Earth's atmosphere stretches outwards from our planet’s surface, with the most distant part extending to well beyond the lunar orbit – almost twice the distance to the Moon - where it then blends into space. The majority of our atmosphere comprises Nitrogen (78%) and Oxygen (21%), followed by Argon (0.9%), as well as a variable amount of water vapour, which is, on average, around 1% at sea level and 0.4% over the entire atmosphere. Gases like carbon dioxide, oxides of nitrogen, methane, and ozone are trace gases that together account for about a tenth of one percent of the atmosphere. The atmosphere becomes thinner and thinner with increasing altitude, with no definite hard boundary between it and outer space.

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Earth’s Atmosphere is divided into several layers based on their temperature. These are the troposphere, stratosphere, mesosphere, and thermosphere.

• The troposphere is the lower most atmospheric layer in direct contact with Earth's surface and containing about 80% of the atmospheres total mass. Most weather phenomena occur here, and atmospheric temperature decreases strongly with altitude. The troposphere extends out to between 8 and 14.5 km altitude above the surface, with its top being significantly higher at the equator than at the poles.
• The stratosphere is the next atmospheric layer, holding around 19% of the total atmospheric mass and extending to around 50 km altitude. It contains Earth’s protective ozone layer and, unlike the troposphere, here temperature increases with altitude due to the formation and presence of stratospheric ozone, which strongly absorbs UV radiation from the Sun.
• The mesosphere starts just above the stratosphere and extends to around 80 km altitude. Here atmospheric temperature once again decreases with height, and noctilucent clouds, the highest clouds in Earth’s atmosphere, form in this region.
• The thermosphere extends out to around 700 km altitude and is free of clouds and water-vapour. Temperatures once again increase with altitude, because of the absorption of energetic solar ultraviolet and X-ray radiation. Air density is so low that most of the thermosphere is what is normally thought of as outer space, and most polar orbiting Earth Observation satellites orbit in or close to this layer.

Above the thermosphere, the exosphere extends further outwards, being composed of particles of hydrogen and helium so widely separated they rarely collide.

In terms of Earth’s atmospheric composition and its probing through Earth observation, the troposphere and stratosphere are generally considered by far the most important layers to target.

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