Methane is a chemical compound with the formula CH₄, meaning one carbon atom is bonded to four hydrogen atoms. It is a type of hydride from group 14, the simplest alkane, and the main part of natural gas. Methane is common on Earth, making it a valuable fuel. However, it is difficult to capture and store because it is a gas at normal temperatures and pressures. In Earth's atmosphere, methane allows visible light to pass through but absorbs infrared radiation, acting as a greenhouse gas. Methane is an organic hydrocarbon and one of the simplest organic compounds.
Natural methane is found underground and beneath the seafloor, formed by both geological and biological processes. The largest methane reserve is under the seafloor in the form of methane clathrates. When methane reaches Earth's surface and atmosphere, it is called atmospheric methane.
Since 1750, methane levels in Earth's atmosphere have increased by about 160%, mostly due to human activities. According to the 2021 Intergovernmental Panel on Climate Change report, methane contributes 20% of the total heat-trapping effect from long-lived greenhouse gases. Reducing methane emissions strongly and quickly could slow near-term global warming and improve air quality by lowering ozone levels.
Methane has also been found on other planets, such as Mars, which is important for research about the possibility of life beyond Earth.
Properties and bonding
Methane is a molecule shaped like a tetrahedron, with four identical bonds connecting carbon to hydrogen. Its structure is formed by four bonding areas created when the outer electrons of carbon and hydrogen overlap. The lowest-energy bonding area forms when the 2s orbital of carbon overlaps with the combined 1s orbitals of the four hydrogen atoms. Above this, there are three similar bonding areas formed by the 2p orbitals of carbon overlapping with different combinations of the 1s orbitals of hydrogen. This bonding pattern, called "three-over-one," matches results from scientific tests called photoelectron spectroscopy.
Methane is a colorless, odorless, and clear gas at normal temperature and pressure. It absorbs some visible light, especially red light, because of special absorption patterns. However, this effect is only visible when light travels a very long path through methane. This is why planets like Uranus and Neptune appear blue or bluish-green: their atmospheres contain methane, which absorbs red light and scatters blue light back into space.
Natural gas, which is mostly methane, is made to smell like sulfur by adding a chemical called tert-butylthiol. This helps people detect leaks. Methane boils at −161.5 °C under normal atmospheric pressure. It is flammable when mixed with air at concentrations between 5.4% and 17%.
Solid methane has several forms, with nine known types. When cooled under normal pressure, methane forms a solid called methane I. This solid has a cube-like structure and belongs to a specific crystal arrangement (space group Fm 3 m). In methane I, hydrogen atoms are not fixed in place, allowing methane molecules to rotate freely. Because of this, methane I is classified as a plastic crystal.
Chemical reactions
The main chemical reactions of methane are combustion, steam reforming to syngas, and halogenation. In general, methane reactions are hard to control.
Partial oxidation of methane to methanol (CH₃OH), a liquid fuel that is easier to use, is difficult because the reaction often continues to produce carbon dioxide and water even when there is not enough oxygen. The enzyme methane monooxygenase can create methanol from methane, but it cannot be used for large-scale industrial processes. Some types of chemical processes, both homogeneous and heterogeneous, have been developed, but each has major problems. These methods usually work by creating protected products that are shielded from further oxidation. Examples include the Catalytica system, copper zeolites, and iron zeolites that help stabilize the alpha-oxygen active site.
A group of bacteria uses nitrite as an oxidant to break down methane without oxygen, a process called anaerobic oxidation of methane.
Like other hydrocarbons, methane is a very weak acid. Its pKa in DMSO is estimated to be 56. Methane cannot be deprotonated in solution, but its conjugate base is known in forms such as methyllithium.
Many positive ions formed from methane have been observed, mostly as unstable species in low-pressure gas mixtures. These include methenium or methyl cation (CH₃), methane cation (CH₄), and methanium or protonated methane (CH₅). Some of these ions have been found in space. Methanium can also be made in diluted solutions using methane and superacids. Ions with higher charges, such as CH₆ and CH₇, have been studied theoretically and are thought to be stable.
Despite the strong C–H bonds in methane, scientists are very interested in catalysts that can help break these bonds in methane and other small alkanes.
Methane’s heat of combustion is 55.5 MJ/kg. Combustion of methane is a multi-step reaction summarized as follows:
Peters four-step chemistry is a simplified four-step process that explains how methane burns.
Under the right conditions, methane reacts with halogen radicals as follows:
where X is a halogen: fluorine (F), chlorine (Cl), bromine (Br), or iodine (I). This process is called free radical halogenation. It begins when UV light or another radical initiator, such as peroxides, creates a halogen atom. A two-step chain reaction then occurs, where the halogen atom takes a hydrogen atom from a methane molecule, forming a hydrogen halide and a methyl radical (•CH₃). The methyl radical then reacts with a halogen molecule to form a halomethane, with a new halogen atom as a byproduct. Similar reactions can happen on the halogenated product, leading to the replacement of more hydrogen atoms with halogen atoms, forming dihalomethane, trihalomethane, and finally tetrahalomethane, depending on the reaction conditions and the ratio of halogen to methane.
This reaction is often used with chlorine to produce dichloromethane and chloroform from chloromethane. Excess chlorine can be used to make carbon tetrachloride.
Uses
Methane can be transported as a refrigerated liquid, called liquefied natural gas (LNG). When methane leaks from a refrigerated container, it is initially heavier than air because the cold gas is denser. However, at normal temperatures, methane gas becomes lighter than air. Gas pipelines carry large amounts of natural gas, of which methane is the main component.
Methane is used as fuel for ovens, homes, water heaters, kilns, automobiles, rockets, turbines, and other devices.
As the main part of natural gas, methane is important for producing electricity. It is burned in gas turbines or steam generators to create energy. Compared to other hydrocarbon fuels, methane produces less carbon dioxide for each unit of heat released. Methane has a heat of combustion of about 891 kJ/mol, which is lower than other hydrocarbons. However, the ratio of its heat of combustion (891 kJ/mol) to its molecular mass (16.0 g/mol) shows that methane produces more heat per unit of mass (55.7 kJ/g) than other complex hydrocarbons. In areas with many people, methane is piped into homes and businesses for heating, cooking, and industrial use. In this context, it is called natural gas, which has an energy content of 39 megajoules per cubic meter or 1,000 BTU per standard cubic foot. Liquefied natural gas (LNG) is mostly methane that has been turned into a liquid for easier storage or transport.
Refined liquid methane and LNG are used as rocket fuel when combined with liquid oxygen, as in the TQ-12, BE-4, Raptor, YF-215, and Aeon engines. These engines are often grouped under the term "methalox" because of the similarities between methane and LNG.
As a liquid rocket propellant, a methane/liquid oxygen combination has advantages over a kerosene/liquid oxygen combination. Methane produces smaller exhaust molecules, reducing soot buildup on engine parts. Methane is easier to store than hydrogen because it has a higher boiling point and density, and it does not cause hydrogen embrittlement. The lower molecular weight of methane’s exhaust increases the fraction of heat energy in the form of kinetic energy, which improves the rocket’s specific impulse. Compared to liquid hydrogen, methane has lower specific energy, but this is balanced by its greater density and temperature range, allowing for smaller and lighter fuel tanks. Liquid methane has a temperature range (91–112 K) that is nearly compatible with liquid oxygen (54–90 K). Methane is currently used in operational launch vehicles like Zhuque-2, Vulcan, and New Glenn, as well as in development projects like Starship, Neutron, Terran R, Nova, and Long March 9.
Natural gas, which is mostly methane, is used to produce hydrogen gas on an industrial scale. Steam methane reforming (SMR), or steam reforming, is the standard method for making hydrogen. More than 50 million metric tons of hydrogen are produced annually worldwide, mainly through SMR of natural gas. Much of this hydrogen is used in petroleum refineries, chemical production, and food processing. Large amounts of hydrogen are also used to make ammonia.
At high temperatures (700–1100 °C) and with a nickel-based catalyst, steam reacts with methane to create a mixture of carbon monoxide and hydrogen, called "water gas" or "syngas." This reaction requires heat and is strongly endothermic (Δ r H = 206 kJ/mol). Additional hydrogen is made when carbon monoxide reacts with water in the water-gas shift reaction, which produces heat (Δ r H = −41 kJ/mol).
Methane is also used in free-radical chlorination to make chloromethanes, though methanol is more commonly used as a starting material.
Hydrogen can also be made by directly decomposing methane, a process called methane pyrolysis. Unlike steam reforming, this method produces no greenhouse gases (GHG). The heat needed for the reaction can come from renewable energy or burning some of the produced hydrogen. If the methane comes from biogas, the process can remove carbon from the atmosphere. Temperatures above 1200 °C are needed to break methane’s bonds, creating hydrogen gas and solid carbon. A suitable catalyst can lower the reaction temperature to 550–900 °C. Many catalysts have been tested, including metal and carbon-based options. This reaction is moderately endothermic.
As a refrigerant, methane has the ASHRAE designation R-50.
Generation
Methane can be created through geological, biological, or industrial methods.
The two main geological ways methane is formed are (i) organic (thermally generated, or thermogenic) and (ii) inorganic (abiotic). Thermogenic methane forms when organic matter breaks apart under high temperatures and pressures in deep sediment layers. Most methane in sedimentary basins is thermogenic, making it the primary source of natural gas. Thermogenic methane is usually considered to be from earlier times. Thermogenic methane can form either by breaking down organic matter or by creating new organic matter. Both processes may involve microorganisms (methanogenesis), but can also occur without them. These processes can also consume methane, with or without microorganisms.
At great depths, such as in crystalline bedrock, the most important source of methane is abiotic. Abiotic methane forms from inorganic compounds without biological activity. This can happen through magmatic processes or through water-rock reactions at low temperatures and pressures, such as serpentinization.
Most methane on Earth is biogenic and is produced by methanogenesis, a type of anaerobic respiration done by certain Archaea. Methanogens live in landfills, soils, ruminants (like cattle), the guts of termites, and in anoxic sediments below the seafloor and lake bottoms.
This multistep process provides energy for these microorganisms. The overall reaction of methanogenesis is:
The final step is catalyzed by the enzyme methyl coenzyme M reductase (MCR).
Wetlands are the largest natural source of methane in the atmosphere, contributing about 20–30% of all atmospheric methane. Climate change increases methane release from wetlands because rising temperatures and changing rainfall patterns alter wetland conditions. This is called the wetland methane feedback.
Rice farming produces about 12% of global methane emissions due to the long-term flooding of rice fields.
Ruminants like cattle release methane through belching, contributing about 22% of U.S. annual methane emissions. One study found that the livestock industry (mainly cattle, chickens, and pigs) produces 37% of all human-caused methane. A 2013 study estimated that livestock accounted for 44% of human-caused methane and about 15% of human-caused greenhouse gas emissions. Many efforts are being made to reduce methane from livestock, such as medical treatments, dietary changes, and capturing methane for energy use.
Most of the seafloor is anoxic because oxygen is used up by aerobic microorganisms in the top layers of sediment. Below the oxygen-rich seafloor, methanogens produce methane that is either used by other organisms or trapped in gas hydrates. Other organisms that use methane for energy are called methanotrophs ("methane-eaters") and are the main reason little methane from deep layers reaches the ocean surface. Groups of Archaea and Bacteria work together to break down methane through anaerobic oxidation of methane (AOM). These organisms include anaerobic methanotrophic Archaea (ANME) and sulfate-reducing bacteria (SRB).
Because methane is cheap and abundant in natural gas, there is little need to make it industrially. Methane can be made by hydrogenating carbon dioxide through the Sabatier process. Methane is also a byproduct of hydrogenating carbon monoxide in the Fischer–Tropsch process, which is used to create longer molecules like gasoline or diesel.
An example of large-scale coal-to-methane gasification is the Great Plains Synfuels plant, started in 1984 in Beulah, North Dakota. This plant was created to use local low-grade lignite, a resource that is hard to transport due to its weight, ash content, low energy value, and tendency to catch fire during storage and transport. Similar plants exist worldwide, though most focus on making longer-chain alkanes for fuel or other uses.
Power to methane is a technology that uses electricity to split water into hydrogen through electrolysis. This hydrogen is then combined with carbon dioxide using the Sabatier reaction to create methane.
Methane can be made by reacting methyl lithium or a methyl Grignard reagent, such as methylmagnesium chloride, with protons. It can also be produced by mixing anhydrous sodium acetate with dry sodium hydroxide and heating the mixture above 300°C (with sodium carbonate as a byproduct). In practice, pure methane can be obtained from standard gas suppliers using steel gas bottles.
Occurrence
Methane is the main part of natural gas, making up about 87% of its volume. The main way methane is obtained is by removing it from underground areas called natural gas fields. Another important source is coal seam gas, which is taken from coal deposits. This includes a method called coal bed methane extraction, and another method called enhanced coal bed methane recovery, which gets methane from coal that cannot be mined. Methane is often found with other fuels like oil, and sometimes with helium and nitrogen. Methane forms in shallow areas (where pressure is low) through the breakdown of organic matter without oxygen, and from methane that has moved up from deep underground. In general, the rock layers that create natural gas are buried deeper and are hotter than the layers that hold oil. Methane is usually moved in large amounts through pipelines as natural gas, or by special ships called LNG carriers when it is turned into a liquid. Very few countries use trucks to transport methane.
Atmospheric methane and climate change
Methane is a greenhouse gas that plays a major role in increasing global temperatures. It is responsible for about 30% of the temperature rise since the start of the industrial revolution.
Methane has a global warming potential (GWP) of 29.8 compared to carbon dioxide (which has a GWP of 1) over 100 years, and 82.5 over 20 years. This means that one tonne of methane released into the atmosphere has the same warming effect as 82.5 tonnes of carbon dioxide. Burning methane and turning it into carbon dioxide reduces its impact compared to letting methane escape into the air.
Over time, methane in the atmosphere changes into carbon dioxide and water. The warming effects of both methane and the carbon dioxide it becomes are included in the GWP values.
Each year, about 580 million tonnes of methane are released globally. Of this, 40% comes from natural sources, and 60% from human activities, such as farming and energy production. Farming is the largest human-caused source, contributing about 25% of all methane emissions. The energy sector, including coal, oil, natural gas, and biofuels, is the next largest source.
Historically, methane levels in the atmosphere ranged between 300 and 400 nmol/mol during cold ice age periods and between 600 and 700 nmol/mol during warm periods between ice ages. A 2012 NASA study suggested the oceans might be a significant source of Arctic methane, but recent research points to human activities as the main cause of rising methane levels.
Monitoring methane levels in the atmosphere began in the 1980s. Since the mid-1700s, methane concentrations have increased by 160%. In 2013, methane contributed 20% of the warming effect from long-lasting greenhouse gases. Between 2011 and 2019, methane levels in the atmosphere rose by an average of 1,866 parts per billion each year. Sharp increases were recorded from 2015 to 2019.
In 2019, methane levels were higher than at any time in the past 800,000 years. According to the IPCC’s 2021 report, methane concentrations have risen by 156% since 1750, far exceeding natural changes between ice ages and warm periods.
In 2020, studies found that methane leaks from the fossil fuel industry might be underreported. The largest increase in methane levels occurred in 2021, with most of the rise linked to human activities.
Climate change can increase methane levels by boosting methane production in natural environments, creating a feedback loop that worsens warming. Another possible reason for rising methane levels is a slower rate of chemical reactions that remove methane from the atmosphere.
Over 100 countries have joined the Global Methane Pledge, launched in 2021, to reduce methane emissions by 30% by 2030. This effort could prevent 0.2°C of global warming by 2050. However, some experts argue stronger actions are needed to meet this goal. A 2022 report by the International Energy Agency states that the most cost-effective ways to reduce methane emissions are in the energy sector, especially in oil and gas operations.
Methane clathrates, also called methane hydrates, are solid structures made of water molecules that trap methane inside. Large amounts of methane clathrates are found in Arctic permafrost and under the ocean floor. These structures form under high pressure and low temperatures. Methane clathrates can be made from natural methane sources or a mix of sources. They are both a potential energy resource and a potential contributor to global warming.
The total amount of carbon stored in methane clathrates is uncertain, with estimates ranging from 500 to 12,500 gigatonnes. Recent studies suggest the amount is closer to 1,800 gigatonnes. Uncertainty remains due to gaps in understanding methane sources, sinks, and the global distribution of clathrates. For example, a new methane source was recently discovered in an Arctic region. Some climate models suggest that methane emissions from the ocean floor today may resemble those during a period of extreme global warming 55.5 million years ago, though no evidence shows methane from clathrates currently reaches the atmosphere.
Methane release from Arctic permafrost and ocean floor clathrates could worsen global warming and may be caused by warming itself. This idea is called the clathrate gun hypothesis. Data from 2016 show that Arctic permafrost is melting faster than scientists predicted.
Public safety and the environment
Methane harms air quality and negatively affects human health, farming results, and the ability of ecosystems to produce life.
The methane gas leak at Aliso Canyon, California, between 2015 and 2016 was the largest in American history in terms of environmental damage. This event was described as more harmful to the environment than the oil spill from the Deepwater Horizon in the Gulf of Mexico.
In May 2023, The Guardian reported that Turkmenistan is the world’s worst offender for methane super emitters. Studies by Kayrros found that two large fossil fuel fields in Turkmenistan leaked 2.6 million and 1.8 million metric tonnes of methane in 2022. These leaks released the same amount of carbon dioxide as 366 million tonnes of CO₂, which is more than the annual CO₂ emissions of the United Kingdom.
Extraterrestrial methane
Methane is found in many places throughout the Solar System and could be collected from the surface of another Solar System body, such as Mars or Titan, using materials found there. This methane could be used as fuel for returning to Earth.
Methane has been found on all planets and most large moons in the Solar System. Except possibly on Mars, it is believed to have formed through non-living processes.
Infrared astronomy has discovered methane in molecular clouds within the space between stars.
The Curiosity rover has recorded changes in the amount of methane in Mars' atmosphere over time. These changes reached their highest level at the end of the Martian summer, with methane levels reaching 0.6 parts per billion. Methane on Mars may form through a non-living process called serpentinization, which involves water, carbon dioxide, and a mineral called olivine, which is common on Mars. Some scientists have suggested that microbes living underground might also produce methane on Mars, but there is no evidence to support this idea.
Methane has been considered as a possible fuel for rockets on future Mars missions because it might be created on Mars using resources found there. A method called the Sabatier methanation reaction could be adapted to produce methane and oxygen from Martian soil water and atmospheric carbon dioxide using a single reactor.
Methane is found in large amounts on Titan, the largest moon of Saturn. It makes up a major part of Titan's atmosphere and exists as liquid on its surface, forming the main component of Titan's lakes of hydrocarbons. One of Titan's largest lakes is nearly entirely made of methane.
The presence of stable liquid methane lakes on Titan, along with Titan's chemically active surface and abundance of organic compounds, has led scientists to consider the possibility of life existing in Titan's lakes. This life might use methane instead of water as a solvent and use hydrogen from Titan's atmosphere to generate energy with acetylene.
History
The discovery of methane is credited to Italian physicist Alessandro Volta. He studied many properties of methane, including how easily it can catch fire and its source from decaying plants and animals.
Volta was inspired by reports from his friend, Father Carlo Giuseppe Campi, who described "inflammable air" found in marshes. During a fishing trip near Lake Maggiore in Italy and Switzerland in November 1776, Volta noticed bubbles rising from nearby marshes and decided to investigate. He collected the gas from the marsh and demonstrated that it could ignite when exposed to a flame or spark.
Volta noted that earlier scientific writings, such as a letter by Benjamin Franklin, also described similar observations of "inflammable air."
After the Felling mine disaster of 1812, in which 92 people died, Sir Humphry Davy determined that the dangerous gas known as firedamp was mostly methane.
The name "methane" was created in 1866 by German chemist August Wilhelm von Hofmann. The name was based on the chemical methanol.
Etymology
The word "methane" comes from two parts. The ending "-ane" is a chemical suffix used for substances in the alkane family. The other part, "methyl," comes from the German word "Methyl" (1840) or the French "méthyle," which was created from the French "méthylène." This term was first used in 1834 by scientists Jean-Baptiste Dumas and Eugène Péligot. It combines parts of the Greek words "méthy" (meaning "wine") and "hýlē" (meaning "wood"). The name "methyl" is linked to methanol, an alcohol first found by distilling wood. The chemical suffix "-ane" comes from "-ine," which is based on the Latin feminine suffix "-ina" used for abstract concepts. In 1866, German chemist August Wilhelm von Hofmann suggested using suffixes like "-ane," "-ene," and "-one" for chemical names.
The abbreviation CH₄-C refers to the mass of carbon in methane. Methane's mass is always 1.33 times the mass of CH₄-C. This also describes the methane-carbon ratio, which is 1.33 by mass. Methane in the atmosphere is often measured in teragrams (Tg CH₄) or millions of metric tons (MMT CH₄), which are the same. Other units used include nanomole (nmol, one billionth of a mole), mole (mol), kilogram, and gram.
Safety
Methane is a gas that can cause suffocation because it does not harm the body directly but can replace oxygen in the air. When there is enough methane, it may reduce oxygen levels to dangerous amounts, which can lead to death by suffocation. Studies have shown that at a concentration of 5% in the air, methane does not cause harm to the body's systems.
Methane is very flammable at normal temperatures and can mix with air to create explosive conditions. Methane gas explosions have caused many deadly accidents in mines. For example, a methane explosion led to the Upper Big Branch coal mine disaster in West Virginia on April 5, 2010, which resulted in 29 deaths. Accidental releases of natural gas have also been a major concern in safety engineering because past incidents have caused jet fire disasters, which are extremely dangerous fires that spread quickly and burn intensely.
Cited sources
Haynes, William M., edited by (2016). CRC Handbook of Chemistry and Physics (97th edition). CRC Press. ISBN 978-1-4987-5429-3.