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The origin of water on Earth is the subject of a significant body of research in the fields of planetary science, astronomy, and astrobiology. Earth is unique among the rocky planets in the Solar System in that it is the only planet with oceans of liquid water on its surface. Liquid water, which is necessary for life, continues to exist on the surface of Earth because the planet is distant enough from the Sun that it does not lose its water to the runaway greenhouse effect, but not so far that low temperatures cause all water on the planet to freeze.

Earth could not have started with its current oceans of water because the early inner Solar System was far too hot for water to condense. Instead, water and other volatiles must have been delivered to Earth from the outer Solar System later in its history. Modern geochemical evidence suggests that water was delivered to Earth by impacts from icy planetesimals similar in composition to modern asteroids in the outer edges of the asteroid belt. However, when and how that water was delivered to Earth is the subject of ongoing research.

History of water on Earth
One challenge in estimating when water appeared on Earth is that water is continually being lost to space. H2O molecules in the atmosphere are broken up via photolysis, and the resulting H atoms are sometimes able to escape Earth's gravitational pull (see: Atmospheric escape). When the Earth was younger and less massive, water would have been lost to space more easily. Lighter elements like hydrogen and helium are expected to leak from the atmosphere continually, but isotopic ratios of heavier noble gases in the modern atmosphere suggests that even the heavier elements in the early atmosphere were subjected to significant loss processes. In particular, Xenon useful for calculations of water loss over time. Not only is it a noble gas (and therefore isn't removed from the atmosphere through chemical reactions with other elements), but comparisons between abundances of its seven stable isotopes in the modern atmosphere reveal that the Earth lost at least one ocean of water early in its history, between the Hadean and Archean eras.

Any water on Earth during the later part of its accretion would have been disrupted by the Moon-forming impact (~4.5 billion years ago), which likely vaporized much of Earth's crust and upper mantle and created 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 majority carbon dioxide atmosphere with hydrogen and water vapor. Afterwards, liquid water oceans may have existed despite the surface temperature of 230 C due to the increased atmospheric pressure of the CO2 atmosphere. As cooling continued, most CO2 was removed from the atmosphere by subduction and dissolution into ocean water, but levels oscillated wildly as new surface and mantle cycles appeared. There is also geological evidence that helps constrain the time frame for liquid water existing on Earth. A sample of pillow basalt (a type of rock formed during an underwater eruption) was recovered from the Isua Greenstone Belt and provides evidence that water existed on Earth 3.8 billion years ago. In the Nuvvuagittuq Greenstone Belt, Quebec, Canada, rocks dated at 3.8 billion years old by one study and 4.28 billion years old by another show evidence of the presence of water at these ages. If oceans existed earlier than this, any geological evidence has either yet to be discovered or has since been destroyed by geological processes like crustal recycling.

Unlike rocks, minerals called zircons are highly resistant to weathering and geological processes and so are used to understand conditions on the very early Earth. Mineralogical evidence from zircons has found that liquid water and an atmosphere must have existed 4.404 ± 0.008 billion years ago, very soon after the formation of Earth. This presents somewhat of a paradox, as the cool early Earth hypothesis suggests temperatures were cold enough to freeze water between about 4.4 billion to 4.0 billion years ago. Other studies of zircons found in Australian Hadean rock hold minerals that point to the existence of plate tectonics as early as 4 billion years ago. If true, that implies that rather than a hot, molten surface and atmosphere full of carbon dioxide, early Earth's surface would be much like it is today. The action of plate tectonics traps vast amounts of carbon dioxide, thereby reducing greenhouse effects, and leading to a much cooler surface temperature, and the formation of solid rock and liquid water.

Earth's water inventory
While the majority of Earth's surface is covered by oceans, those oceans make up just a small fraction of the mass of the planet. The mass of Earth's oceans is estimated to be 1.37 x 1021 kg, which is 0.023% of the total mass of Earth, 6.0 x 1024 kg. An additional 5 x 1020 kg of water (approximately 1/2 of the mass of Earth's oceans) is estimated to exist in ice, lakes, rivers, groundwater, and atmospheric water vapor. A significant amount of water is also stored in Earth's crust, mantle, and core. Unlike molecular H2O that is found on the surface, water in the interior exists primarily in hydrated minerals or as trace amounts of hydrogen bonded to oxygen atoms in anhydrous minerals. Hydrated silicates on the surface transport water into the mantle at convergent plate boundaries, where oceanic crust is subducted underneath continental crust. While it is difficult to estimate the total water content of the mantle due to limited samples, approximately three times the mass of the Earth's oceans could be stored there. Similarly, the Earth's core could contain four to five oceans worth of hydrogen.

Extraplanetary source
Water has a much lower condensation temperature than other materials that compose the terrestrial planets in the Solar System, such as iron or silicates. The region of the protoplanetary disk closest to the Sun was very hot early in the history of the Solar System, and it is not feasible that oceans of water condensed with the Earth as it formed. Further from the young Sun where temperatures were cooler, water was able to condense and form icy planetesimals. This boundary where ice was able to form in the early Solar System is known as the frost line (or snow line), and is located in the modern asteroid belt approximately between 2.7 and 3.1 astronomical units (AU). While there is some evidence in the isotope ratios of Osmium that suggest a sizeable quantity of water would have been in the material that formed Earth, the prevailing theory in the field is that objects forming beyond the frost line–such as comets, trans-Neptunian objects, and water-rich meteoroids (protoplanets)–delivered water to Earth. However, the timing of this delivery is still debated.

One theory claims that Earth accreted (gradually grew by accumulation) icy planetesimals about 4.5 billion years ago, when it was 60 to 90% of its current size. In this scenario, Earth was able to retain water in some form throughout accretion and major impact events. This hypothesis is supported by similarities in the abundance and the isotope ratios of water between the oldest known carbonaceous chondrite meteorites and meteorites from Vesta, both of which originate from the Solar System's asteroid belt. Measurements of the chemical composition of lunar samples collected by the Apollo 15 and 17 missions support this, and indicate that water was already present on Earth before the Moon was formed.

One issue with this hypothesis is that the noble gas isotope ratios of Earths' atmosphere are different from those of its mantle, which suggests they are formed from different sources. To explain this observation, a so called "late veneer" theory has been proposed in which water was deliveredd much later in Earth's history, after the Moon-forming impact. However, our current understanding of Earth's formation allows for less than 1% of Earth's material accreting after the moon formed, implying that the material accreted later must have been very water-rich. Models of early Solar System dynamics have shown that icy asteroids could have been delivered to the inner Solar System (and therefore to Earth) during this period if Jupiter migrated closer to the Sun.

Yet a third hypothesis, supported by evidence from molybdenum isotope ratios, suggests that the Earth gained most of its water from the same interplanetary collision that caused the formation of the Moon.

Geochemical analysis of water in the Solar System
Isotopic ratios provide a unique "chemical fingerprint" that is used to compare Earth's water with reservoirs elsewhere in the Solar System. One such isotopic ratio, deuterium to hydrogen (D/H), is particularly useful in the search for the origin of water on Earth. Hydrogen is the most abundant element in the universe, and its heavier isotope deuterium can sometimes take the place of a hydrogen atom in molecules like H2O. Most deuterium was created in the Big Bang or in supernovae, so its uneven distribution throughout the protosolar nebula was effectively "locked in" early in the formation of the Solar System. By studying the unique isotopic ratios of Earth and other icy bodies in the Solar System, we can search for the likely origins of Earth's water.

Earth
The deuterium to hydrogen ratio for ocean water on Earth is known very precisely to be (1.5576 ± 0.0005) × 10-4. This value represents a mixture of all of the sources that contributed to Earth's reservoirs, and is used to identify the source or sources of Earth's water. The ratio of deuterium to hydrogen may have increased over the Earth's lifetime as the lighter isotope is more likely to leak to space in atmospheric loss processes. However no known processes are able to decrease Earth's D/H ratio over time. This loss of the lighter isotope is one explanation for why Venus has such a high D/H ratio, as that planet's water was vaporized during the runaway greenhouse effect and subsequently lost much of its hydrogen to space. Because Earth's D/H ratio has increased significantly over time, the D/H ratio of water originally delivered to the planet was lower than what we currently measure. This is consistent with a scenario where a significant amount of the water on Earth was already present during the planet's early evolution.

Asteroids
Multiple geochemical studies have concluded that asteroids are most likely the primary source of Earth's water. Carbonaceous chondrites–which are a subclass of the oldest meteorites in the Solar System–have isotopic levels most similar to ocean water. The CI and CM subclasses of carbonaceous chondrites specifically have hydrogen and nitrogen isotope levels that closely match Earth's seawater, which suggests water in these meteorites could be the source of Earth's oceans. Two 4.5 billion-year-old meteorites found on Earth that contained liquid water alongside a wide diversity of deuterium-poor organic compounds further support this. Earth's current deuterium to hydrogen ratio also matches ancient eucrite chondrites, which originate from the asteroid Vesta in the outer asteroid belt. CI, CM, eucrite chondrties are believed to have the same water content and isotope ratios as ancient icy protoplanets from the outer asteroid belt that later delivered water to Earth.

Comets
Comets are kilometer-sized bodies made of dust and ice that originate from the Kuiper Belt (20-50 AU) and the Oort Cloud (>5,000 AU), but have highly elliptical orbits which bring them into the inner solar system. Their icy composition and trajectories which bring them into the inner solar system make them a target for remote and in situ measurements of D/H ratios.

It is implausible that Earth's water originated only from comets, since isotope measurements of the deuterium to hydrogen (D/H) ratio in comets Halley, Hyakutake, Hale–Bopp, 2002T7, and Tuttle, yield values approximately twice that of oceanic water. Using this cometary D/H ratio, models predict that less than 10% of Earth's water was supplied from comets.

Other, shorter period comets (<20 years) called Jupiter family comets likely originate from the Kuiper belt, but have had their orbital paths influenced through gravitational interactions with Jupiter or Neptune. 67P/Churyumov–Gerasimenko is one such comet that was the subject of isotopic measurements by the Rosetta spacecraft, which found the comet has a D/H ratio three times that of Earth's seawater. Another Jupiter family comet, 103P/Hartley 2, has a D/H ratio which is consistent with Earth's seawater, but its nitrogen isotope levels do not match Earth's.