The movement of water around, over, and through the Earth is called the water cycle, a key process of the hydrosphere.
A
hydrosphere (from
Greek ὕδωρ -
hydor, "water" and σφαῖρα -
sphaira, "sphere") in
physical geography describes the combined mass of water found on, under, and over the surface of a
planet.
The total mass of the Earth's hydrosphere is about 1.4 × 10
18 tonnes, which is about 0.023% of the Earth's total mass. About 20 × 10
12 tonnes of this is in the
Earth's atmosphere (the volume of one
tonne of water is approximately 1 cubic metre). Approximately 71% of the
Earth's
surface, an area of some 361 million square kilometres (139.5 million square miles), is covered by
ocean.
Other hydrospheres
A thick hydrosphere is thought to exist around the
Jovian moon
Europa. The outer layer of this hydrosphere is almost entirely ice, but current models predict that there is an ocean up to 100
km in depth underneath the ice. This ocean remains in a liquid form because of
tidal flexing of the moon in its
orbit around Jupiter. The volume of Europa's hydrosphere is 3 × 10
18 m
3, 2.3 times that of Earth.
It has been suggested that the Jovian moon
Ganymede and the Saturnian moon
Enceladus may also possess sub-surface oceans. However the ice covering is expected to be thicker on Jupiter's Ganymede than on Europa.
Hydrological cycle
The sun provides the energy necessary to cause evaporation from all wet surfaces including oceans, rivers, lakes, soil and the leaves of plants. Water vapour is further released as
transpiration from vegetation and from animals and humans.
See also
