Knowra Exosphere Exosphere The outermost region of an atmosphere, where gas molecules are so sparse that they can travel long distances between collisions and may escape into space.
Exobase : The lower boundary of an exosphere, where a molecule’s mean free path becomes comparable to the atmospheric scale height. This boundary marks the transition from collision-dominated gas to the sparse exosphere.
Earth's exosphere : The outermost region of Earth's atmosphere, extending from the exobase into space. It is the familiar example, with hydrogen and helium among its escaping constituents.
Atmosphere : A layer of gases held around a celestial body by gravity. An exosphere is the outermost region of an atmosphere.
Magnetosphere : The region around a celestial body in which its magnetic field controls the motion of charged particles. The magnetosphere is defined by magnetic influence, not by atmospheric density or altitude.
Mean free path : The average distance a particle travels between collisions with other particles. Increasing mean free path explains why exospheric molecules can travel far between collisions.
Lunar exosphere : The Moon's extremely tenuous, collisionless envelope of atoms and molecules. The Moon's weak gravity and lack of a dense atmosphere make its exosphere especially exposed to space.
Thermosphere : The atmospheric layer above the mesosphere and below the exosphere in Earth's conventional layer scheme. In Earth's atmosphere, the exosphere lies above the thermosphere.
Interplanetary space : The space between planets, containing plasma, dust, and fields from the Sun and other sources. The exosphere transitions into space, but remains gravitationally associated with its body.
Jeans escape : The thermal escape of atmospheric particles whose speeds exceed a body's escape velocity. It describes how fast-moving exospheric particles can leave a planet without further collisions.
Mercury's exosphere : Mercury's tenuous, transient envelope of atoms released from its surface. Its exosphere is continually replenished by surface impacts, solar radiation, and solar-wind interactions.
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