I have carried the idea around for a while, to put a world with two surfaces in a science fiction work. I had Titan and Europa in mind when I created it. I pictured a larger Titan size version of Europa.

Its top surface has a dirty water ice crust. On this surface liquid methane flows. There is methane rain, rivers, lakes and ocean. There is methane based life.

The second surface is below the ice. It is a rocky crust with liquid water ocean covering it. Here, life based on water, has evolved.

By mere chance, both the methane life and water life evolve intelligence and one day they meet each other.

Anyway, this news is fascinating. A world with two surfaces and two different environments.