<P> The origin of water on Earth, or the reason that there is clearly more liquid water on Earth than on the other rocky planets of the Solar System, is not completely understood . There exist numerous more or less mutually compatible hypotheses as to how water may have accumulated on Earth's surface over the past 4.5 billion years in sufficient quantity to form oceans . </P> <P> Comets, trans - Neptunian objects or water - rich meteoroids (protoplanets) from the outer reaches of the asteroid belt colliding with Earth may have brought water to the world's oceans . Measurements of the ratio of the hydrogen isotopes deuterium and protium point to asteroids, since similar percentage impurities in carbon - rich chondrites were found in oceanic water, whereas previous measurement of the isotopes' concentrations in comets and trans - Neptunian objects correspond only slightly to water on Earth . </P>

How old is the water that is found on earth today
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