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I've seen that in some videos or photos of the moon mission, the boot prints appear brighter relative to the untouched ground:

Some image from an Apollo mission, 1

Some image from an Apollo mission, 2

Some image from an Apollo mission, 3

But in some other cases, they are the same brightness as the ground:

Some image from an Apollo mission, 4

Is there a specific name for this phenomenon?

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    $\begingroup$ You don't need to go all the way to the moon for this. I'd say this is the same phenomenon that you see in some rugs where sweeping your hand across the surface changes the "shininess" of that part as the fibers change alignment. $\endgroup$
    – muru
    Nov 7, 2023 at 2:08
  • $\begingroup$ Change the colors of the second and third to brown and it would not look so different from boot prints in the mud $\endgroup$
    – Taladris
    Nov 9, 2023 at 1:12

3 Answers 3

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If you take a good look at contrast between the boot print and the unaffected/untouched dust on the lunar surface the unaffected/untouched dust is loose, undulating and has a matt (or matte, in some locales) appearance reflecting light in all directions.

The lunar dust that has been trodden by the boots has been compressed into a series of "stepped" and flattened surfaces. These better reflect the light, a bit like a mirror, and they also reflect the light in particular direction. Thus, from certain perspectives, the cameras are getting more light from the boot impressions than they are from the untouched lunar dust.

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    $\begingroup$ Some portion of the surface of the loose dust is also self-shadowed. $\endgroup$
    – Vaelus
    Nov 6, 2023 at 14:10
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    $\begingroup$ To use the relevant terms: there is a mixture of specular reflection and diffuse reflection in the bootprints whereas it is purely diffuse reflection elsewhere. $\endgroup$
    – g.kertesz
    Nov 6, 2023 at 17:12
  • $\begingroup$ This begs the question of why boot prints reflect light better than undisturbed regolith. Reflecting light "like a mirror" is specular reflection. Supposedly, this only occurs when the surface smoothness is better than the wavelength of the light being reflected. How does a foot fall convert regolith (particle size 70 microns) to a surface smoother than the wavelength of light (0,7 microns)? $\endgroup$
    – Woody
    Nov 8, 2023 at 2:46
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    $\begingroup$ @woody "A bit like a mirror" not "actually exactly the same as a mirror." $\endgroup$
    – barbecue
    Nov 8, 2023 at 15:37
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Reflection

Yes, as Fred points out, the boot prints act like crude mirrors. If you shine a flashlight at a mirror in a random direction, and place yourself at another random direction, the mirror will appear "bright" with low probability. But if you stand in a direction "opposite" the flashlight, the mirror will appear very bright, because you see many of the reflected rays directly.

If you look at the shadows in the photos, the ones where the boot prints appear shiny also have shadows directly ahead of the camera, indicating that the sun is almost in front of the camera. In the darker photo, the shadows are cast to the right, indicating the sun is almost directly left.

You could almost certainly compute the position of the sun by using photogrammetry to reconstruct a 3D model of the boot print orientations and then use their brightness to infer the sun's location.

The shiny boot prints are an image of the sun as rendered by moderately compacted lunar regolith. You don't see this effect on earth so much because there's far more diffuse light due to the atmosphere, and also because we don't have many surfaces that compact as nicely and are as reflective as regolith. The closest you might get here are footprints with some water in them.

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  • $\begingroup$ This begs the question of why boot prints reflect light better than undisturbed regolith. Reflecting light "like a mirror" is specular reflection. Supposedly, this only occurs when the surface smoothness is better than the wavelength of the light being reflected. How does a foot fall convert regolith (particle size 70 microns) to a surface smoother than the wavelength of light (0,7 microns)? $\endgroup$
    – Woody
    Nov 8, 2023 at 2:47
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    $\begingroup$ @Woody "Supposedly" according to whom? This is an easy experiment to demonstrate with a piece of white cardboard (which I think you will agree is not smooth to the nanometre level) and a lightbulb. $\endgroup$ Nov 9, 2023 at 7:05
  • $\begingroup$ @Oddthinking ... according to the Wikipedia article on specular reflection. en.wikipedia.org/wiki/Diffuse_reflection $\endgroup$
    – Woody
    Nov 9, 2023 at 13:43
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Answer: Levitating Moon Dust and a walking capacitor.

Previously submitted answers explain the brightness of footprints as specular (mirror-like) reflection, This begs the question of how a bootprint converts regolith from a diffuse reflector to a specular reflector.

According to Wikipedia, for a surface to be a specular reflector, the surface must be flat to within a fraction of the wavelength of the light being reflected. Visible light has wavelengths of 0.4-0.7micron. Lunar regolith has an average particle size of 70 micron, (there is a continuum of sizes from 500 micron down to 2.0 micron) https://curator.jsc.nasa.gov/lunar/letss/regolith.pdf

With particles this big, there is no surprise that undisturbed regolith is a diffuse reflector... particularly because unpacked regolith is very “fluffy” (up to 90% empty space) https://www.sciencedirect.com/science/article/abs/pii/0032063373900238 The micrograph below demonstrates the irregularity and roughness of regolith particles

enter image description here

Lunar regolith particle Image Credit: David S. McKay, NASA/JSC

The micrograph is copied below with the addition of a reference line (thin red line) indicating the scale of smoothness needed for specular reflection of visible light:

enter image description here

How can such large particles pack to the required smooth surface under the pressure of a boot? Especially considering the silicone sole of the boot was not designed to have a mirror-smooth surface?

enter image description here

Boots are from the A7LB spacesuit

It’s as if your footprints on the beach sand reflected the sky like a mirror… which is possible if something (like water) flows into the footprint to provide the smoothness needed. There is obviously no water on the Moon, but perhaps electrically charged dust provides the same function.

The Moon has an atmosphere, of sorts: the dusty plasma. This produced the “horizon glow” noted by Apollo astronauts.

enter image description here

"Horizon glow" above the surface of the Moon. (Courtesy NASA)

The dusty plasma consists of dust particles which have become electrically charged by solar wind and radiation. The charged dust particles effectively levitate above the negatively charged Lunar surface. These dust particles form the near-surface dusty plasma exosphere of the Moon. https://oxfordre.com/planetaryscience/display/10.1093/acrefore/9780190647926.001.0001/acrefore-9780190647926-e-23

At Lunar “noon”, particles up to 0.1 micron diameter levitate up to 10meters altitude, producing concentrations of up to 10^6/m^3. Astronauts in this environment act as a capacitor, attracting the negatively charged dust which adheres to the astronaut's suit. https://www.lpi.usra.edu/meetings/LEA/presentations/thurs_am/3_Stubbs_Interaction_of_Dust_and_Plasma.pdf

I’m going to stick my neck out and speculate how a bootprint produces a specular surface on regolith: The pressure of the boot sole compresses the regolith (up to 90% empty space), fracturing the dust grains’ brittle fractal structure into rough polygons about 50 micron diameter and producing a surface with the equivalent smoothness of the silicon sole. (silicone can be manufactured with a surface roughness as fine as 0.2micron) https://pubmed.ncbi.nlm.nih.gov/18306290 . The capacitor effect of the astronaut neutralizes charged dust particles, which are then no longer repelled by the surface of the regolith. The dust’s relative positive charge attracts it to the regolith, effectively filling in the rough spots like seawater filling in the rough spots between sand grains in a beach footprint.

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    $\begingroup$ Sounds plausible... $\endgroup$
    – PM 2Ring
    Nov 7, 2023 at 11:24
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    $\begingroup$ I suspect that the diffuse / specular issue has some effect, but mainly it's just compaction causing improved diffuse reflection. A porous grey surface loses light in the pores, so when you fill in the pores with finer powder the surface will be brighter. $\endgroup$
    – PM 2Ring
    Nov 9, 2023 at 4:01
  • $\begingroup$ @PM2Ring ... good point. But diffuse reflection, no mater how good, is omnidirectional. As the OP illustrates with the photographs, bootprints enhance reflection only in the direction of the incident rays ... like specular reflection. $\endgroup$
    – Woody
    Nov 15, 2023 at 15:46

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