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.
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