Hydrodynamic escape
This mechanism may explain why some planetary atmospheres are depleted in oxygen, nitrogen, and heavier noble gases, such as xenon.
[3] 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 (solar EUV heating), momentum transfer from impacting meteoroids or asteroids, or the heat input from planetary accretion processes[4] may provide the requisite energy for hydrodynamic escape.
Such conditions may have been reached in H- or He-rich thermospheres heated by the strong extreme ultraviolet radiation flux of the young Sun.
Estimating the rate of hydrodynamic escape is important in analyzing both the history and current state of a planet's atmosphere.
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.
can be expressed as: where (in SI units): 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.
Matching these ratios can also be used to validate or verify computational models seeking to describe atmospheric evolution.
[14] Exoplanets that are extremely close to their parent star, such as hot Jupiters can experience significant hydrodynamic escape[15][16] to the point where the star "burns off" their atmosphere upon which they cease to be gas giants and are left with just the core, at which point they would be called Chthonian planets.
Hydrodynamic escape has been observed for exoplanets close to their host star, including the hot Jupiters HD 209458b.
Younger stars produce more EUV, and the early protoatmospheres of Earth, Mars, and Venus likely underwent hydrodynamic escape, which accounts for the noble gas isotope fractionation present in their atmospheres.
, which can only be achieved during the first 100 Ma of Earth’s history when the EUV flux from the young Sun was sufficiently strong.
[19] However, from the analysis of ancient atmospheric gases trapped in fluid inclusions contained in minerals of Archean (3.3 Ga) to Paleozoic (404 Ma) rocks, it has been observed that the fractionation of atmospheric Xe was still ongoing at about 2.1 Ga before.
[20] Ionized Xe+ can interact with H+ protons via the strong Coulomb force, which effectively decreases the binary diffusion coefficient b(Xe+, H+) by several orders of magnitude compared to the case of neutral Xe.
