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Marlayna
2006-Oct-14, 04:49 PM
Hi all. I was wondering about what makes the surface of the Moon so reflective. Do we know what it consists of, exactly? What chemical compounds can be found there? Does the fact that the dust there is so finely powdered play a part?

I know there's silicon and iron in it, but do we know anything more specific?

grant hutchison
2006-Oct-14, 05:00 PM
It's not very reflective at all: on average it reflects only about 12% of the light that falls on it, which would make it the equivalent of pretty dark dust on Earth.
But are you referring to the "opposition brightening" of lunar dust when seen in a direction directly opposite the sun? (It accounts for the bright patches surrounding the astronauts' shadows, seen in photographs.) That's more of a physical than a chemical effect: because the dust isn't eroded because of wind movement, it's very jagged and produces an effect called dry heiligenschein (http://www.meteoros.de/oppo/oppoe.htm).

Grant Hutchison

max8166
2006-Oct-14, 05:58 PM
Lunar dust is composed of bits of rock pulveriesed by inpact damage
so the compostion is a bit like sand although I believe the pulverisation makes it alot smaller.
http://en.wikipedia.org/wiki/Lunar_dust#On_the_Moon
http://epsc.wustl.edu/admin/resources/meteorites/regolith_breccia.html

It's also actually very dark I believe it reflects about the same amount of light as a piece of coal.

Marlayna
2006-Oct-14, 06:51 PM
Oh. Well that was a bit of a downer.

Thanks anyway :)

Jeff Root
2006-Oct-14, 06:58 PM
I've seen about half a dozen large lunar rocks, and a few small
containers of lunar regolith, or "soil". The regolith I saw was not
quite black, but very dark gray. Most of the rocks I've seen
were dark, but at least one was a slightly dirty white. I think
the white rocks are anorthosite from the lunar highlands. Dark
rocks are generally basalt from the lunar maria.

I have a couple of pounds of lunar simulant, made from rocks
found in Minnesota and ground up to the same particle size
range as lunar regolith. The main component of the simulant
is gray basalt.

-- Jeff, in Minneapolis

cozzman
2006-Oct-15, 03:39 PM
Hi all. I was wondering about what makes the surface of the Moon so reflective. Do we know what it consists of, exactly?

Cheese.

<sorry>

PhantomWolf
2006-Oct-16, 02:05 AM
it's very jagged and produces an effect called dry heiligenschein.

Probably not what you mean, but from what I understand, it is believed that the glass sphericals in it are what causes the heiligenschein effect and high reflectivity, and they are smooth. This tiny glass spheres are created during the impact of huge rocks and the melting of the silicon. They occur on earth too, but are smaller due to our gravity, and last a lot less time as well because the weather and elements decompose the glass.

grant hutchison
2006-Oct-16, 09:57 AM
Probably not what you mean, but from what I understand, it is believed that the glass sphericals in it are what causes the heiligenschein effect and high reflectivity, and they are smooth.Ah, interesting: so in that case more like a "glass bow" generated by transparent beads (http://antwrp.gsfc.nasa.gov/apod/ap040913.html), than conventional (self shadowing) dry heiligenschein. Thanks for that. :)
But if internal reflection in glass is the main cause, I wonder why there's no evident rainbow fringe to the bright down-sun patches the astronauts photographed (like the one that shows up clearly in the APOD image I link to above).

To judge from your posts on the "Apollo hoax", you seem to know the Apollo image archive well, Phantom Wolf; have you noticed any down-sun rainbow effects in there?

Grant Hutchison

PhantomWolf
2006-Oct-17, 02:28 AM
To judge from your posts on the "Apollo hoax", you seem to know the Apollo image archive well, Phantom Wolf; have you noticed any down-sun rainbow effects in there?

None come instantly to mind, though the quest also would be, are said sphericals large enough to cause diffraction, or would they merely act as a form of corner cube? The beads that created the link you linked too appear to be quite big, while the lunar soil ones, though larger in size that Earth formed ones are still microscopic.

AGN Fuel
2006-Oct-17, 03:29 AM
I once read a paper from someone who argued that the Heiligenschein effect was caused by the direct light more efficiently illuminating the interstitial spaces between the jagged regolithic particles.

This had the effect of making a given area of regolith in direct sunlight appear less 'shadowed' and hence brighter to an observer looking directly away from the light source.

I will try and track down a reference. :think:

grant hutchison
2006-Oct-17, 10:38 AM
The beads that created the link you linked too appear to be quite big, while the lunar soil ones, though larger in size that Earth formed ones are still microscopic.Hmmm. Good point. The colours of a rainbow wash out as the drops get smaller, so that it turns into a more-or-less white "fogbow".

Grant Hutchison

grant hutchison
2006-Oct-17, 10:41 AM
I once read a paper from someone who argued that the Heiligenschein effect was caused by the direct light more efficiently illuminating the interstitial spaces between the jagged regolithic particles.Yes, this is what my original post was about: the uneroded jaggedness of lunar dust creating a high level of self-shadowing.
But Phantom Wolf's point about the spherules is interesting. I wonder what their numerical density is in a typical sample of moondust?
Perhaps both mechanisms are responsible, to different degrees.

Grant Hutchison

gwiz
2006-Oct-17, 11:06 AM
I once read a paper from someone who argued that the Heiligenschein effect was caused by the direct light more efficiently illuminating the interstitial spaces between the jagged regolithic particles.

This had the effect of making a given area of regolith in direct sunlight appear less 'shadowed' and hence brighter to an observer looking directly away from the light source.

I will try and track down a reference. :think:
Or maybe it's both that and the beads.

Jeff Root
2006-Oct-17, 11:43 AM
Perhaps both mechanisms are responsible, to different degrees.
I find that sort of thing really annoying. Why can't nature decide
to work one way or another, and just stick with one mechanism?

I'd like to see some counts of glass spherules in lunar regolith.
Also I wonder how many spherules are on the surface of the
Moon, as opposed to buried. Mixing stuff in the kitchen I know
that you can have a huge number of small objects mixed in a
powder, and almost none will be visible. Also I wonder how
transparent the spherules are, and why they are transparent.

-- Jeff, in Minneapolis

PhantomWolf
2006-Oct-20, 12:55 AM
I found this (http://fti.neep.wisc.edu/neep602/LEC12/IMAGES/soil_comp.JPG) while looking. Ansdwwers the OP exactly in pie form.

and here's a photo of a transparant lunar spherical (http://www.union.edu/PUBLIC/GEODEPT/COURSES/petrology/moon_rocks/lunar_images/SD-HS-64.jpg) at x64 mag