Orange skies, icy dunes and methane clouds—no, it’s not some post-apocalyptic picture of Earth. It’s what Saturn’s moon Titan might look like to explorers. While it doesn’t sound like paradise, scientists, including my colleagues in chemistry at the University of Colorado Boulder, think Titan has enough in common with Earth that it’s worth studying. But is there life on Titan? Right now, we can’t say for sure.
Cassini-Huygens, the mission that discovered most of what’s known about Titan, ended in 2017. Data from Cassini, which orbited Saturn, and Huygens, Cassini’s surface lander on Titan, confirmed that Titan has water as well as numerous reserves of carbon-containing chemicals. Life on Earth is based on complex carbon-based chemicals, like DNA. Based on what’s known about Earth’s history, finding water and carbon-containing chemicals mixed up in a primordial soup could be the right recipe for life. Enter Dragonfly, a proposed mission to explore Titan’s surface using a lander equipped with helicopter-like propellers. Dragonfly’s mission could help us understand our own primordial evolution—or even discover extraterrestrial life.
Titan has some features reminiscent of Earth. There are clouds, lakes (albeit filled with oil, not water) and even tides. Storms form on Titan’s surface, just like on Earth. But Titan’s storms are made of methane, not water vapor. And although Titan’s temperature is well above absolute zero, it’s still absolutely freezing, with an average surface temperature of minus 290 degrees Fahrenheit, it has a dense atmosphere, so you wouldn’t need a pressurized space suit to walk around—just oxygen and many, many layers of very warm clothes.
While scientists don’t know yet if life could exist on Titan, Cassini-Huygens’ discovery of a buried saltwater ocean on Titan was a promising sign for primordial soup. Cassini-Huygens managed to find the ocean without ever getting its feet wet. Electromagnetic waves, like the ones that let your car stereo pick up your favorite country music, can tell scientists a lot about the materials they pass through.
On Titan, electromagnetic waves were sensed that could only be formed by bouncing around between two conductive layers, like, say, a bunch of salty water buried under Titan’s surface and a charged atmosphere. The signal generated by these waves is similar to that formed by lightning activity on Earth; while Titan has no lightning, similar waves could be generated by variations in its magnetic field as it orbits Saturn. This ocean lies about 65 kilometers below the surface of Titan, or six times deeper than the deepest point of the Mariana Trench, the lowest spot on Earth.
Could water in the buried ocean discovered by Cassini-Huygens reach the surface? The mission found evidence that suggests cryovolcanoes—volcanoes that spew ice instead of lava—could be the missing link between Titan’s buried ocean and surface. Water expelled from an erupting cryovolcano could mix with complex carbons at Titan’s surface. Scientists have already shown that similar interactions could produce amino acids, essential to life as we know it. But they need to get closer to confirm that cryovolcanoes exist and to search for other routes for water to reach the surface, too. Cassini was only able to take pictures of suspected cryovolcanoes, while Huygens didn’t land near one.
Dragonfly, on the other hand, could explore sites where cryovolcanoes might exist. Dragonfly would search for remnants of ice deposited on the surface by cryovolcanic eruptions. These could mix with different carbon-containing chemicals from Titan’s hazy atmosphere, creating a primordial soup that could have the right chemistry for life.
There are various sources of carbon, the other ingredient in primordial soup, on Titan’s surface.
One source is methane, a simple carbon-based chemical. On Titan, methane makes up a significant fraction of the atmosphere, creating a thick haze. Cassini-Huygens discovered that the lakes on Titan’s surface are also primarily filled with methane. Titan has a methane cycle, analogous to Earth’s water cycle: oily methane rain resupplies Titan’s lakes, while evaporation creates hazy methane clouds. Cassini also took pictures of dried river channels, which could potentially transport methane on the surface.
But there’s a mystery surrounding Titan’s methane haze: it simply shouldn’t be there. Evaporation from Titan’s surface isn’t sufficient to restore the methane in the atmosphere, which is constantly undergoing reactions to form other chemicals. The amount of methane present in Titan’s haze should have been destroyed millions of years ago.
Scientists have come up with several theories that could explain Titan’s methane haze. One theory is that Titan’s atmosphere could be periodically collapsing, then regenerating every 10 million years or so. Cassini-Huygens also found evidence for seas filled with carbon-containing chemicals, which could evaporate and supply the haze. But unless Cassini-Huygens missed some, there aren’t enough to create the amount of haze existing on Titan.
Cryovolcanoes have been suggested as another possible source for gaseous methane. They could connect underground reservoirs of carbon-containing chemicals with the atmosphere. Methane is largely produced by bacteria on Earth (think cow farts), so there’s also a very distant possibility that the methane observed on Titan comes from some life-form.
Dragonfly’s mobility on the surface would give it a big advantage over Cassini-Huygens in the search for methane. Dragonfly might find cryovolcanoes. It could also look for deeper deposits of carbon and signs of methane-producing life. By looking at the soil and air composition at different sites on Titan, scientists could identify any geologic features that could help explain the mystery of methane.
A life on Titan—with its buried oceans and freezing temperature—might not sound like a walk in the park. But Titan does have the building blocks for life: water and complex carbons. And while Cassini-Huygens was a start, there are still plenty of mysteries left to solve, from the question of Titan’s methane haze to the existence of cryovolcanoes. If the right kind of primordial soup to produce life does exist on Titan, let’s hope Dragonfly can find it.