Though both the inner and outer core are made of the same material, the melting point of inner core is increased due to the increased pressure, so the inner core is in a solid state and outer is a liquid . read more
To prove why earth’s outer core is in liquid state, researchers say that the earth cools over time and its interior temperature drops, while its pressure remains constant. In other words, when the earth was born, it is likely that the entire core was molten. read more