The OUTER core is liquid. Even if the INNER core is even hotter than the outer core, the pressure is higher - so the inner core is solid. We know that it is solid because of the way it refracts seismic waves. read more
Latent heat released as the solid inner core gradually grows at the boundary with the liquid outer core. (1) can account for a great deal of initial heat, but the major uncertainty is how much of this heat would have been immediately radiated into space. (2) could easily have melted the outer few thousand kilometers of the planet. read more
The layer outside the inner core is the outer core, with a thickness of approximately 1,430 miles. The outer core consists primarily of iron and nickel, two valuable metals found in abundant amounts on the planet. read more