Layer of gases surrounding the planet Mars
The atmosphere of Mars is the layer of gases surrounding Mars. It is primarily composed of carbon dioxide (95%), molecular nitrogen (2.85%), and argon (2%). It also contains trace levels of water vapor, oxygen, carbon monoxide, hydrogen, and noble gases. The atmosphere of Mars is much thinner and colder than Earth's having a max density 20g/m3 (about 2% of Earth’s value) with a temperature generally below zero down to -60 Celsius. The average surface pressure is about which is 0.6% of the Earth's value.
The currently thin Martian atmosphere prohibits the existence of liquid water on the surface of Mars, but many studies suggest that the Martian atmosphere was much thicker in the past. The higher density during spring and fall is reduced by 25% during the winter when carbon dioxide partly freezes at the pole caps. The highest atmospheric density on Mars is equal to the density found above the Earth's surface and is ≈0.020 kg/m3. The atmosphere of Mars has been losing mass to space since the planet's core slowed down, and the leakage of gases still continues today.
The atmosphere of Mars is colder than Earth’s owing to the larger distance from the Sun, receiving less solar energy and has a lower effective temperature, which is about . The average surface emission temperature of Mars is just , which is comparable to inland Antarctica. Although Mars' atmosphere consists primarily of carbon dioxide, the greenhouse effect in the Martian atmosphere is much weaker than Earth's: on Mars, versus on Earth due to the much lower density of carbon dioxide, leading to less greenhouse warming. The daily range of temperature in the lower atmosphere presents ample variation due to the low thermal inertia; it can range from to near near the surface in some regions. 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 telescopes from Earth, and in 2018 even with the naked eye as a change in colour and brightness of the planet. Planet-encircling dust storms (global dust storms) occur on average every 5.5 Earth years (every 3 Martian 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. It has been suggested to be loosely related to gravitational influence of both moons, somewhat similar to the creation of tides on Earth.
The Martian atmosphere is an oxidized atmosphere. The photochemical reactions in the atmosphere tend to oxidize the organic species and turn them into carbon dioxide or carbon monoxide. Although the most sensitive methane probe on the recently launched ExoMars Trace Gas Orbiter failed to find methane in the atmosphere over the whole of Mars, several previous missions and ground-based telescopes detected unexpected levels of methane in the Martian atmosphere, which may even be a biosignature for life on Mars. However, the interpretation of the measurements is still highly controversial and lacks a scientific consensus.
Atmospheric evolution.
The mass and composition of the 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 the Martian upper atmosphere, measurements of isotopic composition and analyses of Martian meteorites, provide evidence of the long-term changes of the atmosphere and constraints for the relative importance of different processes.
Atmosphere in the early history.
In general, the gases 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 conditions of the Martian atmosphere in the past.
While Mars and Earth have similar 12C / 13C and 16O / 18O ratios, 14N is much more depleted in the Martian atmosphere. It is thought that the photochemical escape processes are responsible for the isotopic fractionation and has caused a significant loss of nitrogen on geological timescales. 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 leaking these two noble gases to outer space owing to their heavier mass. However, the higher abundance of hydrogen in the Martian atmosphere and the high fluxes of extreme UV from the young Sun, together could have driven a hydrodynamic outflow and dragged away these heavy gases. Hydrodynamic escape also contributed to the loss of carbon, and models suggest that it is possible to lose 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 the MAVEN orbiter 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.
The Martian atmosphere is particularly prone to impact erosion owing to the low escape velocity of Mars. An 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 terms of relative abundance of carbon, the C / 84Kr ratio on Mars is only 10% of that on Earth and Venus. Assuming the three rocky planets have the same initial volatile inventory, then this low C / 84Kr ratio implies the mass of CO2 in the early Martian atmosphere should have been 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 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 during early Mars' history, a much stronger greenhouse effect must have occurred in the Martian atmosphere to warm the surface up above freezing point of water. Carl Sagan first proposed that 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 modeling 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, other studies suggested that high solubility of SO2, efficient formation of H2SO4 aerosol and surface deposition prohibit the long-term build-up of SO2 in the Martian atmosphere, and hence reduce the potential warming effect of SO2.
Atmospheric escape on modern Mars.
Despite the lower gravity, Jeans escape is not efficient in the 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 have sufficient thermal energy can escape from the gravitation of Mars (Jeans escape). The escape of atomic hydrogen 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:
 + hν ⟶ 
 + hν ⟶ 
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 the 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:
 + hν (λ ; 116  nm) ⟶ .
Other potentially important mechanisms include the 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:
 + hν ⟶ 
Nitrogen escape rate is very sensitive to the 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 are not enough fast oxygen atoms the 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 this model suggests that they are less important than dissociative recombination at present. 
Ionospheric escape.
The interaction of the solar wind and the interplanetary magnetic field with the Martian conductive ionosphere induces electrodynamic currents, that have been mapped and studied in detail, using MAVEN. These currents can drive the ionospheric species to high altitudes, where the solar wind is able to sweep them away from the planet, resulting to global scale ion outflows. They are however not sufficient to explain the atmospheric and ionospheric losses of Mars over its lifetime.
Current chemical composition.
Carbon dioxide.
CO2 is the main component of the Martian atmosphere. It has a mean volume (molar) 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 undergo sublimation 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.
There also exists the potential for adsorption of CO2 into and out of the regolith to contribute to the annual atmospheric variability. Although the sublimation and deposition of CO2 ice in the polar caps is the driving force behind seasonal cycles, other processes such as dust storms, atmospheric tides, and transient eddies also play a role. Understanding each of these more minor processes and how they contribute to the overall atmospheric cycle will give a clearer picture as to how the Martian atmosphere works as a whole. It has been suggested that the regolith on Mars has high internal surface area, implying that it might have a relatively high capacity for the storage of adsorbed gas. Since adsorption works through the adhesion of a film of molecules onto a surface, the amount of surface area for any given volume of material is the main contributor for how much adsorption can occur. A solid block of material, for example, would have no internal surface area, but a porous material, like a sponge, would have high internal surface area. Given the loose, finely grained nature of the Martian regolith, there is the possibility of significant levels of CO2 adsorption into it from the atmosphere. Adsorption from the atmosphere into the regolith has previously