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I am trying to find what would be a stable atmosphere for a planet, using this https://docs.google.com/spreadsheets/d/1cW7BIWlBUscqS9MVqs5gSbPH0OGzyE-j2cz6ADBSVzE/edit#gid=0 (you have to make a copy to use it, as editing is disabled for obvious reasons.)

It calculated the stability of an atmosphere of a certain compostion depending on the gravity of a planet (which it calculates from mass and radius, of course).

When I make the atmosphere of a planet at 0.7g, a large amount of elements become unstable. H, H2, He, N, O, CH4, NH3, H2O, and Ne become unstable. As compared to the H, H2, and He of it based on Earth's gravity. I tried to change values to find what would be stable, but I am having difficulty.

I am not sure what formulae it actually uses, so I am confused on how I would be able to find atmospheric stability at different gravities, with what percentages I should do for it. Because, from what I can find, a planet of 0.7g would be able to hold in such an atmosphere. I just can't find how exactly to find how I should adjust the percentages to make it such.

This isn't just about finding the way it works with this sheet, but just a general idea of how I would work to find what atmospheric composition is needed to retain stability at non-Earth gravities. Based on actual science. It is this latter part of the fact I am trying to find deeper knowledge of how to find stable atmospheric compositions that leads me to use Hard Science.

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    $\begingroup$ There is a beautiful graph floating around, for example shown at What is this atmospheric escape chart actually showing? on Physics Stack Exchange. Note that what gas can escape and what gas is trapped depends as much on temperature as on the escape velocity of the planet. $\endgroup$
    – AlexP
    Sep 25 at 19:25
  • $\begingroup$ @AlexP I am sorry, but this doesn't exactly help as it just says that everything I have works. When the graph I displayed says it very much doesn't and a very large amount of elements would escape. As I stated, H, H2, He, N, O, CH4, NH3, H2O, and Ne all escape. $\endgroup$
    – DanceroftheStars
    Sep 25 at 20:32
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    $\begingroup$ Well, there can be no H, N, or O in any kind of resonable atmosphere, only H2, N2, and O2. Those are twice as heavy as their monoatomic estranged siblings. H2 and CH4 burn in oxygen, so that you can either have O2, or CH4 and H2. And Ne escaping is irrelevant -- there can be no sizeable amount of neon in a rocky planet's atmosphere anyway. $\endgroup$
    – AlexP
    Sep 25 at 20:45
  • $\begingroup$ P.S. I have downloaded the worksheet, and the only gas of interest which might escape is water vapor. Which is fine, as there is no water vapor at high altitude in the atmosphere of an Earth-like planet, because it condenses to liquid water and ice looong before that. (The upper layers of the atmosphere are really cold.) (It says that monoatomic N and O can escape, but not molecular N2 and O2, which are what is of interest.) $\endgroup$
    – AlexP
    Sep 25 at 20:51
  • $\begingroup$ I see, I am just concerned as I put in the ratios for Earth's atmosphere, with Earth's mass and such, and it returned it as everything but H, H2, and He being stable. But in my own calculation for 0.7g, far, far more is released into space. Which doesn't seem like it would make for any sort of breathable atmosphere. $\endgroup$
    – DanceroftheStars
    Sep 25 at 20:56

1 Answer 1

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In order to retain gases, as shown in the chart in the comments by AlexP:

enter image description here

To increase the molecular weight retained, it is necessary to reduce the speed of the molecules' brownian motion by reducing the planetary temperature, or increase the escape velocity by increasing the planetary mass. Surface gravity and escape velocity are not necessarily directly linked. By changing the density and diameter of a body, escape velocity may be minimised or maximised while maintaining a constant surface gravity.

The reason for this is that lighter molecules travel faster for a given temperature than a heavier molecule. If the molecule is at the edge of the atmosphere and exceeds the escape velocity away from the planet, it will escape. Colder molecules travel slower, and hence are less likely to escape, and heavier planets have higher escape velocities.

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