Origin of water on Earth

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Water covers about 71% of Earth's surface

 

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 that it is the only planet known to have 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 at a distance far 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 condensed from the protoplanetary disk 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.

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. 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 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 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 (446 °F) due to the increased atmospheric pressure of the CO₂ atmosphere. As 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.

This pillow basalt on the seafloor near Hawaii was formed when magma extruded underwater. Other, much older pillow basalt formations provide evidence for large bodies of water long ago in Earth's history.

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 either has 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 shown 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 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. The action of plate tectonics traps vast amounts of CO₂, 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 × 10²¹ kg, which is 0.023% of the total mass of Earth, 6.0 × 10²⁴ kg. An additional 0.5 × 10²¹ kg of water 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.

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 cooler, 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. 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 theory 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. 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. 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. 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.

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. 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.

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

Carbonaceous chondrites such as the Allende Meteorite (above) likely delivered much of the Earths water, as evidenced by their isotopic similarities to ocean water.

 

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. 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. 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. 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 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.

Asteroids

Comet Halley as imaged by the European Space Agency's Giotto probe in 1986. Giotto flew by Halley's Comet and analyzed the isotopic levels of ice sublimating from the comet's surface using a mass spectrometer.

 

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, 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.

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.

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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.

Theia

Additional evidence from the University of Münster from 2019 shows that the molybdenum isotopic composition of the Earth's core originates from the outer Solar System, likely having brought water to Earth. Their 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.

Extraterrestrial liquid water

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Extraterrestrial liquid water (from the Latin words: extra ["outside of, beyond"] and terrestris ["of or belonging to Earth"]) is water in its liquid state that naturally occurs outside Earth. It is a subject of wide interest because it is recognized as one of the key prerequisites for life as we know it and thus surmised as essential for extraterrestrial life.

With oceanic water covering 71% of its surface, Earth is the only planet known to have stable bodies of liquid water on its surface, and liquid water is essential to all known life forms on Earth. The presence of water on the surface of Earth is a product of its atmospheric pressure and a stable orbit in the Sun's circumstellar habitable zone, though the origin of Earth's water remains unknown.

The main methods currently used for confirmation are absorption spectroscopy and geochemistry. These techniques have proven effective for atmospheric water vapour and ice. However, using current methods of astronomical spectroscopy it is substantially more difficult to detect liquid water on terrestrial planets, especially in the case of subsurface water. Due to this, astronomers, astrobiologists and planetary scientists use habitable zone, gravitational and tidal theory, models of planetary differentiation and radiometry to determine potential for liquid water. Water observed in volcanic activity can provide more compelling indirect evidence, as can fluvial features and the presence of antifreeze agents, such as salts or ammonia.

Using such methods, many scientists infer that liquid water once covered large areas of Mars and Venus. Water is thought to exist as liquid beneath the surface of some planetary bodies, similar to groundwater on Earth. Water vapour is sometimes considered conclusive evidence for the presence of liquid water, although atmospheric water vapour may be found to exist in many places where liquid water does not. Similar indirect evidence, however, supports the existence of liquids below the surface of several moons and dwarf planets elsewhere in the Solar System. Some are speculated to be large extraterrestrial "oceans". Liquid water is thought to be common in other planetary systems, despite the lack of conclusive evidence, and there is a growing list of extrasolar candidates for liquid water.

Liquid water in the Solar System

As of December 2015, the confirmed liquid water in the Solar System outside Earth is 25–50 times the volume of Earth's water (1.3 billion cubic kilometers).

Mars

A cross-section of Mars underground ice is exposed at the steep slope that appears bright blue in this enhanced-color view from the MRO. The scene is about 500 meters wide. The scarp drops about 128 meters from the level ground in the upper third of the image

Water on Mars exists today almost exclusively as ice, with a small amount present in the atmosphere as vapour. Some liquid water may occur transiently on the Martian surface today but only under certain conditions. No large standing bodies of liquid water exist because the atmospheric pressure at the surface averages just 600 pascals (0.087 psi)—about 0.6% of Earth's mean sea level pressure—and because the global average temperature is far too low (210 K (−63 °C)), leading to either rapid evaporation or freezing. Features called recurring slope lineae are thought to be caused by flows of brine — hydrated salts.

In July 2018, scientists from the Italian Space Agency reported the detection of a subglacial lake on Mars, 1.5 kilometres (0.93 mi) below the southern polar ice cap, and spanning 20 kilometres (12 mi) horizontally, the first evidence for a stable body of liquid water on the planet. Because the temperature at the base of the polar cap is estimated at 205 K (−68 °C; −91 °F), scientists assume that the water may remain liquid by the antifreeze effect of magnesium and calcium perchlorates. The 1.5-kilometre (0.93 mi) ice layer covering the lake is composed of water ice with 10 to 20% admixed dust, and seasonally covered by a 1-metre (3 ft 3 in)-thick layer of CO
2 ice.

Europa

Scientific consensus is that a layer of liquid water exists beneath Europa's (moon of Jupiter) surface, and that heat from tidal flexing allows the subsurface ocean to remain liquid. It is estimated that the outer crust of solid ice is approximately 10–30 km (6–19 mi) thick, including a ductile "warm ice" layer, which could mean that the liquid ocean underneath may be about 100 km (60 mi) deep. This leads to a volume of Europa's oceans of 3 × 10¹⁸ m³, slightly more than two times the volume of Earth's oceans. 

Enceladus

Enceladus, a moon of Saturn, has shown geysers of water, confirmed by the Cassini spacecraft in 2005 and analyzed more deeply in 2008. Gravimetric data in 2010–2011 confirmed a subsurface ocean. While previously believed to be localized, most likely in a portion of the southern hemisphere, evidence revealed in 2015 now suggests the subsurface ocean is global in nature.

In addition to water, these geysers from vents near the south pole contained small amounts of salt, nitrogen, carbon dioxide, and volatile hydrocarbons. The melting of the ocean water and the geysers appear to be driven by tidal flux from Saturn.

Ganymede

A subsurface saline ocean is theorized to exist on Ganymede, a moon of Jupiter, following observation by the Hubble Space Telescope in 2015. Patterns in auroral belts and rocking of the magnetic field suggest the presence of an ocean. It is estimated to be 100 km deep with the surface lying below a crust of 150 km of ice.

Ceres

Ceres appears to be differentiated into a rocky core and icy mantle, and may have a remnant internal ocean of liquid water under the layer of ice. The surface is probably a mixture of water ice and various hydrated minerals such as carbonates and clay. In January 2014, emissions of water vapor were detected from several regions of Ceres. This was unexpected, because large bodies in the asteroid belt do not typically emit vapor, a hallmark of comets. Ceres also features a mountain called Ahuna Mons that is thought to be a cryovolcanic dome that facilitates the movement of high viscosity cryovolcanic magma consisting of water ice softened by its content of salts.

Ice giants

The "ice giant" (sometimes known as "water giant") planets Uranus and Neptune are thought to have a supercritical water ocean beneath their clouds, which accounts for about two-thirds of their total mass, most likely surrounding small rocky cores. This kind of planet is thought to be common in extrasolar planetary systems.

Indicators, methods of detection and confirmation

Most known extrasolar planetary systems appear to have very different compositions to the Solar System, though there is probably sample bias arising from the detection methods. 

Spectroscopy

Absorption spectrum of liquid water

 

Liquid water has not been detected in spectroscopic analysis of suspected seasonal Martian flows.

 

Liquid water has a distinct absorption spectroscopy signature compared to other states of water due to the state of its hydrogen bonds. Despite the confirmation of extraterrestrial water vapor and ice, however, the spectral signature of liquid water is yet to be confirmed outside of Earth. The signatures of surface water on terrestrial planets may be undetectable through thick atmospheres across the vast distances of space using current technology.

Seasonal flows on warm Martian slopes, though strongly suggestive of briny liquid water, have yet to indicate this in spectroscopic analysis.

Water vapor has been confirmed in numerous objects via spectroscopy, though it does not by itself confirm the presence of liquid water. However, when combined with other observations, the possibility might be inferred. For example, the density of GJ 1214 b would suggest that a large fraction of its mass is water and follow-up detection by the Hubble telescope of the presence if water vapor strongly suggests that exotic materials like 'hot ice' or 'superfluid water' may be present.

Magnetic fields

For the Jovian moons Ganymede and Europa, the existence of a sub-ice ocean is inferred from the measurements of the magnetic field of Jupiter. Since conductors moving through a magnetic field produce a counter-electromotive field, the presence of the water below the surface was deduced from the change in magnetic field as the moon passed from the northern to southern magnetic hemisphere of Jupiter.

Geological indicators

Thomas Gold has posited that many Solar System bodies could potentially hold groundwater below the surface.

It is thought that liquid water may exist in the Martian subsurface. Research suggests that in the past there was liquid water flowing on the surface, creating large areas similar to Earth's oceans. However, the question remains as to where the water has gone. There are a number of direct and indirect proofs of water's presence either on or under the surface, e.g. stream beds, polar caps, spectroscopic measurement, eroded craters or minerals directly connected to the existence of liquid water (such as Goethite). In an article in the Journal of Geophysical Research, scientists studied Lake Vostok in Antarctica and discovered that it may have implications for liquid water still being on Mars. Through their research, scientists came to the conclusion that if Lake Vostok existed before the perennial glaciation began, that it is likely that the lake did not freeze all the way to the bottom. Due to this hypothesis, scientists say that if water had existed before the polar ice caps on Mars, it is likely that there is still liquid water below the ice caps that may even contain evidence of life.

"Chaos terrain", a common feature on Europa's surface, is interpreted by some as regions where the subsurface ocean has melted through the icy crust.

Volcanic observation

A possible mechanism for cryovolcanism on bodies like Enceladus

Geysers have been found on Enceladus, a moon of Saturn, and Europa, moon of Jupiter. These contain water vapour and could be indicators of liquid water deeper down. It could also be just ice. In June 2009, evidence was put forward for salty subterranean oceans on Enceladus. On 3 April 2014, NASA reported that evidence for a large underground ocean of liquid water on Enceladus, moon of planet Saturn, had been found by the Cassini spacecraft. According to the scientists, evidence of an underground ocean suggests that Enceladus is one of the most likely places in the solar system to "host microbial life". Emissions of water vapor have been detected from several regions of the dwarf planet Ceres. combined with evidence of ongoing cryovalcanic activity.

Gravitational evidence

Scientists' consensus is that a layer of liquid water exists beneath Europa's surface, and that heat energy from tidal flexing allows the subsurface ocean to remain liquid. The first hints of a subsurface ocean came from theoretical considerations of tidal heating (a consequence of Europa's slightly eccentric orbit and orbital resonance with the other Galilean moons).

Scientists used gravitational measurements from the Cassini spacecraft to confirm a water ocean under the crust of Enceladus.  Such tidal models have been used as theories for water layers in other Solar System moons. According to at least one gravitational study on Cassini data, Dione has an ocean 100 kilometers below the surface.

Ground penetrating radio

Site of south polar Martian subglacial water body (reported July 2018)

 

Scientists have detected liquid water using radio signals. The Radar Detection And Ranging (RADAR) instrument of the Cassini probe was used to detect the existence of a layer of liquid water and ammonia beneath the surface of Saturn's moon Titan that are consistent with calculations of the moon's density. Ground penetrating radar and dielectric permittivity data from the MARSIS instrument on Mars Express indicates a 20-kilometer-wide stable body of briny liquid water in the Planum Australe region of planet Mars.

Density calculation

Artists conception of the subsurface water ocean confirmed on Enceladus.

 

Planetary scientists can use calculations of density to determine the composition of planets and their potential to possess liquid water, though the method is not highly accurate as the combination of many compounds and states can produce similar densities.

Models of Saturn's moon Titan density indicate the presence of a subsurface ocean layer. Similar density estimations are strong indicators of an subsurface ocean on Enceladus.

Initial analysis of 55 Cancri e's low density indicated that it consisted 30% supercritical fluid which Diana Valencia of the Massachusetts Institute of Technology proposed could be in the form of salty supercritical water, though follow-up analysis of its transit failed to detect traces of either water or hydrogen.

GJ 1214 b was the second exoplanet (after CoRoT-7b) to have an established mass and radius less than those of the giant Solar System planets. It is three times the size of Earth and about 6.5 times as massive. Its low density indicated that it is likely a mix of rock and water, and follow-up observations using the Hubble telescope now seem to confirm that a large fraction of its mass is water, so it is a large waterworld. The high temperatures and pressures would form exotic materials like 'hot ice' or 'superfluid water'.

Models of radioactive decay

Models of heat retention and heating via radioactive decay in smaller icy Solar System bodies suggest that Rhea, Titania, Oberon, Triton, Pluto, Eris, Sedna, and Orcus may have oceans underneath solid icy crusts approximately 100 km thick. Of particular interest in these cases is the fact that the models indicate that the liquid layers are in direct contact with the rocky core, which allows efficient mixing of minerals and salts into the water. This is in contrast with the oceans that may be inside larger icy satellites like Ganymede, Callisto, or Titan, where layers of high-pressure phases of ice are thought to underlie the liquid water layer.

Models of radioactive decay suggest that MOA-2007-BLG-192Lb, a small planet orbiting a small star could be as warm as the Earth and completely covered by a very deep ocean.

Internal differentiation models

Diagram showing a possible internal structure of Ceres

 

Two models for the composition of Europa suggest a large subsurface ocean of liquid water. Similar models have been proposed for other celestial bodies in the Solar System

Models of Solar System objects indicate the presence of liquid water in their internal differentiation.

Some models of the dwarf planet Ceres, largest object in the asteroid belt indicate the possibility of a wet interior layer. Water vapor detected to be emitted by the dwarf planet may be an indicator, through sublimation of surface ice. 

A global layer of liquid water thick enough to decouple the crust from the mantle is thought to be present on Titan, Europa and, with less certainty, Callisto, Ganymede and Triton. Other icy moons may also have internal oceans, or have once had internal oceans that have now frozen.

Habitable zone

Artist's impression of a class II planet with water vapor clouds, as seen from a hypothetical large moon with surface liquid water

 

A planet's orbit in the circumstellar habitable zone is a popular method used to predict its potential for surface water at its surface. Habitable zone theory has put forward several extrasolar candidates for liquid water, though they are highly speculative as a planet's orbit around a star alone does not guarantee that a planet it has liquid water. In addition to its orbit, a planetary mass object must have the potential for sufficient atmospheric pressure to support liquid water and a sufficient supply of hydrogen and oxygen at or near its surface.

The Gliese 581 planetary system contains multiple planets that may be candidates for surface water, including Gliese 581c, Gliese 581d, which might be warm enough for oceans if a greenhouse effect was operating, and Gliese 581e.

Gliese 667 C has three of them are in the habitable zone including Gliese 667 Cc is estimated to have surface temperatures similar to Earth and a strong chance of liquid water.

Kepler-22b one of the first 54 candidates found by the Kepler telescope and reported is 2.4 times the size of the Earth, with an estimated temperature of 22 °C. It is described as having the potential for surface water, though its composition is currently unknown.

Among the 1,235 possible extrasolar planet candidates detected by NASA's planet-hunting Kepler space telescope during its first four months of operation, 54 are orbiting in the parent star's habitable 'Goldilocks' zone where liquid water could exist. Five of these are near Earth-size.

On 6 January 2015, NASA announced further observations conducted from May 2009 to April 2013 which included eight candidates between one and two times the size of Earth, orbiting in a habitable zone. Of these eight, six orbit stars that are similar to the Sun in size and temperature. Three of the newly confirmed exoplanets were found to orbit within habitable zones of stars similar to the Sun: two of the three, Kepler-438b and Kepler-442b, are near-Earth-size and likely rocky; the third, Kepler-440b, is a super-Earth.

Water rich circumstellar disks

Long before the discovery of water on asteroids on comets and dwarf planets beyond Neptune, the Solar System's circumstellar disks, beyond the snow line, including the asteroid belt and the Kuiper Belt were thought to contain large amounts of water and these were believed to be the Origin of water on Earth. Given that many types of stars are thought to blow volatiles from the system through the photoevaporation effect, water content in circumstellar disks and rocky material in other planetary systems are very good indicators of a planetary system's potential for liquid water and a potential for organic chemistry, especially if detected within the planet forming regions or the habitable zone. Techniques such as interferometry can be used for this. 

 

In 2007, such a disk was found in the habitable zone of MWC 480. In 2008, such a disk was found around the star AA Tauri. In 2009, a similar disk was discovered around the young star HD 142527.

In 2013, a water-rich debris disk around GD 61 accompanied by a confirmed rocky object consisting of magnesium, silicon, iron, and oxygen. The same year, another water rich disk was spotted around HD 100546 has ices close to the star.

There is, of course, no guarantee that the other conditions will be found that allow liquid water to be present on a planetary surface. Should planetary mass objects be present, a single, gas giant planet, with or without planetary mass moons, orbiting close to the circumstellar habitable zone, could prevent the necessary conditions from occurring in the system. However, it would mean that planetary mass objects, such as the icy bodies of the solar system, could have abundant quantities of liquid within them.

History

Lunar maria are vast basaltic plains on the Moon that were thought to be bodies of water by early astronomers, who referred to them as "seas". Galileo expressed some doubt about the lunar 'seas' in his Dialogue Concerning the Two Chief World Systems.

Before space probes were landed, the idea of oceans on Venus was credible science, but the planet was discovered to be much too hot. 

Telescopic observations from the time of Galileo onward have shown that Mars has no features resembling watery oceans. Mars' dryness was long recognized, and gave credibility to the spurious Martian canals. 

Ancient water on Venus

NASA's Goddard Institute for Space Studies and others have postulated that Venus may have had a shallow ocean in the past for up to 2 billion years, with as much water as Earth. Depending on the parameters used in their theoretical model, the last liquid water could have evaporated as recently as 715 million years ago. Currently, the only known water on Venus is in the form of a tiny amount of atmospheric vapor (20 ppm). Hydrogen, a component of water, is still being lost to space nowadays as detected by ESA's Venus Express spacecraft.

Evidence of past surface water

An artist's impression of ancient Mars and its hypothesized oceans based on geological data

 

Assuming that the Giant impact hypothesis is correct, there were never real seas or oceans on the Moon, only perhaps a little moisture (liquid or ice) in some places, when the Moon had a thin atmosphere created by degassing of volcanoes or impacts of icy bodies.

The Dawn space probe found possible evidence of past water flow on the asteroid Vesta, leading to speculation of underground reservoirs of water-ice.

Astronomers speculate that Venus had liquid water and perhaps oceans in its very early history. Given that Venus has been completely resurfaced by its own active geology, the idea of a primeval ocean is hard to test. Rock samples may one day give the answer.

It was once thought that Mars might have dried up from something more Earth-like. The initial discovery of a cratered surface made this seem unlikely, but further evidence has changed this view. Liquid water may have existed on the surface of Mars in the distant past, and several basins on Mars have been proposed as dry sea beds. The largest is Vastitas Borealis; others include Hellas Planitia and Argyre Planitia.

There is currently much debate over whether Mars once had an ocean of water in its northern hemisphere, and over what happened to it if it did. Recent findings by the Mars Exploration Rover mission indicate it had some long-term standing water in at least one location, but its extent is not known. The Opportunity Mars rover photographed bright veins of a mineral leading to conclusive confirmation of deposition by liquid water.

On 9 December 2013, NASA reported that the planet Mars had a large freshwater lake (which could have been a hospitable environment for microbial life) based on evidence from the Curiosity rover studying Aeolis Palus near Mount Sharp in Gale Crater.

Liquid water on comets and asteroids

Comets contain large proportions of water ice, but are generally thought to be completely frozen due to their small size and large distance from the Sun. However, studies on dust collected from comet Wild-2 show evidence for liquid water inside the comet at some point in the past. It is yet unclear what source of heat may have caused melting of some of the comet's water ice.

Nevertheless, on 10 December 2014, scientists reported that the composition of water vapor from comet Churyumov–Gerasimenko, as determined by the Rosetta spacecraft, is substantially different from that found on Earth. That is, the ratio of deuterium to hydrogen in the water from the comet was determined to be three times that found for terrestrial water. This makes it very unlikely that water found on Earth came from comets such as comet Churyumov–Gerasimenko according to the scientists.

The asteroid 24 Themis was the first found to have water, including liquid pressurised by non-atmospheric means, dissolved into mineral through ionising radiation. Water has also been found to flow on the large asteroid 4 Vesta heated through periodic impacts.

Extrasolar habitable zone candidates for water

Most known extrasolar planetary systems appear to have very different compositions to the Solar System, though there is probably sample bias arising from the detection methods.

The goal of current searches is to find Earth-sized planets in the habitable zone of their planetary systems (also sometimes called the Goldilocks zone). Planets with oceans could include Earth-sized moons of giant planets, though it remains speculative whether such 'moons' really exist. The Kepler telescope might be sensitive enough to detect them. There is speculation that rocky planets hosting water may be commonplace throughout the Milky Way.

Exoplanets containing water (artist concept; 17 August 2018)

Educational data mining

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Educational_data_mining

Educational data mining (EDM) describes a research field concerned with the application of data mining, machine learning and statistics to information generated from educational settings (e.g., universities and intelligent tutoring systems). At a high level, the field seeks to develop and improve methods for exploring this data, which often has multiple levels of meaningful hierarchy, in order to discover new insights about how people learn in the context of such settings. In doing so, EDM has contributed to theories of learning investigated by researchers in educational psychology and the learning sciences. The field is closely tied to that of learning analytics, and the two have been compared and contrasted.

 

Definition

Educational data mining refers to techniques, tools, and research designed for automatically extracting meaning from large repositories of data generated by or related to people's learning activities in educational settings. Quite often, this data is extensive, fine-grained, and precise. For example, several learning management systems (LMSs) track information such as when each student accessed each learning object, how many times they accessed it, and how many minutes the learning object was displayed on the user's computer screen. As another example, intelligent tutoring systems record data every time a learner submits a solution to a problem; they may collect the time of the submission, whether or not the solution matches the expected solution, the amount of time that has passed since the last submission, the order in which solution components were entered into the interface, etc. The precision of this data is such that even a fairly short session with a computer-based learning environment (e.g., 30 minutes) may produce a large amount of process data for analysis.

In other cases, the data is less fine-grained. For example, a student's university transcript may contain a temporally ordered list of courses taken by the student, the grade that the student earned in each course, and when the student selected or changed his or her academic major. EDM leverages both types of data to discover meaningful information about different types of learners and how they learn, the structure of domain knowledge, and the effect of instructional strategies embedded within various learning environments. These analyses provide new information that would be difficult to discern by looking at the raw data. For example, analyzing data from an LMS may reveal a relationship between the learning objects that a student accessed during the course and their final course grade. Similarly, analyzing student transcript data may reveal a relationship between a student's grade in a particular course and their decision to change their academic major. Such information provides insight into the design of learning environments, which allows students, teachers, school administrators, and educational policy makers to make informed decisions about how to interact with, provide, and manage educational resources. 

History

While the analysis of educational data is not itself a new practice, recent advances in educational technology, including the increase in computing power and the ability to log fine-grained data about students' use of a computer-based learning environment, have led to an increased interest in developing techniques for analyzing the large amounts of data generated in educational settings. This interest translated into a series of EDM workshops held from 2000 to 2007 as part of several international research conferences. In 2008, a group of researchers established what has become an annual international research conference on EDM, the first of which took place in Montreal, Quebec, Canada.

As interest in EDM continued to increase, EDM researchers established an academic journal in 2009, the Journal of Educational Data Mining, for sharing and disseminating research results. In 2011, EDM researchers established the International Educational Data Mining Society to connect EDM researchers and continue to grow the field.

With the introduction of public educational data repositories in 2008, such as the Pittsburgh Science of Learning Centre's (PSLC) DataShop and the National Center for Education Statistics (NCES), public data sets have made educational data mining more accessible and feasible, contributing to its growth.

Goals

Ryan S. Baker and Kalina Yacef  identified the following four goals of EDM:

1. Predicting students' future learning behavior – With the use of student modeling, this goal can be achieved by creating student models that incorporate the learner's characteristics, including detailed information such as their knowledge, behaviours and motivation to learn. The user experience of the learner and their overall satisfaction with learning are also measured.
2. Discovering or improving domain models – Through the various methods and applications of EDM, discovery of new and improvements to existing models is possible. Examples include illustrating the educational content to engage learners and determining optimal instructional sequences to support the student's learning style.
3. Studying the effects of educational support that can be achieved through learning systems.
4. Advancing scientific knowledge about learning and learners by building and incorporating student models, the field of EDM research and the technology and software used.

Users and stakeholders

There are four main users and stakeholders involved with educational data mining. These include:

1. Learners – Learners are interested in understanding student needs and methods to improve the learner's experience and performance. For example, learners can also benefit from the discovered knowledge by using the EDM tools to suggest activities and resources that they can use based on their interactions with the online learning tool and insights from past or similar learners. For younger learners, educational data mining can also inform parents about their child's learning progress. It is also necessary to effectively group learners in an online environment. The challenge is to learn these groups based on the complex data as well as develop actionable models to interpret these groups.
2. Educators – Educators attempt to understand the learning process and the methods they can use to improve their teaching methods. Educators can use the applications of EDM to determine how to organize and structure the curriculum, the best methods to deliver course information and the tools to use to engage their learners for optimal learning outcomes. In particular, the distillation of data for human judgment technique provides an opportunity for educators to benefit from EDM because it enables educators to quickly identify behavioural patterns, which can support their teaching methods during the duration of the course or to improve future courses. Educators can determine indicators that show student satisfaction and engagement of course material, and also monitor learning progress.
3. Researchers – Researchers focus on the development and the evaluation of data mining techniques for effectiveness. A yearly international conference for researchers began in 2008, followed by the establishment of the Journal of Educational Data Mining in 2009. The wide range of topics in EDM ranges from using data mining to improve institutional effectiveness to student performance.
4. Administrators – Administrators are responsible for allocating the resources for implementation in institutions. As institutions are increasingly held responsible for student success, the administering of EDM applications are becoming more common in educational settings. Faculty and advisors are becoming more proactive in identifying and addressing at-risk students. However, it is sometimes a challenge to get the information to the decision makers to administer the application in a timely and efficient manner.

Phases

As research in the field of educational data mining has continued to grow, a myriad of data mining techniques have been applied to a variety of educational contexts. In each case, the goal is to translate raw data into meaningful information about the learning process in order to make better decisions about the design and trajectory of a learning environment. Thus, EDM generally consists of four phases:

1. The first phase of the EDM process (not counting pre-processing) is discovering relationships in data. This involves searching through a repository of data from an educational environment with the goal of finding consistent relationships between variables. Several algorithms for identifying such relationships have been utilized, including classification, regression, clustering, factor analysis, social network analysis, association rule mining, and sequential pattern mining.
2. Discovered relationships must then be validated in order to avoid overfitting.
3. Validated relationships are applied to make predictions about future events in the learning environment.
4. Predictions are used to support decision-making processes and policy decisions.

During phases 3 and 4, data is often visualized or in some other way distilled for human judgment. A large amount of research has been conducted in best practices for visualizing data.

Main approaches

Of the general categories of methods mentioned, prediction, clustering and relationship mining are considered universal methods across all types of data mining; however, Discovery with Models and Distillation of Data for Human Judgment are considered more prominent approaches within educational data mining.

Discovery with models

In the Discovery with Model method, a model is developed via prediction, clustering or by human reasoning knowledge engineering and then used as a component in another analysis, namely in prediction and relationship mining. In the prediction method use, the created model's predictions are used to predict a new variable. For the use of relationship mining, the created model enables the analysis between new predictions and additional variables in the study. In many cases, discovery with models uses validated prediction models that have proven generalizability across contexts.

Key applications of this method include discovering relationships between student behaviors, characteristics and contextual variables in the learning environment. Further discovery of broad and specific research questions across a wide range of contexts can also be explored using this method.

Distillation of data for human judgment

Humans can make inferences about data that may be beyond the scope in which an automated data mining method provides. For the use of education data mining, data is distilled for human judgment for two key purposes, identification and classification.

For the purpose of identification, data is distilled to enable humans to identify well-known patterns, which may otherwise be difficult to interpret. For example, the learning curve, classic to educational studies, is a pattern that clearly reflects the relationship between learning and experience over time.

Data is also distilled for the purposes of classifying features of data, which for educational data mining, is used to support the development of the prediction model. Classification helps expedite the development of the prediction model, tremendously.

The goal of this method is to summarize and present the information in a useful, interactive and visually appealing way in order to understand the large amounts of education data and to support decision making. In particular, this method is beneficial to educators in understanding usage information and effectiveness in course activities. Key applications for the distillation of data for human judgment include identifying patterns in student learning, behavior, opportunities for collaboration and labeling data for future uses in prediction models.

Applications

A list of the primary applications of EDM is provided by Cristobal Romero and Sebastian Ventura. In their taxonomy, the areas of EDM application are:

• Analysis and visualization of data
• Providing feedback for supporting instructors
• Recommendations for students
• Predicting student performance
• Student modeling
• Detecting undesirable student behaviors
• Grouping students
• Social network analysis
• Developing concept maps
• Constructing courseware – EDM can be applied to course management systems such as open source Moodle. Moodle contains usage data that includes various activities by users such as test results, amount of readings completed and participation in discussion forums. Data mining tools can be used to customize learning activities for each user and adapt the pace in which the student completes the course. This is in particularly beneficial for online courses with varying levels of competency.
• Planning and scheduling

New research on mobile learning environments also suggests that data mining can be useful. Data mining can be used to help provide personalized content to mobile users, despite the differences in managing content between mobile devices and standard PCs and web browsers.

New EDM applications will focus on allowing non-technical users use and engage in data mining tools and activities, making data collection and processing more accessible for all users of EDM. Examples include statistical and visualization tools that analyzes social networks and their influence on learning outcomes and productivity.

Courses

• In October 2013, Coursera offered a free online course on "Big Data in Education" that taught how and when to use key methods for EDM. This course moved to edX in the summer of 2015, and has continued to run on edX annually since then. A course archive is now available online.
• Teachers College, Columbia University offers a MS in Learning Analytics.

Publication venues

Considerable amounts of EDM work are published at the peer-reviewed International Conference on Educational Data Mining, organized by the International Educational Data Mining Society.

• 1st International Conference on Educational Data Mining (2008) – Montreal, Canada
• 2nd International Conference on Educational Data Mining (2009) – Cordoba, Spain
• 3rd International Conference on Educational Data Mining (2010) – Pittsburgh, PA, USA
• 4th International Conference on Educational Data Mining (2011) – Eindhoven, Netherlands
• 5th International Conference on Educational Data Mining (2012) – Chania, Greece
• 6th International Conference on Educational Data Mining (2013) – Memphis, TN, USA
• 7th International Conference on Educational Data Mining (2014) – London, UK
• 8th International Conference on Educational Data Mining (2015) – Madrid, Spain
• 9th International Conference on Educational Data Mining (2016) – Raleigh, NC, USA
• 10th International Conference on Educational Data Mining (2017) – Wuhan, China
• 11th International Conference on Educational Data Mining (2018) – Buffalo, NY, USA

EDM papers are also published in the Journal of Educational Data Mining (JEDM). 

Many EDM papers are routinely published in related conferences, such as Artificial Intelligence and Education, Intelligent Tutoring Systems, and User Modeling, Adaptation, and Personalization.

In 2011, Chapman & Hall/CRC Press, Taylor and Francis Group published the first Handbook of Educational Data Mining. This resource was created for those that are interested in participating in the educational data mining community.

Contests

In 2010, the Association for Computing Machinery's KDD Cup was conducted using data from an educational setting. The data set was provided by the Pittsburgh Science of Learning Center's DataShop, and it consisted of over 1,000,000 data points from students using a cognitive tutor. Six hundred teams competed for over 8,000 USD in prize money (which was donated by Facebook). The goal for contestants was to design an algorithm that, after learning from the provided data, would make the most accurate predictions from new data. The winners submitted an algorithm that utilized feature generation (a form of representation learning), random forests, and Bayesian networks.

Costs and challenges

Along with technological advancements are costs and challenges associated with implementing EDM applications. These include the costs to store logged data and the cost associated with hiring staff dedicated to managing data systems. Moreover, data systems may not always integrate seamlessly with one another and even with the support of statistical and visualization tools, creating one simplified version of the data can be difficult. Furthermore, choosing which data to mine and analyze can also be challenging, making the initial stages very time consuming and labor-intensive. From beginning to end, the EDM strategy and implementation requires one to uphold privacy and ethics for all stakeholders involved. 

Criticisms

• Generalizability – Research in EDM may be specific to the particular educational setting and time in which the research was conducted, and as such, may not be generalizable to other institutions. Research also indicates that the field of educational data mining is concentrated in North America and western cultures and subsequently, other countries and cultures may not be represented in the research and findings. Development of future models should consider applications across multiple contexts.
• Privacy – Individual privacy is a continued concern for the application of data mining tools. With free, accessible and user-friendly tools in the market, students and their families may be at risk from the information that learners provide to the learning system, in hopes to receive feedback that will benefit their future performance. As users become savvy in their understanding of online privacy, administrators of educational data mining tools need to be proactive in protecting the privacy of their users and be transparent about how and with whom the information will be used and shared. Development of EDM tools should consider protecting individual privacy while still advancing the research in this field.
• Plagiarism – Plagiarism detection is an ongoing challenge for educators and faculty whether in the classroom or online. However, due to the complexities associated with detecting and preventing digital plagiarism in particular, educational data mining tools are not currently sophisticated enough to accurately address this issue. Thus, the development of predictive capability in plagiarism-related issues should be an area of focus in future research.
• Adoption – It is unknown how widespread the adoption of EDM is and the extent to which institutions have applied and considered implementing an EDM strategy. As such, it is unclear whether there are any barriers that prevent users from adopting EDM in their educational settings.