What is Origin of water on Earth?
The '''origin of water on Earth''' is only barely known. All [[life]] on [[Earth]] needs [[water]], and most types of life started in the [[sea]]. Earth is the only planet known with liquid water on its [[surface]].<ref>{{Cite web|last=US Department of Commerce|first=National Oceanic and Atmospheric Administration|title=Are there oceans on other planets?|url=https://oceanservice.noaa.gov/facts/et-oceans.html|access-date=2020-07-16|website=oceanservice.noaa.gov|language=EN-US}}</ref> The water on Earth may have come from the [[Solar System]] as it formed, or it may have formed on Earth. Both are possible.<ref name=Nola>{{Cite web|last1=Monday|first1=Nola Taylor Redd |year =2019|title=Where did Earth's water come from|url=https://astronomy.com/magazine/2019/04/where-did-earths-water-come-from|access-date=2020-07-16|website=Astronomy.com}}</ref> We do know that [[hydrogen]] and [[oxygen]], the elements that constitute water come from stars. Oxygen is created through [[Nuclear fusion|fusion]] of [[helium]] and [[carbon]] atoms inside the core of [[Red giant|red giants]]. Earth is in the [[habitable zone]], it is not so far from the [[Sun]] that all water [[Freezing|freezes]], and not so close to the Sun that all water [[Evaporation|evaporates]]. This is why liquid water, important for all life, still exists on the surface of Earth. A long time ago, [[Scientist|scientists]] thought that water on Earth did not come from the planet’s region of the [[protoplanetary disk]]. Instead, it was thought water must have been delivered to Earth from the outer Solar System later in its history. However, recent research suggests that hydrogen inside the Earth played a role in the formation of the ocean.<ref name=Nola/> The two ideas are not mutually exclusive: there is evidence water was delivered to Earth by impacts from icy [[planetesimal]]s similar in composition to [[asteroid]]s in the outer edges of the [[asteroid belt]].<ref>Pepin, Robert O. 1991. On the origin and early evolution of terrestrial planet atmospheres and mete