Theory Planetary Atmospheres

with its atmosphere of Nitrogen and Methane, Titan is a good example of a type of planetary atmosphere in the solar system. picture courtesy NASA/JPL/Space Science Institute

Atmospheres are part of any planet formation process. Oxygen ions, for example, are found at a number of one for every 11 cubic centimeters of space. At the very first any planet or planetary body -like the moons of the gas giants- have an atmosphere of hydrogen and helium, that is made of these elements which are the fundamentals of the protoplanetary disc. Then, planets get secondary atmospheres, that is atmosphere which are the product of outgassing. Once in the process of differentiation, the planet interior is heating. At a moment, this heat is evacuated. As part of the evacuation, an atmosphere forms. Volcanoes e.g. are rejecting important masses of gas into the planet's immediate surrounding. Strictly the outer gas giants of the solar system are small solid cores with very large atmospheres. These atmospheres however are not due to any differentiation/outgassing process. They are simply an accretion of hydrogen, helium, and other gases when the planets formed. Such gases were those found in the protoplanetary disc. Some large moons of the outer gas giants got an atmosphere due to the differentiation/outgassing process. A majority of bodies in the solar system are small and are considered airless, with 'exospheres' in place of dense atmospheres, like our Moon, icy moons within our solar system, the planet Mercury, asteroids and even Pluto are examples of small bodies with known surface-boundary exospheres, which start from their surface that is. Larger bodies, such as Earth, also have tenuous exospheres as the outermost layer of their atmospheres. Exospheres are created by a erosion of the surface by the solar wind and solar storms. Evidence was found, for example, for a exosphere around Europa, as well as around other Jovian moons Ganymede and Callisto, a thin atmosphere surrounding the moons where molecules remain gravitationally trapped. Strong solar inputs, like those of CMEs, for example, can significantly erode the lunar surface according to a set of computer simulations by NASA scientists. This also could be a major method of atmospheric loss for planets like Mars that are unprotected by a global magnetic field. The solar wind can penetrate deep into the Martian atmosphere as a stream of it is not deflected but penetrate deep the upper atmosphere and ionosphere as that is allowed by interactions in the upper atmosphere transforming this stream of ions into a neutral -still ionic- form. The plasma from CMEs impacts the lunar surface, and atoms from the surface are ejected in a process called 'sputtering.' A model predicts 100 to 200 tons of lunar material could be stripped off the lunar surface during the typical 2-day passage of a CME, leading to the creation of the Moon's exosphere. On exposed small bodies like asteroids, the CMEs should create a sputtered-enhanced exosphere about the object, similar to that expected at the Moon. Water, methane and ammonia are collectively referred to as "volatiles" and the fact that there are different amounts in the atmosphere on different planets is a tantalizing clue to the way the planets formed. Accurate descriptions of planetary atmospheres might also help shed light on how the evolution of the solar system left the outer planets with a high percentage of volatiles, but not the inner planets. Studying from close the so-called Schumann Resonance, a wave yielded high in a atmosphere by lightnings could be a technique. A dense atmosphere, generally, like Earth's is relatively rare in our solar system because an object has to be sufficiently massive to have enough gravity to hold onto it. Exospheres, generally, which are very thin atmospheres the atoms of which do not collide, are the most common type of atmosphere in our solar system. Nightglow is a common planetary phenomenon in which the sky faintly glows even in the complete absence of external light. At Mars the glow originates from reactions that start on Mars' dayside. Ultraviolet light from the Sun breaks down molecules of carbon dioxide and nitrogen as resulting atoms are carried around the planet by high-altitude wind patterns and, on the nightside, those winds bring the atoms down to lower altitudes where nitrogen and oxygen atoms collide to form nitric oxide molecules. That recombination comes out as ultraviolet light

Venus possesses a 'electric wind' strong enough to remove the components of water from its upper atmosphere, which may have played a significant role in stripping its atmosphere. Just as every planet has a gravity field, it is believed that every planet with a atmosphere is also surrounded by a weak electric field. Counteracting the planet's gravity, that force can help to push the upper layers of the atmosphere off into space, generally. Another planet where the electric wind may play an important role is Mars like the prime suspect for the atmosphere escape

Further, a atmosphere endures evolution. Chemical reactions can lock a planet's atmosphere away in surface rocks as the early Sun had far more intense ultraviolet radiation and solar wind, which likely impacted how early atmospheres evolved in the solar system. In our solar system, either side of Venus, a area is extant at the inferior limit of which the solar wind -like at Mercury- is blowing any atmosphere out. Up to the superior limit, which is ending just inside the Earth's orbit, the runaway effect of greenhouse gases which rendered the Venus' atmosphere what it turned to be, can exist. A planet may loose its atmosphere. It's mainly seen now like such atmospheres being eroded by the solar wind or solar storms. Magnetic fields can protect a planet atmosphere against the solar wind but they can also create opportunities for escape, like the giant globs cut loose from Saturn and Jupiter when magnetic field lines become tangled. A planetary magnetosphere may thus have a role in the atmospheric loss, particularly as it relates to solar wind. It was the case at Mars, atmosphere of which ceased to be protected when the magnetic field there disappeared and was eroded in a significant proportion since about 5 billion years bringing to turn Mars from wet to dry. 2/3 or more of the Martian atmosphere has been lost to space since Mars origins as solar ultraviolet light and the solar wind were more intense in the past. Measurements of heavy-versus-light variants of elements in the Martian atmosphere indicate that much of Mars' early atmosphere disappeared by processes favoring loss of lighter atoms, such as from the top of the atmosphere. Lighter isotopes are more readily removed than heavier ones. In a process termed 'sputtering,' particles of the solar wind ionized Mars' atmosphere and the ions linked to the lines of the solar magnetic field which stretches all over the solar system. Those ions are also slammed by the solar wind into the top of the atmosphere and knocking other atoms out. A outward flow of oxygen ions triggered from the upper atmosphere in a link with solar activiy, like still observed nowadays at Earth, likely helps especially for planets like Mars which eventually got a weaker magnetic field. What is known like the 'charge-exchange phenomenon', a result of solar wind's interaction with interplanetary neutral atoms likely might have been a factor too into the radiation which contributed to atmospheric loss on the Red Planet. Solar wind gusts may have been prevalent in Sun's past history and the escape of Martian atmosphere larger. Both ions and electrons in the solar wind could start the process of particle escape by transforming the atmosphere's neutral particles into charged ions. This can occur through processes called charge exchange and impact ionization, or the ultraviolet sunlight also transforms many atmospheric particles into ions as once the atmospheric particles charged, they can interact with the solar wind's magnetic field and be accelerated and carried away from the planet, or the 'pickup ions.' An instrument onboard the SOFIA flying observatory in 2016 detected atomic oxygen in the upper layers of the Martian atmosphere known as the mesosphere. The escape rate of water from the atmosphere of Mars is peaking when Mars is at its closest point to the Sun and conversely, with a tenfold difference. The hydrogen in Mars’ upper atmosphere comes from water vapor in the lower one atmosphere as atmospheric water molecules can be broken apart by sunlight, releasing the two hydrogen atoms from the oxygen atom that they had been bound to. Atomic oxygen affects how other gases escape Mars. The Viking and Mariner missions of the 1970s made the last measurements of atomic oxygen in the Martian atmosphere. The ionized gases in the solar wind can interact with the wind's magnetic field to form an electric field, and accelerate the newly charged particles. Such magnetic field lines can connect with the planet's own magnetic fields, providing different routes for particles to travel either towards or away from Mars. The escape of the atmospheric ions likely also is facilitated through the magnetic reconnection which occurs at Mars between the solar wind and the pockets of magnetic fields. Ions are propelled down the magnetotail into space. Particles at Earth may originate on the night side before being energized and accelerated through interactions with Earth’s magnetic field and reappear at the dayside, with lighter hydrogen particles lost in collisions with the atmosphere and leaving an oxygen-rich plasma only. Or a atmosphere may change in composition. It was the case at Earth where first organisms using photosynthesis turned a mostly nitrogen, methane and ammonia atmosphere into a mostly oxygen one, about 2.4 billion years (the move has been called the 'Great Oxydation Event'). Venus, with a thick atmosphere replenished by volcanoes had its atmosphere's losses insignificant. Plasma tails resulting from the atmosphere's escape are observed at planets like Venus and Mars, or at Pluto like recently observed

The role of the atmosphere is mostly to shield the planet from Sun's harmful radiations like the UV or the X-rays. The shield is acting too against other high-energy events like the cosmic rays. Any planetary atmosphere is interacting with the solar radiation though. An atmosphere is moderating the energy balance at a planet. First, it's moderating the infall of solar energy to the surface. The planet's surface is moderately heated only. On the other hand the atmosphere is forbidding the diurnal heat to radiate back into space at night. The planet is retaining a part of the heat of the day. The difference between the diurnal, and nocturnal, temperatures is feeble, like at Earth. On bodies like Mercury or Moon, which have no atmosphere, the temperature gradient is much more high. A stratosphere is a layer of atmosphere in which temperature increases with higher altitudes. A stratosphere is found, generally, at all planets, the exoplanets included. Sunlight is able to penetrate deep into a planet's atmosphere, where it raises the temperature of the gas there which radiates its heat as infrared light. In the Earth's stratosphere, ozone gas traps ultraviolet radiation from the Sun, which raises the temperature as methane is responsible for heating in the stratospheres of Jupiter and Saturn's moon Titan, for example. A stratosphere generally acts like a sunscreen layer to a planet. 'Energy crisis,' in terms of planets, is a problem in which upper-atmospheric temperatures are measured hundreds of degrees hotter than can be explained by sunlight alone. On the other hand, from what is presumed at Venus, and from what is about certain at Titan, the main Saturn's moon, it seems that some interaction between Sun radiation and atmosphere are manufacturing there either primordial organisms, or prebiotic compounds which, falling to the surface, form the basis to a further possible apparition of life. Abundant atmospheric oxygen at a planet has often been treated as a so-called biosignature, or a sign of extant life, but this process does not require life as, like at Mars, it may form from ionizing solar radiation splitting water molecules into hydrogen and oxygen. In terms of what role a haze in a atmosphere has, it could serve up a smorgasbord of carbon-rich, or organic, molecules that could be transformed by chemical reactions into precursor molecules for life. Haze also might screen out much of the harmful UV radiation that can break down DNA. In the worst, haze could become so thick that very little light gets through surface, to get so cold it freezes completely

The atmosphere of a planet too is determining the weather found there. The weather in turn may help shape the terrain through the wind erosion processes, for example, or rains and other factors. Global circulation model of the planetary atmospheres are modeled from ones developed for Earth and Mars, adding in new data on topography and shape, and gravity data. As planetary climate models become more sophisticated, they will include the radiative effects of the clouds seen in data from the Mars Climate Sounder on NASA's Mars Reconnaissance Orbiter. Temperature difference in a atmosphere is a major contributor to global weather at a planet, with the energy for that coming either from the Sun, or from the planet self. Weather at Earth is mainly driven by the Sun's heating and do not mainly occur because of the condensation of water. On a gas giant, at the contrary, it is the condensation heating which drives the storms as the solar influence is lesser. Round, swirling vortices are part of the general circulation in the atmospheres of all four giant, outer planets and many mobile ones rolling through Saturn’s clouds are extant. While vortices are often informally referred to as storms, scientists generally reserve that term for bright, short-lived bursts of convection that punch though the clouds, often accompanied by lightnings. A Kelvin wave is a fundamental part of a planetary atmosphere. In Earth’s atmosphere, Kelvin waves are involved in a tropical wind pattern, or the 'quasi-biennial oscillation,' which can also be a quasi-quadriennal pattern, whose influence can reach as far as the polar vortex. The structure of a Kelvin wave is determined by a balance between the Coriolis force generated by the planet’s rotation and a boundary of some kind. In Earth’s oceans, that boundary could be the coastline. A Kelvin wave exists in Jupiter’s stratosphere as the zone near the equator serves as a boundary. A Kelvin waves exists at Saturn and it repeats roughly every 15 Earth years. 'Atmospheric gravity waves,' which are buoyancy waves, are known to occur on Earth, Mars and likely Pluto as well as they are due to air flowing upon a mountain range. Rossby waves were discovered in Earth’s atmosphere in the late 1930's as they help to steer the planet's jet streams and weather patterns. Driven by a planet's rotation, they’ve been seen in the atmospheres of other planets, as well as in Earth’s oceans. In theory, these waves can form in any rotating fluid as they also exist at our Sun

check a table of data about planets' atmospheres