Atmosphere of Mars

The atmosphere of Mars is the layer of gases surrounding Mars. It is primarily composed of carbon dioxide, molecular nitrogen, and argon. 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 20 g/m³ with a temperature generally below zero down to −60 °C. 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's 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. Furthermore the Martian atmosphere contains much less water vapor than earth's atmosphere and water vapor is another important contributor to the greenhouse effect. 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 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. 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 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 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's 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's 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 H 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. 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 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. 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:
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:
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:
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.