I thought that the millions of mini rocks that hit the moon would cause it to be entirely cratered and not flat.

Why is the moon not like this:

enter image description here

why aren't craters visible in the footprint photo?

enter image description here

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    $\begingroup$ Could you explain how the top photograph was made? I doubt it accurately simulates particles sequentially impacting the lunar surface at ~30 km/s from a variety of angles in a low gravity environment. You also have to consider the spectrum of sizes of the impactors and whether any resurfacing has occured since the impacts. $\endgroup$
    – ProfRob
    Dec 17, 2020 at 13:37
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    $\begingroup$ There are many craters on the moon. I would suspect that the second photo doesn't show an area large enough for craters to be visible $\endgroup$
    – jng224
    Dec 17, 2020 at 14:08
  • $\begingroup$ The top photograph is what I naively imagine the moon would look like based on the size distribution of space rocks. Account taken of the dispersion of dust by big meteorites, I suppose that 1000 tiny meteorites land after every sizeable one... I'm wondering why the moon surface wouldnt easily present a random array of exotic rocks as those that occur on asteroids. $\endgroup$ Dec 17, 2020 at 15:19

1 Answer 1


Nice question, unfortunately your analoguous experiment is not as analoguous as you might hope.

In your comparison image you are using both, not enough impactors, and a too tiny size distribution. Impactors on the Moon (or any other objects) follow a reasonably well-known size distribution where the smaller an impactor is, the more frequent collisions with those do occur.

The visible and measured / counted size distribution of craters on the Moon follows this probability law quite well (e.g. this paper (Cross, 1966), but many others get about identical results). The main difference is that in your comparison you miss all the very tiny impactors which smooth-out the surface again.

Further, the Moon is covered with loose regolith of about 45µm diameter. In the sunlight it is subject to charging and thus the Moon surface is covered by a tiny sheet of statically elevated particles. This leads to a re-distribution of the surface regolith particles, smoothing out the tiniest craters even more.

Even further, the landing sites of Apollo were carefully chosen to be the most level and crater-free places to ensure a safe landing. The amount of cratering in a particular place is also random, even when all the surface is covered by several layers of craters (this is a study for asteroids, but applies to other bodies, too).

And further more, your comparison experiment is conducted in air. That affects the formation of craters and in particular the distribution of ejecta due to friction with air, more so, the smaller the impactors get and the slower the ejecta are.

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    $\begingroup$ Great answer. Also surface gravity on the Moon is much lower than surface gravity on Earth, so the ejecta tend to travel further, and hence be distributed more evenly. $\endgroup$
    – Connor Garcia
    Dec 17, 2020 at 16:41
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    $\begingroup$ There's also topographic diffusion and seismic shaking which will remove small craters. $\endgroup$ Dec 18, 2020 at 7:44

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