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marsbug
I read this article on electrostatic transport of dust on the lunar surface a few days ago, and I was wondering if it had ever been discussed as an erosive or constructive force? What effect might it have had on lunar geology?
dvandorn
Honestly, the effect of electrostatic suspension and movement of dust in the lunar environment is just barely detectable in empirical measurements taken during Apollo. It's way down in the noise on experiments that were designed to notice movement of gases and of charged particles, which ought to have seen it clearly were it an effect large enough to have an impact on lunar geology.

Put it this way -- on the maria, there are a lot of rocks that are dust-free, that have been lying on the surface for hundreds of millions of years. Heck, there are even ancient rocks in the highlands that are relatively dust-free. From what we saw when we went there, I'd have to say that impact scattering of dust is a far more substantial effect in the lunar environment than electrostatic dust levitation has ever been.

-the other Doug
marsbug
Aye, thats the answer I was expecting.

Reading this NASA article though has made me wonder if we understand this phenomena as well as we think. I'll quote the part that made me curious:
QUOTE
"To everyone's surprise," says Olhoeft, "LEAM saw a large number of particles every morning, mostly coming from the east or west--rather than above or below--and mostly slower than speeds expected for lunar ejecta."

(my bolding)

I expect this material would be orders of magnitude more tenous than the material coming out of a clean car exhaust, but on the moon, and over geological time, it might still add up to relatively large amounts of material moving. I only persist asking because lunar processes take place over billions of years, enough time perhaps for even the puniest forces to make their presence felt.

dvandorn, you mention that the number of dust particles detected was way down in the noise in the apollo experiments, which seems to contradict the quote I have above. Could you point me towards your source?
dvandorn
The reason I say that the Apollo experiment results (including LEAM, which was designed to look for dust particles) didn't get conclusive results in re electrostatic dust levitation is that we are just now seeing anything substantive published on the subject. It has taken a more advanced fields and particles understanding of the lunar environment than we had at the time to eliminate various other hypotheses and determine that the observed results were due to electrostatic levitation. Besides, the kinds of fields and energetic particles thought to be required for dust levitation weren't particularly evident in other experimental results, in particular the SIDE and CPLEE results. (The exhaust from the Apollo rocket engines, for example, had a much more noticeable impact on the lunar environment than did the terminator effects that cause the dust levitation.)

Again -- we simply don't see the kind of global dust cover on the tops of rocks, etc. that we would if dust levitation over eons had significant erosional or constructional effects. At least, IMHO. The dust deposited on the tops of rocks is well accounted for by the amount of dust and debris flung about by impact events, and in fact is very minor considering the great amounts of time these rocks have been lying on the surface.

I think the best comparison would be to say that electrostatic dust levitation has probably had the same erosional and constructional impact on lunar geology as the infall of myriad tons of meteoric dust per day on Earth has had on terrestrial geology. Yes, there is a very minor though observable effect, but its overall effect is by far overwhelmed by early orogenic and continuing impact processes.

-the other Doug
marsbug
Thanks for the information dvandorn! smile.gif I imagine that meteorite impacts over billions of years would put quite a bit of dust about, given that they are a sparse but steady phenomena, so I suppose the logical question (and I do realise I'm drifting off my own topic, so I won't pursue this beyond this post) is why are the rocks so clean?
marsbug
Re-reading the article I linked in post three; it suggests that enough dust could have accumulated on the LEAM experiment to significantly decrease its reflectivity in just a few hours. If true than thats a significant transport of material, but the temperature dropped again thereafter so perhaps the effect is almost like frost- a thin layer can build relatively quickly, but it is a strictly temporary phenomena. The wikipedea article mentions that the electrostatic charge on the dust causes it to stick to most things though, so I'm a little confused as to how this works.

On a more than academic; level if significant amounts of dust do move about and attach themselves to things this could be a hazard for temperature control future lunar surface missions.
dvandorn
QUOTE (marsbug @ Sep 7 2009, 02:55 PM) *
Thanks for the information dvandorn! smile.gif I imagine that meteorite impacts over billions of years would put quite a bit of dust about, given that they are a sparse but steady phenomena, so I suppose the logical question (and I do realise I'm drifting off my own topic, so I won't pursue this beyond this post) is why are the rocks so clean?


That is an extremely good question, one that still incites a certain amount of discussion (if not controversy).

We have a fairly good feel for the rate at which a rocky surface is "tilled" by impact into a soil-like regolith (a process that will literally form a thin layer of soil on the tops of rocks that sit on the surface long enough). And analysis of rocks and soil taken from the very surficial layer of the regolith for solar wind particles and irradiation gives us a pretty good idea of how long any given rock or soil layer has been on the surface.

At the mare locations, it appeared that most of the rocks had been on the surface anywhere from a few hundred thousand to a few hundred million years. The cleanest of the rocks, as you might expect, were the ones that had been on the surface the least amount of time. However, many more of the rocks collected at highland sites (Fra Mauro and Descartes) were dust covered as they sat on the ground than you found in the mare regions.

Now, the exposure (and sometimes creation, in the case of regolith breccia) of new rocks on the surface, their subsequent weathering and burial, and in many cases exhumation and re-exposure, is a process that exhibits itself in its every phase on every square kilometer of the lunar surface. Perhaps due to the much thicker layer of regolith, though, this process seems to deliver far fewer angular rocks to the surface in highland areas than it does in mare areas; perhaps this accounts for the difference in the dust covering on highland vs mare rocks.

In another vein, one of the theories I've heard for seeing clean rocks is that micrometeor bombardment "sandblasts" the surface rocks, literally blowing the rocks clean of dust accumulated due to any depositional process (impact or dust levitation). This theory is somewhat borne out by a close examination of lunar rocks, which exhibit "zap pits" on their exposed surfaces, tiny craters caused by micrometeor impacts. You would think, though, that this process would work equally well at highland as at mare sites (assuming the micrometeor impact rate would be the same at both), and yet you see more dust-covered rocks at highland sites. So, as I say, there is still discussion about the theory, mostly centering around compositional differences between the two types of terrains.

The other constructional effect of dust movement is the construction of fillets around sitting rocks. In general, fillets around lunar rocks tend to form around more rounded or sub-rounded rocks; you rarely saw fillets around angular rocks. This has been interpreted to mean that the fillets are being formed by the slow weathering of the sitting rock, the weathering being caused by long-duration exposure to the thermal, radiation and micrometeor environment. However -- and this is a big however -- in a few cases the regolith from the fillet tested out as significantly *older* than samples from the rock itself. This was explained by the admixture of other materials into the fillet soil, but that means that, at least in some cases, the fillets are being emplaced with material that didn't come from the rock the fillet contacts. The best current theory, of course, is that such admixtures come from impact transport of soil into the fillets. However, you don't tend to see any average orientation of fillets within a field of filleted rocks, unless the rocks are on a slope and it's obvious that the filleting is slope-controlled.

So, you see, there has been discussion of the phenomenon of clean rocks going on since Surveyor I showed us what the surface above a thin layer of regolith looks like. And discussion of related phenomena. It just seems that, instead of needing additional mechanisms to account for the amount of dust and soil we see on top of exposed rocks, we need to find a cleaning mechanism (that still is not clearly understood) to account for what we think ought to be deposited by impact processes alone. (I'm sure that there are more processes in play, to account for the things we've seen, than we yet know to apply to the question... wink.gif )

I will point out that electrostatic dust levitation has the potential to remove dust from rocks. However, I would think it has the equal potential for re-deposition, so I can't conceive of a process that isn't a break-even... *sigh*...

-the other Doug
marsbug
Thank you dvandorn, that pretty much answers my questions! smile.gif
Phil Stooke
One other point to add here, the relatively sharp albedo boundaries on the Moon, some of them over 3.5 billion years old, show that large scale horizontal movement of dust is not occurring. Even impact redistribution of dust is not enough to blur them very much. If dust levitation happens it involves a tiny amount of material and/or not much horizontal movement. I think the issue is being grossly exaggerated at the moment.

Phil
marsbug
I think that begs the same question I asked dvandorn, but on a bigger scale: Why has 3.5 billion years of meteorite and micrometeorite bombardment not been enough to significantly blur those boundaries?
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