Inner core composition and thickness?

• A. A solid iron core with 3000 km diameter
• B. A molten iron core
• C. A solid mass of iron with a temperature of 3870 °C, 2400 km in diameter
• D. A liquid iron core

Answer: (C)

Earth’s inner core is a solid sphere of iron (with some nickel) at the planet’s center. The enormous pressures there keep iron in a solid state even though temperatures are extremely hot, so the inner core remains solid while the outer core is liquid. Its diameter is about 2400 km, which matches the description of a solid mass of iron roughly across 2400 km. The high temperature is real, but the surrounding pressure is what keeps this material from melting. Other options either describe a larger solid core than is supported by measurements or refer to a molten/liquid core, which describes the outer region rather than the inner core.