Talk:Hydrodynamic escape

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Wiki Education Foundation-supported course assignment

This article was the subject of a Wiki Education Foundation-supported course assignment, between 2 April 2019 and 28 June 2019. Further details are available on the course page. Student editor(s): Atmospheric Anna. Peer reviewers: Andrew Shumway.

Above undated message substituted from Template:Dashboard.wikiedu.org assignment by PrimeBOT (talk) 22:44, 17 January 2022 (UTC)[reply]

Not sure this is quite right

The page suggests that hydrodynamic escape is only the escape of larger molecules caught up in the flow of hydrogen. But this article suggests that the flow of hydrogen itself is increased due to flow of the atmosphere en masse rather than molecule by molecule - and this in turn is powered by UV absorption of heat - presumably by the other molecules - and it in turn does drag along the heavier molecules with it as the page suggests.

Anyone expert on this care to review?

From page 4 of this paper [1]

"A second type of thermal escape is far more dramatic than Jeans’ mechanism. Jeans’ escape applies when a gas evaporates molecule by molecule from an exobase. But if conditions favor faster escape, the air flows into the vacuum of space, pushed along by pressure from below. This can occur if the bulk gas in the upper atmosphere is a good absorber of ultraviolet light and the heated air flows en masse. Unlike Jeans’ case, the bulk atmosphere is no longer static. The atmosphere flows, accelerates smoothly through the sound speed, and then attains the escape speed and higher. This form of thermal escape is called “hydrodynamic escape” or the “planetary wind,” the latter by analogy to the solar wind, the thermal wind of charged particles blown from the Sun into interstellar space."

"As the lightest gas, hydrogen is the one that most easily overcomes a planet’s gravity, so atmospheres rich with hydrogen are the most prone to hydrodynamic escape. During hydrodynamic escape, hydrogen can drag along heavier molecules and atoms. A loose analogy is to wind-blown dust: without the wind the dust goes nowhere, but a strong wind can lift the dust. Another point of analogy is that the hydrogen wind preferentially carries off molecules and atoms of low mass, the lighter the better, much as the desert wind can blow dust across an ocean and sand grains from dune to dune, while leaving cobbles and boulders behind. This analogy helps us understand that during hydrodynamic escape of hydrogen, atoms heavier than hydrogen will be dragged upwards at a rate depending upon a competition with gravity. For isotopes (atoms of the same chemical element with different masses), ancient episodes of hydrodynamic escape will leave behind telltale traces by removing lighter isotopes preferentially."

Robert Walker (talk) 23:49, 9 September 2017 (UTC)[reply]