How Windy would it get on the Moon if it had an Atmosphere?

[Warren Platts]

If the Moon had an atmosphere in the collisional regime, I figure the wind conditions would be extreme because the long day night periods would cause massive temperature/pressure differences between lit and unlit hemispheres. On the other hand, the Coriolis effect would probably be minimal compared to Earth; and the Moon's axis practically vertical relative to the Sun. So you might not a lot of north-south wind patterns.

The canonical references are Vondrak's paper on creating an artificial atmosphere, and Alan Stern's old paper on transient lunar atmospheres. In particular, Stern said we should expect every few million years that an asteroid or comet impact would create an atmosphere as thick as Mars'...

Strong lunar winds are the only explanation I can see for the strange fact noticed by Harrison Schmidt that large boulders on the Moon are not covered with blankets of regolith. To a first order estimate, what actual wind speeds should we expect if there was a 0.001 bar transient atmosphere, or even if there was a terraformed atmosphere of 1 bar?

Do you know of a publicly accessible simulation program that could model the Moon under such conditions?

[glappkaeft]

In particular, Stern said we should expect every few million years that an asteroid or comet impact would create an atmosphere as thick as Mars'...

Do you mean Sterns paper, "The Lunar Atmosphere: History, Status, Current Problems, and Context"?

If so I think you are misreading it, I can't even find any mention of Mars in it at all.

[Warren Platts]

Do you mean Sterns paper, "The Lunar Atmosphere: History, Status, Current Problems, and Context"?

If so I think you are misreading it, I can't even find any mention of Mars in it at all.

The point is a 10 mbar atmosphere is comparable in thickness to the atmosphere of Mars...

[Warren Platts]

I found this: Modeling a Transient Secondary Paleo‐Lunar Atmosphere: 3‐D Simulations and Analysis.

Fascinating, and they have nice charts showing the average surface temperature under various scenarios, but no mention of wind speeds.

[CJSF]

Strong lunar winds are the only explanation I can see for the strange fact noticed by Harrison Schmidt that large boulders on the Moon are not covered with blankets of regolith.

Really? No other observed phenomena that could transport particles on the Lunar surface? Nothing at all springs to mind that could account for this over the spans of perhaps millions of years?

CJSF

[Warren Platts]

Really? No other observed phenomena that could transport particles on the Lunar surface? Nothing at all springs to mind that could account for this over the spans of perhaps millions of years?

CJSF

Yeah. Really. Unlike you, I have studied electrostatic dust transport extensively. It only works on the smallest particles. Just ***LOOK*** at the photo I provided...

[tusenfem]

The point is a 10 mbar atmosphere is comparable in thickness to the atmosphere of Mars...

Yeah. Really. Unlike you, I have studied electrostatic dust transport extensively. It only works on the smallest particles. Just ***LOOK*** at the photo I provided...

Okay Warren Platts, that is more than enough of your tone. If you cannot discuss politely then please refrain.
One more time and you will get an infraction.

[Warren Platts]

Sorry. I will try again: the fact that boulders on the Moon are, for the most part not covered in dust happens to be one of those persistent mysteries in the lunar lore for which there is no good scientific explanation that has been published (at least as far as I know).

Yes, electrostatic dust transport has been proposed as a possible mechanism, but the forces involved are just too small. In fact, I once personally asked Mihaly Horanyi (Laboratory for Atmospheric and Space Physics & Department of Physics University of Colorado), the world's foremost expert on electrostatic dust transport, about this and he told me the same thing: that electrostatic dust transport will not work to clean the boulders.

Now, if we look at the famous picture of Harrison Schmitt standing next the split boulder (better image here) we can see the Sun is shining from the east, and, therefore, we are looking north. The boulder is mostly clear of dust and small rocks, but in the foreground, we can see there is in fact layer of dust and small rocks. There is a scrape mark where a sample was taken that gives a sense of the depth of the dust deposit--an inch or two at the most.

So, why is there this layer of dust and pebbles on the south side of the boulder but not elsewhere? Well, it seems to me (and yes this could be crazy) that if, sometime in the past, there was a strong wind from the north (due to a transient atmosphere formed by an impact or outgassing event as described in Alan Stern's paper), then the south side of the boulder would be in the lee with respect to the breeze, and I can see how some dust and debris could collect there.

Thus, it could be the case that we are looking at direct, physical evidence of a past, lunar, transient atmosphere!

But why should we expect a north wind? Shouldn't winds on the Moon on the be mainly easterly or westerly? I honestly don't have a clue; that is why I am these questions on this forum. Any and all comments are very welcome!

[Van Rijn]

Sorry. I will try again: the fact that boulders on the Moon are, for the most part not covered in dust happens to be one of those persistent mysteries in the lunar lore for which there is no good scientific explanation that has been published (at least as far as I know).

Really? The argument I read long ago involved micrometeor impacts. Micrometeor impacts create much of the dust and impart considerable energy to new and existing dust, sending some of it flying or moving. Dust will tend to collect at the lowest local low angle areas (usually the “ground” not the rock). Rocks are irregularly shaped, often with significant vertical angles so much of the dust will be knocked off, though some can collect in basins or in other areas that would tend to slow or prevent movement. It looks to me that some dust has collected in a basin in the large rock in your image, but not significantly elsewhere. Has there been some established scientific argument against this explanation?

[Van Rijn]

I think there was also an effect due to larger impacts - the reason there are large irregular rocks on a world without active geology in the first place is due to impacts throwing them about. Large impacts too can send much dust in about it, so you would see evidence of dust and rock thrown from a central point. They also cause moonquakes that can send rocks rolling (a number of examples have been found) or that could dislodge dust.

[Van Rijn]

One other note - I believe rocks with sharp features are thought to be relatively recent compared to the regular surface, as they haven’t yet been so visibly softened by micrometeor impacts. So probably due to more recent (even if ancient by human standards) large impacts rather than very early lunar geological processes.

[Jim]

Warren Platts, this is a reminder, Q&A is for providing mainstream science answers to questions. If you want to speculate, use another forum (S&T?). If you want to push something not mainstream, go use ATM.

[Warren Platts]

@Van Rijn: Thank you for your comments. They actually make sense. However, given Jim's comment above, perhaps we should not discuss boulder cleaning as I am not in the mood for the ATM snarkfest.

@Jim: Your point is well appreciated. However, my original question was regarding wind speeds in lunar transient atmospheres. There have been several papers recently on the topic of lunar transient atmospheres, but I am still trying to find an actual estimate of wind speeds. Besides boulder cleaning, the one practical thing I worry about is lunar mining and settlement. There are proposals for giant solar arrays that have a 1 km^2 "sail area". If a 10 mbar atmosphere appeared for some reason, would that be an engineering consideration?

Again I ask: What sort of wind speeds should we typically expect on the Moon if there was a temporary Mars level of atmospheric pressure?

I don't know.

On Mars, winds speeds of only 60 km/hour (37 mph--a gale force wind if on Earth) are responsible for the global dust storms. Nonetheless, there are 400 km/hr wind speeds on Earth and Venus jet streams, at least. And of course, something like 1000 km/hr wind speeds on places like Neptune that are practically supersonic.

However, I would figure that with the extreme temperature variation on the Moon caused by its slow rotation must cause some winds more extreme than Mars. Again, I don't know. That's why I ask.