Atmosphere of mars

The atmosphere of Mars was open even before the flights of automatic interplanetary stations to this planet. Thanks to the spectral analysis and the confrontations of Mars with the Earth, which occur once every 3 years, astronomers already in the 19th century knew that it had a very uniform composition, more than 95% of which came from carbon dioxide ^[6] .

Back in the early 1920s, the first measurements of the temperature of Mars were carried out using a thermometer placed at the focus of the reflector telescope . The measurements of W. Lampland in 1922 gave the average surface temperature of Mars 245 K (−28 ° C ), E. Pettit and S. Nicholson in 260 obtained 260 K (−13 ° C). A lower value was obtained in 1960 by W. Sinton and J. Strong: 230 K (−43 ° C) ^{[4] [3]} . The first estimates of pressure — averaged — were obtained only in the 1960s using ground-based IR spectroscopes: a pressure of 25 ± 15 hPa obtained from the Lorentz broadening of carbon dioxide lines meant that it was the main component of the atmosphere ^[2] .

After the beginning of the era of spacecraft launches to Mars, it became possible to directly measure the atmospheric parameters of Mars. So, the dynamics of braking of descent vehicles is determined by the density of the atmosphere and thus provides information on changes in temperature and pressure with height ^[7] . Atmospheric temperature profiles up to 85 km altitude were also obtained by spectroscopic method - measurements in the IR range where the 15 μm carbon dioxide absorption band is located - using InfraRed Imaging Spectrometer (IRIS) IR spectroscopes on a Mariner 9 and InfraRed Thermal apparatus Mapper (IRTM) at the Viking , then Thermal Emission Spectrometer (TES) at the Mars Global Surveyor station, Thermal Emission Imaging System (THEMIS) at the Odyssey , Planetary Fourier Spectrometer (PFS) at Mars Express , and finally, Mars Climate Sounder (MCS) on the Mars Reconnaissance Orbiter . In addition, temperatures in the lower atmosphere (up to 45 km) were determined by eclipse sensing by all spacecraft, starting from Mariner-9, using transmitted by them through the atmosphere, and using the SPICAM instrument to Mars Express ”, Using the UV radiation of stars passing through the planet’s limb , data were also obtained on the upper layer up to 100 km high ^[2] . The eclipsing soundings performed by the Vikings ^[8] , Mars Express ^[9] from 2004, and the Mars Global Surveyor from 1998 to 2005, became an important source of information about the upper atmosphere; it is also studied by the Mars Express apparatus using ASPERA3 and MARSIS instruments - the properties of the plasma constituting the ionosphere at high altitudes are investigated^{[2] [10]} .

Wind speed can be determined by the Doppler shift of spectral lines. So, for this, the shift of the CO lines in the millimeter and submillimeter ranges was measured, and measurements on the interferometer make it possible to obtain the velocity distribution in the whole layer of large thickness ^[11] .

The most detailed and accurate data on the temperature of the atmosphere and surface, pressure, relative humidity and wind speed are continuously obtained by the Rover Environmental Monitoring Station (REMS) instrument kit aboard the Curiosity rover operating in the Gale crater since 2012 ^[2] . And the MAVEN apparatus, which has been in the orbit of Mars since 2014, is intended for a detailed study of the upper atmosphere, their interaction with particles of the solar wind, and especially the scattering dynamics ^[12] .

The chemical constituents of the atmosphere and their contents were determined mainly by spectroscopic methods — using instruments both on the Earth and on spacecraft — as well as using mass spectrometry ^{[13] [8] [14]} .

A number of processes, complex or not yet possible for direct observation, are subject only to theoretical modeling, however, it is also an important research method.

Due to the lower gravity force in comparison with the Earth, Mars is characterized by smaller gradients of density and pressure of its atmosphere, and therefore the Martian atmosphere is much longer than the Earth. The height of the homogeneous atmosphere on Mars is greater than on Earth, and is about 11 km. Despite the strong sparseness of the Martian atmosphere, in it, according to various signs, the same concentric layers stand out as in the earth ^[15] .

In general, the atmosphere of Mars is divided into lower and upper; the latter is considered to be the region above 80 km above the surface ^[2] , where the processes of ionization and dissociation play an active role. Her section is devoted to what is commonly called aeronomy ^{[16] [10]} . Usually, when they talk about the atmosphere of Mars, they mean the lower atmosphere.

Also, some researchers distinguish two large shells - the homosphere and the heterosphere. In the homosphere, the chemical composition does not depend on height, since the processes of heat and moisture transfer in the atmosphere and their vertical exchange are entirely determined by turbulent mixing. Since molecular diffusion in the atmosphere is inversely proportional to its density, from a certain height this process becomes predominant and is the main feature of the upper shell - the heterosphere, where molecular diffusion separation occurs. The interface between these shells, which is located at altitudes from 120 to 140 km, is called a turbopause ^{[15] [8]} .

Lower Atmosphere

The troposphere stretches from the surface to an altitude of 20-30 km, where the temperature drops with height. The upper boundary of the troposphere varies depending on the season (the temperature gradient in the tropopause varies from 1 to 3 deg / km with an average value of 2.5 deg / km) ^[15] .

Above the tropopause is the isothermal region of the atmosphere - the stratomesosphere , stretching to an altitude of 100 km. The average temperature of the stratomesosphere is extremely low and amounts to -133 ° С. Unlike the Earth, where the stratosphere contains mainly all atmospheric ozone , its concentration on Mars is negligible (it is distributed from heights of 50-60 km to the surface itself, where it is maximum) ^[15] .

Upper atmosphere

Above the stratomesosphere, the upper atmosphere — the thermosphere — extends. It is characterized by an increase in temperature with a height up to a maximum value (200-350 K), after which it remains constant up to the upper limit (200 km) ^{[15] [2]} . In this layer, the presence of atomic oxygen is recorded; its density at an altitude of 200 km reaches 5-6⋅10 ⁷ cm ^{– 3 [2]} . The presence of a layer with a predominance of atomic oxygen (as well as the fact that the main neutral component is carbon dioxide) combines the atmosphere of Mars with the atmosphere of Venus ^[10] .

The ionosphere , a region with a high degree of ionization, is in the altitude range from about 80-100 to about 500-600 km. The ion content is minimal at night and maximally during the day ^[15] , when the main layer is formed at an altitude of 120-140 km due to photoionization of carbon dioxide by the radiation of the Sun ^{[2] [9]} СО ₂ + hν → СО ₂ ⁺ + e ^- as well as reactions between ions and neutral substances СО ₂ ⁺ + O → О ₂ ⁺ + CO and О ⁺ + СО ₂ → О ₂ ⁺ + CO. The concentration of ions, of which 90% O ₂ ⁺ and 10% CO ₂ ⁺ , reaches 10 ⁵ per cubic centimeter (in other regions of the ionosphere it is 1-2 orders of magnitude lower) ^{[2] [8] [10]} . It is noteworthy that O ₂ ⁺ ions predominate with the almost complete absence of molecular oxygen proper in the Martian atmosphere ^[10] . The secondary layer is formed in the region of 110-115 km due to soft X-ray radiation and knocked out fast electrons ^[9] . At an altitude of 80-100 km, some researchers have identified a third layer, sometimes manifested under the influence of cosmic dust particles, introducing metal ions ^[2] Fe ⁺ , Mg ⁺ , Na ⁺ into the atmosphere. However, later it was not only confirmed the appearance of the latter (and practically throughout the entire volume of the upper atmosphere) due to the ablation of matter falling into the atmosphere of Mars, meteorites and other cosmic bodies ^[17] , but also their constant presence. Moreover, due to the absence of a magnetic field in Mars, their distribution and behavior are significantly different from what is observed in the Earth’s atmosphere ^[18] . Above the main maximum, other additional layers can also appear due to interaction with the solar wind. So, the layer of O ⁺ ions is most pronounced at an altitude of 225 km. In addition to the three main types of ions (O ₂ ⁺ , CO ₂ ⁺ and O ⁺ ), H ₂ ⁺ , H ₃ ⁺ , He ⁺ , C ⁺ , CH ⁺ , N ⁺ , NH ⁺ , OH ⁺ , H were also relatively recently registered ₂ O ⁺ , H ₃ O ⁺ , N ₂ ⁺ / CO ⁺ , HCO ⁺ / HOC ⁺ / N ₂ H ⁺ , NO ⁺ , HNO ⁺ , HO ₂ ⁺ , Ar ⁺ , ArH ⁺ , Ne ⁺ , CO ₂ ⁺⁺ and HCO ₂ ⁺ . Above 400 km, some authors distinguish "ionopause", but there is no consensus on this subject ^[2] .

As for the plasma temperature, near the main maximum the ion temperature is 150 K, increasing to 210 K at an altitude of 175 km. Above, the thermodynamic equilibrium of ions with a neutral gas is substantially violated, and their temperature rises sharply to 1000 K at an altitude of 250 km. The electron temperature can be several thousand Kelvin, most likely due to the magnetic field in the ionosphere, and it increases with an increase in the zenith angle of the Sun and varies in the northern and southern hemispheres, which is probably due to the asymmetry of the residual magnetic field of the Martian crust. In general, three populations of high-energy electrons with different temperature profiles can even be distinguished. The magnetic field also affects the horizontal distribution of ions: over magnetic anomalies, flows of high-energy particles are formed, swirling along the field lines, which increases the ionization intensity, and an increased ion density and local formations are observed ^[2] .

At an altitude of 200-230 km is the upper boundary of the thermosphere - the exobase, above which the exosphere of Mars begins at about 250 km. It consists of light substances - hydrogen , carbon , oxygen - which appear as a result of photochemical reactions in the underlying ionosphere, for example, dissociative recombination of O ₂ ⁺ with electrons ^[2] . The continuous supply of atomic hydrogen to the upper atmosphere of Mars occurs due to the photodissociation of water vapor at the Martian surface. Due to the very slow decrease in the concentration of hydrogen with height, this element is the main component of the outermost layers of the planet’s atmosphere and forms a hydrogen corona that extends over a distance of about 20,000 km ^[15] , although there is no strict boundary, and particles from this region simply gradually scatter into the surrounding cosmic space ^[2] .

In the atmosphere of Mars, the chemosphere also sometimes stands out - a layer where photochemical reactions occur, and since due to the lack of an ozone screen, like Earth, ultraviolet radiation reaches the very surface of the planet, they are possible even there. The Martian chemosphere extends from the surface to an altitude of about 120 km ^[15] .

• Carbon dioxide is the main component (95.32%). It is the only and most stable heavy gas with low Martian gravity, which constantly replenishes the atmosphere during volcanic eruptions, continuous for millions of years due to the lack of plate tectonics; in fact, this is actually the only reason for the existence of the atmosphere of Mars. Moreover, lighter gases, since the planet lost its magnetic field, were carried away by the solar wind ^[6] . A dense atmosphere can exist stably under low gravity only if the planet has a significant magnetic field, or is located far from the Sun. Gases from nitrogen and lighter (hydrogen, helium) are exposed to active dissipation.

Despite the strong sparseness of the Martian atmosphere, the concentration of carbon dioxide in it is about 23 times higher than in the Earth ^{[6] [3]} .

• Nitrogen (2.7%) is currently actively dissipating into space. In the form of a diatomic molecule, nitrogen is stably retained by the attraction of the planet, but is split by solar radiation into single atoms, easily leaving the atmosphere.
• Argon (1.6%) is represented by the argon-40 heavy isotope relatively resistant to dissipation. Light ³⁶ Ar and ³⁸ Ar are available only in parts per million
• Other noble gases : neon , krypton , xenon (ppm) ^[13]
• Carbon monoxide (CO) - is a product of CO ₂ photodissociation and is 7.5–10 ^{-4 of the} concentration of the latter ^[15] - this is an inexplicably small value, since the reverse reaction CO + O + M → CO ₂ + M is forbidden, and should accumulate much more CO. Various theories have been proposed how carbon monoxide can still be oxidized to carbon dioxide, but all of them have one or another disadvantage ^[2] .
• Molecular oxygen (O ₂ ) - appears as a result of photodissociation of both CO ₂ and H ₂ O in the upper atmosphere of Mars. In this case, oxygen diffuses into the lower layers of the atmosphere, where its concentration reaches 1.3⋅10 ^-3 of the near-surface concentration of С0 ₂ ^[15] . Like Ar, CO and N ₂ , it belongs to non-condensable substances on Mars, therefore its concentration also undergoes seasonal variations. In the upper atmosphere, at an altitude of 90–130 km, the O ₂ content (fraction relative to CO ₂ ) is 3-4 times higher than the corresponding value for the lower atmosphere and averages 4–10 ^–3 , varying from 3.1–10 ^-3 to 5.8⋅10 ^{-3 [2]} . In ancient times, the atmosphere of Mars contained, however, a greater amount of oxygen, comparable to its share on the young Earth ^{[3] [19]} . Oxygen, even in the form of individual atoms, is not as actively dissipating as nitrogen, due to the greater atomic weight, which allows it to accumulate.
• Ozone - its amount varies greatly depending on the surface temperature ^[15] : it is minimum during the equinox at all latitudes and maximum at the pole where winter is, in addition, inversely proportional to the concentration of water vapor. There is one pronounced ozone layer at an altitude of about 30 km and another between 30 and 60 km ^[2] .
• Water. The content of H ₂ O in the atmosphere of Mars is approximately 100-200 times less than in the atmosphere of the driest regions of the Earth, and averages 10-20 microns of the precipitated column of water. The concentration of water vapor undergoes significant seasonal and daily variations.   ^{[15] [7]} . The degree of saturation of air with water vapor is inversely proportional to the content of dust particles, which are the centers of condensation, and in some areas (in winter, at an altitude of 20-50 km) steam was detected, the pressure of which exceeds the saturated vapor pressure by 10 times - much more than in the Earth’s atmosphere ^{[2] [20]} .
• Methane Since 2003, there are reports of the registration of methane emissions of an unknown nature ^[21] , however, none of them can be considered reliable due to one or another shortcoming of registration methods. In this case, we are talking about extremely small values ​​- 0.7 ppbv (the upper limit is 1.3 ppbv) as the background value and 7 ppbv for episodic bursts, which is on the verge of solvability. Since along with this information was also published on the absence of CH ₄ confirmed by other studies ^[22] , this may indicate some inconsistent source of methane, as well as the existence of some mechanism of its rapid destruction, while the duration of the photochemical destruction of this substance is estimated at 300 years . The discussion on this issue is currently open, and it is of particular interest in the context of astrobiology , due to the fact that this substance has a biogenic origin on Earth ^[2] .
  In 2013, the Curiosity rover discovered methane in the atmosphere of Mars. In 2019, new data were recorded, and these observations are three times higher than the gas level recorded six years ago. ^[23]

• Traces of some organic compounds ^[24] . The most important are the upper limits on H ₂ CO, HCl, and SO ₂ , which indicate the absence, respectively, of reactions involving chlorine , as well as volcanic activity, in particular, the non-volcanic origin of methane, if its existence is confirmed ^[2] .

The composition and pressure of the atmosphere of Mars make it impossible for humans to breathe ^[25] and other terrestrial organisms ^[6] . To work on the surface of the planet, a spacesuit is needed, although not as bulky and protected as for the Moon and outer space. The atmosphere of Mars is not poisonous in itself and consists of chemically inert gases. The atmosphere somewhat slows down the meteorite bodies, so there are fewer craters on Mars than on the Moon and they are less deep. And micrometeorites burn completely, not reaching the surface.

Low density does not prevent the atmosphere from forming large-scale phenomena that affect the climate ^[3] .

Water vapor in the Martian atmosphere is not more than one thousandth of a percent, however, according to recent (2013) studies, this is still more than previously expected, and more than in the upper layers of the Earth’s atmosphere ^[26] , and at low pressure and temperature it is in a state close to saturation, therefore it often gathers in clouds. As a rule, water clouds form at heights of 10-30 km above the surface. They are mainly concentrated at the equator and are observed almost throughout the year ^[3] . Clouds observed at high atmospheric levels (more than 20 km) are formed as a result of CO ₂ condensation. The same process is responsible for the formation of low (at an altitude of less than 10 km) clouds of the polar regions in the winter, when the temperature of the atmosphere drops below the freezing point of CO ₂ (-126 ° С); in the summer, similar thin formations are formed from ice Н ₂ О ^[15]

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  Animation of the movement of clouds, photos from the device Phoenix

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  Animation of cloud movement from images of the Curiosity rover .

Condensation formations are also represented by fogs (or haze). They often stand over lowlands - canyons, valleys - and at the bottom of craters in the cold season ^{[15] [4]} .

One of the interesting and rare atmospheric phenomena on Mars ( Viking-1 ) was discovered when photographing the northern polar region in 1978. These are cyclonic structures clearly identified in photographs by vortex-shaped cloud systems with counterclockwise circulation. They were found in the latitudinal zone of 65-80 ° C. w. during the “warm” period of the year, from spring to early autumn, when the polar front is established here. Its occurrence is due to the sharp contrast of surface temperatures existing at this time of the year between the edge of the ice cap and the surrounding plains. The wave movements of air masses associated with such a front also lead to the appearance of cyclonic vortices, so familiar to us on Earth. The systems of vortex clouds found on Mars range in size from 200 to 500 km, their speed of movement is about 5 km / h, and the wind speed at the periphery of these systems is about 20 m / s. The duration of the existence of a separate cyclonic vortex varies from 3 to 6 days. The temperatures in the central part of Martian cyclones indicate that the clouds consist of water ice crystals ^[15] .

In 2008, the Phoenix rover ^{[27] [28]} observed in the circumpolar regions of Mars phenomena unexpected for an almost devoid of planetary atmosphere - virgu (this is a strip of precipitation under clouds that evaporate not reaching the surface of the planet). According to the first estimates of scientists, the rate of rainfall in the virgin was very low. However, in 2017, a simulation of ^[29] Martian atmospheric phenomena showed that in reality the speed of particles during snowstorms can reach 10 m / s. This is due to the sharp cooling of the Martian clouds after sunset - at a speed of about four degrees per hour. So during Martian nights, a couple of hours after midnight, intense snowstorms can be expected. It was previously believed that a “slow” blizzard would necessarily lead to the formation of a virga - particles will evaporate in the air, not reaching the surface. The authors of the new work admit that strong winds combined with low cloudiness can lead to snow falling on the surface of Mars. This phenomenon resembles terrestrial micropoles - squalls from a descending wind at a speed of up to 35 m / s, often associated with thunderstorms. The new mechanism may not reflect the cause of the blizzard recorded by the Phoenix rover, since it was located in polar latitudes, where the Sun hardly sets, and in this situation the necessary night conditions causing the blizzards practically do not arise. However, the mechanism may well be realized at the mid latitudes of the red planet ^[30] .

Snow was indeed observed repeatedly ^[6] . So, in the winter of 1979, a thin layer of snow fell over the Viking-2 landing area, which lay for several months ^[4] .

A characteristic feature of the atmosphere of Mars is the constant presence of dust; according to spectral measurements, the size of dust particles is estimated at 1.5 μm ^{[15] [7] [31]} . Low gravity allows even rarefied air currents to lift huge clouds of dust to a height of 50 km. And winds, which are one of the manifestations of temperature differences, often blow over the planet’s surface ^[6] (especially in late spring - early summer in the southern hemisphere, when the temperature difference between the hemispheres is especially sharp   ), and their speed reaches 100 m / s. In this way, extensive dust storms are formed that have long been observed in the form of separate yellow clouds, and sometimes in the form of a continuous yellow shroud covering the entire planet. Most often, dust storms occur near polar caps, their duration can reach 50-100 days. A weak yellow haze in the atmosphere, as a rule, is observed after large dust storms and is easily detected by photometric and polarimetric methods ^{[15] [4] [2]} .

Dust storms, which were well observed in photographs taken from orbital vehicles, were inconspicuous when shooting from landing vehicles. The passage of dust storms at the landing sites of these space stations was noted only by a sharp change in temperature, pressure and a very slight darkening of the general sky background. The layer of dust that settled after the storm in the vicinity of the Viking landing sites was only a few micrometers. All this indicates a rather low bearing capacity of the Martian atmosphere ^[15] .

From September 1971 to January 1972, a global dust storm occurred on Mars, which even interfered with photographing the surface from the Mariner-9 probe ^[4] . The mass of dust in the atmospheric column (with optical thickness from 0.1 to 10), estimated during this period, ranged from 7.8⋅10 ^-5 to 1.66⋅10 ^-3 g / cm ² . Thus, the total weight of dust particles in the Martian atmosphere during the period of global dust storms can reach 10 ⁸ - 10 ⁹ t, which is comparable with the total amount of dust in the Earth’s atmosphere ^[15] .

Dust tornadoes are another example of the processes of raising dust into the air due to diurnal temperature variations.   ^[4] near the surface of Mars. Due to the very low density of the atmosphere of the red planet, tornadoes there are more like tornadoes , towering several kilometers in height and hundreds of meters across. They are formed so rapidly that once inside it, a hypothetical observer would suddenly not be able to see more than a few centimeters in front of him. The wind reaches 30 m / s. Dust tornadoes on Mars will be a serious problem for astronauts who will have to face them upon arrival on the planet; An additional difficulty is that the friction of dust in the air creates electricity. Due to the lack of erosion on the planet’s surface, traces of these phenomena remain on it, and the rovers managed to photograph the traces left by dust devils earlier ^[6] .

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  A global dust storm recorded by the Hubble telescope in 2001. A continuous veil hides the entire surface of Mars.

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  A complete map of the Martian surface with the dynamics of atmospheric processes, including two local dust storms, from February 18 to March 6, 2017. Based on images of the Mars Reconnaissance Orbiter.

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  The passage of a dust vortex along the surface of Mars, captured by the Spirit rover, 2005

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  Traces of dusty vortices on the surface of Mars.

Due to the absence of a global magnetic field, high-energy particles of the solar wind freely enter the atmosphere of Mars, causing auroras in the ultraviolet range during solar flares. This concentrated highly localized radiation, determined by the magnetic anomalies of the crust, is a type of aurora that is unique in the solar system precisely because of the specifics of the Martian magnetic field ^[2] . Its lines form cusps, but not at the poles, but on separate parts of the surface that are not attached to latitudes (mainly in the mountainous regions of the southern hemisphere), and along them electrons with kinetic energies from several tens to 300 eV move - their impacts cause luminescence . It is formed under special conditions near the boundary between the “open” and “closed” magnetic field lines ^[32] , and the field lines along which the electrons move are deflected from the vertical. The phenomenon lasts only a few seconds, and the average height of its occurrence is 137 km ^[33] .

Aurora was first detected by a SPICAM UV spectrometer aboard the Mars Express apparatus ^[34] . Then it was repeatedly observed by the MAVEN apparatus, for example, in March 2015 ^[35] , and in September 2017, a much more powerful event was recorded by the Radiosity Assessment Detector (RAD) on the Curiosity rover ^{[36] [37]} . An analysis of the data of the MAVEN apparatus also revealed auroras of a fundamentally different type — diffuse auroras, which occur at low latitudes, in areas not attached to magnetic field anomalies and caused by the penetration of very high energy particles of about 200 keV into the atmosphere ^[38] .

In addition, the extreme ultraviolet radiation of the Sun causes the so-called self-emission of the atmosphere ( English airglow ).

Registration of optical transitions at auroras and intrinsic luminescence provides important information on the composition of the upper atmosphere, its temperature and dynamics. Thus, the study of the γ- and δ-bands of radiation of nitric oxide in the night period helps to characterize the circulation between the illuminated and unlit regions. And the registration of radiation at a frequency of 130.4 nm with its own luminescence helped to identify the presence of atomic oxygen of high temperature, which was an important step in understanding the behavior of atmospheric exospheres and corona in general ^[2] .

The dust particles that fill the atmosphere of Mars consist mainly of iron oxide, and it gives it a reddish-red tint ^{[6] [15]} .

According to measurement data, the atmosphere has an optical thickness of 0.9 ^[31] - this means that only 40% of the incident solar radiation reaches the surface of Mars through its atmosphere, and the remaining 60% is absorbed by dust hanging in the air. Without it, the Martian skies would have approximately the same color as the Earth’s sky at an altitude of 35 kilometers ^[39] , where the pressure and density of the Earth’s atmosphere are comparable to those on the surface of Mars. Without dust, the sky of Mars would be almost black, perhaps with a pale blue haze near the horizon. It should be noted that in this case the human eye would adapt to these colors, and the white balance would automatically adjust so that the sky would appear the same as under terrestrial lighting conditions.

The color of the sky is very heterogeneous, and in the absence of clouds or dust storms from relatively light on the horizon, it darkens sharply and gradient towards the zenith. In a relatively calm and calm season, when there is less dust, at the zenith the sky can be completely black.

Nevertheless - thanks to the pictures of the rovers, it became known that at sunset and sunrise around the sun the sky turns blue. The reason for this is RELAY scattering - light is scattered on gas particles and colors the sky, but if the effect is weak and invisible to the naked eye due to the rarefied atmosphere and dust, the sun shines through a much thicker layer of air at sunset, due to which blue and violet begin to scatter. components. The same mechanism is responsible for the blue sky on Earth during the day and yellow-orange at sunset .

The general circulation of the atmosphere occurs according to the classical Hadley scheme: the flow rises in the hemisphere, where it is currently summer, and drops back in the opposite hemisphere. Such Hadley cells can extend up to 60 km in height - much higher than on Earth, where the convective zone is limited by the tropopause (up to 12 km). At an altitude of up to 50 km, this process is well described by the general circulation model ^[2] , although it is possible that it gives somewhat lower temperatures for the middle atmosphere (20–50 km) and high temperatures for the region above 50 km. The main zonal circulation is determined by winds blowing in the opposite direction to the planet’s rotation, with high speeds of 70–170 m / s, which vary depending on the time of year, latitude and longitude (especially strongly between morning and evening hours) ^[11] .

Changes in the upper atmosphere are quite complex, as they are related to each other and to the underlying layers. Atmospheric waves and tides propagating upward can have a significant effect on the structure and dynamics of the thermosphere and, as a consequence, the ionosphere, for example, the height of the upper boundary of the ionosphere. During dust storms   in the lower atmosphere, its transparency decreases, it heats up and expands. Then the density of the thermosphere increases - it can even vary by an order of magnitude - and the height of the maximum electron concentration can rise by up to 30 km. Changes in the upper atmosphere caused by dust storms can be global, affecting areas up to 160 km above the planet's surface. The response of the upper atmosphere to these phenomena takes several days, and it returns to its previous state much longer - several months. Another manifestation of the relationship between the upper and lower atmosphere is that water vapor, which, as it turned out, is oversaturated with the lower atmosphere, can undergo photodissociation into lighter components H and O, which increase the density of the exosphere and the rate of water loss in the Martian atmosphere. External factors causing changes in the upper atmosphere are the extreme ultraviolet and soft X-rays of the Sun, solar wind particles, cosmic dust and larger bodies such as meteorites . The task is complicated by the fact that their effect, as a rule, is random, and its intensity and duration cannot be predicted, and cyclical processes associated with changes in the time of day, time of year, and also the solar cycle are superimposed on episodic events. At present, the dynamics of atmospheric parameters at best have accumulated statistics of events, but a theoretical description of the laws has not yet been completed. Direct proportionality between the concentration of plasma particles in the ionosphere and solar activity has definitely been established. This is confirmed by the fact that a similar regularity was actually fixed ^[40] according to the results of observations in 2007–2009 for the Earth’s ionosphere , despite the fundamental difference in the magnetic field of these planets, which directly affects the ionosphere. And emissions of particles of the solar corona, causing a change in the pressure of the solar wind, also entail a characteristic compression of the magnetosphere and ionosphere ^[2] : the maximum plasma density drops to 90 km ^[9] .

Daily fluctuations

Since the atmosphere of Mars is very thin, it does not smooth out daily fluctuations in surface temperature. Under the most favorable conditions, in the summer, on the daytime half of the planet, the air warms up to 20 ° C (and at the equator - up to +27 ° C) - a quite acceptable temperature for the inhabitants of the Earth. But on a winter night, frost can even reach −80 ° C at the equator to −125 ° C, and at the poles the night temperature can drop to −143 ° C ^{[4] [6]} . However, diurnal temperature fluctuations are not so significant as on the atmosphereless Moon and Mercury ^[3] . On Mars, there are temperature oases, in the areas of the “lake” Phoenix (plateau of the Sun) and Noah’s land, the temperature difference is from −53 ° C to + 22 ° C in the summer and from −103 ° C to −43 ° C in the winter. Thus, Mars is a very cold world, but the climate there is not much harsher than in Antarctica ^[4] .

Despite its sparseness, the atmosphere nonetheless responds to changes in the flow of solar heat more slowly than the surface of the planet. So, in the morning, the temperature varies greatly with altitude: a difference of 20 ° was recorded at an altitude of 25 cm to 1 m above the surface of the planet. With the rising of the sun, cold air heats up from the surface and rises in the form of a characteristic swirl upward, raising dust into the air - this is how dust devils are formed. In the near-surface layer (up to 500 m high), temperature inversion takes place. After the atmosphere has already warmed up by noon, this effect is no longer observed. The maximum is reached at about 2 hours in the afternoon. Then the surface cools faster than the atmosphere, and an inverse temperature gradient is observed. Before sunset, the temperature again decreases with height ^{[7] [2]} .

The change of day and night affects the upper atmosphere. First of all, ionization by solar radiation ceases at night, but the plasma continues to replenish for the first time after sunset due to the flux from the day side, and then is formed due to impacts of electrons moving down along the lines of the magnetic field (the so-called electron invasion) - then the maximum observed at an altitude of 130-170 km. Therefore, the density of electrons and ions from the night side is much lower and is characterized by a complex profile, which also depends on the local magnetic field and varies in a non-trivial way, the regularity of which is not yet fully understood and described theoretically ^[9] . During the day, the state of the ionosphere also varies depending on the zenith angle of the Sun ^{[2] [8]} .

Annual cycle

As on Earth, on Mars, the seasons change due to the inclination of the axis of rotation to the orbit plane, so in winter the polar cap grows in the northern hemisphere and almost disappears in the southern hemisphere, and after six months the hemispheres change places. Moreover, due to the rather large eccentricity of the planet’s orbit in perihelion (winter solstice in the northern hemisphere), it receives up to 40% more solar radiation than in aphelion ^[2] , and in the northern hemisphere winter is short and relatively mild, and summer is long, but it’s cool, in the south, on the contrary, summer is short and relatively warm, and winter is long and cold. In this regard, the southern cap in winter grows to half the pole-equator distance, and the northern cap only up to one third. When summer begins at one of the poles, carbon dioxide from the corresponding polar cap evaporates and enters the atmosphere; the winds carry him to the opposite hat, where he freezes again. Thus, the carbon dioxide cycle occurs, which, along with different sizes of the polar caps, causes a change in the atmospheric pressure of Mars as it revolves around the Sun ^{[3] [4] [6]} . Due to the fact that in winter up to 20-30% of the entire atmosphere freezes in the polar cap, the pressure in the corresponding region accordingly drops ^[7] .

Seasonal variations (as well as daily variations) also undergo water vapor concentration - they are in the range of 1-100 microns. So, in winter the atmosphere is almost “dry”. Water vapor appears in it in spring, and by mid-summer its amount reaches a maximum, following changes in surface temperature. During the summer - autumn period, water vapor is gradually redistributed, and its maximum content moves from the northern polar region to equatorial latitudes. Moreover, the total global vapor content in the atmosphere (according to Viking-1) remains approximately constant and equivalent to 1.3 km ^{3 of} ice. The maximum H ₂ O content (100 μm of deposited water, equal to 0.2 volume%) was recorded in summer over a dark area surrounding the northern residual polar cap - at this time of the year the atmosphere above the ice of the polar cap is usually close to saturation ^[15] .

In the spring-summer period in the southern hemisphere, when dust storms are most actively formed, diurnal or semidiurnal atmospheric tides are observed - an increase in surface pressure and thermal expansion of the atmosphere in response to its heating ^[2] .

The change of seasons affects both the upper atmosphere - both the neutral component (thermosphere) and the plasma (ionosphere), and this factor must be taken into account together with the solar cycle, and this complicates the task of describing the dynamics of the upper atmosphere ^[2] .

Long-Term Changes

• Atmosphere
• Atmosphere pressure
• Carbon dioxide
• Mars Climate
• Colonization of Mars
• Terraforming