Diagram showing the interior of the terrestrial planet Venus. The outermost layer is the crust, which is about 30 miles (50 km) deep - quite thin, to account for the planet's recent volcanic activity. The mantle, immediately below the crust, is a thick shell of silicate rocks. Lastly there is a substantial core which occupies up to 40 per cent of the planet's radius. Like the Earth's it's probably made up mostly of nickel-iron, but unlike Earth's it is probably entirely solid.