The Moon currently has a very thin layer of gases that is not stable over time and that we can hardly consider as a true atmosphere . Of course we will use this word to refer to it, but this atmosphere will have little to do with that of Earth, Mars or even Titan, Saturn’s moon.
The lunar “atmosphere” exerts a pressure on the surface of the satellite one thousand billion times less than the Earth’s and in fact it is estimated that its total mass is around 20 tons , like a truck loaded with goods. In comparison , the Earth’s atmosphere accumulates a mass of about 5 thousand billion tons . The Moon’s atmosphere is composed mainly of Argon, Neon and Helium , which are produced as a result of nuclear disintegrations inside the moon or after interaction of other atoms with the wind or sunlight r.
We could think that the Moon could be like Mars [link to “This is how Mars lost its oceans”], that it had a dense atmosphere in the past and that it lost it when the magnetic field that protected it from the solar wind disappeared, but that this atmosphere could resurface and endure with proper planetary engineering. And although it is true that we think that in its origins our satellite had an atmosphere twice as dense as the current Martian atmosphere, which lasted for about 70 million years but ended up being swept away, the similarities with Mars end there.
At present it would be impossible for us to provide the Moon with an atmosphere again , for several reasons. The first is, as we have already mentioned, due to the absence of a magnetic field that repels the solar wind . However, this could be fixed relatively easily. Mechanisms have already been studied to create an artificial field around Mars, with some kind of coil that creates this magnetic field in space , protecting the Martian atmosphere.
But even with the solar wind out of the equation, the Moon would have another problem retaining a possible atmosphere, one we can’t remedy. This problem is a result of its small size and close proximity to the Sun. Basically, the Moon has too little gravity and is too hot (when hit by the Sun) to retain any gas lighter than Xenon . That is, all the gases that interest us.
This idea is reflected very elegantly in this graph. The colored bands show us the speed of the particles of a gas as a function of the temperature of that gas . Furthermore, we have the major planets and moons of the solar system placed according to two criteria: the temperature and the escape velocity at their surface . The escape velocity is nothing more than the minimum speed needed by any object (or particle) trying to escape the gravity of a planet to actually do so .
Looking closely at the image we can see, first looking at the planets and satellites represented, that the more massive objects will have a higher escape velocity. It will be harder to leave Jupiter or Saturn than Earth , but much easier to leave Mercury, Callisto or Pluto. On the other hand, bodies closer to the Sun will have a higher temperature. Mercury will be the hottest , followed by Venus (because here we are considering the average temperature of its atmosphere, which is very high on the surface, but lower in upper layers), Earth and Mars . The Moon will be slightly cooler than Earth (because it lacks an atmosphere), but much warmer than Ganymede, Titan, or Eris.
Looking now at the colored bands we will see that the lighter gases (hydrogen and helium) will have higher speeds than the heavier ones for the same temperature. Therefore, this graph answers our question: the Moon could not have an atmosphere composed of gases lighter than xenon . But let’s understand why.
As we can see, the giant planets have no problem retaining helium and hydrogen . During their formation they accumulated enough mass to retain these elements and that is why today they abound in their atmosphere. Earth and Venus , however, are considerably less massive and can hold methane, ammonia, and water , relatively light compounds (with molecular masses of approximately 16, 17, and 18 grams per mole, respectively), but not hydrogen and helium.
Mars , with 15% of the mass of our planet, can only hold oxygen and nitrogen molecules (32 and 28 grams per mole), albeit just barely. It is for this reason that its atmosphere is mainly composed of carbon dioxide (44 g/mol). The nitrogen, being lighter, will have gradually escaped from its surface, while the oxygen will have reacted with its rocks, oxidizing the Martian soil.
Ganymede and Titan , which have a similar mass and somewhat less than Mercury, would be able to retain an atmosphere , while the planet does not. Ganymede currently has no detectable atmosphere , probably due to interaction with the Jovian magnetic field, which will have dragged it with it, while Titan does have an atmosphere . Atmosphere composed of 95% nitrogen (with no trace of oxygen for the same reason as Mars) and 6% methane . This second gas, as we can see from the graph, should not be able to survive on Titan , and its presence has been confusing astrophysicists ever since it was detected.
Therefore, to ensure that the Moon could have an atmosphere with at least nitrogen, oxygen and carbon dioxide (like the Earth’s) we have two options: enlarge it or cool it . Neither is really feasible, nor attractive. We would have to give the Moon a mass similar to that of Mars (almost 9 times greater) or lower its temperature by 200 ºC . As you can see, neither option is a good idea. Therefore, any atmosphere that we try to create on the Moon, to facilitate its colonization and to protect us from solar radiation, will be doomed to disappear in a short time.
REFERENCES:
Needham, D. Kring, 2017, Lunar volcanism produced a transient atmosphere around the ancient Moon, Earth and Planetary Science Letters, 478: 175 DOI: 10.1016/j.epsl.2017.09.002
