Hypotheses for the possible sources of the water on Earth
The origin of water on Earth is the subject of a body of research in the fields of
planetary science,
astronomy, and
astrobiology.
Earth is unique among the
rocky planets in the
Solar System in having
oceans of liquid
water on its surface.[2] Liquid water, which is necessary for all known forms of
life, continues to exist on the surface of Earth because the planet is at a far enough distance (known as the
habitable zone) from the
Sun that it does not
lose its water, but not so far that low temperatures cause all water on the planet to freeze.
It was long thought that Earth's water did not originate from the planet's region of the
protoplanetary disk. Instead, it was hypothesized water and other
volatiles must have been delivered to Earth from the outer Solar System later in its history. Recent research, however, indicates that hydrogen inside the Earth played a role in the formation of the ocean.[3] The two ideas are not mutually exclusive, as there is also evidence that water was delivered to Earth by impacts from icy
planetesimals similar in composition to
asteroids in the outer edges of the
asteroid belt.[4]
History of water on Earth
One factor in estimating when water appeared on Earth is that water is continually being lost to space. H2O molecules in the atmosphere are broken up by
photolysis, and the resulting free
hydrogen atoms can sometimes
escape Earth's gravitational pull. 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 suggest that even the heavier elements in the early atmosphere were subject to significant losses.[4] In particular,
xenon is useful for calculations of water loss over time. Not only is it a noble gas (and therefore is not removed from the atmosphere through chemical reactions with other elements), but comparisons between the abundances of its nine 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 eons.[5][clarification needed]
Any water on Earth during the latter 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.[6][7] 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. Afterward, liquid water oceans may have existed despite the surface temperature of 230 °C (446 °F) due to the increased atmospheric pressure of the CO2 atmosphere. As the cooling continued, most CO2 was removed from the atmosphere by
subduction and dissolution in ocean water, but levels oscillated wildly as new surface and
mantle cycles appeared.[8]
Geological evidence also 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.[9] In the
Nuvvuagittuq Greenstone Belt, Quebec, Canada, rocks dated at 3.8 billion years old by one study[10] and 4.28 billion years old by another[11] show evidence of the presence of water at these ages.[9] If oceans existed earlier than this, any geological evidence has yet to be discovered (which may be because such potential evidence has been destroyed by geological processes like
crustal recycling). More recently, in August 2020, researchers reported that sufficient water to fill the oceans may have always been on the
Earth since the beginning of the
planet's formation.[12][13][14]
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 shown that liquid water and an atmosphere must have existed 4.404 ± 0.008 billion years ago, very soon after the formation of Earth.[15][16][17][18] 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 and 4.0 billion years ago. Other studies of zircons found in Australian Hadean rock 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 an atmosphere full of carbon dioxide, early Earth's surface was much as it is today (in terms of
thermal insulation). The action of plate tectonics traps vast amounts of CO2, thereby reducing
greenhouse effects, leading to a much cooler surface temperature and the formation of solid rock and liquid water.[19]
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 × 1021 kg, which is 0.023% of the total mass of Earth, 6.0 × 1024 kg. An additional 5.0 × 1020 kg of water is estimated to exist in ice, lakes, rivers, groundwater, and atmospheric water vapor.[20] 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.[21] 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.[21] Similarly, the Earth's core could contain four to five oceans' worth of hydrogen.[20][22]
Hypotheses for the origins of Earth's water
Extraplanetary sources
Water has a much lower condensation temperature than other materials that compose the terrestrial planets in the Solar System, such as iron and 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 lower, water could condense and form icy
planetesimals. The boundary of the region where ice could form in the early Solar System is known as the
frost line (or snow line), and is located in the modern asteroid belt, between about 2.7 and 3.1
astronomical units (AU) from the Sun.[23][24] It is therefore necessary 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 in question.
One hypothesis claims that Earth
accreted (gradually grew by accumulation of) icy planetesimals about 4.5 billion years ago, when it was 60 to 90% of its current size.[21] 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 chondritemeteorites and meteorites from
Vesta, both of which originate from the Solar System's
asteroid belt.[25][26] It is also supported by studies of
osmium isotope ratios, which suggest that a sizeable quantity of water was contained in the material that Earth accreted early on.[27][28] Measurements of the chemical composition of lunar samples collected by the
Apollo 15 and
17 missions further support this, and indicate that water was already present on Earth before the Moon was formed.[29]
One problem with this hypothesis is that the
noble gas isotope ratios of Earth's atmosphere are different from those of its mantle, which suggests they were formed from different sources.[30][31] To explain this observation, a so-called "late veneer" theory has been proposed in which water was delivered much later in Earth's history, after the Moon-forming impact. However, the 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 (including Earth) during this period if Jupiter migrated closer to the Sun.[32]
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.[33]
The evidence from 2019 shows that the molybdenum isotopic composition of the Earth's mantle originates from the outer Solar System, likely having brought water to Earth. The explanation is that
Theia, the planet said in the
giant-impact hypothesis to have collided with Earth 4.5 billion years ago forming the
Moon, may have originated in the outer Solar System rather than in the inner Solar System, bringing water and carbon-based materials with it.[33]
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, that of
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.[34] By studying the different isotopic ratios of Earth and of other icy bodies in the Solar System, the likely origins of Earth's water can be researched.
Earth
The deuterium to hydrogen ratio for ocean water on Earth is known very precisely to be (1.5576 ± 0.0005) × 10−4.[35] 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 process is known that can decrease Earth's D/H ratio over time.[36] 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.[37] 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 at present. This is consistent with a scenario in which a significant proportion of the water on Earth was already present during the planet's early evolution.[20]
Asteroids
Multiple geochemical studies have concluded that asteroids are most likely the primary source of Earth's water.[38]Carbonaceous chondrites–which are a subclass of the oldest meteorites in the Solar System–have isotopic levels most similar to ocean water.[39][40] 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.[41] 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.[42] Earth's current deuterium to hydrogen ratio also matches ancient
eucrite chondrites, which originate from the asteroid
Vesta in the outer asteroid belt.[43] CI, CM, and eucrite chondrites 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.[44]
A further asteroid particle study supported the theory that a large source of earth's water has come from hydrogen atoms carried on particles in the
solar wind which combine with oxygen on asteroids and then arrive on earth in space dust. Using atom probe tomography the study found hydroxide and water molecules on the surface of a single grain from particles retrieved from the asteroid
25143 Itokawa by the Japanese space probe
Hayabusa.[45][46]
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.[47][48][49][50] Using this cometary D/H ratio, models predict that less than 10% of Earth's water was supplied from comets.[51]
Other, shorter period comets (<20 years) called Jupiter family comets likely originate from the Kuiper belt, but have had their orbital paths influenced by gravitational interactions with Jupiter or Neptune.[52]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.[53] 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.[50][54]
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