Earth is the only world we know that has liquid water on its surface. Oceans cover seven tenths of the planet's surface. Yet, in many ways, they remain a mystery.
The oceans have a major impact on the weather and atmosphere. They act as storage heaters in the winter and air conditioners in the summer.
Ocean currents act like enormous conveyor belts, carrying heat from the equator toward the poles. Western Europe would be as cold as Canada in the winter if the Gulf Stream current disappeared.
There is more life in the oceans than anywhere else on Earth. Even plankton, the tiniest forms of ocean life, soak up huge amounts of carbon dioxide. However, dumping of oil and poisonous waste is a major problem.
Satellites are ideal for studying and mapping the vast seas. One instrument on ESA’s Envisat can measure sea surface temperature to within a fraction of a degree. Other sensors pick out ocean colour and pollutants entering the sea.
Space-based radar can detect “hills” and “valleys” in the sea. Also revealed are wave size, wind speed and wind direction. Ships benefit greatly from images of storms, dangerous icebergs and sheets of sea ice in winter.