I understand that this situation is hypothetical -- I just need a piece of advice to make it more realistic.
In my story, there is a superearth of 4,5 Earth masses and 1,35 Earth's radius, rotating around star similar to the Sun, except little lighter and dimmer. The planet has a moon... Composed mostly of iron.
Now, I've already made some calculations. For storytelling reasons (planet-moon physical transfer supposed to be much faster than Earth-Moon' is) the moon is supposed to rotate very close to the Superearth - I've estimated around 50,000 kilometers in average, considering Roche limit of roughly 28,000 kilometers due to the moon's enormous density. As itself, it is pretty small and light, but maintains a high surface gravity, again, due to the high density.
Now, here is one thing I am trying to understand right now the geological activity. It is clear that planet-moon system with only 50,000 km orbit will result in dramatic tidal heating, and, subsequently, plate tectonics and volcanism. In my case, first is countered by the planet's proximity from it's host star - it resides on cooler edge of habitable zone, and technically thick atmosphere and tidal heating are the only things that keep it warm enough for cold-acceptable vegetation and prevention of oceanic freezing. On the other hand, the major geological activity is diminished by superearth's incredibly thick crust without large "cracks", at least on surface level. I'm not sure what will be happening on the moon in that scenario, because tidal effect on it, caused by superearth's gravity, will be even stronger. I'm aware that cannonball planets probably get geologically dead pretty fast, but wouldn't this process stumble into said tidal effects? I just couldn't find any theories about iron MOONS, not planets.
How much of this has an actual scientific basis and what should I do to make it better?
Thank you in advance.
With my concerns about Roche limit, I totally forgot about the importance of mass - so there is some more calculations to make, but like I said, the moon is still relatively light. Also, in the initial idea most of heating of planet is atmospheric and not tidal - the latter might only contribute for a little oceanic heating.