An atmosphere is an envelope of gases around a planet or moon. In the case of gas giants, the atmosphere makes up most of the volume of the planet. By contrast, Earth's atmosphere is generally considered to be well under 1000 kilometers (600 miles) thick, although its upper reaches are so thin that it's difficult to draw a clear dividing line between the upper atmosphere and the interplanetary vacuum. Above 300 kilometers or so (200 miles), the atmosphere is so thin that satellites can orbit through it for decades without being slowed down significantly by friction with the air.
Earth's atmosphere serves as one of main transport systems for nutrient cycles within Gaia, along with the hydrosphere and possibly the mantle. James Lovelock argues that Mars's atmosphere is so thin and so close to chemical equilibrium that it indicates an absence of life on that planet.
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