Atmospheric escape

Atmospheric escape is the loss of atmospheric particles from a celestial body into space through thermal or nonthermal processes. It changes atmospheric composition and can determine whether a world retains a substantial atmosphere.

Jeans escape: Thermal escape in which particles from the high-speed tail of a gas distribution exceed escape velocity from the exobase. This is the classic thermal route for light particles that can outrun gravity without collisions.
Exobase: The atmospheric level above which particles are unlikely to collide with one another before traveling substantial distances. Thermal escape is commonly assessed at this boundary, where particle speeds become decisive.
Mars atmosphere: The thin envelope of gases surrounding Mars, composed mainly of carbon dioxide and shaped by billions of years of loss and surface exchange. Mars's depleted atmosphere provides a nearby case for studying escape over planetary time.
Thermal escape: Atmospheric loss caused by the thermal motion of particles in a body's upper atmosphere. It groups Jeans escape and heated bulk outflow by their dependence on atmospheric heat.
Hydrodynamic escape: Bulk atmospheric outflow driven by intense heating, capable of carrying heavier species along with lighter gases. Strong heating can make the atmosphere flow outward rather than lose particles one by one.
Escape velocity: The minimum speed an object needs to escape a body's gravitational field without further propulsion. It sets the speed threshold that escaping atmospheric particles must overcome.
Venus atmosphere: The dense, carbon-dioxide-dominated atmosphere surrounding Venus, with a small amount of water vapor and a high surface pressure. Its retained heavy atmosphere contrasts with the escape of lighter hydrogen from water.
Nonthermal escape: Atmospheric loss driven by processes other than the random thermal motion of particles. It distinguishes escape powered by chemistry, plasma interactions, or impacts from thermal loss.
Photochemical escape: Loss of atmospheric particles energized by chemical reactions initiated by sunlight or energetic radiation. Chemical reactions can give atoms enough energy to escape without heating the whole atmosphere.
Scale height: The vertical distance over which atmospheric pressure decreases by a factor of e in an approximately isothermal atmosphere. It helps determine how much gas reaches the thin upper atmosphere where escape occurs.

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