<p>One of the means of atmospheric escape is the <a href="https://en.wikipedia.org/wiki/Hydrodynamic_escape" rel="nofollow noreferrer">hydrodynamic escape</a>. <br>
I read someplace that this doesn't cause a change in the isotopic ratio of elements in the atmosphere. But if you heat a group of molecules with a certain amount of thermal energy, the velocities they acquire will be inversely related to their masses, and one could say that eventually, more lighter isotopes will acquire the escape velocity than heavier ones and the heavier isotopes would remain.</p>

CC BY-SA 4.0
2021-12-19T17:35:51.793Z
Re *I read someplace that this doesn't cause a change in the isotopic ratio of elements in the atmosphere.* Wherever you read that, either you misunderstood what was written or the article you read was completely wrong.
David Hammen
CC BY-SA 4.0
2021-12-20T07:44:28.133Z
@DavidHammen It was given as a question on a quiz in an online course by Caltech. Quite credible, I should think. But I am certain about my reasoning as well.
Ambica Govind
CC BY-SA 4.0
2021-12-20T20:11:14.493Z
It would depend on what mechanism they exactly meant by hydrodynamic escape. There are two limits of hydro escape, the energy-limited regime and the parker-wind like regime. In the energy-limited regime, if the fractionating species is also the one absorbing all the solar energy, then the Caltech quiz might be right. Otherwise, if the fractionating species is dragged along, by a different, but escaping species, then no matter if we are energy-limited or Parker-like, the drag parameter depends on the mass and will always lead to a change in isotopic ratio.
AtmosphericPrisonEscape
CC BY-SA 4.0
2021-12-21T07:28:04.797Z
@AtmosphericPrisonEscape Could you please post an answer?
Ambica Govind
<atmosphere>
Why does hydrodynamic escape not change isotopic ratio of elements in the atmosphere?
Ambica Govind
# Why does hydrodynamic escape not change isotopic ratio of elements in the atmosphere?

(Asked by: username_0 on 2021-12-19)

One of the means of atmospheric escape is the [hydrodynamic escape](https://en.wikipedia.org/wiki/Hydrodynamic_escape).   
I read someplace that this doesn't cause a change in the isotopic ratio of elements in the atmosphere. But if you heat a group of molecules with a certain amount of thermal energy, the velocities they acquire will be inversely related to their masses, and one could say that eventually, more lighter isotopes will acquire the escape velocity than heavier ones and the heavier isotopes would remain.

**Tags:** atmosphere

> **Comments:**
> **username_1**: Re *I read someplace that this doesn't cause a change in the isotopic ratio of elements in the atmosphere.* Wherever you read that, either you misunderstood what was written or the article you read was completely wrong.  
> **username_0**: @username_1 It was given as a question on a quiz in an online course by Caltech. Quite credible, I should think. But I am certain about my reasoning as well.  
> **username_2**: It would depend on what mechanism they exactly meant by hydrodynamic escape. There are two limits of hydro escape, the energy-limited regime and the parker-wind like regime. In the energy-limited regime, if the fractionating species is also the one absorbing all the solar energy, then the Caltech quiz might be right. Otherwise, if the fractionating species is dragged along, by a different, but escaping species, then no matter if we are energy-limited or Parker-like, the drag parameter depends on the mass and will always lead to a change in isotopic ratio.  
> **username_0**: @username_2 Could you please post an answer?  

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