Methane
Methane is a chemical compound with the formula CH4. It consists of one carbon atom bonded to four hydrogen atoms. The molecule forms a tetrahedral shape. Four equivalent bonds connect the central carbon to each hydrogen. This electronic structure involves four bonding molecular orbitals. These orbitals result from overlapping valence electrons on carbon and hydrogen. The lowest energy orbital comes from the 2s orbital of carbon. It overlaps with the in-phase combination of 1s orbitals from the four hydrogens. Above this level lies a triply degenerate set of MOs. These involve the 2p orbitals of carbon. They combine with various linear combinations of the 1s orbitals on hydrogen. Photoelectron spectroscopic measurements confirm this three-over-one bonding scheme. Methane exists as an odorless, colorless gas at standard temperature and pressure. It absorbs infrared radiation while remaining transparent to visible light. A faint absorption occurs at the red end of the spectrum due to overtone bands. This effect becomes noticeable only when the light path extends very far. Such scattering gives Uranus and Neptune their blue or bluish-green colors.
Naturally occurring methane resides below ground and under the seafloor. It forms through geological and biological processes. Thermogenic methane arises from the breakup of organic matter. This occurs at elevated temperatures and pressures within deep sedimentary strata. Most methane found in sedimentary basins is thermogenic. Abiotic sources create methane without biological activity. Magmatic processes generate some abiotic methane. Water-rock reactions like serpentinization produce it at low temperatures and pressures. Biogenic methane results from methanogenesis. This form of anaerobic respiration happens among members of the domain Archaea. Methanogens inhabit landfills, soils, ruminant guts, termite intestines, and anoxic sediments beneath lakes. Wetlands account for approximately 20, 30% of atmospheric methane. Rice cultivation generates up to 12% of total global emissions. Ruminants such as cattle belch out about 22% of U.S. annual methane emissions. The livestock sector produces roughly 37% of all human-induced methane. Subseafloor sediments are generally anoxic. Oxygen disappears within the first few centimeters due to aerobic microorganisms. Below this layer, methanogens produce methane trapped in gas hydrates. These deposits exist along continental margins within the gas clathrate stability zone.
Methane serves as a primary component of natural gas. It powers ovens, homes, water heaters, kilns, automobiles, rockets, and turbines. Gas pipelines distribute large amounts of natural gas globally. Liquefied natural gas converts methane into liquid form for storage or transport. Refined liquid methane combines with liquid oxygen in rocket engines. Engines like TQ-12, BE-4, Raptor, YF-215, and Aeon use this mixture. This combination is known as methalox. Methane offers advantages over kerosene by producing smaller exhaust molecules. Reduced soot deposition occurs on engine components. Liquid methane stores more easily than hydrogen due to its higher boiling point. Its density allows for smaller tankage compared to liquid hydrogen. Operational launch vehicles include Zhuque-2, Vulcan, and New Glenn. In-development launchers feature Starship, Neutron, Terran R, Nova, and Long March 9. Steam methane reforming produces commercial bulk hydrogen gas. More than 50 million metric tons are produced annually worldwide. The Great Plains Synfuels plant started in 1984 in Beulah, North Dakota. It processed low-grade lignite coal to develop local resources.
Methane acts as a greenhouse gas responsible for around 30% of global temperature rise since the industrial revolution. Its global warming potential equals 29.8 ± 11 times that of carbon dioxide over 100 years. Over a 20-year period, it reaches 82.5 ± 25.8 times the impact of CO2. Annual global emissions currently stand at approximately 580 megatonnes. Natural sources contribute 40% while human activity accounts for 60%. Agriculture represents one quarter of anthropogenic emissions. The energy sector follows closely behind. Atmospheric concentration has increased by about 160% since preindustrial levels in the mid-18th century. Levels reached their highest point in 2019 compared to the last 800,000 years. Between 2011 and 2019, the annual average increase was 1866 parts per billion. Sharp rises occurred from 2015 to 2019. Fugitive emissions from the fossil fuel industry may have been significantly underestimated. A report published in May 2023 identified Turkmenistan as the world's worst methane super emitter. Two large fields leaked 2.6 million and 1.8 million metric tonnes in 2022 alone. This surpassed the annual CO2 emissions of the United Kingdom. Over 100 countries signed the Global Methane Pledge in 2021. They promised to cut emissions by 30% by 2030.
Methane exists abundantly throughout many parts of the Solar System. It appears on all planets and most larger moons. The Curiosity rover documented seasonal fluctuations of atmospheric methane on Mars. These peaks reached 0.6 parts per billion at the end of the Martian summer. Scientists propose synthesizing methane on Mars for future missions. In situ resource utilization could produce it using water from subsoil and carbon dioxide from the atmosphere. Titan, Saturn's largest moon, holds vast amounts of liquid methane. It comprises a significant portion of Titan's atmosphere. Liquid methane forms the majority of hydrocarbon lakes on its surface. One lake is believed to be almost pure methane. Stable lakes exist alongside highly chemically active surfaces rich in organic compounds. Researchers consider life possibilities within these lakes. Such organisms might use methane as a solvent instead of water. They could derive energy with hydrogen and acetylene. Negative methane ions also exist in interstellar space. Their formation mechanism remains partially understood. Harvesting methane on another body provides fuel for return journeys.
Italian physicist Alessandro Volta identified methane in November 1776. He investigated bubbles rising from marshes near Lake Maggiore. This location straddles Italy and Switzerland. Volta collected the gas and demonstrated its flammability. His friend Father Carlo Giuseppe Campi had reported inflammable air present in marshes previously. Benjamin Franklin wrote letters about similar observations earlier. Sir Humphry Davy established that firedamp was largely methane following the Felling mine disaster of 1812. Ninety-two men perished in that tragedy. The name methane originated in 1866. German chemist August Wilhelm von Hofmann coined it. The term derives from methanol. Etymologically, the suffix -ane denotes substances belonging to the alkane family. Methyl comes from the Greek word for wine or mead. Jean-Baptiste Dumas and Eugène Péligot created the root in 1834. It relates to wood distillation which first isolated methanol. The coordination of chemical suffixes like -ane, -ene, and -one was proposed by Hofmann in 1866.
Common questions
What is the chemical formula and molecular structure of methane?
Methane has the chemical formula CH4. It consists of one carbon atom bonded to four hydrogen atoms in a tetrahedral shape with four equivalent bonds.
When was methane first identified by Alessandro Volta?
Italian physicist Alessandro Volta identified methane in November 1776. He investigated bubbles rising from marshes near Lake Maggiore which straddles Italy and Switzerland.
How much does methane contribute to global temperature rise since the industrial revolution?
Methane acts as a greenhouse gas responsible for around 30% of global temperature rise since the industrial revolution. Its global warming potential equals 29.8 ± 11 times that of carbon dioxide over 100 years.
Where are natural sources of methane found below ground and under the seafloor?
Naturally occurring methane resides below ground and under the seafloor where it forms through geological and biological processes. Subseafloor sediments are generally anoxic and contain methanogens that produce methane trapped in gas hydrates along continental margins within the gas clathrate stability zone.
Who coined the name methane and when did this occur?
The name methane originated in 1866. German chemist August Wilhelm von Hofmann coined it based on the term methanol with the suffix -ane denoting substances belonging to the alkane family.