Water on Mars exists almost exclusively as water ice, located in the Martian polar ice caps and under the shallow Martian surface even at more temperate latitudes.[65][66] A small amount of water vapor is present in the atmosphere.[67] There are no bodies of liquid water on the Martian surface because its atmospheric pressure at the surface averages 600 pascals (0.087 psi)—about 0.6% of Earth's mean sea level pressure—and because the temperature is far too low, (210 K (−63 °C)) leading to immediate freezing. Despite this, about 3.8 billion years ago,[68] there was a denser atmosphere, higher temperature, and vast amounts of liquid water flowed on the surface,[69][70][71][72] including large oceans.[73][74][75][76][77] It has been estimated that the primordial oceans on Mars would have covered between 36%[78] and 75% of the planet.[79]