Titan, moon of Saturn

The density suggests that it is rock and ice.

What is the escape velocity2 compared to Earth?

It has a big atmosphere. About 10 times as much per square meter of surface as Earth. It is mostly nitrogen. How can that be, with so little gravity? It helps a lot that Titan is so cold, so the gas molecules aren't moving very fast. That makes it just possible for Titan to keep its atmosphere.

Still, I have not seen a convincing explanation of why Titan got so much atmosphere to start with. Presumably, there was a considerable amount of ammonia ice, NH3 in the original Titan. Then a little warming released ammonia gas into the atmosphere, where ultraviolet radiation from the sun broke it into nitrogen and hydrogen (which mostly escaped).

What is the atmosphere made of?

Not many details are known, but it would clearly be an interesting destination for a probe.

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Davison E. Soper, Institute of Theoretical Science, University of Oregon, Eugene OR 97403 USA soper@bovine.uoregon.edu