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The Jeans parameter is a measure of an atmosphere's tendency to escape (atmospheric escape) through a thermal escape mechanism (Jeans escape or hydrodynamic escape). It is a ratio of the gravitational potential energy of particles in the atmosphere to their kinetic energy. In a (virtually) spherically-symmetric situation, it is a function of distance from the center of the body. The smaller the Jeans parameter, the more likely the escape, and thresholds have been devised to indicate whether such escape is a significant possibility.