.
Titan sounds inhospitable, but it鈥檚 possible that conditions there may be conducive to life relying on different chemistry and forms in ways beyond our current understanding, according to NASA.
