Hydrodynamic escape

In atmospheric science, hydrodynamic escape refers to a thermal atmospheric escape mechanism that can lead to the escape of heavier atoms of a planetary atmosphere through numerous collisions with lighter atoms. Hydrodynamic escape occurs if there is a strong thermally driven atmospheric escape of light atoms which, through drag effects (collisions), also drive off heavier atoms. The heaviest species of atom that can be removed in this manner is called the cross-over mass. In order to maintain a significant hydrodynamic escape, a large source of energy at a certain altitude is required. Soft X-ray or extreme ultraviolet radiation, momentum transfer from impacting meteoroids or asteroids, or the heat input from planetary accretion processes may provide the requisite energy for hydrodynamic escape. Estimating the rate of hydrodynamic escape is important in analyzing both the history and current state of a planet's atmosphere. In 1981, Watson et al. published calculations that describe energy-limited escape, where all incoming energy is balanced by escape to space. Recent numerical simulations on exoplanets have suggested that this calculation overestimates the hydrodynamic flux by 20 - 100 times.[30] However, as a special case and upper limit approximation on the atmospheric escape, it is worth noting here. Hydrodynamic escape flux (Φ, [m^-2s^-1]) in an energy-limited escape can be calculated, assuming (1) an atmosphere composed of non-viscous, (2) constant-molecular-weight gas, with (3) isotropic pressure, (4) fixed temperature, (5) perfect extreme ultraviolet (XUV) absorption, and that (6) pressure decreases to zero as distance from the planet increases. where (in SI units): F_XUV is the photon flux [J m^-2s^-1] over the wavelengths of interest, R_p is the radius of the planet [m], G is the gravitational constant [ms^-2], M_p is the mass of the planet [kg], R_XUV is the effective radius where the XUV absorption occurs [m]. Corrections to this model have been proposed over the years to account for the Roche lobe of a planet and efficiency in absorbing photon flux.