This Cassini image of Saturn’s moon Titan shows dark-colored dunes near its equator and dark-colored lakes of liquid ethane and methane near the north pole. In the center there is also one of the few impact craters on the Moon. Credit: NASA/Cassini VIMS
The second-largest moon in our solar system may have a secret ingredient that explains many of its mysteries and perhaps increases the chances of life there.
In research published on September 30 in The Journal of Planetary Science, a team led by the University of Hawaii modeled the potential for methane clathrate (a kind of methane-infused water ice) in the crust of Titan, Saturn’s largest moon. Titan is a moon unlike any other in the solar system: It has a thick atmosphere and its surface is dotted with hydrocarbon lakes, making it the only world in our solar system other than Earth known to have liquid on its surface.
Strange chemistry
But some of Titan’s chemistry shouldn’t be: Methane is a volatile chemical that is easily broken down by sunlight, so its presence in the atmosphere indicates some source of supply. The surface is relatively free of craters, meaning something must continually fill the impact sites. Additionally, Titan has evidence of an ocean of liquid water beneath its surface, which requires some sort of heating mechanism to stay liquid in the icy outer solar system.
Methane clathrate solves this problem: It would help keep the ice shell around Titan warm and convective, which also helps drive the methane cycle. A clathrate is a substance that can trap another chemical inside it, due to its molecular shape. In particular, Titan’s methane clathrate comprises water ice surrounding the methane molecules. Methane clathrate, the study found, likely creates a unique upper crust around Titan, about 3.1 to 6.2 miles (5 to 10 kilometers) thick.
This would provide a source for the replenishment of methane in the atmosphere, as well as explaining how the surface exhibits characteristics associated with relative heat. Essentially, much of the trapped methane can be released slowly, allowing for atmospheric replenishment, and a warm surface would help erase evidence of most meteor impacts, as the crust relaxes and collapses to cover the holes. It could also help the ocean beneath the surface stay liquid, perhaps providing a place for life to grow.
“Methane clathrates keep Titan’s interior warm, helping to keep liquid subsurface ocean water from freezing and promoting convection and movement in the ice shell,” says Lauren Schurmeier, a researcher at the University of Hawaii and lead author of the article. “If Titan’s ocean were habitable, organic materials would have to make their way from the surface into the ocean, through the ice shell.”
Schurmeier adds that the convection of the ice sheets could also help transport any biosignatures – the chemical fingerprints of life – from the ocean to the world above.
Looking for answers
The last spacecraft to visit Titan was Cassini, which performed several close flybys and launched the Huygens probe into Titan before NASA intentionally crashed the spacecraft into Saturn at the end of its operations in 2017. But there are currently plans to return to Titan via the Dragonfly mission. , which received confirmation from NASA earlier this year and is in development for a 2028 launch.
Related: How we landed a probe on another planet’s moon
Dragonfly, which would arrive on Titan in 2034, is a unique spacecraft. Because of Titan’s thick, foggy atmosphere, a drone could fly there. Dragonfly will have eight rotors, and NASA will send it on multiple flights covering about 50 miles (80 km) of the lunar surface, exploring Titan’s potential for life.
The mission will also help confirm the models set out in this paper. Gwendolyn Brouwer, a Ph.D. candidate at the University of Hawaii Manoa and author of the paper, says the seismometer on the rotorcraft would be able to examine the thickness of the crust and, through the way the signal travels, its composition compared to pure ice , which would show whether or not, contains methane clathrate.
The flight path will also take it to Selk Crater, one of the few craters on Titan’s surface, also examined in this study. Selk, like Titan’s other craters, is strangely shallow, and Dragonfly could help ascertain how much methane clathrates are responsible for this.
“If I can image the inside of the crater bowl, we should be able to see whether the crater appears eroded or filled with material such as sand from nearby sand dunes,” Brouwer says.
If confirmed, the presence of methane clathrate would help explain many of the strange processes on Titan, although Dragonfly will almost certainly provide fodder for a new generation of mysteries.
