[kenhikage (OP)]

So, I was just listening to the lastest podcast and someone mentioned that Mercury's core is roughly 1/3 of the planet. But, if there is no tectonic activity, how can this claim be made. Also, currently what reason is there to bother differentiating between a solid planet's core and the rest of it?

[Soul Surfer]

The main source of information is the planet's size, shape, gravity and surface composition.  This gives the mean density, the density of the surface material and the total mass.  In a differentiated rocky planet the rocky surface is silicates and the dense material is assumed to be iron group elements.  On this assumption the size of an iron core can be estimated.

[Lamprey5]

You can theorize what the core composition is by studying the magnetic field around the planet. However more up-close study is required for more accurate guesses at what it is made up of. That is partly why the Juno Spacecraft was launched to Jupiter: to study the composition of the planet. Once you find out how much of the substance is inside the planet, you can determine its volume and the percent composition of the planet as a whole.

[kenhikage (OP)]

Sorry to revive this after so long, I've been away for a while.

So, in Mercury's case, we aren't certain if there is a clear division between silicates and ferrous material, right? I mean, it could be somewhat graduated. This leaves me with a new concept of "core."

[evan_au]

If there is a differentiated iron core and silicate surface, that tells a lot about the history of the planet(oid). It suggests that at one time it was a molten mass, which separated out according to density, even if today there is no textonic activity.

On the other hand, it is possible to imagine a planet(oid) which is a jumbled mass of iron and silicate, which would tell us that the pieces cooled separately, and that the whole object was not molten as a unit.