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I am trying to figure out ways to determine the density and composition of an Earth-like planet. For the sake of the "experiment", let's assume that the planet is identical to Earth in every physical way except it is 1.25 times the mass. If the Earth's atmospheric composition is 78% Nitrogen, 21% Oxygen and the rest various other gases, what would the atmospheric composition be if the planet were more massive?

Let's further assume that the planet formed in the same way and the moon came about the same way.

What I am curious about is: does this difference cause far lighter elements to be in the atmosphere due to a higher escape velocity? And if so, would the gravity cause a greater atmospheric pressure?

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    $\begingroup$ (1) Nope, the mass of the planet does not determine the composition of its atmosphere. Earth and Venus have just about the same mass, but very different atmospheres. And even Earth alone started out with a very very different atmosphere; the current atmospheric composition is the work of billions of years of biological activity. Four billion years ago, Earth's atmosphere consisted mostly of nitrogen and carbon dioxide. (2) The most important factor for atmopsheric pressure is simply how much atmosphere a planet has. Venus and Earth have the same mass, but Venus has a lot more atmosphere. $\endgroup$
    – AlexP
    Jan 23, 2021 at 9:40
  • $\begingroup$ @AlexP thanks, that's more or less what I was expecting to see for an answer. Out of curiosity, are you aware of any programs that can be used to play around with atmospheric composition? $\endgroup$
    – Markitect
    Jan 23, 2021 at 15:13
  • $\begingroup$ I agree life would profoundly alter planetary gasses. Anyone know if the Argon on Mars is simply an effect of so much less other gasses, or particular to local conditions? How about the effect of proximity to the sun blowing off gasses? Would a bigger Mars have more gasses than Earth (and thus be potentially cold but habitable, or even retain more heat and possibly be warmer than it is now)? $\endgroup$
    – DWKraus
    Jan 23, 2021 at 16:10

2 Answers 2

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Take Earth, Venus and Mars.

Earth:

By volume, dry air contains 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, and small amounts of other gases.

Venus:

The atmosphere of Venus is the layer of gases surrounding Venus. It is composed primarily of carbon dioxide and is much denser and hotter than that of Earth. The temperature at the surface is 740 K (467 °C, 872 °F), and the pressure is 93 bar (1,350 psi),

Mars:

It is primarily composed of carbon dioxide (95.32%), molecular nitrogen (2.6%) and argon (1.9%). It also contains trace levels of water vapor, oxygen, carbon monoxide, hydrogen and other noble gases. The atmosphere of Mars is much thinner than Earth's. The average surface pressure is only about 610 pascals

Surface gravity can have some effect on the species trapped in the atmosphere of a planet, but that's only one factor. Life on Earth for example has dramatically altered its atmospheric composition, introducing all the oxygen we breathe today and removing most of carbon dioxide.

enter image description here

As you can see from the chart above, for the same surface temperature a slightly more massive Earth could start holding some helium in its potential atmosphere.

The real atmosphere would depend also on additional local processes, like presence/absence of life, volcanism and so on.

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For the first question, The atmosphere would not be influenced by the mass of a planet, because you can see that the atmosphere of Venus is much denser than Earth's.

The composition being the same as the Earth's, but the mass different. Of course, there are other important factors, like if life ever developed, in which case the atmosphere would change and add oxygen or carbon depending on the type of organism.

Since there is a higher density, the atmosphere gasses will compact and get heavier, because of the gravitational effect, and there would be a higher atmosphere density.

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