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The atmosphere of Mars is the layer of gas surrounding Mars. It is mostly composed of carbon dioxide (94.9%), molecular nitrogen (2.6%) and argon (1.9%). It also contains trace levels of water vapor, oxygen, carbon monoxide, hydrogen and other noble gases. The atmosphere of Mars is much thinner than Earth's. The surface pressure is only about 610 Pascal (0.088 psi; 6.1 mbar), which is less than 1 % of the Earth's value. The currently thin Martian atmosphere prohibits the existence of liquid water at the surface of Mars, but many studies suggested that the Martian atmosphere had been much thicker in the past. The atmosphere of Mars has been losing mass to space throughout history, and the leakage of gases still continues today.

The atmosphere of Mars is colder the Earth's. Owing to the larger distance from Sun, Mars receives less solar energy and has a lower effective temperature (about 210 K). The average surface emission temperature of Mars is just 215 K, which is comparable to inland Antarctica. The weaker greenhouse effect in the Martian atmosphere (5 °C, versus 33 °C on Earth) can be explained by the low abundance of other greenhouse gases. The daily range of temperature in the lower atmosphere is huge (can exceeds 100°C near the surface in some region) due to the low thermal inertia. The temperature of the upper part of the Martian atmosphere is also significantly lower than Earth's because of the absence of stratospheric ozone and the radiative cooling effect of carbon dioxide at higher altitudes.

Dust devils and dust storms are prevalent on Mars, which are sometimes observable by the telescopes on Earth. Planet-encircling dust storms (global dust storms) occur on average every 5.5 earth years on Mars and can threaten the operation of Mars rovers. However, the mechanism responsible for the development of large dust storms is still not well understood.

Martian atmosphere is an oxidizing atmosphere. The photochemical reactions in the atmosphere tend to oxidize the organic species and turn them into inorganic carbon dioxide or carbon monoxide. Several missions detected unexpected levels of methane in the Martian atmosphere, which may be a biosignature for the life on Mars. However, the interpretation of the measurements is still highly controversial and lacks a scientific consensus.

History of observation of Martian atmosphere
In 1784, German-born British astronomer William Herschel published an article about his observations of Martian atmosphere in Philosophical Transactions and mentioned about the occasional movement of the brighter region on Mars, which he attributed to clouds and vapors. In 1809, French astronomer Honoré Flaugergues wrote about his observation of "yellow clouds" on Mars, which is believed to be dust events. In 1864, William Rutter Dawes observed that "the ruddy tint of the planet does not arise from any peculiarity of its atmosphere seems to be fully proved by the fact that the redness is always deepest near the centre, where the atmosphere is thinnest." Spectroscopic observations in the 1860s and 1870s led many to think the atmosphere of Mars is similar to Earth's. In 1894, though, spectral analysis and other qualitative observations by William Wallace Campbell suggested Mars resembles the Moon, which has no appreciable atmosphere, in many respects. In 1926, photographic observations by William Hammond Wright at the Lick Observatory allowed Donald Howard Menzel to discover quantitative evidence of Mars's atmosphere.

With an enhanced understanding of optical properties of atmospheric gases and advancement in spectrometer technology, scientists started to measure the composition of Martian atmosphere in the mid-20th Century. Lewis David Kaplan and his team detected the signals of water vapor and carbon dioxide in the spectrogram of Mars in 1964, as well as carbon monoxide in 1969. In 1965, the measurements made during Mariner 4' s flyby confirmed that the Martian atmosphere is constituted mostly of carbon dioxide, and the surface pressure is about 400 to 700 Pa.

In the 1970s, landers of the Viking program provided the first ever in-situ measurements of the Martian atmosphere. Since then, many orbiters and landers have been sent to Mars to study various processes in Martian atmosphere.

Carbon dioxide
CO2 is the main component of Martian atmosphere. It has a mean volume ratio of 94.9%. In winter polar regions, the surface temperature can be lower than the frost point of CO2. CO2 gas in the atmosphere can condense on the surface to form 1-2 m thick solid dry ice. In summer, the polar dry ice cap can melt and release the CO2 back to the atmosphere. As a result, significant annual variability in atmospheric pressure (~25%) and atmospheric composition can be observed on Mars. The condensation process can be approximated by the Clausius–Clapeyron relation for CO2.

Despite of the high concentration of CO2 in Martian atmosphere, greenhouse effect is relatively weak on Mars (about 5 °C) because of the low concentration of water vapor and low atmospheric pressure. While water vapor in Earth's atmosphere has the largest contribution to greenhouse effect on modern Earth, it is present in only very low concentration in Martian atmosphere. Moreover, under low atmospheric pressure, greenhouse gases cannot absorb infrared radiation effectively because the pressure-broadening effect is weak.

In the presence of solar UV radiation, CO2 in Martian atmosphere can be photolyzed via the following reaction:

If there is no chemical production of CO2, all the CO2 in the current Martian atmosphere would be removed by photolysis in about 3500 years. The hydroxyl radicals (OH) produced from the photolysis of water vapor, together with the other odd hydrogen species (e.g. H, HO2), can convert carbon monoxide (CO) back to CO2. The reaction cycle can be described as:

Mixing also plays a role in regenerating CO2 by bringing the O, CO and O2 in the upper atmosphere downward. The balance between photolysis and redox production keeps the average concentration of CO2 stable in the modern Martian atmosphere.

CO2 ice clouds can form in winter polar regions and very high altitude (>50km) in tropical regions, where the air temperature is lower than the frost point of CO2.

Nitrogen
N2 is the second most abundant gas in Martian atmosphere. It has a mean volume ratio of 2.6%. Various measurements showed that Martian atmosphere is enriched in 15N. The enrichment of heavy isotope of nitrogen is possibly caused by mass-selective escape processes.

Argon
Argon is the third most abundant gas in the Martian atmosphere. It has a mean volume ratio of 1.9%. In term of stable isotopes, Mars is enriched in 38Ar relative to 36Ar, which can be attributed to hydrodynamic escape.

Argon has a radiogenic isotope 40Ar, which is produced from the radioactive decay of 40K. In contrast, 36Ar is primordial and was acquired by Martian Atmosphere during the formation of Mars. Observation suggested that Mars is enriched in 40Ar relative to 36Ar, which cannot be attributed to mass-selective loss processes. A possible explanation for the enrichment is that significant amount of primordial atmosphere, including 36Ar, was lost by impact erosion in the early history of Mars, while 40Ar was emitted to the atmosphere after the impact.

Oxygen and ozone
The estimated mean volume ratio of molecular oxygen (O2) in Martian atmosphere is 0.174 %. It is one of the products of the photolysis of CO2, water vapor and ozone. It can react with atomic oxygen (O) to re-form ozone (O3). In 2010, the Herschel Space Observatory detected molecular oxygen in Martian atmosphere.

Atomic oxygen is produced by photolysis of CO2 in the upper atmosphere and can escape the atmosphere via dissociative recombination or ion pickup. In early 2016, Stratospheric Observatory for Infrared Astronomy (SOFIA) detected atomic oxygen in the atmosphere of Mars, which has not been found since the Viking and Mariner mission in the 1970s.

Similar to stratospheric ozone in Earth's atmosphere, the ozone in Martian atmosphere can be destroyed by catalytic cycles involving odd hydrogen species:

Since water is an important source of these old hydrogen species, higher abundance of ozone is usually observed in the regions with lower water vapor content. Measurements showed that the total column of ozone can reach 2-30 µm-atm around the poles in winter and spring, where the air is cold and has low water saturation ratio. The actual reactions between ozone and odd hydrogen species may be further complicated by the heterogeneous reactions that take place in water-ice clouds.

It is believed that the vertical distribution and seasonality of ozone in Martian atmosphere is driven by the complex interactions between chemistry and transport. The UV/IR spectrometer on Mars Express (SPICAM) has shown the presence of two distinct ozone layers at low-to-mid latitudes. These comprise a persistent, near-surface layer below an altitude of 30 km, a separate layer that is only present in northern spring and summer with an altitude varying from 30 to 60 km, and another separate layer that exists 40–60 km above the southern pole in winter, with no counterpart above the Mars's north pole. This third ozone layer shows an abrupt decrease in elevation between 75 and 50 degrees south. SPICAM detected a gradual increase in ozone concentration at 50 km until midwinter, after which it slowly decreased to very low concentrations, with no layer detectable above 35 km.

Water vapor
Water vapor is a trace gas in Martian atmosphere and has huge spatial, diurnal and seasonal variability. Measurements made by Viking orbiter in the late 1970s suggested that the entire global total mass of water vapor is equivalent to about 1 to 2 km3 of ice. More recent measurements on Mars Express showed that the globally annually-averaged column abundance of water vapor is about 10-20 precipitable microns (pr. µm). Maximum abundance of water vapor (50-70 pr. µm) is found in the northern polar regions in early summer due to the sublimation of water ice in the polar cap.

Different to Earth's atmosphere, liquid-water cloud cannot exist in Martian atmosphere because of the low atmospheric pressure. Cirrus-like water-ice clouds have been captured by the cameras on Opportunity and Phoenix Rovers. Measurements made by Phoenix rover showed that water-ice clouds can form at the top of the planetary boundary layer at night and precipitate back to the surface as ice crystals in the northern polar region.

Dust
Under sufficiently strong wind (> 30 ms-1), dust particles can be mobilized and emitted from land surface to air. Some of the dust particles can suspend in the atmosphere and travel by circulation before depositing back to the ground. Dust particles can attenuate solar radiation and interacts with infrared radiation, which can lead to significant radiative effect on Mars. Orbiter measurements suggest that the globally-averaged dust optical depth has a background level of 0.15 and peaks in the perihelion season (southern spring and summer). The local abundance of dust varies greatly by seasons and years. During global dust events, Mars rovers can observe optical depth that is over 4. Surface measurements also showed the effective radius of dust particles ranges from 0.6 μm to 2 μm and has considerable seasonality.

Dust has an uneven vertical distribution on Mars. Apart from the planetary boundary layer, sounding data showed that there are other peaks of dust mixing ratio at the higher altitude (e.g. 15-30 km above the surface).

Methane
As a volcanic and biogenic species, methane is of interest to many geologists and astrobiologists. However, methane is chemically unstable in an oxidizing atmosphere with UV radiation. The lifetime of methane in Martian atmosphere is about 400 years. The detection of methane in a planetary atmosphere may indicate the presence of recent geological activities or living organisms. Since 2004, trace amounts of methane (range from 60 ppb to under detection limit (0.05 ppb)) 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 under active debate.

See also the section "detection of methane in the atmosphere" for more details.

Sulfur dioxide
Sulfur dioxide (SO2) in the atmosphere is thought to be a tracer of current volcanic activity. It has become especially interesting due to the long-standing controversy of methane on Mars. If volcanoes have been active in recent Martian history, we would expect to find SO2 together with methane in the modern Martian atmosphere. No SO2 was detected in these studies, but they were able to place stringent upper limits on the atmospheric concentration of 0.2 ppb. However, a team led by scientists at NASA Goddard Space Flight Center reported detection of SO2 in Rocknest soil samples analyzed by the Curiosity rover in March 2013.

Other trace gases
Carbon monoxide (CO) is produced by the photolysis of CO2 and quickly reacts with the oxidants in Martian atmosphere to re-form CO2. The estimated mean volume ratio of CO in Martian atmosphere is 0.0747 %.

Noble gases other than helium are present at trace level (~ 10 - 0.01 ppmv) in Martian atmosphere. The concentration of Helium, neon, krypton and xenon in Martian atmosphere has been measured in different missions. The isotopic ratios of noble gases reveal information about the early geological activities on Mars and the evolution of its atmosphere.

Molecular hydrogen (H2) is produced by the reaction between odd hydrogen species in the middle atmosphere. It can be delivered to the upper atmosphere by mixing or diffusion, decompose to atomic hydrogen (H) by solar radiation and escape the Martian atmosphere. Photochemical model estimated that the mixing ratio of H2 in the lower atmosphere is about 15±5 ppmv.

Vertical structure
The vertical temperature structure of the Martian atmosphere differs from Earth's atmosphere in many ways. Information about the vertical structure is usually inferred by using the observations from thermal infrared soundings, radio occultation, aerobraking, landers' entry profiles. Mars's atmosphere can be classified into three layers according to the average temperature profile:


 * Troposphere ( ~0-40 km): The layer where most of the weather phenomena (e.g. convection and dust storms) take place. Mars has a higher scale height of 11.1 km than Earth (8.5 km) because of its weaker gravity. Hence, the troposphere can extend further upward. Its dynamics is heavily driven by the daytime surface heating and the amount of suspended dust. The theoretical dry adiabatic lapse rate of Mars is 4.3 °C km-1, but the measured average lapse rate is about 2.5 °C km-1 because the suspended dust particles absorb solar radiation and heat the air . The planetary boundary layer can extend to over 10 km thick during the daytime. The near-surface diurnal temperature range is huge (60 °C ) due to the low thermal inertia. Under dusty conditions, the suspended dust particles can reduce the surface diurnal temperature range to only 5 °C. The temperature above 15 km is controlled by radiative processes instead of convection. Mars is also a rare exception to the "0.1-bar tropopause" rule found in the other atmospheres in solar system.
 * Mesosphere ( ~40-100 km): The layer that has the lowest temperature. CO2 in the mesosphere acts as a cooling agent by efficiently radiating heat into space. Stellar occultation observations show that the mesopause of Mars locates at about 100 km (around 0.01 to 0.001 Pa level) and has a temperature of 100-120 K. The temperature can sometimes be lower than the frost point of CO2, and detections of CO2 ice clouds in the Martian mesosphere have been reported.
 * Thermosphere ( ~100-200 km): The layer is mainly controlled by extreme UV heating. The temperature in Martian thermosphere increases with altitude and varies by season. The daytime temperature of the upper thermosphere ranges from 175 K (at aphelion) to 240 K (at perihelion) and can reach up to 390 K, but it is still significantly lower than the temperature of Earth's thermosphere. The higher concentration of CO2 in Martian thermosphere may explain part of the discrepancy because of the cooling effects of CO2 in high altitude. It is believed that auroral heating processes is not important in Martian thermosphere because of the absence of a strong magnetic field in Mars, but MAVEN orbiter has detected several aurora events.

Mars does not have a persistent stratosphere due to the lack of shortwave-absorbing species in its middle atmosphere (e.g. stratospheric ozone in Earth's atmosphere and organic haze in Jupiter's atmosphere) for creating a temperature inversion. However, a seasonal ozone layer and a strong temperature inversion in the middle atmosphere have been observed over the Martian South Pole. The altitude of the turbopause of Mars varies greatly from 60 to 140 km, and the variability is driven by the CO2 density in the lower thermosphere. Mars also has a complicated ionosphere that interacts with the solar wind particles, extreme UV radiation and X-rays from Sun, and the magnetic field of its crust. The exosphere of Mars starts at about 230 km and gradually merges with interplanetary space.



Dust devils and dust storms
Dust devils are common on Mars. Like their counterparts on Earth, dust devils form when the convective vortices driven by strong surface heating are loaded with dust particles. Dust devils on Mars usually have a diameter of tens of meter and height of several kilometers, which are much taller than the ones observed on Earth. Study of dust devils' tracks showed that most of Martian dust devils occurs at around 60°N and 60°S in spring and summer. They lift about 2.3 × 1011 kg of dust from land surface to atmosphere annually, which is comparable to the contribution from local and regional dust storm.

Local and regional dust storms are not rare on Mars. Local storms have a size of about 103 km2 and occurrence of about 2000 events per Martian year, while regional storms of 106 km2 large are observed frequently in southern spring and summer. Near the polar cap, dust storms sometimes can be generated by frontal activities and extratropical cyclones. Global dust storms (area > 106 km2 ) occur on average once every 5.5 Earth years (3 Martial years). Observations showed that larger dust storms are usually the result of merging smaller dust storms, but the growth mechanism of the storm and the role of atmospheric feedbacks are still not well understood. Model study suggested the growth of dust storm may be caused by Although it is believed that Martian dust can be entrained into the atmosphere by processes similar to Earth's (e.g. saltation), the actual mechanisms are yet to be verified, and electrostatic or magnetic forces may also play in modulating dust emission. Researchers reported that the largest single source of dust on Mars comes from the Medusae Fossae Formation.

On 1 June 2018, NASA scientists detected signs of a dust storm (see image) on Mars which resulted in the end of the solar-powered Opportunity rover's mission since the dust blocked the sunlight (see image) needed to operate. By 12 June, the storm was the most extensive recorded at the surface of the planet, and spanned an area about the size of North America and Russia combined (about a quarter of the planet). By 13 June, Opportunity rover began experiencing serious communication problems due to the dust storm.

Thermal tides
Solar heating on the day side and radiative cooling on night side of a planet can induce pressure difference. Thermal tides, which are the wind circulation and waves driven by such a daily-varying pressure field, can explain a lot of variability of the Martian atmosphere. Compared to Earth's atmosphere, thermal tides have large influence on Martian atmosphere because of the stronger diurnal temperature contrast. The surface pressure measured by Mars rovers showed clear signal of thermal tides, although the variation also depend on the shape of the planet's surface and the amount of suspended dust in the atmosphere. The atmospheric waves can also travel vertically and affect the temperature and water-ice content in the middle atmosphere of Mars.

Orographic clouds
On Earth, mountain ranges sometimes force an air mass to rise and cool down. As a result, water vapor becomes saturated and clouds are formed during the lifting process. On Mars, orbiters have observed seasonally recurrent formation of a huge water-ice clouds around the downwind side of the 20 km-high volcanoes Arsia Mons, which is likely caused by the same mechanism.

Wind modification of surface
On Mars, the near-surface wind is not only emitting dust but also modifying the geomorphology of Mars at different time scale. Although it was thought that the atmosphere of Mars is too thin for mobilizing the sandy features, observation made by HiRSE showed that the migration of dunes is not rare on Mars. The global average migration rate of dunes (2 - 120 m tall) is about 0.5 meter per year. Atmospheric circulation model suggested repeated cycles of wind erosion and dust deposition can possibly lead to a net transport of soil materials from the lowlands to the uplands at geological timescale.

Atmospheric evolution
The mass and composition of Martian atmosphere are thought to have changed over the course of the planet's lifetime. A thicker, warmer and wetter atmosphere is required to explain several apparent features in the earlier history of Mars, such as the existence of liquid water bodies. Observations of Martian upper atmosphere, measurement of isotopic composition and analysis of Martian meteorites provide evidence of the long-term changes in atmosphere and constraints for the relative importance of different processes.

Atmospheric escape on modern Mars
Despite the lower gravity, Jeans escape is not efficient in modern Martian atmosphere due to the relatively low temperature at the exobase (~ 200 K at 200 km altitude). It can only explain the escape of hydrogen from Mars. Other non-thermal processes are needed to explain the observed escape of oxygen, carbon and nitrogen.

Hydrogen escape
Molecular hydrogen (H2) is produced from the dissociation of H2O or other hydrogen-containing compounds in the lower atmosphere and diffuses to the exosphere. The exospheric H2 then decomposes into hydrogen atoms, and the atoms that has sufficient thermal energy can escape from the gravitation of Mars (Jean escape). The escape of hydrogen atom is evident from the UV spectrometers on different orbiters. While most studies suggested that the escape of hydrogen is close to diffusion-limited on Mars, more recent studies suggest that the escape rate is modulated by dust storms and has a large seasonality. The estimated escape flux of hydrogen range from 107 cm-2 s-1 to 109 cm-2 s-1.

Carbon escape
Photochemistry of CO2 and CO in ionosphere can produce CO2+ and CO+ ions, respectively:

An ion and an electron can recombine and produce electronic-neutral products. The products gain extra kinetic energy due to the Coulomb attraction between ions and electrons. This process is called dissociative recombination. Dissociative recombination can produce carbon atoms that travel faster than escape velocity of Mars, and those moving upward can then escape the Martian atmosphere:

UV Photolysis of carbon monoxide is another crucial mechanism for the carbon escape on Mars:

Other potentially important mechanisms include sputtering escape of CO2 and collision of carbon with fast oxygen atoms. The estimated overall escape flux is about 0.6×107 cm-2 s-1 to 2.2×107 cm-2 s-1 and depends heavily on solar activity.

Nitrogen escape
Like carbon, dissociative recombination of N2+ is important for the nitrogen escape on Mars. In addition, other photochemical escape mechanism also play an important role:

Nitrogen escape rate is very sensitive to mass of the atom and solar activity. The overall estimated escape rate of 14N is 4.8 ×105 cm-2 s-1.

Oxygen escape
Dissociative recombination of CO2+ and O2+ (produced from CO2+ reaction as well) can generate the oxygen atoms that travel fast enough to escape:

However, the observations showed that there is not enough fast oxygen atoms in Martian exosphere as predicted by the dissociative recombination mechanism. Model estimations of oxygen escape rate suggested it can be over 10 times lower than the hydrogen escape rate. Ion pick and sputtering have been suggested as the alternative mechanisms for the oxygen escape, but model showed that they are less important than dissociative recombination at present.

Atmosphere in earlier history
In general, the violates found on modern Mars are depleted in lighter stable isotopes, indicating the Martian atmosphere has changed by some mass-selected processes over its history. Scientists often rely on these measurements of isotope composition to reconstruct the conditions of the Martian atmosphere in the past.

While Mars and Earth have similar 12C/13C and 16O/18O, 14N is much more depleted in Martian atmosphere. It is believed that the photochemical escape processes is responsible for the isotopic fractionation and has caused a significant loss of nitrogen in geological timescale. Estimates suggest that the initial partial pressure of N2 may have been up to 30 hPa.

Hydrodynamic escape in the early history of mars may explain the isotopic fractionation of Argon and Xenon. On modern Mars, the atmosphere is not not leaking these two noble gases to the space owing to their heavier masses. However, the higher abundance of hydrogen in The Martian atmosphere and the high fluxes of extreme UV from young Sun together can drive a hydrodynamic outflow and dragged away these heavy gases. Hydrodynamic escape also contributed to the loss of carbon, and model showed that it is possible to lose 1 bar of CO2 by hydrodynamic escape in one to ten million years under much stronger solar extreme UV on Mars. Meanwhile, more recent observations made by MAVEN suggested that sputtering escape is very important for the escape of heavy gases on the nightside of Mars and could have contributed to 65% loss of argon in the history of Mars.

Martian atmosphere is particular prone to impact erosion owing to the low escape velocity of Mars. Early computer model suggested that Mars could have lost 99% of its initial atmosphere by the end of late heavy bombardment period based on a hypothetical bombardment flux estimated from lunar crater density. In term of relative abundance of carbon, the C/84Kr ratio on Mars is only 10% of that on Earth and Venus. If we assume the three rocky planets has the same initial volatile inventory, then this low C/84Kr ratio implies the mass of CO2 in early Martian atmosphere should be ten times higher than the present value. The huge enrichment of radiogenic 40Ar over primordial 36Ar is also consistent with the impact erosion theory.

One of the ways to estimate the amount of water lost by hydrogen escape in the upper atmosphere is to examine the enrichment of deuterium over hydrogen. Isotope-based studies estimate that 12 m to over 30 m of global equivalent layer of water has been lost to space via hydrogen escape in Mars' history. It is noted that atmospheric-escape-based approach only provides the lower limit for the estimated early water inventory.

To explain the coexistence of liquid water and faint young sun in early Mars' history, a much stronger greenhouse effect is needed in Martian atmosphere to warm the surface up above freezing point of water. Carl Sagan first proposed that the a 1 bar H2 atmosphere can produce enough warming for Mars. The hydrogen can be produced by the vigorous outgassing from a highly reduced early martian mantle and the presence of CO2 and water vapor can lower the required abundance of H2 to generate such a greenhouse effect. Nevertheless, photochemical modelling showed that maintaining an atmosphere with this high level of H2 is difficult. SO2 has also been one of the proposed effective greenhouse gases in the early history of Mars. However, more recent studies suggested that high solubility of SO2, efficient formation of H2SO4 aerosol and surface deposition prohibit the long-term build-up of SO2 in Martian atmosphere, and hence reduce the potential warming effect of SO2.

Detection of methane in the atmosphere
Methane (CH4) is chemically unstable in the current oxidizing atmosphere of Mars. It would quickly break down due to ultraviolet radiation from the Sun and chemical reactions with other gases. Therefore, a persistent presence of methane in the atmosphere may imply the existence of a source to continually replenish the gas. Trace amounts of methane, at the level of several parts per billion (ppb), were first reported in Mars's atmosphere by a team at the NASA Goddard Space Flight Center in 2003. In March 2004, the Mars Express Orbiter and ground-based observations by three groups also suggested the presence of methane in the atmosphere at a concentration of about 10 ppb (parts per billion). Large differences in the abundances were measured between observations taken in 2003 and 2006, which suggested that the methane was locally concentrated and probably seasonal.

In 2011, NASA scientists reported a comprehensive search using ground-based high-resolution infrared spectroscopy 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 limits at ppbv levels).

In August 2012, the Curiosity rover landed on Mars. The rover's instruments are capable of making precise abundance measurements, which can be used to distinguish between different isotopologues of methane. The first measurements with Curiosity Tunable Laser Spectrometer (TLS) in 2012 indicated that there was no methane or less than 5 ppb of methane at the landing site,   later calculated to a baseline of 0.3 to 0.7 ppb. On 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 ppb, implying that Mars is episodically producing or releasing methane from an unknown source. Before and after that, readings averaged around one-tenth that level. On 7 June 2018, NASA announced a cyclical seasonal variation in the background level of atmospheric methane. The Indian Mars Orbiter Mission, which entered orbit around Mars on 24 September 2014, is equipped with a Fabry–Pérot interferometer to measure atmospheric methane, but after entering Mars orbit it was determined that it was not capable of detecting methane,  so the instrument was repurposed as an albedo mapper. The latest measurements made by ExoMars Trace Gas Orbiter also showed that the concentration of methane is under detectable level (0.05 ppbv).

The principal candidates for the origin of Mars' methane include non-biological processes such as water-rock reactions, radiolysis of water, and pyrite 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, 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 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 that may be released occasionally. Under the assumption of a cold early Mars environment, a cryosphere could trap such methane as clathrates in a stable form at depth, that 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 unlikely to be the source of methane.

Living microorganisms, such as methanogens, 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. The long-term ground-based spectroscopic observation did not find these organic species in Martian atmosphere. Given the expected long lifetimes for some of these species, emission of biogenic organics seem to be extremely rare or currently non-existent. Measuring the ratio of hydrogen and methane levels on Mars may help determine the likelihood of life on Mars. A low H2/CH4 ratio in the atmosphere (less than approximately 40) may indicate that a large part of atmospheric methane can be attributed to biological activities. 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." The scientists suggested measuring the H2 and CH4 flux at the Martian surface for a more accurate assessment.

It had been proposed that the methane might be replenished by meteorites entering the atmosphere of Mars, but researchers from Imperial College London found that the volumes of methane released this way are too low to sustain the measured levels of the gas. A group of Mexican scientists found that bursts of methane can be produced when a discharge interacts with water ice by performing plasma experiments in a synthetic Mars atmosphere. The discharges from the electrification of dust particles from sand storms and dust devils may produce about 1.41×1016 molecules of methane per joule of applied energy. There are also suggestions that the source of methane is the rover itself.

Current photochemical models cannot explain a apparent rapid variability of the methane levels in 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. In 2014, it was concluded that the presence of strong methane sinks that are not subject to atmospheric oxidation. A possibility is that the methane is not consumed at all, but rather condenses and evaporates seasonally from clathrates. Another possibility is that methane reacts with tumbling surface sand quartz and olivine to form covalent Si– bonds.

Lightning events
In 2009, an Earth-based observational study reported detection of large-scale electric discharge events on Mars and proposed that they are related to lightning discharge in Martian dust storms. However, later observation studies showed that the result is not reproducible using the radar receiver on Mars Express and Earth-based Allen telescope array. A laboratory study showed that the air pressure on Mars is not favorable for charging the dust grains, and thus it is difficult to generate lightning in Martian atmosphere.

Super-rotating jet over the equator
Super-rotation refers to the phenomenon that air mass has a higher angular velocity than the surface of the planet at the equator, which in principle cannot be driven by inviscid axisymmetric circulations. Assimilated data and general circulation model (GCM) simulation suggest that super-rotating jet can be found in Martian atmosphere during global dust storms, but it is much weaker than the ones observed on slow-rotating planets like Venus and Titan. GCM experiments showed that the thermal tides can play a role in inducing the super-rotating jet. Nevertheless, modeling super-rotation still remains as a challenging topic for planetary scientists.