QUOTE (marsbug @ Sep 7 2009, 02:55 PM)
Thanks for the information dvandorn!
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...
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