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.

How would you write a Lunar Cultural Heritage Management Plan (LCHMP)?

This is Appendix 8 from a reference document prepared for the Global Expert Group on Sustainable Lunar Activity (GEGSLA). 

Gorman, A.C. 2023 The Sustainable Management of Lunar Natural and Cultural Heritage: Suggested Principles and Guidelines. Reference document, Global Expert Group on Sustainable Lunar Activity 

The full document can be downloaded here. 

Apollo 17 panorama. Credit: NASA

This draft has adapted standard components of terrestrial Cultural Heritage Management Plans with a view to their applicability to the unique circumstances of lunar activities. It is intended as a first approximation which could be further developed. 

Standard components of a Lunar Cultural Heritage Management Plan (LCHMP) could include the following: 

Introduction 

• The reasons for preparing the Management Plan (eg voluntary, required by regulation) 

• A brief description of the location of the activity area or safety zone, including relevant coordinates 

• The time frame for application of the LCHMP, in terms of the duration of the activity or the safety zone 

• The name of the lunar operator (space agency, private company, consortium etc) with all contact details for enquiries or reporting 

• The name of the heritage expert who undertook the work and their qualifications and experience 

Activity description 

• Clear, relevant and detailed information about the nature and extent of the proposed activity to be covered by the LCHMP, including ancillary works, in order to assess the scope for potential impact on lunar cultural heritage. 

 • A description of the likely impact on the surface from the activity and how this relates to impacts on heritage sites 

• Appropriate images of the activity area. 

Documentation of consultation 

• The names and roles of any persons or parties consulted in the process of creating the LCHMP 

• Records of formal consultation meetings or processes, including date, location, agenda items 

• Outcomes of consultation meetings, including the documentation of disagreements 

• Details of informal consultations (eg personal communications) 

• Details of meetings of any advisory groups established for the purposes of the project 

• If the proponent of the development and the LCHMP is different to the launching state of the heritage site, official representatives of the launching state may need to be signatories to the LCHMP as a way of avoiding disputes and ensuring agreement to the mitigation measures. 

Dispute resolution 

Procedures for dispute resolution are a typical feature of terrestrial CHMPs. The LCHMP may set out time frames for communications regarding the dispute, and preferred methods, for example, mediation or negotiation, or the appointment of a neutral evaluator. Any mechanisms which have been established for more general disputes in lunar governance systems would be appropriate to use. 

Results of cultural heritage assessments 

Cultural heritage sites should be identified by unique designators to avoid confusion. If a lunar heritage register has been established, these designators should be used. 

Desktop assessment 

• Search of relevant international or national heritage registers to locate registered lunar objects and registered terrestrial heritage sites which are related to the lunar site 

• Search of literature and UN Register of Space Objects to identify sites not present on national heritage registers or international space heritage registers 

• Search of relevant museum collections to identify material culture related to heritage sites in the activity area 

• Literature review of previous reports, academic literature, and archives where applicable 

• Satellite imagery of the activity area 

• Assessment of the likelihood that previously unknown heritage locations or objects might be present 

• Identification of relevant stakeholders. A cultural heritage site may exist across more than one safety zone or activity area. 

Field assessment 

Where a field assessment, either using human personnel or robotic means, can be undertaken without creating harm to a heritage site, it should include: 

• Survey methods eg remote sensing, instruments used, location of transects, scale of observation 

• Maps, images or new data obtained about the location and condition of existing sites 

• Maps, images or new data obtained about the location and condition of previously unknown sites 

• Obstacles and limitations of the survey 

• Details of any samples removed or any other disturbance of the site, whether deliberate or accidental 

Details of cultural heritage in the activity area (if any) 

The aim of these sections is to provide sufficient information to make evidence-based management decisions. 

 • Details of the assessments undertaken to determine the nature and significance of each place or object, including analysis of site formation processes; 

• Results of the assessments 

• Precise coordinates of location and extent of the site 

• A detailed plan of the site showing the relationship between objects and traces 

• A detailed description of the material remains at the site, including any catalogues of data recorded. 

• Historical background of the site 

• An assessment of the significance of the place, site or objects. It is recommended that the significance criteria of the Burra Charter be used for consistency. 

• Any images of the site 

• Impact assessment, including the cumulative impact of ongoing activities in the area 

        • Whether the activity will be conducted in a way that avoids harm to the place or object 

        • If there is potential harm, whether the activity will be conducted in a way that minimises                harm to the place or object 

        • What aspects of cultural significance will be affected by the activity 

        • Any specific measures required for the management of the place or object, before,                       during  and after the activity. 

        • Any contingency plans required in relation to disputes, delays and other obstacles that                may affect the conduct of the activity 

Specific management and mitigation measures 

The Management Plan should clearly explain why the activity cannot be conducted to avoid harm to cultural heritage if this is the case. If harm is likely to be caused, then mitigation measures to minimise the harm should be outlined. 

Based on the significance assessment, specific management measures should be identified. They could include: 

• Avoidance of the site as the preferred management strategy in the first instance ie locate the activity as far away as possible 

• If a heritage site is not going to be impacted by the activity, then no action should be taken that will create unnecessary disturbance. 

• Adjust the design of the activity (eg location of specific elements, construction methods, operations methods) to minimise harm 

• A salvage strategy to recover information only when it is not possible for that cultural heritage to be preserved in situ. 

• Note that disturbance and salvage is destructive and should only be carried out when necessary to identify and document the extent, nature and significance of the cultural heritage that may be threatened by the proposed activity. Disturbance or salvage should not occur if it causes more harm to the heritage than the activity. 

• Removal and curation of heritage objects. A plan should be provided specifying secure storage location (whether that is on the Moon or Earth), resourcing, and any relevant factors relating to the long-term survival and safety of the objects. Potential repositories should be identified in advance. For example, the Smithsonian Institution has a Memorandum of Understanding with NASA for the deposition of materials related to US space activity. 

• Removal of human biological material, for example, from the Apollo missions or cremated ashes, must be handled sensitively, with a view to preserving the dignity of the people to whom they belong (or their family and descendants). Protocols established for ethically dealing with human remains in other disciplines can provide guidance here. 

• Any removed objects must be catalogued, labelled and documented to the fullest extent possible. 

• A monitoring plan to collect information on the condition of sites at regular intervals during the activity. This can be done by remote sensing. 

• At the end of the activity or safety zone, an audit of the impacts of the activity on heritage places 

• Any combination of the above measures.

Contingency plans 

A Management Plan must include contingency plans for the discovery of previously unknown lunar heritage during works. This could include: 

• Stop works for a specified time and/or within a specified distance (ie buffer zone), leaving the remains in situ, until an assessment can be prepared and appropriate management recommended. 

 • Verification of the identity of the material and the launching state 

• Notification to the legal owner of the object/s (Liability Convention 1972) 

• A plan for consultation with the legal owner and other stakeholders 

• Consultation with a heritage expert to provide a significance assessment 

• Dispute resolution in relation to the cultural heritage eg between the lunar operator and the legal owner of the heritage objects, where these are different, about how the heritage place is to be managed; or between different stakeholders. Dispute resolution should specify appropriate time frames and processes, using, for example, any mechanisms which have been established for more general disputes in lunar governance systems. 

Other Considerations 

A LCHMP may also provide for the following: 

• Disaster management provisions 

• Protocols for handling sensitive information 

• Cultural heritage training or inclusion of heritage in induction procedures for employees or contractors 

• Evaluation of the LCHMP by an independent expert prior to adoption or implementation 

• Evaluation of the LCHMP by relevant stakeholders

Who lives on the Moon? What we learn from Ambrose Bierce and The Devil's Dictionary

'LUNARIAN, n. An inhabitant of the moon, as distinguished from Lunatic, one whom the moon inhabits. The Lunarians have been described by Lucian, Locke and other observers, but without much agreement. For example, Bragellos avers their anatomical identity with Man, but Professor Newcomb says they are more like the hill tribes of Vermont'.

Somehow I have started a practice of posting on Bluesky every day a dictionary definition from one of the many interesting and/or satirical dictionaries that are around. First of all I did Flaubert's Dictionary of Received Ideas. Then I moved on to Ambrose Bierce's The Devil's Dictionary, published in 1911. (I'm grateful to @Fionnbharr for recommending this to me). Some of Bierce's definitions are definitely old school racist and misogynist, so I've been quite selective in what I've chosen to put on social media. But of course, when I came across this definition for Lunarians, I was immediately captivated.

We'll leave aside any analysis of the Moon and madness; I'm just going to focus on the Moon itself. 

First of all, Bierce mentions Lucian. Lucian of Samosata's account of voyages to the Moon, written in the 2nd century CE, is a work of satire. This quote about the lunar inhabitants will give you the flavour of it:

'I am afraid to mention their eyes, lest, from the incredibility of the thing, you should not believe me. I must, however, inform you that they have eyes which they take in and out whenever they please: so that they can preserve them anywhere till occasion serves, and then make use of them; many who have lost their own, borrow from others; and there are several rich men who keep a stock of eyes by them'

You can read the full work for yourself here.

Lucian crater, on the Moon. Source: NASA

Now, I was unaware that philosopher John Locke (1632 -1704) had written about the Moon so of course I had to look this up straight away. It turns out I had the wrong Locke; but in tumbling down this wild rabbit hole, I did find this. In the fourth book of An Essay Concerning Human Understanding, according to the Stanford Encyclopaedia of Philosophy,

Locke distinguishes two sorts of probable propositions. The first of these have to do with particular existences or matters of fact, and the second that are beyond the testimony of the senses. Matters of fact are open to observation and experience, and so all of the tests noted above for determining rational assent to propositions about them are available to us. Things are quite otherwise with matters that are beyond the testimony of the senses. These include the knowledge of finite immaterial spirits such as angels or things such as atoms that are too small to be sensed, or the plants, animals or inhabitants of other planets that are beyond our range of sensation because of their distance from us.

So other planets could be inhabited, but we'll never know!

Bierce actually means Richard Locke, who was the perpetrator of the Great Moon Hoax of 1835. This was also a work of satire, so this makes sense. The illustration below shows how he portrayed the lunar inhabitants.

A lithograph of the hoax's "ruby amphitheater", as printed in The Sun. Source: Wikimedia

Bierce says that 'Bragellos avers their anatomical identity with Man', which is funny when you consider the Lunarians described by Lucian, with their detachable eyes and many other absurdities, and by Locke, as you can see in the illustration above, with bat wings stretching between their limbs (Homo vespertilio). Bargellos, however, is a made-up person; Bierce has such people scattered throughout the Dictionary, with hilarious names like Opoline Jones and Jex Wopley.

Professor Newcomb is not made up. I'm pretty sure he's astronomer Simon Newcomb (1835 – 1909). Newcomb wrote three serious works on the Moon: Research on the Motion of the Moon, Part I (1878), Investigation of Inequalities in the Motion of the Moon Produced by the Action of the Planets (1907) and Research on the Motion of the Moon, Part II (1912). I had a look at them and frankly they're too full of equations and not very satirical.

He also wrote The Philosophy of Hyperspace and The Fairyland of Geometry, which I am very much more interested in. I suppose I'll just have to read between the equations.

Bierce might be alluding to the fact that in 1884 Newcomb became the inaugural president of the American Society for Psychical Research (ASPR). In this period, there were psychics, such as Catherine Müller, who visited other planets and met the inhabitants. In the course of his association with the Society, Newcomb decided that psychic research was all bunkum, which doesn't provide much to mock. 

By coincidence, one of the founding members of the ASPR was psychologist G. Stanley Hall.  I had come across him before. He invited Freud and Jung to the US in 1909 - enough alone to make him interesting! Some time before the turn of the 19th century, he conducted a study on what children thought about the Moon. It's fascinating, and I talk more about it in Dr Space Junk vs the Universe: Archaeology and the Future.

Back to Newcomb, though. Bierce says Newcomb thought the Lunarians were 'more like the hill tribes of Vermont'. Vermont is a very mountainous US state. I feel that this means something, but I don't know what. The hill tribes of Vermont could refer to a First Nations people, or to some type of white people. Bierce seems to be saying that Newcomb thought the hill tribes of Vermont did not resemble 'Man' [sic], so might as well be alien, I guess, or unevolved or devolved or something. I haven't really found anything else about this one.

So there you have it, a little discursion on Ambrose Bierce's satirical interplanetary musings.