Earth has a large amount of water on its surface. 97% of it is in the ocean that covers 70% of the planet’s surface. The rest of the water is in ice, groundwater, lakes, rivers, and water vapor.

But Earth is mostly a huge rock in space; with just a thin layer of water covering the surface. If we gather all of that water together into a single sphere it would appear like a very small moon, just 1,380 km in diameter. Water is the key ingredient to life as we know it. This tiny ball of water sustains all of life on Earth. In fact it sustains all the known life in the Universe.

Earth is currently the only world in the Solar System with liquid water on its surface. There are several places known to have subsurface oceans of water, like Jupiter’s moon, Europa or Saturn’s moon Enceladus. There is strong evidence that both Venus and Mars had global surface oceans like Earth when they were billions of years younger. But both worlds evolved in diverging ways that ended with the loss of their oceans.

With the great majority of Earth’s surface covered by it, and water’s significant heat capacity, the ocean controls the climate and weather on the surface. The ocean is also the biggest component of the planet’s carbon cycle, which significantly impacts the composition of the atmosphere.