Are there rivers of liquid hydrogen outside of our solar system?
Extraterrestrial liquid water
The Gliese 581 planetary system contains multiple planets that may be candidates for surface water, including Gliese 581c, Gliese 581d, might be warm enough for oceans if a greenhouse effect was operating, and Gliese 581e.
A Beautiful Planet
After spending almost the entire film scrutinizing the Earth and the ISS (with its astronauts) in low Earth orbit, one of the final scenes of "A Beautiful Planet" briefly examines an important extrasolar planet (a planet outside of our solar system) which was discovered in 2014. The planet, Kepler-186f, was the first approximately-Earth-sized planet found to be orbiting within its star's habitable zone, or orbital area where liquid water could conceivably exist without freezing or vaporizing. In other words, it was the first discovery of an Earth-sized planet on which life could reside.
Solar Lake
Solar Lake ( "Birkat aš-Šams") is a saline desert lake located on the edge of the Red Sea, about 18 km south of Eilat in the Sinai Peninsula, Taba, Egypt, close to its borders with Israel. A small lake of high salinity, it is the site of complex biochemical phenomena, linked to cycles of evaporation and of infiltration of waters.
List of extrasolar candidates for liquid water
Further work published in June 2013 suggests that the system has six planets, and that three of them are in the habitable zone.
Hydrogen cyanide
In February 2016, it was announced that traces of hydrogen cyanide were found in the atmosphere of the hot Super-Earth 55 Cancri e with NASA's Hubble Space Telescope.
List of ecoregions in North America (CEC)
Many watersheds are dominated by large rivers that dump huge amounts of freshwater into the ocean such as the Lena river in Central Siberia. This exportation of freshwater helps control the thermohaline circulation and the global climate. Flow rates of taiga rivers are variable and "flashy" due to the presence of a permafrost that keeps water from percolating deep into the soil. Due to global warming, flow rates have increased as more of the permafrost melts every year. In addition to "flashy" flow levels, the permafrost in the taiga allows dissolved inorganic nitrogen and organic carbon levels in the water to be higher while calcium, magnesium, sulfate, and hydrogen bicarbonate levels are shown to be much lower. As a dominant characteristic in the soil, the permafrost also influences the degree to which water percolates into the soil. Where there is a year-long permafrost, the water table is located much deeper in the soil and is less available to organisms, while a discontinuous permafrost provides much shallower access.
Period 1 element
Hydrogen is the most abundant of the chemical elements, constituting roughly 75% of the universe's elemental mass. Stars in the main sequence are mainly composed of hydrogen in its plasma state. Elemental hydrogen is relatively rare on Earth, and is industrially produced from hydrocarbons such as methane, after which most elemental hydrogen is used "captively" (meaning locally at the production site), with the largest markets almost equally divided between fossil fuel upgrading, such as hydrocracking, and ammonia production, mostly for the fertilizer market. Hydrogen may be produced from water using the process of electrolysis, but this process is significantly more expensive commercially than hydrogen production from natural gas.
Cassini–Huygens
On April 3, 2014, nearly ten years after "Cassini" entered Saturn's orbit, NASA reported evidence of a large salty internal ocean of liquid water in Enceladus. The presence of an internal salty ocean in contact with the moon's rocky core, places Enceladus "among the most likely places in the Solar System to host alien microbial life". On June 30, 2014, NASA celebrated ten years of "Cassini" exploring Saturn and its moons, highlighting the discovery of water activity on Enceladus among other findings.
Interplanetary spaceflight
Oxygen is a common constituent of the moon's crust, and is probably abundant in most other bodies in the Solar System. Non-terrestrial oxygen would be valuable as a source of water ice only if an adequate source of hydrogen can be found. Possible uses include:Unfortunately hydrogen, along with other volatiles like carbon and nitrogen, are much less abundant than oxygen in the inner Solar System.
List of potentially habitable exoplanets
In astronomy and astrobiology, the circumstellar habitable zone (CHZ or sometimes "ecosphere", "liquid-water belt", "HZ", "life zone" or "Goldilocks zone") is the region around a star where a planet with sufficient atmospheric pressure can maintain liquid water on its surface.
Galán
The caldera contains a lake in its southwestern corner, which is known as Laguna Diamante. Laguna Diamante has gained attention among scientists for the extreme environmental conditions that life within the lake has to withstand, including high arsenic contents of the waters and high insolation with ultraviolet radiation. The water is hyper-alkaline and five times as salty as the sea but supports microorganisms which form microbial mats and provide food for a colony of flamingos. A smaller lake known as Laguna Pabellon lies just south of Laguna Diamante. North of the resurgent dome, the Rio Aguas Calientes drains the caldera northward, while east of it the Rio Leon Muerto runs eastward out of the caldera. Rivers in the caldera and neighbourhood display river terraces which may reflect pre-caldera formation uplift of the terrain and uplift associated with the resurgent dome. These drainages eventually converge in the Rio de Los Patos and end into the Salar del Hombre Muerto north of Galán. The western flanks of the caldera drain into the Antofagasta de la Sierra valley through a number of drainages such as Rio Punilla, Rio Toconquis, Rio Miriguaca, Rio Las Pitas; the waters eventually end into the Laguna Antofagasta south of Antofagasta de la Sierra. Two hot springs are found within the caldera, the first close to its northern end and the second on the southwestern foot of the resurgent dome, both emitting water with temperatures of about . The first one is known as the Aguas Calientes hydrothermal spring and features deposits of tufa and boiling water. Another geothermal system is known as La Colcha and includes fumaroles as well as boiling water and sinter deposits; it has been prospected for the possibility of geothermal power generation.
Extraterrestrial liquid 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.
List of extrasolar candidates for liquid water
The unconfirmed planet Gliese 581 g is another good candidate. This planet is estimated to be between three and four times as massive as the earth, and as such it is too small to be a gas giant. The orbital period is estimated at 37 days, which places its orbit right in the middle of the habitable zone of the star Gliese 581.
Lick–Carnegie Exoplanet Survey
In September 2010, the team announced the discovery of Gliese 581g in orbit within the Gliese 581 planetary system. The observations place the planet in an area where liquid water could exist on the planet's surface, that is, a habitable zone. If confirmed, this would be the first strong case for a potentially habitable Earth-like exoplanet yet discovered. One contributor to the team was Peter Jalowiczor, an amateur astronomer who analyzed some of the data the team released to the public. His work contributed to the discovery of four exoplanets.
Extraterrestrial liquid water
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.
Metallic hydrogen
At high pressure and temperatures, metallic hydrogen can exist as a liquid rather than a solid, and researchers think it might be present in large quantities in the hot and gravitationally compressed interiors of Jupiter, Saturn, and in some exoplanets.
Hydrogen
Throughout the universe, hydrogen is mostly found in the atomic and plasma states, with properties quite different from those of molecular hydrogen. As a plasma, hydrogen's electron and proton are not bound together, resulting in very high electrical conductivity and high emissivity (producing the light from the Sun and other stars). The charged particles are highly influenced by magnetic and electric fields. For example, in the solar wind they interact with the Earth's magnetosphere giving rise to Birkeland currents and the aurora. Hydrogen is found in the neutral atomic state in the interstellar medium. The large amount of neutral hydrogen found in the damped Lyman-alpha systems is thought to dominate the cosmological baryonic density of the Universe up to redshift "z"=4.
Red dwarf
Observations with HARPS further indicate 40% of red dwarfs have a "super-Earth" class planet orbiting in the habitable zone where liquid water can exist on the surface. Computer simulations of the formation of planets around low mass stars predict that Earth-sized planets are most abundant, but more than 90% of the simulated planets are at least 10% water by mass, suggesting that many Earth-sized planets orbiting red dwarf stars are covered in deep oceans.
At least four and possibly up to six exoplanets were discovered orbiting within the Gliese 581 planetary system between 2005 and 2010. One planet has about the mass of Neptune, or 16 Earth masses (). It orbits just 6 million kilometers (0.04 AU) from its star, and is estimated to have a surface temperature of 150 °C, despite the dimness of its star. In 2006, an even smaller exoplanet (only ) was found orbiting the red dwarf OGLE-2005-BLG-390L; it lies 390 million km (2.6 AU) from the star and its surface temperature is −220 °C (53 K).
Extraterrestrial 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.
Extraterrestrial liquid water
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.
Lakes of Titan
According to Cassini data, scientists announced on February 13, 2008, that Titan hosts within its polar lakes "hundreds of times more natural gas and other liquid hydrocarbons than all the known oil and natural gas reserves on Earth." The desert sand dunes along the equator, while devoid of open liquid, nonetheless hold more organics than all of Earth's coal reserves. It has been estimated that the visible lakes and seas of Titan contain about 300 times the volume of Earth's proven oil reserves. In June 2008, "Cassini" Visible and Infrared Mapping Spectrometer confirmed the presence of liquid ethane beyond doubt in a lake in Titan's southern hemisphere. The exact blend of hydrocarbons in the lakes is unknown. According to a computer model, 3/4 of an average polar lake is ethane, with 10 percent methane, 7 percent propane and smaller amounts of hydrogen cyanide, butane, nitrogen and argon. Benzene is expected to fall like snow and quickly dissolve into the lakes, although the lakes may become saturated just as the Dead Sea on Earth is packed with salt. The excess benzene would then build up in a mud-like sludge on the shores and on the lake floors before eventually being eroded by ethane rain, forming a complex cave-riddled landscape.Salt-like compounds composed of ammonia and acetylene are also predicted to form. However, the chemical composition and physical properties of the lakes probably varies from one lake to another (Cassini observations in 2013 indicate Ligeia Mare is filled with a ternary mixture of methane, ethane, and nitrogen and consequently the probe's radar signals were able to detect the sea floor 170 m (557'9") below the liquid surface).
Extraterrestrial liquid water
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.
Interstellar communication
The SETI project has for the past several decades been conducting a search for signals being transmitted by extraterrestrial life located outside the Solar System, primarily in the radio frequencies of the electromagnetic spectrum. Special attention has been given to the Water Hole, the frequency of one of neutral hydrogen's absorption lines, due to the low background noise at this frequency and its symbolic association with the basis for what is likely to be the most common system of biochemistry (see Alternative biochemistry).
Interstellar ice
Research published in the journal "Science" estimates that about 30–50% of the water in the solar system, like the water on Earth, the discs around Saturn, and the meteorites of other planets was already around even before the birth of the sun.
Hypothetical types of biochemistry
In addition to carbon compounds, all currently known terrestrial life also requires water as a solvent. This has led to discussions about whether water is the only liquid capable of filling that role. The idea that an extraterrestrial life-form might be based on a solvent other than water has been taken seriously in recent scientific literature by the biochemist Steven Benner, and by the astrobiological committee chaired by John A. Baross. Solvents discussed by the Baross committee include ammonia, sulfuric acid, formamide, hydrocarbons,
and (at temperatures much lower than Earth's) liquid nitrogen, or hydrogen in the form of a supercritical fluid.
Extraterrestrial liquid water
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.
Mallik gas hydrate site
The Mallik Methane Hydrate Site (or "Mallik Gas Hydrate Production Research Well", "Mallik test well") is located in the Beaufort Sea, Canada.The Mackenzie Delta region on the coast of the Beaufort Sea is not only known for its remarkable Arctic scenery and iconic flora and fauna, but also for its large natural gas and oil deposits. Due to this combination of factors, natural gas occurrences in the Canadian Arctic Ocean region are increasingly the focus of energy resource and climate studies, which aim to develop knowledge of the formation, occurrence, and amount of natural gas hydrates in the area, its impacts on the environment, and what kind of natural and human induced changes may lead to the dissociation of these hydrates. The Delta lies at the mouth of the Mackenzie River where it enters the Beaufort Sea, approximately 2,200 kilometers northwest of Calgary, Alberta. The most important natural gas hydrate research site in the area, the Mallik gas hydrate field, was discovered in 1971 through an exploration well by Imperial Oil Ltd., and is located 50 kilometers north of the Arctic Circle. As natural gas hydrates are assumed to underlie large portions of the world’s Arctic continental areas and marine continental shelf and are a tremendous potential future hydrocarbon resource, the Mallik drill site is a location of major interest. Consequently, it has been an area of ongoing scientific and technical research on gas hydrates for more than thirty years.
Lakes of Titan
The possibility remained that liquid ethane and methane might be found on Titan's polar regions, where they were expected to be abundant and stable. In Titan's south polar region, an enigmatic dark feature named Ontario Lacus was the first suspected lake identified, possibly created by clouds that are observed to cluster in the area. A possible shoreline was also identified near the pole via radar imagery. Following a flyby on July 22, 2006, in which the "Cassini" spacecraft's radar imaged the northern latitudes (which were at the time in winter), a number of large, smooth (and thus dark to radar) patches were seen dotting the surface near the pole. Based on the observations, scientists announced "definitive evidence of lakes filled with methane on Saturn's moon Titan" in January 2007. The "Cassini–Huygens" team concluded that the imaged features are almost certainly the long-sought hydrocarbon lakes, the first stable bodies of surface liquid found off Earth. Some appear to have channels associated with liquid and lie in topographical depressions. Channels in some regions have created surprisingly little erosion, suggesting erosion on Titan is extremely slow, or some other recent phenomena may have wiped out older riverbeds and landforms. Overall, the Cassini radar observations have shown that lakes cover only a few percent of the surface and are concentrated near the poles, making Titan much drier than Earth. The high relative humidity of methane in Titan's lower atmosphere could be maintained by evaporation from lakes covering only 0.002–0.02% of the whole surface.
Circumstellar habitable zone
Subsequently, some astrobiologists propose that the concept be extended to other solvents, including dihydrogen, sulfuric acid, dinitrogen, formamide, and methane, among others, which would support hypothetical life forms that use an alternative biochemistry. In 2013, further developments in habitable zone theory were made with the proposal of a circum"planetary" habitable zone, also known as the "habitable edge", to encompass the region around a planet where the orbits of natural satellites would not be disrupted, and at the same time tidal heating from the planet would not cause liquid water to boil away.
Circumstellar habitable zone
An estimate of the range of distances from the Sun allowing the existence of liquid water appears in Newton's "Principia" (Book III, Section 1, corol. 4). The concept of a circumstellar habitable zone was first introduced in 1953 by Hubertus Strughold, who in his treatise "The Green and the Red Planet: A Physiological Study of the Possibility of Life on Mars", coined the term "ecosphere" and referred to various "zones" in which life could emerge. In the same year, Harlow Shapley wrote "Liquid Water Belt", which described the same theory in further scientific detail. Both works stressed the importance of liquid water to life. Su-Shu Huang, an American astrophysicist, first introduced the term "habitable zone" in 1959 to refer to the area around a star where liquid water could exist on a sufficiently large body, and was the first to introduce it in the context of planetary habitability and extraterrestrial life. A major early contributor to habitable zone theory, Huang argued in 1960 that circumstellar habitable zones, and by extension extraterrestrial life, would be uncommon in multiple star systems, given the gravitational instabilities of those systems.
List of extrasolar candidates for liquid water
Gliese 667 Cc was originally described as one of two 'super-Earth' planets around Gliese 667 C, a dim red star that is part of a triple star system. The stars of this system have a concentration of heavy elements only 25% that of our Sun's. Such elements are the building blocks of terrestrial planets so it was thought to be unusual for such star systems to have an abundance of low mass planets. It seems that habitable planets can form in a greater variety of environments than previously believed.
Extraterrestrial liquid water
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.
Water
The existence of liquid water, and to a lesser extent its gaseous and solid forms, on Earth are vital to the existence of life on Earth as we know it. The Earth is located in the habitable zone of the solar system; if it were slightly closer to or farther from the Sun (about 5%, or about 8 million kilometers), the conditions which allow the three forms to be present simultaneously would be far less likely to exist.
Kepler-1229b
The exoplanet, along with eight others, was announced to be orbiting in the habitable zone of its parent star, the region where, with the correct conditions and atmospheric properties, liquid water may exist on the surface of the planet. Kepler-1229b has a radius of 1.4 , so it is likely rocky. Its host star is a red dwarf, with about half as much mass than the Sun does. As a result, stars like Kepler-1229 have the ability to live up to 50–60 billion years, 5–6 times longer than the Sun will live.
Fresh water
Out of all the water on Earth, saline water in oceans, seas and saline groundwater make up about 97% of it. Only 2.5–2.75% is fresh water, including 1.75–2% frozen in glaciers, ice and snow, 0.5–0.75% as fresh groundwater and soil moisture, and less than 0.01% of it as surface water in lakes, swamps and rivers. Freshwater lakes contain about 87% of this fresh surface water, including 29% in the African Great Lakes, 22% in Lake Baikal in Russia, 21% in the North American Great Lakes, and 14% in other lakes. Swamps have most of the balance with only a small amount in rivers, most notably the Amazon River. The atmosphere contains 0.04% water. In areas with no fresh water on the ground surface, fresh water derived from precipitation may, because of its lower density, overlie saline ground water in lenses or layers. Most of the world's fresh water is frozen in ice sheets. Many areas suffer from lack of distribution of fresh water, such as deserts.
Habitability of natural satellites
An atmosphere is considered by astrobiologists to be important in developing prebiotic chemistry, sustaining life and for surface water to exist. Most natural satellites in the Solar System lack significant atmospheres, the sole exception being Saturn's moon Titan.
Extraterrestrial liquid water
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.
Extraterrestrial liquid water
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).
Liquid hydrogen
Liquid hydrogen is a common liquid rocket fuel for rocketry applications — both NASA and the United States Air Force operate a large number of liquid hydrogen tanks with an individual capacity up to 3.8 million liters (1 million U.S. gallons). In most rocket engines fueled by liquid hydrogen, it first cools the nozzle and other parts before being mixed with the oxidizer — usually liquid oxygen (LOX) — and burned to produce water with traces of ozone and hydrogen peroxide. Practical H–O rocket engines run fuel-rich so that the exhaust contains some unburned hydrogen. This reduces combustion chamber and nozzle erosion. It also reduces the molecular weight of the exhaust, which can actually increase specific impulse, despite the incomplete combustion.
Liquid hydrogen can be used as the fuel for an internal combustion engine or fuel cell. Various submarines (Type 212 submarine, Type 214 submarine) and concept hydrogen vehicles have been built using this form of hydrogen (see DeepC, BMW H2R). Due to its similarity, builders can sometimes modify and share equipment with systems designed for liquefied natural gas (LNG). However, because of the lower volumetric energy, the hydrogen volumes needed for combustion are large. Unless direct injection is used, a severe gas-displacement effect also hampers maximum breathing and increases pumping losses.
K2-72e
The exoplanet, along with K2-72c, were announced to be orbiting in the habitable zone of its parent star, the region where, with the correct conditions and atmospheric properties, liquid water may exist on the surface of the planet. K2-72e has a radius of 0.82 , so it is likely rocky. Its host star is a red dwarf, with about a fifth as much mass than the Sun does. As a result, stars like K2-72 have the ability to live up to 500–600 billion years, 40–50 times longer than the Sun will live.
Proxima Centauri b
The exoplanet is orbiting within the habitable zone of Proxima Centauri, the region where, with the correct planetary conditions and atmospheric properties, liquid water may exist on the surface of the planet. The host star, with about an eighth of the mass of the Sun, has a habitable zone between ∼0.0423–0.0816 AU. In October 2016, researchers at France's CNRS research institute stated that there is a considerable chance of the planet harboring surface oceans and having a thin atmosphere. However, unless the planet transits in front of its star from the perspective of Earth, it is difficult to test these hypotheses.
Geology of solar terrestrial planets
The physical features of land are remarkably varied. The largest mountain ranges—the Himalayas in Asia and the Andes in South America—extend for thousands of kilometres. The longest rivers are the river Nile in Africa () and the Amazon river in South America (). Deserts cover about 20% of the total land area. The largest is the Sahara, which covers nearly one-third of Africa.
Solar energy
Hydrogen production technologies have been a significant area of solar chemical research since the 1970s. Aside from electrolysis driven by photovoltaic or photochemical cells, several thermochemical processes have also been explored. One such route uses concentrators to split water into oxygen and hydrogen at high temperatures (). Another approach uses the heat from solar concentrators to drive the steam reformation of natural gas thereby increasing the overall hydrogen yield compared to conventional reforming methods. Thermochemical cycles characterized by the decomposition and regeneration of reactants present another avenue for hydrogen production. The Solzinc process under development at the Weizmann Institute of Science uses a 1 MW solar furnace to decompose zinc oxide (ZnO) at temperatures above . This initial reaction produces pure zinc, which can subsequently be reacted with water to produce hydrogen.
Water vapor
Water vapor is a relatively common atmospheric constituent, present even in the solar atmosphere as well as every planet in the Solar System and many astronomical objects including natural satellites, comets and even large asteroids. Likewise the detection of extrasolar water vapor would indicate a similar distribution in other planetary systems. Water vapor is significant in that it can be indirect evidence supporting the presence of extraterrestrial liquid water in the case of some planetary mass objects.
Hydrogen sulfide
Hydrogen sulfide has often been detected in the interstellar medium. It also occurs in the clouds of planets in our solar system.
Lake Whillans
These projects may yield insights into the search for life elsewhere in the Solar System. In particular, the moons Europa (Jupiter) and Enceladus (Saturn) have large amounts of liquid water beneath icy crusts.
Kalahari Desert
In the Kalahari, there are two main mechanisms of atmospheric circulation, dominated by the Kalahari High anticyclone:There are huge subterranean water reserves beneath parts of the Kalahari; the Dragon's Breath Cave, for example, is the largest documented non-subglacial underground lake on the planet. Such reserves may be in part the residues of ancient lakes; the Kalahari Desert was once a much wetter place. The ancient Lake Makgadikgadi dominated the area, covering the Makgadikgadi Pan and surrounding areas, but it drained or dried out some 10,000 years ago. It may have once covered as much as .
Inland waterways of the United States
Most navigable rivers and canals in the United States are in the eastern half of the country, where the terrain is flatter and the climate is wetter. The Mississippi River System, including the Gulf Intracoastal Waterway (GIWW) connects Gulf Coast ports, such as Mobile, New Orleans, Baton Rouge, Houston, and Corpus Christi, with major inland ports, including Memphis, Kansas City, St. Louis, Chicago, St. Paul, Cincinnati, and Pittsburgh. The Lower Mississippi River from Baton Rouge to the Gulf of Mexico allows ocean shipping to connect with the barge traffic, thereby making this segment vital to both the domestic and foreign trade of the United States. The Mississippi River System is connected to the Illinois Waterway, which continues to the Great Lakes Waterway and then to the Saint Lawrence Seaway. Many other eastern rivers are navigable as well, including the Potomac, the Hudson, and the Atchafalaya Rivers, which are all dredged by the Army Corps of Engineers.
Hydrogen economy
Molecular hydrogen is not available on Earth in convenient natural reservoirs. Most hydrogen in the lithosphere is bonded to oxygen in water. Manufacturing elemental hydrogen does require the consumption of a hydrogen carrier such as a fossil fuel or water. The former carrier consumes the fossil resource and produces carbon dioxide, but often requires no further energy input beyond the fossil fuel. Decomposing water, the latter carrier, requires electrical or heat input, generated from some primary energy source (fossil fuel, nuclear power or a renewable energy). Hydrogen can also be produced by refining the effluent from geothermal sources in the lithosphere. Hydrogen produced by zero emission renewable energy sources such as electrolysis of water using wind power, solar power, hydro power, wave power or tidal power is referred to as green hydrogen. Hydrogen produced by non-renewable energy sources may be referred to as brown hydrogen. Hydrogen produced as a waste by-product or industrial by-product is sometimes referred to as grey hydrogen.
Lakes of Titan
The discoveries in the polar regions contrast with the findings of the "Huygens" probe, which landed near Titan's equator on January 14, 2005. The images taken by the probe during its descent showed no open areas of liquid, but strongly indicated the presence of liquids in the recent past, showing pale hills crisscrossed with dark drainage channels that lead into a wide, flat, darker region. It was initially thought that the dark region might be a lake of a fluid or at least tar-like substance, but it is now clear that "Huygens" landed on the dark region, and that it is solid without any indication of liquids. A penetrometer studied the composition of the surface as the craft impacted it, and it was initially reported that the surface was similar to wet clay, or perhaps crème brûlée (that is, a hard crust covering a sticky material). Subsequent analysis of the data suggests that this reading was likely caused by "Huygens" displacing a large pebble as it landed, and that the surface is better described as a "sand" made of ice grains. The images taken after the probe's landing show a flat plain covered in pebbles. The pebbles may be made of water ice and are somewhat rounded, which may indicate the action of fluids. Thermometers indicated that heat was wicked away from Huygens so quickly that the ground must have been damp, and one image shows light reflected by a dewdrop as it falls across the camera's field of view. On Titan, the feeble sunlight allows only about one centimeter of evaporation per year (versus one meter of water on Earth), but the atmosphere can hold the equivalent of about 10 meters (28') of liquid before rain forms (versus about 2 cm [25/32"] on Earth). So Titan's weather is expected to feature downpours of several meters (15-20') causing flash floods, interspersed by decades or centuries of drought (whereas typical weather on Earth includes a little rain most weeks). Cassini has observed equatorial rainstorms only once since 2004. Despite this, a number of long-standing tropical hydrocarbon lakes were unexpectedly discovered in 2012 (including one near the Huygens landing site in the Shangri-La region which is about half the size of Utah's Great Salt Lake, with a depth of at least 1 meter [3'4"]). As on Earth, the likely supplier is probably underground aquifers, in other words the arid equatorial regions of Titan contain "oases".
Hydrogen vehicle
, there are 3 hydrogen cars publicly available in select markets: the Toyota Mirai, the Hyundai Nexo, and the Honda Clarity.
Extraterrestrial liquid water
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.
Lava planet
There are no known lava worlds in the Solar System and the existence of extrasolar lava planets remains theoretical. Several known exoplanets are likely lava worlds, given their small enough masses, sizes and orbits. Likely lava exoplanets include COROT-7b, Kepler-10b, and Kepler-78b.
List of extrasolar candidates for liquid water
Gliese 581 d looks an even better candidate. The orbital period was originally estimated at 83 days and has now been revised to 66 days. This was announced along with another new world, Gliese 581 e, which is next to twice the mass of Earth but too close to its sun for liquid water. In May 2011, a new study suggested that the planet might have a thick atmosphere, oceans and even life.
Extraterrestrial liquid water
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.
Nature
A lake (from Latin "lacus") is a terrain feature (or physical feature), a body of liquid on the surface of a world that is localized to the bottom of basin (another type of landform or terrain feature; that is, it is not global) and moves slowly if it moves at all. On Earth, a body of water is considered a lake when it is inland, not part of the ocean, is larger and deeper than a pond, and is fed by a river. The only world other than Earth known to harbor lakes is Titan, Saturn's largest moon, which has lakes of ethane, most likely mixed with methane. It is not known if Titan's lakes are fed by rivers, though Titan's surface is carved by numerous river beds. Natural lakes on Earth are generally found in mountainous areas, rift zones, and areas with ongoing or recent glaciation. Other lakes are found in endorheic basins or along the courses of mature rivers. In some parts of the world, there are many lakes because of chaotic drainage patterns left over from the last Ice Age. All lakes are temporary over geologic time scales, as they will slowly fill in with sediments or spill out of the basin containing them.
Extraterrestrial atmosphere
In February 2016, it was announced that NASA Hubble Space Telescope had detected hydrogen and helium (and suggestions of hydrogen cyanide), but no water vapor, in the atmosphere of 55 Cancri e, the first time the atmosphere of a super-earth exoplanet was analyzed successfully.
Mu Arae
The gas giant planet "b" is located in the liquid water habitable zone of Mu Arae. This would prevent an Earth-like planet from forming in the habitable zone, however large moons of the gas giant could potentially support liquid water. On the other hand, it is unclear whether such massive moons could actually form around a gas giant planet, thanks to an apparent scaling law between the mass of the planet and its satellite system. In addition, measurements of the star's ultraviolet flux suggest that any potentially habitable planets or moons may not receive enough ultraviolet to trigger the formation of biomolecules. Planet "d" would receive a similar amount of ultraviolet to the Earth and thus lies in the ultraviolet habitable zone, however, it would be too hot for any moons to support surface liquid water.
List of extrasolar candidates for liquid water
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, and the remaining 49 habitable zone candidates range from twice the size of Earth to larger than Jupiter.
HD 69830
The outermost planet discovered appears to be within the system's habitable zone, where liquid water would remain stable (more accurate data on the primary star's luminosity will be required to know for sure where the habitable zone is). HD 69830 is the first extrasolar planetary system around a Sun-like star without any known planets comparable to Jupiter or Saturn in mass.
Extraterrestrial liquid water
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.
Astrobiology
Another planetary body that could potentially sustain extraterrestrial life is Saturn's largest moon, Titan. Titan has been described as having conditions similar to those of early Earth. On its surface, scientists have discovered the first liquid lakes outside Earth, but these lakes seem to be composed of ethane and/or methane, not water. Some scientists think it possible that these liquid hydrocarbons might take the place of water in living cells different from those on Earth. After Cassini data was studied, it was reported on March 2008 that Titan may also have an underground ocean composed of liquid water and ammonia. Additionally, Saturn's moon Enceladus may have an ocean below its icy surface and, according to NASA scientists in May 2011, "is emerging as the most habitable spot beyond Earth in the Solar System for life as we know it". On 27 June 2018, astronomers reported the detection of complex macromolecular organics on Enceladus.
Sustainable energy
Large national and regional research projects on artificial photosynthesis are designing nanotechnology-based systems that use solar energy to split water into hydrogen fuel. and a proposal has been made for a Global Artificial Photosynthesis project In 2011, researchers at the Massachusetts Institute of Technology (MIT) developed what they are calling an "Artificial Leaf", which is capable of splitting water into hydrogen and oxygen directly from solar power when dropped into a glass of water. One side of the "Artificial Leaf" produces bubbles of hydrogen, while the other side produces bubbles of oxygen.
Circumstellar habitable zone
Liquid-water environments have been found to exist in the absence of atmospheric pressure, and at temperatures outside the CHZ temperature range. For example, Saturn's moons Titan and Enceladus and Jupiter's moons Europa and Ganymede, all of which are outside the habitable zone, may hold large volumes of liquid water in subsurface oceans.
Water scarcity
According to a UN climate report, the Himalayan glaciers that are the sources of Asia's biggest rivers – Ganges, Indus, Brahmaputra, Yangtze, Mekong, Salween and Yellow – could disappear by 2035 as temperatures rise. It was later revealed that the source used by the UN climate report actually stated 2350, not 2035. Approximately 2.4 billion people live in the drainage basin of the Himalayan rivers. India, China, Pakistan, Bangladesh, Nepal and Myanmar could experience floods followed by droughts in coming decades. In India alone, the Ganges provides water for drinking and farming for more than 500 million people. The west coast of North America, which gets much of its water from glaciers in mountain ranges such as the Rocky Mountains and Sierra Nevada, also would be affected.
Extraterrestrial liquid water
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.
Gliese 581c
Gliese 581c is likely to lie outside the habitable zone. No direct evidence has been found for water to be present, and it is probably not present in the liquid state. Techniques like the one used to measure the extrasolar planet HD 209458 b may in the future be used to determine the presence of water in the form of vapor in the planet's atmosphere, but only in the rare case of a planet with an orbit aligned so as to transit its star, which Gliese 581c is not known to do.
Asteroidal water
Asteroidal water are water or water precursor deposits such as hydroxide (OH) that exist in asteroids (i.e., small Solar System bodies (SSSBs) not explicitly in the subcategory of comets). The "snow line" of the Solar System lies outside of the main asteroid belt, and the majority of water is expected in Kuiper belt objects (KBOs) and minor planets. Nevertheless, a significant amount of water is also found inside the snow line, including in near-earth objects (NEOs).
55 Cancri e
In February 2016, it was announced that NASA's Hubble Space Telescope had detected hydrogen and helium (and suggestions of hydrogen cyanide), but no water vapor, in the atmosphere of 55 Cancri e, the first time the atmosphere of a super-Earth exoplanet was analyzed successfully. In November 2017, it was announced that infrared observations with the Spitzer Space Telescope indicated the presence of a global lava ocean obscured by an atmosphere with a pressure of about 1.4 bar, slightly thicker than that of Earth. The atmosphere may contain similar chemicals in Earth's atmosphere, such as nitrogen and possibly oxygen, in order to cause the infrared data observed by Spitzer.
Hydrogenics
In January 2010, Hydrogenics began development of a next-generation power system to be used for surface mobility applications on the moon for the Canadian Space Agency. The system includes an electrolyzer that produces both hydrogen and oxygen using solar power, and a fuel cell system that can be used for mobility, auxiliary, and life support systems. Heliocentris and FAUN Umwelttechnick collaborated with Hydrogenics to develop a hybrid waste disposal vehicle for BSR (Berliner Stadtreinigung) in August of that year.
Circumstellar habitable zone
In astronomy and astrobiology, the circumstellar habitable zone (CHZ), or simply the habitable zone, is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressure. The bounds of the CHZ are based on Earth's position in the Solar System and the amount of radiant energy it receives from the Sun. Due to the importance of liquid water to Earth's biosphere, the nature of the CHZ and the objects within it may be instrumental in determining the scope and distribution of Earth-like extraterrestrial life and intelligence.
Extraterrestrial liquid water
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.
Energy Observer
Energy Observer joins a vein of big projects for research and development sharing a calling for technology, society and the environment and carrying a message in favour of renewable energies. In this vein we can also name the polar schooner Tara, that has travelled across the oceans since 2007 for science and the protection of the environment, Solar Impulse, the first solar plane, or even Planet Solar which became the first solar electric vehicle ever to circumnavigate the globe in 2010. In October 2016, the foundation Race for Water Odyssey announced that it was introducing hydrogen technology to the Planet Solar catamaran utilising solely solar energy, for an expedition around the world with similar ambitions to the Energy Observer project.  Planned, but not yet realized ships with hydrogen fuel production aboard with renewable energies are the "orcelle". This ferry should be the "green flagship" of the Wallenius Wilhelmsen Logistics, a Norwegian-Swedish shipping Company. She uses sun and wave energy to produce hydrogen aboard for propulsion of the vessel. Another hydrogen ship, also not realized so far, is the deepsea yacht "eco trimaran", using also the energy of sun and waves. Wind energy is utilized not by sails, but by a wind turbine. So this energy source may also contribute to hydrogen production aboard, even in mooring times.
Extraterrestrial life
Hydrogen sulfide has been proposed as a hypothetical solvent for life and is quite plentiful on Jupiter's moon Io, and may be in liquid form a short distance below the surface.
Extraterrestrial liquid water
Models of Solar System objects indicate the presence of liquid water in their internal differentiation.
Lake
Evidence of extraterrestrial lakes exists; "definitive evidence of lakes filled with methane" was announced by NASA as returned by the Cassini Probe observing the moon Titan, which orbited the planet Saturn.
Extraterrestrial liquid water
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.
Vid Flumina
Vid Flumina is a river of liquid methane and ethane on Saturn's moon Titan. The river has been compared to the Nile. It is more than long and flows into Titan's second largest hydrocarbon sea, Ligeia Mare. The surface of Titan is mostly water ice, so Vid Flumina is a river of methane and ethane flowing across and cutting canyons into ice as though it were bedrock. NASA scientists think that it likely has rapids, whirlpools and waterfalls, just like rivers on Earth.
Alice (spacecraft instrument)
In August 2018, NASA confirmed, based on results by "Alice" on the "New Horizons" spacecraft, the detection of a "hydrogen wall" at the outer edges of the Solar System that was first detected in 1992 by the two Voyager spacecraft which have detected a surplus of ultraviolet light determined to be coming from hydrogen.
Nubian Sandstone Aquifer System
The Nubian Sandstone Aquifer System (NSAS) is the world’s largest known fossil water aquifer system. It is located underground in the Eastern end of the Sahara Desert and spans the political boundaries of four countries in north-eastern Africa. NSAS covers a land area spanning just over two million km, including north-western Sudan, north-eastern Chad, south-eastern Libya, and most of Egypt. Containing an estimated 150,000 km of groundwater, the significance of the NSAS as a potential water resource for future development programs in these countries is extraordinary. Recently the Great Man-made River Project (GMMR) in Libya began extracting substantial amounts of water from this aquifer, removing an estimated 2.4 km per year. This system is primarily used to supply water in the Kufra oasis.
Extraterrestrial liquid 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.
Hydrogen
A molecular form called protonated molecular hydrogen () is found in the interstellar medium, where it is generated by ionization of molecular hydrogen from cosmic rays. This ion has also been observed in the upper atmosphere of the planet Jupiter. The ion is relatively stable in the environment of outer space due to the low temperature and density. is one of the most abundant ions in the Universe, and it plays a notable role in the chemistry of the interstellar medium. Neutral triatomic hydrogen H can exist only in an excited form and is unstable. By contrast, the positive hydrogen molecular ion () is a rare molecule in the universe.
Geology of solar terrestrial planets
In early 2014, using data from the Herschel Space Observatory, it was discovered that there are several localized (not more than 60 km in diameter) mid-latitude sources of water vapor on Ceres, which each give off about 10 molecules (or 3 kg) of water per second. Two potential source regions, designated Piazzi (123°E, 21°N) and Region A (231°E, 23°N), have been visualized in the near infrared as dark areas (Region A also has a bright center) by the W. M. Keck Observatory. Possible mechanisms for the vapor release are sublimation from about 0.6 km2 of exposed surface ice, or cryovolcanic eruptions resulting from radiogenic internal heat or from pressurization of a subsurface ocean due to growth of an overlying layer of ice. Surface sublimation would be expected to decline as Ceres recedes from the Sun in its eccentric orbit, whereas internally powered emissions should not be affected by orbital position. The limited data available are more consistent with cometary-style sublimation. The spacecraft Dawn is approaching Ceres at aphelion, which may constrain Dawn's ability to observe this phenomenon.
Hydrogen isotope biogeochemistry
Water is the primary source of hydrogen to all living organisms, so the isotopic composition of environmental water is a first-order control on that of the biosphere. The hydrological cycle moves water around different reservoirs on the surface of the earth, during which hydrogen isotopes in water are significantly fractionated. As the primary moisture source to the atmosphere, the ocean has a relatively uniform hydrogen isotope composition across the globe around 0‰ (VSMOW). Variations of δD larger than 10‰ in the ocean are generally confined to surface waters due to evaporation, sea ice formation, and addition of meteoric waters by precipitation, rivers or icebergs. In the hydrological cycle, the two major processes that fractionate hydrogen isotopes from ocean water are evaporation and condensation. It should be pointed out that oxygen isotopic composition (O/O) of water is also an important tracer in the hydrological cycle, and cannot be separated from hydrogen isotopes when we talk about isotope fractionation processes associated with water.
Colonization of the outer Solar System
On March 9, 2006, NASA's "Cassini" space probe found possible evidence of liquid water on Enceladus. According to that article, "pockets of liquid water may be no more than tens of meters below the surface." These findings were confirmed in 2014 by NASA. This means liquid water could be collected much more easily and safely on Enceladus than, for instance, on Europa (see above). Discovery of water, especially liquid water, generally makes a celestial body a much more likely candidate for colonization. An alternative model of Enceladus' activity is the decomposition of methane/water clathrates – a process requiring lower temperatures than liquid water eruptions. The higher density of Enceladus indicates a larger than Saturnian average silicate core that could provide materials for base operations.
List of extrasolar candidates for liquid water
Kepler-62f is only 40 percent larger than Earth, making it the exoplanet closest to the size of our planet known in the habitable zone of another star. Kepler-62e orbits on the inner edge of the habitable zone and is roughly 60 percent larger than Earth. Both are assumed to be rocky planets, but since the star is 1200 light-years away, it's hard to be sure.
Water
Water has been detected in interstellar clouds within our galaxy, the Milky Way. Water probably exists in abundance in other galaxies, too, because its components, hydrogen and oxygen, are among the most abundant elements in the universe. Based on models of the formation and evolution of the Solar System and that of other star systems, most other planetary systems are likely to have similar ingredients.
Mississippi River System
From the perspective of natural geography and hydrology, the system consists of the Mississippi River itself and its numerous natural tributaries and distributaries. The major tributaries are the Arkansas, Illinois, Missouri, Ohio and Red rivers. Given their flow volumes, major Ohio River tributaries like the Allegheny, Tennessee, and Wabash rivers are considered important tributaries to the Mississippi system. Before the Mississippi River reaches the Gulf of Mexico, it runs into its distributary, the Atchafalaya River.
List of extrasolar candidates for liquid water
The goal of current searches is to find Earth-sized planets in the habitable zone of their planetary systems (also 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. But there is evidence that rocky planets hosting water may be commonplace throughout the Milky Way.
Extraterrestrial liquid water
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 April 3, 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.
Channel (geography)
Extraterrestrial natural channels are found elsewhere in the Solar System than the Earth and the longest and widest of which are the outflow channels on Mars and the channels of Venus many of which are tens of kilometres wide (the network of channels flowing from Argyre Planitia on Mars for example is 8000 km in length and the Baltis Vallis Venus is 7000 km compared to the 6,650 km Nile, the largest active channel on Earth). The exact formation of these large ancient channels is unknown although it is theorised that those on Mars may have been formed due to catastrophic flooding and on Venus by lava flow. In planetary science the term "rille" is sometimes used for similar formations found on The Moon and Mercury that are of inconclusive origin. Channels have also been recently discovered on Titan. The Saturnian moon has the only known permanently active channels in the Solar System other than Earth, the largest known of which is 400 km in length. These are believed to be formed from flowing hydrocarbons in the hypothesized methanological cycle.
Extraterrestrial liquid water
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.
Circumstellar habitable zone
Outside the CHZ, tidal heating and radioactive decay are two possible heat sources that could contribute to the existence of liquid water. Abbot and Switzer (2011) put forward the possibility that subsurface water could exist on rogue planets as a result of radioactive decay-based heating and insulation by a thick surface layer of ice.
Solar thermal rocket
A solar-thermal propulsion architecture outperforms architectures involving electrolysis and liquification of hydrogen from water by more than an order of magnitude, since electrolysis requires heavy power generators, whereas distillation only requires a simple and compact heat source (either nuclear or solar); so the propellant production rate is correspondingly far higher for any given initial mass of equipment. However its use does rely on having clear ideas of the location of water ice in the solar system, particularly on lunar and asteroidal bodies, and such information is not known, other than that the bodies with the asteroid belt and further from the Sun are expected to be rich in water ice.
List of extrasolar candidates for liquid water
In February 2017, seven planets were discovered in the star system TRAPPIST-1, previously unknown to host any planets. Three of the discovered planets, called TRAPPIST-1e, TRAPPIST-1f and TRAPPIST-1g, are candidates for liquid water. They are all located towards the outer system (with the closest to the star, TRAPPIST-1e, being within or slightly outside the habitable zone), making them cool planets. TRAPPIST-1e and f are probably tidally locked planets, and if liquid water exists in them, it is located in their respective terminator lines. However, if the planets support a thick enough atmosphere to transfer heat to the sides facing away from the star, much larger portions of them may be habitable.
Exoplanetology
If a planet has an eccentric orbit, then tidal heating can provide another source of energy besides stellar radiation. This means that eccentric planets in the radiative habitable zone can be too hot for liquid water. Tides also circularize orbits over time so there could be planets in the habitable zone with circular orbits that have no water because they used to have eccentric orbits. Eccentric planets further out than the habitable zone would still have frozen surfaces but the tidal heating could create a subsurface ocean similar to Europa's. In some planetary systems, such as in the Upsilon Andromedae system, the eccentricity of orbits is maintained or even periodically varied by perturbations from other planets in the system. Tidal heating can cause outgassing from the mantle, contributing to the formation and replenishment of an atmosphere.
List of largest lakes and seas in the Solar System
Listed below are the largest lakes and seas on various worlds in the Solar System. The table includes single bodies of water or other liquid on or near the surface of a solid body (terrestrial planet, planetoid, or moon). All objects on this list are expected to be round, hence anything that is part of a belt or disc is expected to be a dwarf planet.
Rain
Rainfalls of diamonds have been suggested to occur on the gas giant planets, Jupiter and Saturn, as well as on the ice giant planets, Uranus and Neptune. There is likely to be rain of various compositions in the upper atmospheres of the gas giants, as well as precipitation of liquid neon in the deep atmospheres. On Titan, Saturn's largest natural satellite, infrequent methane rain is thought to carve the moon's numerous surface channels. On Venus, sulfuric acid virga evaporates from the surface. Extrasolar planet OGLE-TR-56b in the constellation Sagittarius is hypothesized to have iron rain.
Hydrogen vehicle
Automobiles, buses, forklifts, trains, PHB bicycles, canal boats, cargo bikes, golf carts, motorcycles, wheelchairs, ships, airplanes, submarines, and rockets can already run on hydrogen, in various forms. NASA used hydrogen to launch Space Shuttles into space. A working toy model car runs on solar power, using a regenerative fuel cell to store energy in the form of hydrogen and oxygen gas. It can then convert the fuel back into water to release the solar energy. Since the advent of hydraulic fracturing the key concern for hydrogen fuel cell vehicles is consumer and public policy confusion concerning the adoption of natural gas powered hydrogen vehicles with heavy hidden emissions to the detriment of environmentally friendly transportation.
Lake
Only one world other than Earth is known to harbor large lakes, Saturn's largest moon, Titan. Photographs and spectroscopic analysis by the Cassini–Huygens spacecraft show liquid ethane on the surface, which is thought to be mixed with liquid methane. The largest Titanean lake, Kraken Mare at 400,000 km, is three-times the size of any lake on Earth, and even the second, Ligeia Mare, is estimated to be slightly larger than Earth's Lake Michigan–Huron.
List of extrasolar candidates for liquid water
The star Kepler-62 has five planets, two of which are the right distance from the star to have liquid water and potentially sustain life.