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The context:

There is a population of people surviving on a lunar-analogue's surface, descended from the crew of a crashed spaceship. The rest of their society is not relevant to the question, but their technology includes cobbled together habitats and void suits that protect against some solar radiation but not all (an arbitrary amount that allows them to maintain a population, but not necessarily easily).

The question:

What skin colour would this select for? I'd initially say a pallid white given the lack of UV exposure, but I've recently stumbled upon research which suggest melanin provides at least some protection against gamma radiation: https://www.news-medical.net/amp/news/20110824/Melanin-also-protects-from-ionizing-radiation.aspx

The question is, what skintone would partial protection from gamma radiation on a longstanding permanent lunar culture select for?

For reference, this is for an art project where colour palette will be important, so injecting some realism into the skintone and working from there would be the way to go.

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  • $\begingroup$ (an arbitrary amount that allows them to maintain a population, but not necessarily easily). Then why would them use regularly void suits if they are unsafe? They would just switch to tunnels and vehicles unless for exceptional situations. It is not like people is going to be happy if you tell them "As you are white you can go because you will die of cancer at 50 instead than at 40". $\endgroup$ – SJuan76 Mar 24 '18 at 11:17
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    $\begingroup$ This is one of these questions where nobody has a 100% bulletproof answer, especially since we are talking tens of thousands of years here in a very hostile environment. Basically we lack any data. You should maybe consider choosing one, e.g pink, and ask how to justify it given x $\endgroup$ – Raditz_35 Mar 24 '18 at 15:07
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    $\begingroup$ Given that they're protected against "some solar radiation but not all", why would there be a "lack of UV exposure"? That's solar radiation, too. $\endgroup$ – William McBrine Mar 25 '18 at 10:32
  • $\begingroup$ @SJuan76 The level of knowledge of these people is a little above tribal->medieval, with a dash of higher technology remembered largely through folklore. A complex understanding of their 'natural' environment, but little in the way of theoretical knowledge. For instance, they would know that sometimes people got sick when they got older, but not that this sickness was caused by radiation exposure. Perhaps they might know of the sun as harsh, but equally as a source of warmth they may view it as wholly benign. $\endgroup$ – Ynneadwraith Apr 3 '18 at 13:09
  • $\begingroup$ @Raditz_35 If it makes it easier, the stylistic choices would be either a pallid white or jet black. Either could be justified through either lack of UV exposure for the former, or gamma radiation exposure for the latter. Ultimately we have no direct empirical data for either I can find, but you never know! $\endgroup$ – Ynneadwraith Apr 3 '18 at 13:13

11 Answers 11

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Random.

I am reminded of the teeth of the numbat.

numbat skulls http://animaldiversity.org/collections/contributors/anatomical_images/family_pages/dasyuromorphia/myrmecobiidae/

The numbat has more teeth than any other land mammal. Tooth number and shape vary between individuals. It does not matter to the animal because none of the teeth are used at all. The numbat eats with its tongue exclusively.

The variability in number and form of teeth, as well as the lack of significant tooth wear have been cited as. evidence that the teeth are used very little and so are not subject to intense selection pressure (Calaby 1960). https://www.environment.gov.au/system/files/pages/a117ced5-9a94-4586-afdb-1f333618e1e3/files/22-ind.pdf

So too skin color for your moon people. Skin color for earth humans is influenced by evolutionary pressures that have to do with UV damage / vitamin D synthesis. Absent selection pressures for or against given colors, skin color would evolutionarily drift, like the number and shape of teeth of the numbat. One could invoke this to explain why different individuals were colored differently one to the next: it is random.

Note that it has taken the numbat millions of years for its teeth to reach this state. But with a small population you could have evolution / genetic drift happen faster.

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  • $\begingroup$ I like this answer. Shows a great mid-way point through an evolutionary change. Eventually, the numbat should lose it's teeth simply because teeth are expensive to produce, unless there's something else at play. For the same reasons, I believed that in the absence of UV my people would take a similar path (although as far as I know teeth are more expensive to produce than melanin, so likely a very small effect). However, the suggestion that melanin could protect from gamma rays could produce a significant selection pressure given their effect on fertility, depending on melanin's effectiveness. $\endgroup$ – Ynneadwraith Apr 3 '18 at 14:32
  • $\begingroup$ Actually, this answer is doubly interesting. Teeth are expensive to produce. If they're completely unused, yet the numbat still has them, it suggests that evolutionary 'efficiency savings' might take a long time to realise (unless the numbat has ample calcium resulting in teeth production not being expensive). So, my people would have a very weak selection pressure towards paler skin, but a stronger selection to darker if melanin protects from gamma rays. So, the likely answer seems to be an overall selection pressure towards darker skin. $\endgroup$ – Ynneadwraith Apr 3 '18 at 14:35
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...Realism in the skin tone...

You have only two perspectives that would affect skin color.

  1. The original ethnicity and/or races of the crew. In the 1960's this would have been white people. Today, there is better diversity. Tomorrow, better still.

  2. Time. It takes time for skin color to change. Not years. Not centuries. Possibly not even millenium. It takes eons. The genetics of skin color takes a boatload of time.

If your intrepid crew's descendants haven't experienced at least tens to hundreds of thousands of years, then their location has nothing to do with their skin color. The politics and social mores of the society that launched them into space would have everything (as in 100%) to do with skin color.

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    $\begingroup$ "Not years. Not centuries. Possibly not even millenium. It takes eons." Actually. No. Not if there is a selective fitness or cultural benefit to a skin color. As long as you have some variation in the underlying population genetic skin color mix, you can get shifts in a matter of a few centuries. This has happened at least four times in Europe. (1) Light skinned Neanderthal to dark Cro-Magnon, (2) dark hunter-gatherers to olive skinned in the early Neolithic, (3) olive to pale in the Late Neolithic/early Bronze Age. Also (4) in Nordic areas from dark to pale over about 1ky in the Mesolithic. $\endgroup$ – ohwilleke Mar 25 '18 at 5:49
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    $\begingroup$ Where the fitness benefit is especially high, color change on existing variation can happen in two to three generations. Color change in moths due to pollution from coal took place in two to three moth generations and was one of the first observed cases of natural selection directly documented. $\endgroup$ – ohwilleke Mar 25 '18 at 5:51
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    $\begingroup$ @ohwilleke On the other side, around 15k years weren't sufficient for the native Americans coming from Bering strait to Ecuador and Brazil (and similar places) to become black again. $\endgroup$ – jk - Reinstate Monica Mar 26 '18 at 12:01
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    $\begingroup$ This question is a good example of the Founders Effect. Starting from a small sample size, things like skin tone would be overwhelmingly biased in favor of the original astronauts. If they are all white (or all black), then naturally developing skin tone variations would take a very long time, perhaps never, unless there was an external selection pressure. Same with things like epicanthic folds or hair color. Maybe if the crew was from latin America, with a mix of European and Asiatic DNA, you might have that unsort and allow for distinct skin tones. $\endgroup$ – Jason K Mar 26 '18 at 13:55
  • $\begingroup$ @jknappen If the relevant genes are entirely fixed in the source population and everyone had exactly the same pigmentation genes, then you'd need a de novo mutation to create variation before natural selection could act upon it, and the likelihood of that happening is a function of population size. In the New World, the founding population was on the order of a few hundred or so and it was likely fixed on these loci, and it took many generations for exponential growth to create a reasonable probability of a mutation. One confound for the Moon is whether cosmic rays increase mutation rates. $\endgroup$ – ohwilleke Mar 27 '18 at 4:17
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There would be no evolutionary pressure for a specific skin tone for living on the Moon, because -- even were we to colonize the Moon -- no one lives on the Moon like they live on the Earth.

That's because people will live inside all the time, getting their Vitamin D from either food or the interior lighting.

It's somewhat similar to white people living in Australia. You'd think that Europeans with darker skin would be more genetically successful, but they aren't. Why? Because clothes (and hats) shield them from the excess UV, while allowing enough to get to the exposed body parts.

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    $\begingroup$ Yes. People who go outside in a lunar environment die within seconds or minutes, unless they are protected from it with spacesuits or vehicles. Thus they don't adapt to the lunar environment since experiencing the lunar environment is deadly. $\endgroup$ – M. A. Golding Mar 24 '18 at 19:40
  • $\begingroup$ You're assuming a dedicated colonisation effort with modern understanding of the lunar environment. These people are about as far from that as possible. Think hunter-gatherer level of practical-vs-theoretical knowledge, but tailored to a lunar environment. Besides, in your model there could well be evolutionary pressure selecting against darker skin if their vitamin D intake was limited. Not large, but certainly present. $\endgroup$ – Ynneadwraith Apr 3 '18 at 13:42
  • $\begingroup$ Take the Europeans in Australia example. Technology (clothes and hats) allows fairer-skinned people to compete genetically with darker-skinned people. Shielding from excess UV is easy. Shielding from excess gamma radiation is hard (especially if you have no theoretical knowledge of what gamma radiation is). So, the question is, given decreased UV radiation and increased gamma radiation, what level of melanism would be more adaptive? $\endgroup$ – Ynneadwraith Apr 3 '18 at 13:45
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    $\begingroup$ @Ynneadwraith living on the Moon requires a lot of technology and energy. They'd die long before devolving to stone age technology. $\endgroup$ – RonJohn Apr 3 '18 at 13:52
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    $\begingroup$ @Ynneadwraith The Moon is about as far from a shirtsleeve environment as you can get. If something happens that compromises the technologically-driven stability of a colony, everyone drops dead when the air, food or water run out. If you have the technology to survive on the Moon to begin with, you have the technology to render skin exposure a complete non-issue. $\endgroup$ – Catgut Apr 3 '18 at 15:39
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It won't differ much from the mixture available in the founders' pool.

The reason is simple: while when our ancestors moved out from Africa to colonize the world had the pressure resulting from lower UV exposure that allowed for the selection of paler skins, I am pretty sure your colonist would be assuming integration vitamin D, removing any need for the body to adapt.

In the case they would not be assuming vitamin D integrators they would still keep the original mix for quite some time: it takes lots of generations for a character to spread, and humans are not that fast breeders.

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  • $\begingroup$ As far as I can tell, OP mentioned nothing about the timeframe. Perhaps they intend to represent this lunar society after thousands or even tens of thousands of years. $\endgroup$ – AngelPray Mar 24 '18 at 10:57
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    $\begingroup$ Yes, that’s why some arctic dwellers never lost their pigmentation, sincd tney get vitamin D from their diet. $\endgroup$ – JDługosz Mar 24 '18 at 14:04
  • $\begingroup$ @AngelPray Correct. I specifically avoided mentioning timeframe to try to reduce 'evolution takes a long time' responses. The question was phrased as 'what would be selected for', allowing me to know the future direction of travel for skincolour and pick a suitable timeframe to ensure enough difference. $\endgroup$ – Ynneadwraith Apr 3 '18 at 14:06
  • $\begingroup$ @L.Dutch Interesting. I hadn't considered vitamin D, and to survive my people will have to find an alternative source (food is probably the best bet given their GM crops). $\endgroup$ – Ynneadwraith Apr 3 '18 at 14:09
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Since skin colour affects appearance, sexual selection comes into play. Whatever skin colour their culture finds most attractive is what will be selected for.

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  • $\begingroup$ In societies that practice infanticide or infant abandonment, there is also selection at the time a mother decides whether to keep her newborn baby. This may result in selection for lighter features. $\endgroup$ – Jasper Mar 25 '18 at 18:54
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I do not know if more dangerous rays can trigger the same reaction in the skin as UV light does, but I expect they don't.

These people would most probably suffer lack of melanin as well as vitamin D. So they would need some sort of artificial sunlight source. Expect european people to be a bit paler if they do not attend their artificial sunlight exposures, but otherwise there shouln't be much difference. Tanning and skin color have very little to do with each other. One is a reaction of skin on dangerous environment and the other is a genetical predisposition.

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  • $\begingroup$ Interesting. I hadn't thought about gamma radiation influencing tanning. However, it is the genetic component that I am looking for an answer to, rather than tanning (although that is likely to be influenced). $\endgroup$ – Ynneadwraith Apr 3 '18 at 14:00
  • $\begingroup$ Creating a resistance to dangerous radiation would be impossible or it could take billions of years. So the only answer to that in your scenario is GMOs. But that is something for a completely new discussion. $\endgroup$ – Nuloen The Seeker Apr 3 '18 at 22:32
  • $\begingroup$ Read the link I posted in the OP, which suggests that an already existing and widespread biological mechanism may provide resistance to gamma radiation (barring further research discrediting it). Evolution isn't great at creating new mechanisms, but is very good at using pre-existing mechanisms for alternative purposes. $\endgroup$ – Ynneadwraith Apr 4 '18 at 8:32
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The selection pressures for skin colour are that dark skin copes better with exposure to ultraviolet light and light skin produces more vitamin D.

Your people are protected from UV so there's no pressure towards dark skin. If they're eating a balanced diet, there's also no particular pressure to produce more vitamin D. In that case, no skin colour would be preferred. The people's protection against UV exposure and sufficient vitamin D in their diet means that skin colour will have no influence on whether they live beyond child-bearing age.

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  • $\begingroup$ Initially what I thought, hence the selection pressure towards pale pallid skin. Melanin requires resources to produce, so the selection pressure would be to not produce it if it's not necessary (barring any sexual selection pressures). However, the linked article in my OP suggests that melanin can also provide protection against ionising radiation which will be a significant presence in the environment my people find themselves in. Protection from gamma radiation would have a significant effect on fertility, and thus potentially a significant selection pressure for darker skin. $\endgroup$ – Ynneadwraith Apr 3 '18 at 13:52
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As already pointed out - there would be no real external evolutionary pressure, thus sexual selection would be the way to go.

Short term: -Just mixing up whole gene pool. (so mixed color skin, dark eyes, dark hair etc)

Long term: Survival of the cutest. (but it is based on assumption, that there would be either a lot of time or possibility to pick designer babies)

-light skin (setting all PC aside, its not a recent phenomena, but something more entrenched. According to records in East Asia lighter skin was being perceived as attractive even in times when Europeans were just considered as some distant barbarians; moreover even in Europe there were periods when lead based white makeup was top trendy)

-blue eyes, blond hair (those genes are recessive, so would not manifest easily)

-neonate features of Asian face

-tall (it's also selected in sexual selection, and in low gravity setting it would have less drawbacks)

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    $\begingroup$ Interesting tidbit from my evolutionary psychology degree days, sexual selection for lighter skintone is likely to be a species-wide phenomenon. One of the effects of heightened oestrogen production (and thus greater female fertility) is paler skin, which translates across all skintones. Thus, preference for a lighter skintone in mates has an evolutionary advantage. Not pleasant in this day and age, but a large amount of evolutionary psychology just plain isn't. $\endgroup$ – Ynneadwraith Apr 3 '18 at 13:59
  • $\begingroup$ @Ynneadwraith Linkage between light skin color and oestrogen? That would explain why I've seen a study showing that average skin color of female models is slightly lighter than skin color of male models. Now this expectation concerning sexual dimorphism starts making sense. $\endgroup$ – Shadow1024 Apr 5 '18 at 9:05
  • $\begingroup$ Googled: progesterone makes lighter, estrogen makes darker. $\endgroup$ – Shadow1024 Apr 5 '18 at 9:14
  • $\begingroup$ Ah, that'll be fuzzy recollections of a long-gone university course then ;) thanks for the clarification. Details aside, the outcome of that particular study was that lighter skincolour in women was sexually selected, at least in part. $\endgroup$ – Ynneadwraith Apr 5 '18 at 9:34
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Gamma radiation would have no direct effect on skin color, since no pigment absorbs gamma better or worse than an equivalent mass of flesh -- or of water, for that matter. Gamma radiation is almost entirely due to nuclear energy level transitions, not electron energy level transitions (which are what produce color.)

Gamma absorption (absent resonances which are not relevant to the broad-spectrum gamma you get in space) depends pretty much exclusively on the density of nuclear matter in the way, which translates pretty exactly to the mass of absorber. So to absorb significant gamma, your skin would need to get more massive (a lot more massive), not change color.

It could become thicker or, conceivably, become denser by somehow developing calcium deposits. But never forget that any evolutionary change incurs a fitness cost as well, and evolution would balance the fitness cost of thicker (and hence higher energy cost and also less flexible) skin against the gains from increased gamma radiation resistance.

The only effect that gamma exposure would have on the evolution of pigmentation is to potentially speed the process up by causing a higher rate of mutation. If there was selection pressure for a change in skin color, that process might well be sped up. But where the increased mutation rate took the people would depend on other things.

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    $\begingroup$ Initially that was my understanding, but read the link in the OP for interesting interactions between melanin and gamma radiation in fungi. $\endgroup$ – Ynneadwraith Apr 3 '18 at 19:31
  • $\begingroup$ It looks like what's happening there is that the melanin is acting as a free radical scavenger. Very interesting. Pigments do have some advantage there, because the ability to absorb visible light requires somewhat more active chemistry than most organics (which absorb only in the UV) have. That suggests that there might be a general darkening of all tissue -- since gamma is so penetrating, the skin or the center of a muscle or your gut get the same dosage as an adaptation to radiation in general. It's always worth being a bit skeptical of surprising results until they are replicated. $\endgroup$ – Mark Olson Apr 3 '18 at 19:41
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    $\begingroup$ It is interesting indeed! At basically the same time you've hit upon a similar idea as Lex: melanism throughout the internal organs as gamma radiation is so penetrative. Interestingly this would probably be useful for people thinking of genetic modifications for space-borne people in a near-future setting as well $\endgroup$ – Ynneadwraith Apr 3 '18 at 20:03
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A detail that heavily effects the answer is what temperature is maintained in the colony. Several complex factors will effect temperature, and you can basically choose whatever fits the story. Temperature will effect how much clothing is worn. Amount of clothing then effects melanin levels.

  • hot->little clothing->high melanin to shield from uv
  • cold->thick clothing->low melanin to allow vitamin D production in the little exposed skin

If you are assuming sufficient clothing to block UV, then then assumption that gamma radiation will have the dominating effect is flawed. If opaque clothing has a negligible effect on blocking gamma rays, then so will opaque skin. Since high energy gamma rays penetrate opaque clothing they will penetrate skin as well. Clothing will actually perform better than skin could. Based on the description of the environment, it seems likely that clothing will be made from animal and plant tissue. Many animals and plants will adapt to the environment faster then humans due to shorter life cycles and selective breeding. This means clothing will more quickly adapt to blocking any radiation that can be blocked than humans will.

There is one way melanin might have a greater shielding effect than clothing. If a much thicker layer of opaque tissue could be used for shielding than just the skin. This could lead to the possibility of pale semi translucent skin for vitamin D with melanin rich fat, muscle, and or bone tissue. It sounds like you are wanting a very striking look and a way to justify it, this combination may be fitting.

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  • $\begingroup$ Oh now this I like. Gamma rays penetrate far further than UV (basically causing ionisation events in a little line straight through you), so melanised fat, muscle and bone (basically jet black internal organs) would protect far better than simply melanised skin. Plus, the pale translucent skin would assist with vitamin D production. I also very much like the idea of animals/clothing adapting faster, both from a logical and visual perspective :) $\endgroup$ – Ynneadwraith Apr 3 '18 at 19:49
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I'm not sure if this is the convention, but after some more research and reading all these answers I think we may have come to something approaching an answer.

So, it seems we have a number of factors that influence skin colour:

  1. Original population genetics
  2. UV exposure
  3. Vitamin D production
  4. Sexual selection
  5. Resource cost of producing melanin
  6. Gamma radiation exposure

Original population genetics sets the startpoint and pre-existing genetic variety, but we can split the rest into pale-selecting and dark-selecting pressures:

Pale: 1. Vitamin D production 2. Sexual selection 3. Resource cost of producing melanin

Dark: 1. UV exposure 2. Gamma ray exposure

From these, for our lunar population we can discount Vitamin D production (in order to protect from UV they'd have to avoid direct sun exposure, so vitamin D would likely be sourced from food). We can also probably discount the resource cost of producing melanin given that it's taken so long for numbats to lose their expensive-to-produce teeth (I'd like to find some other data points for that). Sexual selection is an interesting one, but considering the relative stability of skincolours and lack of sexual dimorphism it's probably pretty weak.

So, it basically comes down to relative exposure of UV on the earth's surface to gamma radiation on the moon. If the radiation on the moon is equivalent to northern Europe we might see a gradual slow movement towards paler skin. If it's equivalent to Africa (or higher) then we will likely see a move towards darker skin (potentially rapidly).

Unfortunately, there's a maddening lack of studies comparing the relative damage of gamma ray and UV exposure. Closest I've come to finding something is a load of people stating how difficult it is to compare them and one guy who's actually done something and found that 6J/m² of UV exposure and 4 Grays of gamma exposure killed the same amount of chicken cells (conditions unknown so not the greatest test but it's all we've got).

From this study we can see that in Europe we are around 200J/m² per day. In central Africa we are around 5000J/m² per day.

The highest figure I can find quoted for average radiation on the lunar surface is 120 millirem per day (others hover around 50 millirems), which converts to 0.0012 Grays of gamma radiation. Practically nothing. Wait, why are we scared of gamma radiation on the moon again? Unless they're quoting shielded figures, or the 6-to-4 ratio of that guy was for one layer of cells (so gets multiplied by each layer of cells the gamma rays reach that the UV rays don't).

The only thing I can see that would be a problem gamma-radiation-wise is the recommended maximum radiation dose for fetuses (50 millirems per month plus the 25 millirems background). So, sod all effect on adults but very dangerous for kiddos, unless I'm missing anything major.

Oh, and apparently during an 18-month study on Mars there were 2 events which saw radiation increase to 2000 millirems per day (0.02 grays).

So, all of that weighs out to a very slight selection pressure towards paler skin with a cultural trait of hiding pregnant women within rad-shielded bunkers, or a strong selection pressure towards jet-black skin in order to protect their unborn children.

Edit: apparently the safe level for radiation exposure in US legislature is 5000 millirems per year, or 13.7 millirems per day. Lower than the level our lunites will be receiving. So, leaning towards the jet black option of the two above...

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