Origin of water on Earth
Encyclopedia
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
on the Earth than on the other planets of the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
, has not been clarified. There are several acknowledged theories as to how the world's ocean
Ocean
An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...
s were formed over the past 4.6 billion years
History of Earth
The history of the Earth describes the most important events and fundamental stages in the development of the planet Earth from its formation 4.578 billion years ago to the present day. Nearly all branches of natural science have contributed to the understanding of the main events of the Earth's...
.
Origins
Some of the most likely contributory factors to the origin of the EarthEarth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
's oceans are as follows:
- The cooling of the primordialHadeanThe Hadean is the geologic eon before the Archean. It started with the formation of the Earth about 4.7 Ga and ended roughly 3.8 Ga, though the latter date varies according to different sources. The name "Hadean" derives from Hades, Greek for "Underworld", referring to the "hellish"...
Earth to the point where the outgassedOutgassingOutgassing is the release of a gas that was dissolved, trapped, frozen or absorbed in some material. As an example, research has shown how the concentration of carbon dioxide in the Earth's atmosphere has sometimes been linked to ocean outgassing...
volatile components were held in an atmosphereEarth's atmosphereThe atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
of sufficient pressureAtmospheric pressureAtmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...
for the stabilization and retention of liquid water. - CometCometA comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...
s, trans-Neptunian objectTrans-Neptunian objectA trans-Neptunian object is any minor planet in the Solar System that orbits the Sun at a greater distance on average than Neptune.The first trans-Neptunian object to be discovered was Pluto in 1930...
s or water-rich meteoriteMeteoriteA meteorite is a natural object originating in outer space that survives impact with the Earth's surface. Meteorites can be big or small. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids...
s (protoplanetProtoplanetProtoplanets are large planetary embryos that originate within protoplanetary discs and have undergone internal melting to produce differentiated interiors. They are believed to form out of kilometer-sized planetesimals that attract each other gravitationally and collide...
s) from the outer reaches of the main asteroid beltAsteroid beltThe asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets...
colliding with the Earth may have brought water to the world's oceans. Measurements of the ratio of the hydrogenHydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
isotopeIsotopeIsotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
s deuteriumDeuteriumDeuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...
and protium point to asteroidAsteroidAsteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...
s, since similar percentage impurities in carbon-rich chondrites were found to oceanic water, whereas previous measurement of the isotopes' concentrations in comets and trans-Neptunian objects correspond only slightly to water on the earth. - Biochemically through mineralizationMineralization (geology)In geology, mineralization is the hydrothermal deposition of economically important metals in the formation of ore bodies or "lodes".The first scientific studies of this process took place in Cornwall, United Kingdom by J.W.Henwood FRS and later by R.W...
and photosynthesisPhotosynthesisPhotosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
. - Gradual leakage of water stored in hydrousHydrateHydrate is a term used in inorganic chemistry and organic chemistry to indicate that a substance contains water. The chemical state of the water varies widely between hydrates, some of which were so labeled before their chemical structure was understood....
mineralMineralA mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
s of the Earth's rocksRock (geology)In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...
. - PhotolysisPhotodissociationPhotodissociation, photolysis, or photodecomposition is a chemical reaction in which a chemical compound is broken down by photons. It is defined as the interaction of one or more photons with one target molecule....
: radiationRadiationIn physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...
can break down chemical bonds on the surface
.
Water in the development of the Earth
A sizeable quantity of water would have been in the material which formed the Earth. Water molecules would have escaped Earth's gravity more easily when it was less massMass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
ive during its formation. Hydrogen and helium are expected to continually leak from the atmosphere, but the lack of denser noble gas
Noble gas
The noble gases are a group of chemical elements with very similar properties: under standard conditions, they are all odorless, colorless, monatomic gases, with very low chemical reactivity...
es in the modern atmosphere suggests that something disastrous happened to the early atmosphere.
Part of the young planet is theorized to have been disrupted by the impact which created the Moon
Giant impact hypothesis
The giant impact hypothesis states that the Moon was created out of the debris left over from a collision between the young Earth and a Mars-sized body. The colliding body is sometimes called Theia for the mythical Greek Titan who was the mother of Selene, the goddess of the moon.The giant impact...
, which should have caused melting of one or two large areas. Present composition does not match complete melting and it is hard to completely melt and mix huge rock masses. However, a fair fraction of material should have been vaporized by this impact, creating 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 heavy carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
atmosphere with hydrogen and water vapor
Water vapor
Water vapor or water vapour , also aqueous vapor, is the gas phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Under typical atmospheric conditions, water vapor is continuously...
. Liquid water oceans existed despite the surface temperature of 230°C because of the atmospheric pressure of the heavy CO2 atmosphere. As cooling continued, subduction
Subduction
In geology, subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge. These 3D regions of mantle downwellings are known as "Subduction Zones"...
and dissolving in ocean water removed most CO2 from the atmosphere but levels oscillated wildly as new surface and mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
cycles appeared.
Study of zircon
Zircon
Zircon is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate and its corresponding chemical formula is ZrSiO4. A common empirical formula showing some of the range of substitution in zircon is 1–x4x–y...
s has found that liquid water must have existed as long ago as 4.4 Ga, very soon after the formation of the Earth. This requires the presence of an atmosphere. The Cool Early Earth
Cool Early Earth
The Cool Early Earth theory posits that the early planet Earth had a calm influx of bolides and a cool climate allowing fluid water, after the planetary accretion but before the occurrence of the Late Heavy Bombardment in the Hadean geological eon. The Cool Early Earth is believed to have been the...
theory covers a range from about 4.4 Ga to 4.0 Ga.
In fact, recent studies of zircons (in the fall of 2008) found in Australian Hadean rock hold minerals that point to the existence of plate tectonics
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
as early as 4 billion years ago. If this holds true, the previous beliefs about the Hadean period
Hadean
The Hadean is the geologic eon before the Archean. It started with the formation of the Earth about 4.7 Ga and ended roughly 3.8 Ga, though the latter date varies according to different sources. The name "Hadean" derives from Hades, Greek for "Underworld", referring to the "hellish"...
are far from correct. That is, rather than a hot, molten surface and atmosphere full of carbon dioxide, the Earth's surface would be very much like it is today. The action of plate tectonics traps vast amounts of carbon dioxide, thereby eliminating the greenhouse effects and leading to a much cooler surface temperature and the formation of solid rock, and possibly even life.
Extraterrestrial sources
That the Earth's water originated purely from comets is implausible, as a result of measurements of the isotope ratios of hydrogen in the three comets Halley, Hyakutake and Hale-Bopp by researchers like David Jewitt, as according to this research the ratio of deuteriumDeuterium
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...
to protium (D/H ratio) of the comets is approximately double that of oceanic water. What is however unclear is whether these comets are representative of those from the Kuiper Belt
Kuiper belt
The Kuiper belt , sometimes called the Edgeworth–Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune to approximately 50 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20 to 200 times as massive...
. According to A. Morbidelli the largest part of today's water comes from protoplanets formed in the outer asteroid belt that plunged towards the Earth, as indicated by the D/H proportions in carbon-rich chondrites. The water in carbon-rich chondrites point to a similar D/H ratio as oceanic water. Nevertheless, mechanisms have been proposed to suggest that the D/H-ratio of oceanic water may have increased significantly throughout Earth's history. Such a proposal is consistent with the possibility that a significant amount of the water on Earth was already present during the planet's early evolution.
Role of organisms
In the early 1930s Cornelis Van Niel discovered that sulfide-dependent chemoautotrophic bacteria (purple sulfur bacteriaPurple sulfur bacteria
The purple sulfur bacteria are a group of Proteobacteria capable of photosynthesis, collectively referred to as purple bacteria. They are anaerobic or microaerophilic, and are often found in hot springs or stagnant water. Unlike plants, algae, and cyanobacteria, they do not use water as their...
) fix carbon and synthesize water as a byproduct of a photosynthetic pathway using hydrogen sulfide
Hydrogen sulfide
Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...
and carbon dioxide :
Few organisms utilize this method of photosynthesis today, making the water contribution negligible. But on the hydrogen sulfide
Hydrogen sulfide
Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...
-rich and oxygen-poor early planet, a small but significant portion of the earth's water may have been synthesized biochemically through this pathway.