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Imagine you have a world that is earthlike enough for humans (or nearly human-equivalent creatures) to exist on it, so naturally, this world also has a moon.

I imagine the moon to be roughly the same size as the Earth's moon (since the planet this moon orbits is also roughly Earth-sized), but I have never thought about its composition (the type of rock or material it is made up of), since I don't have enough knowledge about that to make an educated guess, that would still be believable.

Would it be possible for this moon (as opposed to the Earth's moon) to have a breathable atmosphere?

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  • $\begingroup$ The answer depends on your definition of "science". If you allow faster-than-light travel, then you have left the realm of science and are into the realm of science fiction and/or fiction. For the purpose of your story, you could just state that the moon is habitable and has an atmosphere. Perhaps some people are curious as to how it can be, but most people are happy to have a place to live. $\endgroup$
    – DwB
    Commented Apr 7, 2023 at 19:20
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    $\begingroup$ Titan, with only 83% more mass than the Moon, has a very thick atmosphere. On the other hand, Ganymede, more than twice the Moon's mass, has very little. $\endgroup$
    – J.G.
    Commented Apr 7, 2023 at 19:28

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As you probably know, Earth moon has no appreciable atmosphere.

This is because the combination of its low escape velocity, temperature and lack of magnetic field can allow for just traces of xenon to be kept long enough around it.

enter image description here

Giving a magnetic field to a body like the Moon is difficult: it is small and cools down pretty quickly on cosmic scales, and it also has too little iron to have an appreciable dynamo effect.

If you can't stop the leak, you can still think of pumping more in: an active volcanism might help in releasing more gases in the moon atmosphere, and if the net balance is positive, the atmosphere will grow thicker over time.

However, for volcanism we have the same considerations as for the magnetic field: the little body cools down quickly.

Unless you place it closer to the planet so that the tidal effects keep pumping energy into the planet, heating it up. And also this would not last too long, since the energy put into heating the moon is taken away from the orbiting energy, altering the orbital distance.

Moreover, volcanism alone won't give you a breathable atmosphere, not for humans at least. For that you would need the whole shabang of life and carbon cycle we have on Earth.

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  • $\begingroup$ This is a more thorough explanation than mine. Thank you very much! $\endgroup$
    – cconsta1
    Commented Apr 7, 2023 at 8:57
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    $\begingroup$ The chart is very cool but warrants a source and explanation. Which temperature is the x-axis (planet core or surface)? Do the colored bands mean this is the conditions at which this molecule will be retained in the atmosphere (I'm assuming so, but it took some staring to figure it out)? What are the unlabeled bands at the top of the diagram (above hydrogen)? How does a magnetic field affect it? $\endgroup$
    – Zags
    Commented Apr 7, 2023 at 16:53
  • $\begingroup$ @Zags Looks like surface temperature--earth's core is definitely hotter than 300 K. $\endgroup$
    – Hearth
    Commented Apr 7, 2023 at 22:42
  • $\begingroup$ @Hearth: Which makes sense; we're talking about how much energy the atmosphere has to aid in escaping the gravity well; doesn't matter how hot the core is except insofar as it heats the atmosphere. Eyeballing it, Earth is at 290 K, which seems reasonable for surface temp (it's low 60s Fahrenheit, mid-upper teens Celsius). $\endgroup$
    – ShadowRanger
    Commented Apr 8, 2023 at 1:25
  • $\begingroup$ I was thinking: if the species didn't come to exist on there through evolution, then it could be that the magnetic field was a megaproject by the arriving species to set up protection to create a good enough atmosphere i.e. solar powered electromagnets at each pole or even embedded into the core something like the magnetic version of the three gorges dam $\endgroup$
    – Harry Mu
    Commented Apr 9, 2023 at 4:02
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Assuming that:

a) the moon is similar to the Earth's moon in terms of density b) the assumed atmosphere would be Earth-like, that is the temperature would be roughly 20 Celcius and the pressure about 1 atmosphere,

then no it's not possible for the moon to have a breathable atmosphere. This is because the escape velocity of the moon would be about $2300$ m/s (see here), while the distribution of thermal velocity for gases such as oxygen is in general above the escape velocity of the moon. You can check the distribution (called Maxwell-Boltzman) here.

Take into account that gases higher up in this imaginary atmosphere will have high velocities due to the heat from the star. Finally, planets such as Mars do not have a thick atmosphere for the same reason.

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Sure. As per the other answers the thing that stops the moon having an atmosphere is the gravity is not strong enough to hold it. however, the moon is only 3.3g/ cm^3, all we need to do is increase its density by a factor of 5 and it will have a comparable gravitational field as earth, and comparable escape velocity. Make the core of the moon predominantly out of Uranium, (or other very heavy metal) and you will get there, which also takes care of the question as to why it doesn't cool down. How on earth such a moon forms with so much Uranium is another question. Such a heavy moon will have a severe tidal effect on the earth.

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  • $\begingroup$ Exactly how ''severe'' are we talking here? (i might be able to use such an effect) $\endgroup$
    – Blue Devil
    Commented Apr 9, 2023 at 13:25
  • $\begingroup$ Order of magnitude, the mass will be 5x greater, so the half of tidal forces due to the moon would be 5x greater, I would expect tides on the earth would rise and fall approximately 2.5 - 3x as far as they do on the earth. The crust will flex more as well, so I would expect that the earth to be more Geologicially active, but this is going to be impossible to estimate. $\endgroup$
    – camelccc
    Commented Apr 9, 2023 at 15:55
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The earth's core stays warmer due to radioactive decay. Perhaps a moon with a large iron core infused with a high concentration of radioactive elements would

  1. have enough gravity to keep oxygen in the atmosphere
  2. enough radioactivty to stay warm for eons
  3. enough magnetic field to prevent the solar wind from blowing the atmosphere away.
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    $\begingroup$ Mercury (4880 km diameter) is slightly larger than Moon (3476 km), has a larger iron core than Moon and yet it is geologically dead. $\endgroup$
    – L.Dutch ♦
    Commented Apr 8, 2023 at 3:48
  • $\begingroup$ The issue here isn't geological activity but ability to support an oxygen atmosphere. Mercury's core has cooled; which was the purpose of introducing radioactivity to keep the core warm. $\endgroup$
    – Rocky Rococo
    Commented Apr 8, 2023 at 23:22

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