Origin of water on Earth

hypotheses for the possible sources of the water on Earth

The origin of water on Earth is only partly known. Life as we know it depends on water, and most types of life started in the seas. Earth is the only planet known with liquid water on its surface.[1] The water on Earth may have come from the Solar System as it formed, or it may have formed on Earth. Both are possible.[2]

Our planet is in the habitable zone, not so far out that all water freezes, and not so close to the Sun that all water evaporates into space. This is why liquid water, necessary for life as we know it, continues to exist on the surface of Earth.

It was long thought that Earth’s water 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.[2] The two ideas are not mutually exclusive: there is evidence water was delivered to Earth by impacts from icy planetesimals similar in composition to asteroids in the outer edges of the asteroid belt.[3]

One problem, not yet resolved, is that the noble gas isotope ratios of Earth's atmosphere are different from those of its mantle. This suggests they were formed from different sources.[4][5]


  1. US Department of Commerce, National Oceanic and Atmospheric Administration. "Are there oceans on other planets?". oceanservice.noaa.gov. Retrieved 2020-07-16.
  2. 2.0 2.1 Monday, Nola Taylor Redd (2019). "Where did Earth's water come from". Astronomy.com. Retrieved 2020-07-16.
  3. Pepin, Robert O. 1991. On the origin and early evolution of terrestrial planet atmospheres and meteoritic volatiles. Icarus. 92 (1): 2–79. Bibcode:1991Icar...92....2P. doi:10.1016/0019-1035(91)90036-s. ISSN 0019-1035
  4. Dauphas, Nicolas (2003). "The dual origin of the terrestrial atmosphere". Icarus. 165 (2): 326–339. arXiv:astro-ph/0306605. Bibcode:2003Icar..165..326D. doi:10.1016/s0019-1035(03)00198-2. ISSN 0019-1035. S2CID 14982509.
  5. Owen, Tobias; Bar-Nun, Akiva; Kleinfeld, Idit 1992. Possible cometary origin of heavy noble gases in the atmospheres of Venus, Earth and Mars. Nature 358 (#6381): p43–46. [1]