Titan is the only world besides Earth with an active liquid cycle: clouds form, rain falls, rivers carve channels and seas fill and evaporate — but the rain is liquid methane and the ground beneath it is water ice frozen hard as stone.

I wrote a shorter piece earlier asking how a moon about 1.4 billion kilometres from the Sun can have rivers without liquid water. The fuller answer is more interesting, because Titan’s liquid cycle is not merely Earth’s water cycle with one chemical name swapped for another. An active liquid cycle does not mean every channel is always full or every landscape is soaked. In another article, I looked at a 2025 model that tested whether large impacts could keep Titan’s methane atmosphere alive. The evidence for a methane cycle is strong, but it comes from many observations joined together rather than one continuously watched loop.