<P> A sizeable quantity of water would have been in the material that formed the Earth . Water molecules would have escaped Earth's gravity more easily when it was less massive during its formation . Hydrogen and helium are expected to continually escape (even to the present day) due to atmospheric escape . Part of the ancient planet is theorized to have been disrupted by the impact that created the Moon, which should have caused melting of one or two large regions of the Earth . Earth's present composition suggests that there was not complete remelting as it is difficult to completely melt and mix huge rock masses . However, a fair fraction of material should have been vaporized by this impact, creating a rock vapor atmosphere around the young planet . The rock vapor would have condensed within two thousand years, leaving behind hot volatiles which probably resulted in a heavy CO atmosphere with hydrogen and water vapor . Liquid water oceans existed despite the surface temperature of 230 ° C (446 ° F) because at an atmospheric pressure of above 27 atmospheres, caused by the heavy CO atmosphere, water is still liquid . As cooling continued, subduction and dissolving in ocean water removed most CO from the atmosphere but levels oscillated wildly as new surface and mantle cycles appeared . </P> <P> Studies of zircons have found that liquid water must have existed as long ago as 4.4 billion years ago, very soon after the formation of the Earth . This requires the presence of an atmosphere . The cool early Earth theory covers a range from about 4.4 to about 4.1 billion years . </P> <P> A September 2008 study of zircons found that Australian Hadean rock holds minerals pointing to the existence of plate tectonics as early as 4 billion years . If this is true, the time when Earth finished its transition from having a hot, molten surface and atmosphere full of carbon dioxide, to being very much like it is today, can be roughly dated to about 4.0 billion years ago . The actions of plate tectonics and the oceans trapped vast amounts of carbon dioxide, thereby eliminating the greenhouse effect and leading to a much cooler surface temperature and the formation of solid rock, and possibly even life . </P>

During the hadean eon the earth was colder