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Atmosphere, gaseous envelope surrounding a solid or liquid; especially that surrounding the Earth and other planets.
Sometimes it comes to the atmosphere also for the Sun and the stars, which are completely gaseous bodies: in this case are per atmospheres the outer and less dense layers.
The Earth‘s mass is one-millionth that of Earth; 99% is below 50 km altitude and 90% to below 17 km away.
The Earth’s atmosphere is not a “casing” for homogeneous and this is divided into various layers having different characteristics. The Earth’s layers, starting from the ground, are five: the troposphere, stratosphere, mesosphere, thermosphere, exosphere, while in much of the mesosphere and the thermosphere includes the so-called ionosphere. The gas planetary constituents undergo a constant leakage outer space, favored solar radiation and hindered by the force of gravity exerted by the mass of the planet; the atmosphere is also supplied with gas from any new volcanic eruptions that may take place on the planet’s surface. Therefore, the largest planets and/or geologically active and/or more distant from the Sun tend to more easily maintain an atmosphere. The planet Mercury, being small and being a short distance from the Sun currently has only traces; it is the only planet in the solar system that has this feature.
The presence of free oxygen is only the prerogative of the atmosphere of Earth, while in all the other planets of the solar system atmospheres studied so far if they have not found a trace. A planetary atmosphere containing oxygen gas in large quantities is not chemically balanced: in fact, oxygen is an extremely reactive gas, which over time combines completely with rocks and other surface compounds, oxidizing them and disappearing from the atmosphere. Its presence on Earth is a consequence of the biological activity of plants, which produce it in large quantities as a byproduct of photosynthesis: for this reason, we consider the presence of gaseous oxygen in appreciable quantities in the atmosphere of a given planet as an indicator of the presence of life on it. Also, the atmosphere has a “filter” function because it allows the light and heat of the Sun to reach the planet, but it prevents harmful solar radiation to life come in large quantities (X or UV rays).
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