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Where is the Lunar Dust?

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#1 Brian Albin

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Posted 18 January 2013 - 10:32 AM

During the Apollo days Geologists or Astronomers or somebody, measured the amount of meteorites falling onto the Earth daily and calculated how much depth of dust or sand shall have accumulated on the Moon over the eons of time. I have forgotten the number, but it was very deep. I think it was hundreds of feet. NASA then did research on how to keep the Lunar Lander from sinking out of sight when landing on the Moon. They decided it was no different from driving a dune buggy on sand and put pad feet on the legs of the lander module and all was well.

But when they got to the Moon they did not find a great depth of dust; the dust was only a few inches deep over the bedrock.

Either the solar system is nothing like as old as we believe. Considering all the Geologic evidence amassed here on Earth in the last several centuries this does not seem creditable. Or the rate of meteor fall is much more now than in the past. The planets and Moons seem to be formed by mass attracting mass among the gravel & rubble of the early solar system, so with the passage of time the solar system is progressively being swept increasingly clean. So it does not appear that the rate of meteor fall can be increasing. Indeed, it ought to be less each year.

I remember this being a puzzle of interest during the Apollo program, but when we stopped going to the Moon interest in the puzzle faded. Or at least I heard no more of it. Was an answer ever suggested?

#2 Jay_Bird

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Posted 18 January 2013 - 11:35 AM

A hypothetical sea of dust from eons of micrometeorite infall might be a feature imagined without considering the full effects of larger impacts.

Is most of the supposed "Moondust" stabilized by layers or splashes of impact glass from small craters, or filtered into voids between larger particles of the "megaregolith" created by biggest impacts?

#3 llanitedave

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Posted 18 January 2013 - 11:58 AM

I think Jay is right. Most debris from larger impacts isn't dust-sized, but sand-to-cobble sized, and much of it melts into glass on impact. The lunar regolith is a well-pounded mass of rock, sand, glass, and dust. It is hundreds of feet thick in a lot of places -- I don't think the "bedrock" Brian refers to is the same thing we have here on Earth. It's just a depth where the regolith is too compacted to manually drill through.

A single site can accumulate dust for a billion years, but then all it takes is one moderate-sized impact to solidify and weld the whole package together in an instant. Even a more distant impact can send shock waves through loose material and cause settling and compaction.

I don't think the idea of hundreds of feet of soft fluffy dust was ever really that well thought out.

Another thing about impact rates -- they most likely aren't constant. One event elsewhere in the solar system can lead to impact "swarms" that could last for millions of years.

#4 FeynmanFan

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Posted 18 January 2013 - 07:46 PM

I admit to knowing little about the mechanical properties of materials in a vacuum, but could so-called vacuum welding take place? Also, what would be the effect of the ceaseless high intensity bombardment by radiation at all frequencies, and high energy particles from the sun? Could these act to somehow conglomerate dust?

#5 llanitedave

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Posted 18 January 2013 - 10:38 PM

I don't know the affect of radiation or solar wind either. It can change the atomic bonds within grains, but I've never heard what affect it has between grains. As for the welding process, most of where it occurs is in the subsurface, and very quickly under the pressure of the impactor, so I don't think the vacuum is applicable here. In larger impacts, you can certainly have surface melting and the pooling of molten material, but since we aren't talking about water, and molten rock is not very volatile, I don't think there's a huge difference between its cooling history on the lunar surface and that on the Earth's surface. Lunar basalts look, at least texturally, a lot like Earth basalts.

#6 Brian Albin

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Posted 19 January 2013 - 05:57 PM

That all makes good sense. Thanks fellas.

P.S. "The "Eye of Sauron" Observatory Open for Business!"
Did you get your 16" telescope together, Dave?

#7 Matthew Ota

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Posted 13 February 2013 - 04:39 PM

A soil mechanics experiment was run at almost every Apollo landing site. Core samples were also taken. In every case the farther down they dug or drilled, the harder the regolith got.

#8 llanitedave

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Posted 13 February 2013 - 10:02 PM

In the early 90's one of the highlights of my life was the chance to stand in Houston's NASA Lunar Laboratory and look over the shoulder of a technician who was painstakingly removing material from an Apollo 14 core sample. She gently scraped very tiny amounts on to what I think was filter paper for analysis. I don't know what the depth of that particular core section was supposed to be, but it was indeed very fine-grained.

We were all wrapped up in suits and headgear and gloves and masks, but still I would have hated to sneeze just then!

#9 Qwickdraw

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Posted 14 February 2013 - 01:49 PM

" Where is the Lunar Dust? "

I believe it is all over my coffee table

#10 CounterWeight

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Posted 17 February 2013 - 12:47 PM

There are two really good chapters on this concept as envisioned and actual results (Apollo and probes) and further thinking in the book "Volcanoes of the Solar System" by Chrles Frankel. I'm still reading the book, not exactly lite reading but fascinating if you enjoy this sort of thing.

#11 Brian Albin

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Posted 20 February 2013 - 12:58 AM

Thank you for the book recommendation, Jim.

#12 dickbill

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Posted 20 February 2013 - 09:48 AM

I think Jay is right. Most debris from larger impacts isn't dust-sized, but sand-to-cobble sized, and much of it melts into glass on impact. The lunar regolith is a well-pounded mass of rock, sand, glass, and dust. It is hundreds of feet thick in a lot of places -- I don't think the "bedrock" Brian refers to is the same thing we have here on Earth. It's just a depth where the regolith is too compacted to manually drill through.

A single site can accumulate dust for a billion years, but then all it takes is one moderate-sized impact to solidify and weld the whole package together in an instant. Even a more distant impact can send shock waves through loose material and cause settling and compaction.

I don't think the idea of hundreds of feet of soft fluffy dust was ever really that well thought out.

Another thing about impact rates -- they most likely aren't constant. One event elsewhere in the solar system can lead to impact "swarms" that could last for millions of years.


So basically any terrestrian impactor (containing early organics of fossils like bacterial mat) that hit the moon during the 4-3.8 billion years period would have had no chances of survival, without being melted or recristallized, unless it was deeply buried under the regolith.
Is it possible that an impactor hitting Earth could release secondary impact rocks with enough velocity to penetrate the moon for hundred of feet?

#13 llanitedave

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Posted 20 February 2013 - 10:36 PM

I think there's certainly a problem with planetary debris (from Mars or Earth or anyplace else) that impacts the Moon rather than Earth. A piece of Martian crust, for example, that happens to encounter Earth will, if it's not too large or too small, slow down considerably from atmospheric friction before it hits the surface. Hitting at only a few hundred miles per hour, its interior, at least, can survive relatively unscathed. That same object, hitting the Moon, will have an impact velocity of thousands of miles an hour with no prior deceleration. It would be very difficult for any substantial quantity to survive the smack.

#14 dickbill

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Posted 21 February 2013 - 09:59 AM

At that time (4 to 3.8 by) the moon might still have been degazing enough to have an atmosphere, plus since it was so close to the earth, i assume that terrestrian debris with just the earth escape velocity (~11km-7 miles/s) could have landed on the moon.
The huge tidal force that shaked the moon could have also made it less compact, and perhaps spongy enough to decelerate the impactor smoothly. So maybe, we can still hope in some early earth prebiotic fossils cryopreserved deep inside the moon.






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