The existence of liquid water, and to a lesser extent, gaseous and solid dosage forms on Earth is vital to the existence of life on Earth as we know it. The land is located in a residential area of the solar system, if it was slightly closer to or farther from (about 5%, or about 8 million km), the conditions that must be present simultaneously would be much less. Gravity of the earth allows him to retain the atmosphere. Water vapor and carbon dioxide in the atmosphere provides a temperature buffer (greenhouse effect), which allows you to maintain a relatively steady surface temperature. If Earth were smaller, thinner atmosphere allows extreme temperatures, thereby preventing the accumulation of water, except the polar ice caps (as on Mars). Surface temperature was relatively constant through geologic time, despite different levels of incoming solar radiation (insolation), indicating that the dynamic process of regulating the temperature of the Earth through a combination of greenhouse gases and surface or atmospheric albedo. This proposal is known as the Gaia hypothesis.
Water (hydrogen oxide) – clear liquid with no color (in a small volume), and smell. Chemical formula: H2O. In the solid state is ice or snow, as well as a gas – water vapor. About 71% of the Earth's surface is covered with water (oceans, seas, lakes, rivers and ice at the poles). It is a good solvent silnopolyarnym. Under natural conditions, always contains dissolved substances (salts, gases). Water is the key value in creating and sustaining life on Earth, in the chemical structure of living organisms in climate and weather.
The state of water on the planet depends on external pressure, which is determined by gravity of the planet. If planet sufficiently massive, the water it can be hard, even at high temperatures because of the high pressure caused by gravity. There are various theories about the origin of water on Earth. Water ice is present on: Earth – mainly ice sheets of polar ice caps on Mars, Titan Enceladus Europa and comets comet population source (in the Kuiper belt and Oort cloud objects). Water ice may be present on the Moon, Ceres, and Tethys. Water and other volatile components are likely to constitute a significant part of the internal structures of Uranus and Neptune.