The atmosphere (from ancient Greek ἀτμός, atmòs, “vapor” and σφαῖρα, sphàira, “sphere”) is a layer or a set of layers of gases (whose molecules are held in place by the force of gravity of the body itself) surrounding a planet or other material body, that is held in place by the gravity of that body. An atmosphere is more likely to be retained if the gravity it is subject to is high and the temperature of the atmosphere is low.
The Earth’s atmosphere is the envelope of gas that covers the planet Earth, held both by the force of gravity and by the magnetic field (it counteracts the solar wind that would otherwise sweep it away) participating in large part to its rotation. With a chemical composition that varies according to altitude and location, it has a rather complex structure divided into five layers, called spheres, defined according to the inversion of its vertical thermal gradient. Starting from the bottom, these spheres are troposphere, stratosphere, mesosphere, thermosphere and exosphere, while the surface of discontinuity between two layers where the sign inversion takes place is called “break”. The study of the atmosphere in all its aspects falls within the broad scope and disciplines of atmospheric sciences.
The origin of the Earth’s atmosphere is closely related to cosmogonic theories on the origin of the solar system. It can be considered with good probability that the primordial atmosphere was composed mainly of hydrogen, helium, nitrogen, methane, ammonia, water vapor and inert gases. Through the geological eras it would have arrived with slow changes to the current composition: the lighter gases, hydrogen and helium, would have been brought to the upper parts of the atmosphere and largely dispersed into space; oxygen and carbon dioxide would have been introduced mainly through the biological cycles of plants and animals and during volcanic events. The current composition of the atmosphere can be considered, between the ground and a height of 10-12 km, almost constant and formed by a mixture of gases in which nitrogen and oxygen predominate (99% overall) with small amounts of carbon dioxide and water vapor, traces of inert gases and light gases and various impurities (see air).
With the height gradually decreases the heavy gases (oxygen and nitrogen) and increases the percentage of hydrogen and helium; between 15 and 80 km high, with a maximum around 25 km, we encounter small amounts of ozone, resulting from the photodissociation of diatomic oxygen molecules in atomic oxygen that, recombining with the diatomic molecules, forms the triatomic oxygen or ozone. Recent studies of astrophysics have highlighted some zones of the ozonosphere with low percentage of ozone, the so-called holes in the ozone that, according to some hypothesis, would be caused by fluorinated chemical compounds used in aerosols (spray). The lack of ozone that absorbs part of the ultraviolet radiation of the Sun, would result in a slow increase of the average temperature on Earth at sea level. At an altitude of about 70 km has been identified a layer containing sodium; above 1000 km the atmosphere consists almost entirely of helium and hydrogen. The total mass of the atmosphere represents about one millionth of the mass of the Earth.
In the atmosphere the density decreases exponentially with height: at ground level one cubic centimeter of air contains about 2.5⋅1019 molecules that continuously collide, since the average free path of a molecule at the Earth’s surface is about 10-8 m; at 100 km this value is a million times lower and the average path increases to 10-1 m; at 200 km there are 109 molecules per cm3 and a molecule travels even 100 m before meeting a molecule. 10-8 m; at an altitude of 100 km this value is a million times lower and the average path rises to 10-1 m; at 200 km there are 109 molecules per cm3 and a molecule travels even 100 m before meeting another one, while in the upper atmosphere one reaches paths of many kilometers.
The atmosphere is assumed to extend until its density reaches a value equal to that of the surrounding interplanetary gas. The atmosphere exerts with its weight, in each point, a pressure that depends on the density of the air mass above that point and consequently on the height.
It is defined as atmospheric pressure gradient the decrease in pressure for a displacement in height of 100 m (or even 10 m); near the ground for a vertical increase of 100 m there is a decrease of 9 mmHg; for the same displacement at an altitude of 8 km the variation is 3.5 mmHg and the gradient continues to decrease with increasing altitude.
The temperature of the atmosphere is due to solar radiation: one third of this is reflected and diffracted into outer space, mainly by clouds, the rest is absorbed by the air and the earth’s surface. The air is transparent to short wavelength radiation while it absorbs high wavelength radiation, transforming it into heat. The heat stored by the ground is returned to the atmosphere largely in the form of infrared radiation that is absorbed mainly by carbon dioxide and water vapor present in the first 10-12 km of air (greenhouse effect). The transmission of heat in these air layers occurs through vertical convective motions. This continuous mixing allows the air to maintain its uniform constitution.
The temperature of the atmosphere varies with altitude: up to an altitude of 10-12 km it regularly decreases by 0.5-0.7 °C every 100 m. At the ground it is 20 °C, at 12 km it is -55 °C, then it increases until it reaches 0 °C at an altitude of about 50 km; from this point the temperature starts to decrease again, reaching values between -70 and -100 °C at an altitude of 80-90 km. From here begins a rapid increase in temperature that reaches in the highest atmospheric layers 1000-2000 °C (note, however, that in this case the temperature, given the extreme rarefaction, is referred to the kinetic energy of the individual molecules).
Atmospheric humidity is also variable with altitude. In the layers closer to the ground the amount of water vapor depends very much on climatic conditions and geographical location; its volume percentage can reach a maximum of 4%. As altitude increases, humidity decreases: in the first 8 km, for temperate zones, the water vapor content of the air varies on average from 6.8 to 0.1 g/m3. The decrease continues up to an altitude of 15 km where there is a very dry layer; from this point there is a certain increase up to about 30 km (nacreous clouds are formed in fact between 25 and 30 km high). Above this altitude the data are rather uncertain; appreciable moisture is certainly present around an altitude of 80 km, as is evidenced by the formation of noctilucent clouds.
The atmosphere is home to an electric field produced by the negative electricity of the Earth and by the charges (mainly positive ions) present in the air. We can therefore define an atmospheric electric potential that in normal conditions has increasing values with altitude (from zero at ground level – reference value – it rises to about 200 kV at an altitude of 10 km). The electric gradient, that is the variation of potential for each meter of vertical displacement, decreases with altitude, going from 100 V/m near the ground to 4 V/m at 10 km.
The value of the gradient is however subject to significant and abrupt changes depending on atmospheric disturbances (storm clouds, induction phenomena between clouds and Earth, lightning, etc..) and the activity of the Sun that sends swarms of electrically charged particles on Earth. In some cases, during a thunderstorm, between the negatively charged earth and clouds, very high potential differences can be reached (up to 108 V): the intense electric fields between cloud and cloud and between clouds and ground cause lightning, which are discharges of electricity due to the presence in the atmosphere of free electrons and ionized particles that make it a conductor of electricity. It should be added that the first discharge to the ground produces a further ionization of the air (breakdown condition) opening the way for subsequent discharges of increasing intensity. The path of the lightning is zigzag and jerky from the cloud to the ground. When the lightning arrives at 50 m from the ground, the last discharge is reflected and through the same path it can return to the cloud with a much higher velocity, thus triggering a continuous discharge; the intensities of the currents are very high (of the order of 500.000 A).
Considering the atmosphere as a gaseous layer between the negatively charged ground and the positively charged ionosphere, the whole Earth-atmosphere-ionosphere can be considered as an electrical capacitor in which the atmosphere is the dielectric. Under clear weather conditions, the capacitor is charged to 3.6⋅105 V. If the electrical conductivity of the atmosphere is considered, such a capacitor should discharge in about 10 minutes with a discharge current of about 2000 A. In fact many factors intervene to keep constant the potential difference, such as, for example, the transport of electrons from the bottom to the top, also called “good time current”, the hydrological cycle that with the evaporation from seas and lakes causes a further transport of negative charges to the top, the so-called “corona effect” for which negative charges escape from the tips of rocks and trees. In other words, a balance is established between ground discharge and upward currents of negative charges.
The ionosphere, moreover, is subjected to the ionizing action of ultraviolet rays, X-rays and cosmic rays, action that produces a considerable electronic density and therefore great conductivity. Within the ionosphere we can then identify four layers (D, E, F1, F2) characterized by increasing electronic density; in particular the layer F2, which has maximum density, is of great importance for radio communications because it reflects short and very short radio waves.
The atmosphere is also the site of numerous optical and acoustic phenomena. The first are due to the effects of refraction, reflection and diffusion of solar radiation (twilight, rainbows, halos, etc.) or light manifestations of electrical phenomena (flashes, polar auroras, etc.); the second are caused by irregularities in the propagation of sound due to the different density of the layers of air or are manifestations of violent electrical phenomena.
Regional atmospheric division
Because the principal physical and chemical characteristics of the atmosphere are a function of height above the Earth’s surface, it was determined to divide the atmosphere into concentric regions, each defined by uniform properties and characteristics. A first division, made on the basis of chemical constitution, includes two regions: the homosphere and the heterosphere. The homosphere extends to an altitude of about 90-100 km, maintaining a fairly constant chemical composition. The heterosphere, above 100 km, has a composition that varies with altitude, with the final predominance of helium and hydrogen, while carbon dioxide, ozone and water vapor are missing. The two zones are separated by a transition layer called homopause.
The most widely used division and adopted by the International Union of Geodesy and Geophysics in 1951 is that based on the trend of temperature with altitude. In this scheme the following atmospheric regions are recognized: troposphere, tropopause, stratosphere, stratopause, mesosphere, mesopause, thermosphere, thermopause, exosphere.
Zone in which the temperature is regularly decreasing with altitude (average values: 20 ºC at ground level, -55 ºC at 12 km); its thickness varies with latitude: at the poles it is 6-7 km, at the Equator it reaches 18 km. This difference is due both to dynamic causes (greater centrifugal force at the Equator) and to thermodynamic causes, since the strong heating of the equatorial zone produces very intense convection currents. The troposphere is the seat of the main meteorological phenomena and of the physical-chemical cycles that allow animal and plant life; it contains 3/4 of all the air and almost all the water vapor present in the atmosphere. The water vapor plays a very important role in the processes of absorption, exchange (latent heat of vaporization and condensation) and radiation of atmospheric heat that, determining phenomena of turbulence, make uniform the constitution of the troposphere.
Separation layer between the troposphere and stratosphere; the temperature is uniform, while the thickness, a few hundred meters on average, is variable with latitude and seasons. The tropopause is the seat of high-speed air currents (200-300 km/h) that seem to be at the origin of the great perturbations of the troposphere (see jet stream). § Stratosphere. Region extending to an altitude of about 40 km; the temperature, at first almost constant, increases with altitude. The air, which is very rarefied, is at rest and its constituents tend to be distributed according to their weight; there is very little water vapor. In the upper part of the stratosphere the temperature tends to rise due to the formation of ozone, which also absorbs ultraviolet radiation otherwise dangerous to life.
Zone of separation between stratosphere and mesosphere; temperature is around 0 ºC.
According to some authors there would be no distinction between stratopause and mesosphere. In this region, which extends to about 80 km, the temperature, at first slightly increasing due to the presence of ozone, then decreases rapidly to a minimum of -70 ºC. The mesosphere is the site of light emission from the air, of the formation of most of the atmospheric ozone and of the transformation of primary cosmic radiation into secondary radiation, to which we owe the presence of the lowest ionized layer (layer D). There are also observed light trails caused by the burning of meteorites and the lowest polar auroras.
Transitional layer between mesosphere and thermosphere with extremely low temperatures: from -70 ºC to -100 ºC; in this zone can sometimes be observed the nottilucent clouds due to crystallization by freezing of the residual atmospheric water vapor.
Region extending up to an altitude of about 400 km or according to other authors 650 km. The temperature is rising rapidly, the gases are ionized or in the atomic state, carbon dioxide, water vapor and ozone are completely absent. The thermosphere is the seat of the ionized layers with greater electronic density; there occur important electrical and geomagnetic phenomena including polar auroras and those light emissions that produce the weak night glow of the sky.
Layer of separation between thermosphere and exosphere; here the temperature increase ends.
Area above 400 km, or 650 km, in which the temperature is considered constant. Due to extreme and progressive rarefaction, no upper limit can be established.
Planetary atmospheres appear to be a prerogative of the major bodies of the system because it is mainly the force of gravity to keep aggregated the air molecules, while the heating by the ground is the agent that – also in relation to the different molecular weight of chemical species – tends to disperse them in space increasing the thermal agitation up to the limit of the escape velocity.
The above is already sufficient to understand how the planetary atmospheres – in their variety of consistency and composition – represent the product of a process of dynamic differentiation driven simultaneously by the amount of matter collected by the individual planets at the time of their formation, and by the heliocentric distance at which this formation was produced.
The chemical elements present in the preplanetary nebula were found in the normal “solar” proportions, that is with absolute preponderance of hydrogen followed by helium, and with contamination of carbon, oxygen, nitrogen. These were probably also the substances that became part of the planetary protoatmospheres and also underwent – in preference to the heavy elements – a centrifugal thrust towards the peripheral regions of the system due to pressure, radiation and solar wind.
The spaces closer to the Sun were therefore enriched with heavy elements (silicates, metals and their oxides) which, through processes of fusion and differentiation, entered to a very large extent in the constitution of the solid parts of the inner planets, so that today, after 4.5 billion years, we see a very diversified situation.
We notice the so-called terrestrial planets, the closest to the Sun, endowed with modest masses and gravitational forces, that, not having been able to hold back their original atmospheres, have ended up by replacing them thanks to the volatilization of the gaseous content of their superficial layers, for example through volcanic or, however, endogenous activity. On the contrary, the protoatmospheres appear preserved in an almost integral way in the massive planets of the Jovian type where, favored by the distance from the Sun and by the intense gravitational fields, they gave rise to the gigantic mantles of Jupiter, Saturn, Uranus, Neptune, within which the existence of a solid nucleus is completely hidden and subordinate.
Excluding Mercury, the planet that for the modesty of its mass and for the proximity to the Sun has dispersed in space every trace of gaseous element, the major bodies of the system show themselves wrapped by diversified atmospheric envelopes.
Even some large satellites – as the close investigations performed by the probes have revealed – have been found to be equipped with them: on Io (Jupiter’s volcanic moon) hover sulfurous emanations and nitrous vapors; Titan, Saturn’s largest satellite, is enveloped by a consistent atmosphere of nitrogen, argon, hydrogen, with methane in aerosol suspension; Triton (Neptune’s most conspicuous companion) seems to possess a nitrogen-rich atmosphere contaminated by various polymers of a hydrocarbon nature. Pluto, the most distant and least known of the system’s members, may also retain traces of an original ammonia- and methane-rich atmosphere.