I had an idea for a world, or maybe a vision. It is a world somewhere between the size of Mars and Earth. It has the density somewhat like Ganymede and a surface gravity between 0.2 and 0.25 G (Titan = 0.14, Ganymede = 0.15, Moon = 0.16, IO = 0.18, Mars = 0.38 and Mercury = 0.39)

This world orbits a gas giant several times more massive than Jupiter. It orbits at a distance from its star where methane can exist as a liquid, solid or gas under the right atmospheric pressure.

The crust of this world is made of mainly water ice. It has an atmosphere similar in density and composition as Titan. It has oceans, rivers and lakes of methane. It rains and snows methane. There are methane thunderstorms. There are methane polar ice caps. The sky is blue. The clouds are white. The polar caps are white. The oceans are a deep blues. It is a blue world swirling with white clouds- it looks a lot like Earth. In its sky is a gas giant and its sister moons.

There are active plate tectonics on this world. There are volcanoes, but the lava is water.

This world resembles Europa in that it has liquid water under its crust. Tidal heating keeps its innards hot. This mantel of water behaves in a similar way as Earth’s mantel of liquid rock- driving the plate tectonics.

Then there is a rocky core. The surface of this rocky core is solid and it acts as a sea bed for the ocean of liquid water. The inner core, however, is liquid rock. On the surface of this core are hot spots, where water is heated to very high temperatures.

This is a two level world. It has a rocky surface below, and an ice surface above, much like Europa. But is has an atmosphere. Imagine a cross between Europa and Titan, but only much larger.

Now lets throw some life in story. On the upper icy surface there is life based on methane. Below, on the water covered rocky surface, there is life based on water.

This life could have evolved on the icy surface in the methane ocean. This life spread to the icy continents. Slowly, some of its microbes adapted to the water lava. As it became more adapted, its chemistry switched from methane based to water based. It was then able to survive and thrive in the ocean of water below.

Ore perhaps it originated in the hot water springs below and worked its way to the icy surface through water volcanoes.

Or perhaps the methane based life, and water based life evolved independently from each other.

What a fascinating world this would be. Two worlds in one. Even more interesting would be if both the water based life and methane based life evolved intelligence, and one day met up with each other.