How did Earth gets its water?

Answer 1

There are only a few ways Earth could have gotten its water.

1. Water was present when Earth was originally formed from the gas and dust cloud that created the solar system and was part of the original mass that condensed to form Earth.

2. Water was delivered to Earth after the initial formation, primarily in the form of a cometary bombardment. Comets at the time of Earth's early years would have been nearly the same composition, with much water, as they are now. There is much reason to believe that all planets have had significant bombardments.

3. Water, was formed by combining hydrogen and oxygen through chemical processes to form water after the initial formation of Earth. (There are good chemistry reasons to expect this was unlikely.)

Though all three occurred to some degree, the real debate is over how much. Certainly, a lot of water was present during the formation of Earth. The last possibility of additional water being added through a chemical process is believed to have been a very minor factor. The real question that remains is whether extraterrestrial sources, like comets, provided most or only a little water.

A more thorough discussion of these and other possibilities is provided in the related links.

Answer 2

Volcanic outgassing