Every single organism on Earth, no matter the biome, the kingdom, the domain, whether it's an extremophile in a hot spring or some lithotroph buried in the crust, depends on water.

Water is the universal solvent. Life is, ultimately, a series of chemical reactions, and those chemical reactions need to take place in a medium. Water is a great choice because it's abundant, it's liquid in a wide range of temperatures, it's liquid in WARM temperatures, which means there's plenty of energy to go around, it dissolves a wide variety of molecules, and, and….I could spend an episode singing the praises of water.

But water's not the only liquid in the universe. Titan has no liquid water whatsoever on its surface. But it does have liquids. Seas, lakes, streams, rivers…of methane and ethane.

Titan is a world unlike any other. It's the largest moon of Saturn, and it's even bigger than our own Moon. It's the only rocky world in the entire solar system, aside from the Earth, to host a substantial atmosphere. It's so substantial that the air pressure on the surface is 50% greater than the Earth's, despite the fact that Titan is much, much smaller than our planet.

That atmosphere is almost entirely nitrogen, with a sizable fraction of methane. But UV radiation from the sun reacts with that nitrogen and methane to make a wide variety of other molecules, including acetylene, cyanogen, benzene, propane, and propane accessories.

And all of this – all of this – is operating a temperature of around negative 180 degrees Celsius, or around negative 300 degrees Fahrenheit.

Like I said, there's nothing like Titan.

At these temperatures and pressures, the methanes and ethanes remain liquid. And it's possible – and I really want to emphasize that wordpossible– that these liquids can serve as a solvent for life.

Titan has it all. Not water, but a liquid solvent for life's playground. It has an abundant source of energy, namely UV radiation from the Sun. It has an extremely rich chemistry with a wide and diverse array of compounds. And it has the physics: temperatures and pressures that allow complex chemistry to take place and movement of materials, through precipitation and wind, to mix them up.

If there's life on Titan, it's different from any Earth-like life at a deep, fundamental, biochemical level. This isn't water-based life. It's methane-based life. Instead of consuming oxygen or carbon dioxide, methane-based life would consume hydrogen or acetylene. In fact, evidence from NASA's Cassini spacecraft and the Huygens probe, which landed on the surface in 2005, reveal production of both hydrogen and acetylene in the atmosphere, but then a lack of it near the surface.

Sure, there are plenty of ways to scrub those out of the atmosphere at lower altitudes. But maybe…just maybe…it's gettingeaten.

In fact, it might even be possible to build cell membranes using the exotic chemistry on Titan. On Earth, most life uses phospholipids to build membranes, but those are in short supply in the cold, toxic environment of Titan. Instead, life on Titan could build what are azotosomes, which are membranes made of acrylonitrile – although other research suggests that these compounds just form ices instead of flexible, semi-rigid membranes.

Life on Titan wouldn't exactly be…thrilling. I mean, it would be exciting to find, but the extremely low temperatures prevent rapid metabolisms. Almost certainly if there's life, it's so extremely simple and slow-acting that it just might be unrecognizable.

If that's not enough, Titan has a secret. The atmosphere and lakes of methane sit on top of a crust made of water ice. And some computer models suggest that there is a liquid water ocean hiding underneath that crust. This ocean wouldn't be as a nice and pristine as the one on Europa, as it would likely be extremely rich in ammonia. On Earth, this would be a completely toxic environment. So on Titan's possible ocean, life wouldn't have to just overcome the lack of sunlight, but also a challenging environment to support biological functions.

But hey, we can't rule it out, at least not yet. In 2028 NASA plans to launch the Dragonfly mission to Titan, which is a rotorcraft designed to land at sites, study them, and then hop up to another location. As with all the other missions designed to hunt for life, the hopes of actually finding life are, honestly, slim. But no matter what, we're going to learn a lot about chemistries on worlds other than the Earth, and we may even learn about how life got its start on the Earth…but that's another episode.