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Q & A: Why footprints stay on the moon

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Most recent answer: 10/22/2007
Q:
With no surface moisture or atmospheric pressure on the moon, what force or magic holds the dust particles so compactly together to form such distinctive "foot" prints as seen from the NASA Apollo photographs around the luner lander module?
- Mike
Tri County Tech, Bartlesville, OK
A:
Great question!

Whether a footprint will stay nicely formed or will be erased depends on a variety of things.

On earth, you get the clearest footprints which last the longest when the particles (dust, mud, or whatever) adhere to each other much more than they do to the sole of your boot (otherwise you end up pulling up a sticky mess, as in some muds). Furthermore, the self-adhesion of the particles is responsible for the footprint retaining its shape over time. If you make a boot print in sand on the beach, the shape of the impression will immediately change once your boot is gone because sand likes to flow down hills. There is a maximum steepness to a hill of sand, and if a hill is any steeper than that, the sand will spontaneously flow in little or big avalanches until that steepness is attained. That's why when you pour sand on a pile (say in an hourglass) the conical pile you get always has the same steepness.

Sand flows in this way because the grains are large and the cohesive forces between sand grains are small. Gravity pulls down on sand grains and they will rearrange themselves until the forces balance out. If you wet the sand, you increase the cohesive forces between grains because water is electrically polarized and attracts the sand grains electrostatically, "wetting" them. Other fluids may have the opposite effect (pouring, say, freon on a sand pile may actually make it reduce in height).

On the moon, there is a layer a few meters deep of very very fine rocky dust, with the consistency of dry talcum powder. The individual grains are smaller than usual sand grains. The cohesive forces are small, but on the moon, gravity is also quite low. What's important is the relative size of the electrostatic cohesive forces and the weight of each particle of dust, and it appears from the fact that the footprint doesn't collapse under its own weight, that the cohesive forces win out. This may not be the case in another region of the moon where there may be larger grains of rock covering the ground. Although the stuff that's there is very fine powder because it is ejecta from meteor impacts, and the entire moon may be covered in nothing but this stuff.

Tom


(published on 10/22/2007)

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