Meteorites give the Moon its thin atmosphere | Hacker News Reader

Meteorites give the Moon its thin atmosphere

arstechnica.com

36 pointsBrajeshwar13 comments

One thing not clear to me after reading the article: Are these "atmospheric" particles in orbit around the moon, or is there enough collisions between them (actual pressure) to keep them afloat? Or both?

According to https://www.ias.ac.in/article/fulltext/jess/114/06/0637-0644, the mean free path of particles in the moon's “atmosphere” is thousands of km.

This makes me think it's probably a combination of suborbital and out-gassing during daylight, with the occasional collision boosting a molecule to orbital or escape velocities. Which technically would be pressure I guess, but not on normal timescales.

That link talks about gas molecules, but the article is saying ions. Whether or not these are ionized particles would have a big impact on what's happening.

> Whether or not these are ionized particles would have a big impact on what's happening

The lunar atmosphere is theorised to come from two sources: impact vaporisation and ion sputtering. When immediately produced, the latter will produce ionised particles wihle the former will have no preference. Both will be subject to later photo-ionization once...erm...airborne.

> Are these "atmospheric" particles in orbit around the moon

No.

They "hop on ballistic trajectories until they are either returned permanently to the surface" or are "lost to space" [1].

[1] https://www.science.org/doi/10.1126/sciadv.adm7074

Which is why the term "atmosphere" is a misnomer.

There exist molecules is motion above the lunar surface. These molecules are too sparse to be solid or liquid. But also, too sparse to be gas.

> too sparse to be gas

This isn’t a thing. The interstellar medium, for instance, is gaseous. Vacuum vs gas isn’t clearly, uniformly defined because “true” vacuums really don’t exist.

When the mean free path is longer then a typical ballistic trajectory that these molecules and ions are typically on, they do not exhibit any properties associated with gas. Importantly, the property of expanding under pressure to fill a volume is not exhibited.

What if we gave the moon a sulfur hexafluoride atmosphere?

Well, there's not a lot of S or F there so it would be expensive. Also, I think the solar wind would tend to lead to significant charge build up (and plasma discharge?) as it was slowly blown away. Might be pretty?

That was the first heavy gas I could think of - can you think of anything we could make in situ? How bad would siloxane gasses be?

Might also be useful for Mars *

* Maybe not in real life, but in my imagination

Isn't the moon within Earth's atmosphere?

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