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The reported presence of
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Eart ...
in the
atmosphere of Mars The atmosphere of Mars is the layer of gases surrounding Mars. It is primarily composed of carbon dioxide (95%), molecular nitrogen (2.8%), and argon (2%). It also contains trace levels of water vapor, oxygen, carbon monoxide, hydrogen, and no ...
is of interest to many geologists and astrobiologists, as methane may indicate the presence of microbial
life on Mars The possibility of life on Mars is a subject of interest in astrobiology due to the planet's proximity and similarities to Earth. To date, no proof of past or present life has been found on Mars. Cumulative evidence suggests that during the ...
, or a
geochemical Geochemistry is the science that uses the tools and principles of chemistry to explain the mechanisms behind major geological systems such as the Earth's crust and its oceans. The realm of geochemistry extends beyond the Earth, encompassing the e ...
process such as
volcanism Volcanism, vulcanism or volcanicity is the phenomenon of eruption of molten rock (magma) onto the surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics, and volcanic gases erupt through a break in the surface called ...
or
hydrothermal activity Hydrothermal circulation in its most general sense is the circulation of hot water (Ancient Greek ὕδωρ, ''water'',Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with th ...
. Since 2004, trace amounts of methane (range from 60 ppbv to under detection limit (< 0.05 ppbv)) have been reported in various missions and observational studies. The source of methane on Mars and the explanation for the enormous discrepancy in the observed methane concentrations are still unknown and are under study. Whenever methane is detected, it is rapidly removed from the atmosphere by an efficient, yet unknown process.


History of detections

Methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Eart ...
(CH4) is chemically unstable in the current oxidizing atmosphere of Mars. It would quickly break down due to
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...
(UV) radiation from the Sun and chemical reactions with other gases. Therefore, a persistent or episodic presence of methane in the atmosphere may imply the existence of a source to continually replenish the gas. The first evidence of methane in the atmosphere was measured by ESA's ''
Mars Express ''Mars Express'' is a space exploration mission being conducted by the European Space Agency (ESA). The ''Mars Express'' mission is exploring the planet Mars, and is the first planetary mission attempted by the agency. "Express" originally ref ...
'' orbiter with an instrument called the Planetary Fourier Spectrometer.Is Mars' Methane Spike a Sign of Life? Here's How We'll Know.
Daniel Oberhaus, ''Wired''. 24 June 2019.
In March 2004, the ''Mars Express'' science team suggested the presence of methane in the atmosphere at a concentration of about 10 ppbv. This was confirmed soon after by three ground-based telescope teams, although large differences in the abundances were measured between observations taken in 2003 and 2006. This spatial and temporal variability of the gas suggests that the methane was locally concentrated and probably seasonal. It is estimated that Mars produces 270 tons of methane per year. In 2011, NASA scientists reported a comprehensive search using high-resolution infrared spectroscopy from high-altitude Earth ground-based observatories (VLT, Keck-2, NASA-IRTF) for trace species (including methane) on Mars, deriving sensitive upper limits for methane (< 7 ppbv), ethane (< 0.2 ppbv), methanol (< 19 ppbv) and others ( H2CO, C2H2, C2H4, N2O, NH3, HCN, CH3Cl, HCl, HO2 – all with limits at ppbv levels). In August 2012, the ''Curiosity'' rover landed on Mars. The rover's instruments are capable of making precise abundance measurements, but cannot be used to distinguish between different
isotopologues In chemistry, isotopologues are molecules that differ only in their isotopic composition. They have the same chemical formula and bonding arrangement of atoms, but at least one atom has a different number of neutrons than the parent. An exampl ...
of methane and so it cannot determine if it is geophysical or biological in origin. However, the
Trace Gas Orbiter The ExoMars Trace Gas Orbiter (TGO or ExoMars Orbiter) is a collaborative project between the European Space Agency (ESA) and the Russian Roscosmos agency that sent an atmospheric research orbiter and the ''Schiaparelli'' demonstration lander ...
(TGO) can measure these ratios and point to their origin. The first measurements with ''Curiosity'' Tunable Laser Spectrometer (TLS) in 2012 indicated that there was no methane —or less than 5 ppb— at the landing site, later calculated to a baseline of 0.3 to 0.7 ppbv. In 2013, NASA scientists again reported no detection of methane beyond a baseline. But in 2014, NASA reported that the ''Curiosity'' rover detected a tenfold increase ('spike') in methane in the atmosphere around it in late 2013 and early 2014. Four measurements taken over two months in this period averaged 7.2 ppbv, implying that Mars is episodically producing or releasing methane from an unknown source. Before and after, readings averaged around one-tenth that level. On 7 June 2018, NASA announced the confirmation of a cyclical seasonal variation in the background level of atmospheric methane. The largest concentration of methane detected ''in situ'' by the ''Curiosity'' rover showed a spike to 21 ppbv, during an event in late June 2019. The ''Mars Express'' orbiter happened to be performing spot tracking in that area 20 hours before ''Curiosity'' methane detection, as well as 24 and 48 hours after the detection, and the TGO was performing atmospheric observations at around the same time but at a higher latitude. The Indian
Mars Orbiter Mission The Mars Orbiter Mission (MOM), also called ''Mangalyaan'', was a space probe orbiting Mars since 24 September 2014. It was launched on 5 November 2013 by the Indian Space Research Organisation (ISRO). It was India's first interplanetary missi ...
, which entered orbit around Mars on 24 September 2014, is equipped with a
Fabry–Pérot interferometer In optics, a Fabry–Pérot interferometer (FPI) or etalon is an optical cavity made from two parallel reflecting surfaces (i.e.: thin mirrors). Optical waves can pass through the optical cavity only when they are in resonance with it. It is ...
to measure atmospheric methane, but after entering Mars orbit it was determined that it was not capable of detecting methane,India's Mars Orbiter Mission Has a Methane Problem
Irene Klotz, ''Seeker'', 7 December 2016.
so the instrument was repurposed as an albedo mapper. As of April 2019, the TGO showed that the concentration of methane is under the detectable level (< 0.05 ppbv). The ''Perseverance'' rover (landed Feb 2021) and the ''Rosalind Franklin'' rover (due 2023) will not be equipped to analyze the atmospheric methane nor its isotopes, so the proposed
Mars sample-return mission A Mars sample-return (MSR) mission is a proposed mission to collect rock and dust samples on Mars and return them to Earth. Such a mission would allow more extensive analysis than that allowed by onboard sensors. The three most recent concept ...
in the mid-2030s seems the earliest a sample could be analyzed to differentiate a geological from a biological origin.


Potential sources


Geophysical

The principal candidates for the origin of Mars' methane include non-biological processes such as
water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...
-rock reactions,
radiolysis Radiolysis is the dissociation of molecules by ionizing radiation. It is the cleavage of one or several chemical bonds resulting from exposure to high-energy flux. The radiation in this context is associated with ionizing radiation; radiolysis is ...
of water, and
pyrite The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Iron, FeSulfur, S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic Luster (mineralogy), lust ...
formation, all of which produce H2 that could then generate methane and other hydrocarbons via Fischer–Tropsch synthesis with CO and CO2. It has also been shown that methane could be produced by a process involving water, carbon dioxide, and the mineral
olivine The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickl ...
, which is known to be common on Mars. The required conditions for this reaction (i.e. high temperature and pressure) do not exist on the surface but may exist within the crust. Detection of the mineral by-product
serpentinite Serpentinite is a rock composed predominantly of one or more serpentine group minerals, the name originating from the similarity of the texture of the rock to that of the skin of a snake. Serpentinite has been called ''serpentine'' or ''ser ...
would suggest that this process is occurring. An analog on Earth suggests that low-temperature production and exhalation of methane from serpentinized rocks may be possible on Mars. Another possible geophysical source could be ancient methane trapped in
clathrate hydrates Clathrate hydrates, or gas hydrates, clathrates, hydrates, etc., are crystalline water-based solids physically resembling ice, in which small non-polar molecules (typically gases) or polar molecules with large hydrophobic moieties are trapped ins ...
that may be released occasionally. Under the assumption of a cold early Mars environment, a
cryosphere ] The cryosphere (from the Ancient Greek, Greek ''kryos'', "cold", "frost" or "ice" and ''sphaira'', "globe, ball") is an all-encompassing term for those portions of Earth's surface where water is in solid form, including sea ice, lake ice, ri ...
could trap such methane as clathrates in a stable form at depth, which might exhibit sporadic release. On modern Earth, volcanism is a minor source of methane emission, and it is usually accompanied by sulfur dioxide gases. However, several studies of trace gases in the Martian atmosphere have found no evidence for sulfur dioxide in the Martian atmosphere, which makes
volcanism on Mars Volcanic activity, or volcanism, has played a significant role in the geologic evolution of Mars. Scientists have known since the Mariner 9 mission in 1972 that volcanic features cover large portions of the Martian surface. These features in ...
unlikely to be the source of methane. Although geologic sources of methane such as
serpentinization Serpentinization is a hydration and Metamorphic rock, metamorphic transformation of ferromagnesian minerals, such as olivine and pyroxene, in mafic and ultramafic rock to produce serpentinite. Minerals formed by serpentinization include the Serp ...
are possible, the lack of current
volcanism Volcanism, vulcanism or volcanicity is the phenomenon of eruption of molten rock (magma) onto the surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics, and volcanic gases erupt through a break in the surface called ...
,
hydrothermal activity Hydrothermal circulation in its most general sense is the circulation of hot water (Ancient Greek ὕδωρ, ''water'',Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with th ...
or
hotspots Hotspot, Hot Spot or Hot spot may refer to: Places * Hot Spot, Kentucky, a community in the United States Arts, entertainment, and media Fictional entities * Hot Spot (comics), a name for the DC Comics character Isaiah Crockett * Hot Spot (Tra ...
is not favorable for geologic methane. It had also been proposed that the methane might be replenished by meteorites entering the atmosphere of Mars, but researchers from the
Imperial College London Imperial College London (legally Imperial College of Science, Technology and Medicine) is a public research university in London, United Kingdom. Its history began with Prince Albert, consort of Queen Victoria, who developed his vision for a cu ...
found that the volumes of methane released this way are too low to sustain the measured levels of the gas. It has been suggested that the methane was produced by chemical reactions in meteorites, driven by the intense heat during entry through the atmosphere. Although research published in December 2009 ruled out this possibility, research published in 2012 suggested that a source may be
organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. The ...
s on meteorites that are converted to methane by
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...
radiation. Lab tests have demonstrated that bursts of methane can be produced when an electrical discharge interacts with water ice and CO2. The discharges from the electrification of dust particles from sand storms and dust devils in contact with permafrost ice may produce about 1.41×1016 molecules of methane per joule of applied energy. Current
photochemical Photochemistry is the branch of chemistry concerned with the chemical effects of light. Generally, this term is used to describe a chemical reaction caused by absorption of ultraviolet (wavelength from 100 to 400  nm), visible light (400–7 ...
models cannot explain the apparent rapid variability of the methane levels on Mars. Research suggests that the implied methane destruction lifetime is as long as ≈ 4 Earth years and as short as ≈ 0.6 Earth years. This unexplained fast destruction rate also suggests a very active replenishing source. A team from the Italian
National Institute for Astrophysics The National Institute for Astrophysics ( it, Istituto Nazionale di Astrofisica, or INAF) is an Italian research institute in astronomy and astrophysics, founded in 1999. INAF funds and operates twenty separate research facilities, which in turn e ...
suspects that the methane detected by the ''Curiosity'' rover may have been released from a nearby area called
Medusae Fossae Formation The Medusae Fossae Formation is a large geological formation of probable volcanic origin on the planet Mars. It is named for the Medusa of Greek mythology. "Fossae" is Latin for "trenches". The formation is a collection of soft, easily eroded de ...
located about 500 km east of Gale crater. The region is fractured and is likely volcanic in origin.


Biogenic

Living
microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...
s, such as
methanogen Methanogens are microorganisms that produce methane as a metabolic byproduct in hypoxic conditions. They are prokaryotic and belong to the domain Archaea. All known methanogens are members of the archaeal phylum Euryarchaeota. Methanogens are com ...
s, are another possible source, but no evidence for the presence of such organisms has been found on Mars. In Earth's oceans, biological methane production tends to be accompanied by
ethane Ethane ( , ) is an organic chemical compound with chemical formula . At standard temperature and pressure, ethane is a colorless, odorless gas. Like many hydrocarbons, ethane is isolated on an industrial scale from natural gas and as a petr ...
() generation. The long-term ground-based spectroscopic observation did not find these organic molecules in the Martian atmosphere. Given the expected long lifetimes for some of these molecules, emission of biogenic organics seems to be extremely rare or currently non-existent. The reduction of
carbon dioxide Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...
into methane by reaction with
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...
can be expressed as follows: :CO2 + 4 H2 -> CH4 + 2 H2O (∆G˚' = -134 kJ/mol CH4) This reaction of CO2 with the hydrogen to produce methane is coupled with the generation of an
electrochemical gradient An electrochemical gradient is a gradient of electrochemical potential, usually for an ion that can move across a membrane. The gradient consists of two parts, the chemical gradient, or difference in solute concentration across a membrane, and th ...
across the
cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment ( ...
, which is used to generate ATP through
chemiosmosis Chemiosmosis is the movement of ions across a semipermeable membrane bound structure, down their electrochemical gradient. An important example is the formation of adenosine triphosphate (ATP) by the movement of hydrogen ions (H+) across a membra ...
. In contrast,
plant Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclud ...
s and
algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular mic ...
obtain their energy from
sunlight Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared, visible, and ultraviolet light. On Earth, sunlight is scattered and filtered through Earth's atmosphere, and is obvious as daylight when t ...
or nutrients. Measuring the ratio of
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...
and methane levels on Mars may help determine the likelihood of
life on Mars The possibility of life on Mars is a subject of interest in astrobiology due to the planet's proximity and similarities to Earth. To date, no proof of past or present life has been found on Mars. Cumulative evidence suggests that during the ...
. A low H2/CH4 ratio in the atmosphere (less than approximately 40) may indicate that a large part of atmospheric methane could be attributed to biological activities, but the observed ratios in the lower Martian atmosphere were "approximately 10 times" higher "suggesting that biological processes may not be responsible for the observed CH4". Since the 2003 discovery of methane in the atmosphere, some scientists have been designing models and ''in vitro'' experiments testing the growth of
methanogen Methanogens are microorganisms that produce methane as a metabolic byproduct in hypoxic conditions. They are prokaryotic and belong to the domain Archaea. All known methanogens are members of the archaeal phylum Euryarchaeota. Methanogens are com ...
ic bacteria on simulated Martian soil, where all four methanogen strains tested produced substantial levels of methane, even in the presence of 1.0 wt%
perchlorate A perchlorate is a chemical compound containing the perchlorate ion, . The majority of perchlorates are commercially produced salts. They are mainly used as oxidizers for pyrotechnic devices and to control static electricity in food packaging. Per ...
salt. Methanogens do not require oxygen or organic nutrients, are non-photosynthetic, use hydrogen as their energy source, and carbon dioxide (CO2) as their carbon source, so they could exist in subsurface environments on Mars. If microscopic Martian life is producing the methane, it probably resides far below the surface, where it is still warm enough for
liquid water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a s ...
to exist. Research at the University of Arkansas published in 2015 suggested that some methanogens could survive on Mars' low pressure in an environment similar to a subsurface liquid aquifer on Mars. The four species tested were ''
Methanothermobacter In taxonomy, ''Methanothermobacter'' is a genus of the Methanobacteriaceae.See the NCBIbr>webpage on Methanothermobacter Data extracted from the The species within this genus are thermophilic and grow best at temperatures between 55°C and 6 ...
wolfeii'', ''
Methanosarcina ''Methanosarcina'' is a genus of euryarchaeote archaea that produce methane. These single-celled organisms are known as anaerobic methanogens that produce methane using all three metabolic pathways for methanogenesis. They live in diverse envi ...
barkeri'', ''
Methanobacterium In taxonomy, ''Methanobacterium'' is a genus of the Methanobacteriaceae family of Archaea.See the NCBIbr>webpage on Methanobacterium Data extracted from the Despite the name, this genus belongs not to the bacterial domain but the archaeal doma ...
formicicum'', and ''
Methanococcus maripaludis ''Methanococcus maripaludis'' is a species of methanogen. It is anaerobic, weakly motile, non-spore-forming, Gram-negative Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram staining method of ba ...
''. A team led by
Gilbert Levin Gilbert Victor Levin (April 23, 1924 – July 26, 2021) was an American engineer, the founder of Biospherics and the principal investigator of the ''Viking'' mission Labeled Release experiment. He was born in Baltimore, Maryland. In 1997, Levin ...
suggested that both phenomena—methane production and degradation—could be accounted for by an ecology of methane-producing and methane-consuming microorganisms. Even if rover missions determine that microscopic Martian life is the seasonal source of the methane, the life forms probably reside far below the surface, outside of the rover's reach.


Potential sinks

It was initially thought that methane is chemically unstable in an oxidizing atmosphere with UV radiation and so its lifetime in the Martian atmosphere should be about 400 years, but in 2014, it was concluded that the strong methane sinks are not subject to atmospheric oxidation, suggesting an efficient physical-chemical process at the surface that "consumes" methane, generically called a "sink". A hypothesis postulates that the methane is not consumed at all, but rather condenses and evaporates seasonally from
clathrates A clathrate is a chemical substance consisting of a lattice that traps or contains molecules. The word ''clathrate'' is derived from the Latin (), meaning ‘with bars, latticed’. Most clathrate compounds are polymeric and completely envelop t ...
. Another hypothesis is that methane reacts with tumbling surface sand quartz (silicon dioxide ) and
olivine The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickl ...
to form covalent Si –  bonds. The researchers showed that these solids can be oxidized and gases are ionized during the erosion processes. Thus, the ionized methane reacts with the mineral surfaces and bonds to them.Saltation may be contributory in the depletion of methane on Mars.
Per Nørnberg, Jan Thøgersen, Ebbe Nordskov Bak, Kai Finster, Hans Jørgen Jacobsen, and Svend J. Knak Jensen. Geophysical Research Abstracts. Vol. 21, EGU2019-13986, 2019. EGU General Assembly 2019.


Images

File:PIA16463-MarsVolatiles-20121102.jpg, Volatile gases on Mars. File:PIA19087-MarsCuriosityRover-GaleCrater-MethaneChart-20141216.png,
Methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Eart ...
measurements in the atmosphere of Mars by the ''Curiosity'' rover.


See also

*
Atmosphere of Mars The atmosphere of Mars is the layer of gases surrounding Mars. It is primarily composed of carbon dioxide (95%), molecular nitrogen (2.8%), and argon (2%). It also contains trace levels of water vapor, oxygen, carbon monoxide, hydrogen, and no ...
*
Climate of Mars The climate of Mars has been a topic of scientific curiosity for centuries, in part because it is the only terrestrial planet whose surface can be directly observed in detail from the Earth with help from a telescope. Although Mars is smaller t ...
*
Life on Mars The possibility of life on Mars is a subject of interest in astrobiology due to the planet's proximity and similarities to Earth. To date, no proof of past or present life has been found on Mars. Cumulative evidence suggests that during the ...
* Weather of Mars


References

{{Astrobiology Atmosphere of Mars Mars Astrobiology