Question: How would you figure out what's happening in the cores of terrestrial planets?

Lidunka Vocadlo answered on 16 Jun 2015:

@209frcc28 A very good question. Well for a start we have missions which have given us data on something called the moment of inertia of each planet. This tells us in the broadest sense the density distribution within the planet – i.e. it tells us that the heavy stuff is towards the centre, and it has some known density. It turns out that in the case of the terrestrial planets, this fits very well to iron (well actually it’s a bit lighter than iron, so it’s iron alloyed with a few light elements like silicon and sulphur). We also know (from clever physics about self-compression) what the pressure is throughout the planet, and approximately what the temperature might be (or, to be honest, a range of likely temperatures). With this information, confirmed by the knowledge of what elements the universe is made of and how much (called cosmic abundances), we can get a first guess for the composition of each terrestrial planet. We can then do experiments and calculations to find out if, when you heat up and squash iron (and iron alloys) to the estimated pressure and temperatures of the interiors of these planets, how does it behave (for a start, is it solid or liquid, is it runny or stationary, and a bunch of other properties). There are then some other clever physics equations which you can use to then make predictions as to how this material will behave over time, and therefore make some estimate for the dynamics and evolution of the planet. You can do the same for the other layers of the planet (the rock above the iron cores) and end up with a whole-planet model.