Titan's surface seas actually interest me a lot more than the water oceans of the ice shell moons. This is for a couple of reasons. First, they're a lot more accessible. You don't need a probe capable enough to land, drill/melt through miles of ice, etc. You can plop down directly in them from space and start exploring. They're exposed directly to the atmosphere.

But that's not the real reason I'm interested in Titan's hydrocarbon seas. People have come up with speculative models for life that could actually exist in this environment. I'm not talking about microbes hiding out in the liquid water mantle of Titan, I'm talking exotic life that actually uses methane or ethane as its solvent, as Earth life uses water. Biologists have proposed models for such life forms and how their biochemistry could work.

Why is this so interesting? For one, it would just be a really neat discovery; it would prove that our type of life isn't the only type of life possible. But it goes much deeper than that.

Let's say we find some microbes in the waters of Europa or hiding in a briny aquifer in Mars. That would be a neat discovery, but we'll quickly run into a problem; how do we know that this life truly represents a second case of abiogenesis? In other words, how do we know that the microbes or other life we find is actually a truly unique instance of life? If panspermia is in play, then it's entirely possible that life originated on Earth and then was transferred to the other bodies in the Solar System. Or, life could have originated somewhere else and been transferred here. We have examples of Martian meteorites on Earth; we know that the various planets have all contaminated each other with some of their surface rocks. It's hotly debated whether microbes or their more durable spores could survive being launched into space by an asteroid, drifting for years in the vacuum, and then survive crashing onto another planet or moon. But the important part is that is a possibility. It's by no means proven, but it's not an unreasonable hypothesis.

If we find these microbes on Mars, biologists will immediately try to sequence their genomes and see if there is a common ancestor with Earth life. But the big problem is that ambiguity will still exist. We can't for sure know what kind of microbes existed on the early Earth. Even if the life we find seems to be evolutionarily distinct from Earth life, there will always be a possibility that the microbes we find are simply descended from a now-extinct branch of Earth life. Especially if it largely uses the same chemical elements as Earth life, whether such life is truly a second genesis will remain ambiguous. Biologists will debate the topic for generations, arguing for this reason or that reason why Earth life and Mars life do or do not have a common ancestor. We may never get a firm answer.

And this answer matters because what I'm ultimately most curious about is how common life is in the universe. If life on Mars and Earth share a common ancestor, we just go from only knowing that one planet has life to only knowing that one solar system has life. We could just be from one freakishly lucky solar system that happened to have an abiogenesis event, and almost every star in the sky is orbited by completely dead worlds. However, if we had clear evidence that two genesis events happened in one solar system, it would mean life is everywhere. Life cannot be incredibly rare if there are two independent occurrences of it in just our star system.

And that's where the potential of Titan's seas really shines. It may be possible for life to exist in Titan's seas, but it would have to be, from the molecular level up, constructed completely differently from every life form on Earth. I've heard it eloquently describe that, "such life would be as different from us as a stone fish is from a stone." There is zero chance that a microbe that uses methane or ethane as a solvent and can only exist at temperatures cold enough for liquid methane will share any ancestry with Earth life. The discovery of a single microbe in Titan's seas would represent an undeniable, completely unambiguous example of a second abiogenesis event. In an instant, we would know that life is absolutely everywhere in the universe.

I see life in Titan's seas as the hail Mary play of astrobiology. Though we have some conjectural models for how such life might work, we have no way of knowing if such life is truly even possible. No one has managed to assemble such a microbe in a lab. So it's a huge gamble whether such life exists. But if it does, it would provide unambiguous proof that life is everywhere in the universe. It's the ultimate high risk/high reward gamble.

The underground oceans might also be some kind of eutectic - say water and ammonia. It lowers the melting point to such an extent that it stays liquid at temperatures that would otherwise freeze water.

On Titan the "rocks" and mountains are made of mostly water ice. It could mean that Titan has water "magma" at depth with dissolved hydrocarbons. There are lots of possibilities, but of course, all speculation.

So... oil?

Maybe an exotic biochemistry would function in such an environment.