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The thin upper part of the atmosphere surrounding a planet or satellite where the molecules are gravitationally bound to the body but the density is too low for them to behave like a gas. For the Earth, this tenuous layer forms the uppermost layer where the atmosphere thins out and merges with interplanetary space. It lies about 530km above the surface.

Of the other planets in the Solar System, Mercury is the one with no substantial atmosphere, but it does have a very thin layer of gas extending around it that's essentially an exosphere. Several moons, including the Moon and the Galilean satellites of Jupiter have exospheres even though they also have no dense atmosphere underneath. These are referred to as surface boundary exospheres.

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