The existence of liquid water,
and to a lesser extent its gaseous and solid forms, on Earth is vital to the
existence of life on Earth as we know it. The Earth is located in the habitable
zone of the solar system; if it were slightly closer to or further from the Sun
(about 5%, or about 8 million kilometres), the conditions which allow the three
forms to be present simultaneously would be far less likely to exist.
Earth's mass allows gravity to hold an atmosphere.
Water vapor and carbon dioxide in the atmosphere provide a greenhouse effect
which helps maintain a relatively steady surface temperature. If Earth were
smaller, a thinner atmosphere would cause temperature extremes preventing the
accumulation of water except in polar ice caps (as on Mars).
It has been proposed that life itself may maintain
the conditions that have allowed its continued existence. The surface
temperature of Earth has been relatively constant through geologic time despite
varying levels of incoming solar radiation (insolation), indicating that a
dynamic process governs Earth's temperature via a combination of greenhouse
gases and surface or atmospheric albedo. This proposal is known as the Gaia
The state of water also depends on a planet's
gravity. If a planet is sufficiently massive, the water on it may be solid even
at high temperatures, because of the high pressure caused by gravity.
There are various theories about origin of water on
is the medium of life. It is the most abundant compound found in all organisms.
It varies from 65 to 89 % in different organisms. Human tissues contain about
20 % water in bone cells and 85% in brain cells. Almost all the reactions of a
cell occur in the presence of water.