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The Jeans parameter is a measure of a body's atmosphere's gas particles' 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 the particles to their kinetic energy. The smaller the Jeans parameter, the more likely the escape, and thresholds have been devised to indicate whether such escape is a significant possibility. In a (virtually) spherically-symmetric situation (such as surrounds an astronomical body), it is a function of distance from the center of the body, inversely-related. It also is directly-related to the mass of the particle, so a small-mass particle at a high altitude is most likely to escape.