The standard unit of pressure is the pascal, abbreviated Pa, which is equivalent to 1 kg/m2 (1 kilogram per square meter). The pressure at the surface of the Earth is 100,000 Pa. Pressure rises rapidly in a planet because of the immense weight of the overlying rocks. To save using large numbers, pressures inside planets are usually expressed as GPa (giga means billion). At a depth of 200 km in Mars, the pressure is 2 GPa.