Raindrops keep falling on the Moon: circular geometry at different scales

Recently I went through all the released photographs from the US Artemis II mission in April, 2026, and pulled out those I thought would be useful for my research into how we perceive the Moon. This is a lovely one (Figure 1).

 

Figure 1. Image credit: NASA

There's lots of things I like about this photo. It shows the circular topography of the lunar surface, like raindrops falling on mud or soft cement. Compare it to the image below (Figure 2) of fossilised raindrops from the Triassic period, 251 mya to 200 mya, at Great Notch in New Jersey, USA.

Figure 2. Credit: Breck P. Kent

A big difference between these two images is that, unlike the impact craters on the lunar surface, the raindrops are all more or less the same size. But you can see the same superposition in both, with later impacts/raindrops overlying the earlier or older ones. 

The scale and the position of the camera or observer is obviously very different. Artemis II photographs were taken from 8000 km or further above the surface of the Moon. The craters could be hundreds of kilometres in diameter. You can see them clearly because there is no atmosphere, oceans or vegetation to obscure the view. The largest raindrops ever recorded were still under a centimetre in diameter.

A few weeks ago I was in a taxi when it began to rain very heavily. I watched the raindrops falling on the windscreen. They would accumulate and run together, the wipers would remove them, and the next few drops would fall onto clean glass. It was like the surface of the Moon made clear and liquid. I tried to take a photograph but it would never have come out.

Here is another 'lunar' surface, formed by rain falling on sandy mud (Figure 3). This illustration appears in Charles Lyell's Principles of Geology (9th edition, 1853). There is more of a contrast between the sizes of the raindrops.

Figure 3. Credit: artist unknown.

The raindrops in Figure 4 below were found in dried mud at the Lehigh Portland Cement Quarry, Leeds, Alabama. They were collected by W. Edward Osborne (Geological Survey of Alabama). This one does have a scale, in centimetres. The largest crater, showing a spattered rim, is too large to be the impression of a regular raindrop and may be caused by a hailstone. It is overlain by the rain prints from subsequent drops, just as you see on some lunar craters. A great example is the Earthrise crater over which the Apollo 8 crew saw Earth rise. The little craters on the raised edge of the large crater looked like footprints in beach sand to William Anders.

Figure 4. Credit: Andrew K. Rindsberg 

It's just occurred to me that being caught in a hail storm might be similar to being exposed to a meteorite shower on the lunar surface. You could get severely injured in both. 

Years ago, I was working on an excavation in the wilds of the Hunter Valley, when a massive hail storm struck – hail stones the size of golf balls. I can't remember what happened to everyone else, but a couple of us didn't make it to shelter in time. We put the metal geological sieves that we were using to sieve the excavation deposit over our heads to protect them, and crouched down to minimise the amount of surface area we were presenting as a target. There was torrential rain as well as hail. I think only a few stones hit the sieve on my head but it was quite scary all the same. A vehicle came by soon enough to collect us and get us out of there before the roads became impassible.

But I digress!

Do you know what else has the distinctive circular topography we see on the Moon? Bubbles, that's what (Figure 5).

Figure 5. Credit: Shutterstock.

Figure 5 shows bubbles forming in liquid, maybe boiling water, or a liquid under pressure, or even champagne. Bubbles are almost the opposite of craters. Instead of the solid or liquid body striking a hard surface surface, spheres of air rise up through a liquid medium to escape into the atmosphere.

I love soap bubbles, and sometimes I'll sit in the garden with a bubble wand and blow a whole bunch just to be happy with. But these aren't my favourite type of bubbles. 

Figure 6 shows the paths of subatomic particles in the superheated liquid hydrogen of a bubble chamber. The particles create black lines of bubbles as they pass through, which are visible even though the particle itself isn't. The particles lose energy and curl into smaller paths as they fizzle out.

Bubble diagrams also share visual similarities with the curvilinear geometry of the lunar surface.

Figure 6:Fermilab experiment E632. Credit: André Michaud

There's something important that photos don't show you. The Moon is not still and static. Look at the crater with the central peak in Figure 1. It makes me think of the central peak that you see in the slow motion film of a drop falling into milk. Look at the video below and imagine this as the moment when the meteorite slams into the surface of the Moon.

The Moon is constantly bombarded by meteorites of all size, down to grains of interplanetary dust. There's no atmosphere to burn them or slow them down. They churn the surface up in a process called impact gardening. Sometimes we can see it. When the Lunar Reconnaissance Orbiter passes over the surface, sometimes it takes pictures of before and after a meteorite impact.

Credit: Ernie Wright, NASA Scientific Visualisztion Studio

This video shows what it looks like: 

After simulating the distant view of a new impact, the camera zooms up to the surface to show actual before/after images of a new 12-meter crater taken by the Lunar Reconnaissance Orbiter narrow-angle camera. 

Shadows on the Moon are in constant motion too - although they move more slowly than on Earth, because sunlight lasts for two weeks. Everything on the Moon is a sundial.

The video below shows the Apollo 8 spacecraft flying into sunset. You can see how vibrant the shadows are.

We think of the Moon as still, as it is in the photographic representations. But there is constant movement as the light and shadows change, as cosmic rays, interplanetary dust, micrometeorites and meteorites garden the regolith, not to mention moonquakes, landslides, dust levitation and many other factors I haven't talked about here. The Moon is rippling and scintillating in your peripheral vision, through the looking glass.

Since the first human eyes saw the Moon up close in the 1960s, we've learnt to regard it as grey, 'barren' (how I hate that term), and dead, passively pock-marked by craters. But it isn't any of those things. We just haven't learnt yet to see the Moon on its own terms.