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I was thinking through how to portray Martians, and realized that there's really no reason for them to be green that I can think of. Both camouflage and (I assume) availability of pigments in the local biosphere would imply that they would end up reddish, like the soil.

But then I had a thought; is there a plausible reason for Martians to have green skin?

Note that I am not looking for an explanation of why Martians are generally portrayed as having green skin. I want a story element I can use to explain how green skin could develop in a Martian species that has natural selection pressures similar to our own.

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    $\begingroup$ they could do photosynthesis $\endgroup$
    – Nephanth
    Jul 12 at 13:08
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    $\begingroup$ To hide from dogs $\endgroup$
    – Qami
    Jul 12 at 15:40
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    $\begingroup$ So they can hide in xmas trees. Ask yourself, have you ever seen a martian in a xmas tree? See! It works really well. $\endgroup$
    – EDL
    Jul 13 at 6:34
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    $\begingroup$ There are a wide range of different colours in terrestrial animals and these don't always correspond to their surroundings. I see little reason to even question the colour a particular hypothetical being would be. Camouflage is an evolutionary advantage for prey and more passive predators, but it doesn't apply to all species at all stages of evolution and it's far from the only factor in determining colour. If that pigment can't form with Martian biology at all, that would obviously be a problem, but its existence in Martians in itself is sufficient to prove that it can. $\endgroup$
    – NotThatGuy
    Jul 14 at 3:25
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    $\begingroup$ We have iron in our blood. Maybe they have reason to have copper in theirs? $\endgroup$
    – MonkeyZeus
    Jul 14 at 20:14

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It's Not Easy, Being Green:

Why would Martians be green? Lots of strange things can happen to a species, so let's dig into them.

  • Ancient traits: Once, Mars was green. Martians blended into their environment seamlessly. Then things got cold and dry and oxygen dropped off, but Martians were still green. Without a strong pressure to change to a new color, they are still green. A complex series of biochemical processes go into them being green, and change would mean a reworking of Martian biology.
  • Cupric skin toxins/flavorants: Green skin comes from toxic copper compounds in the skin of Martians. Martians taste bitter, are visibly green (a signal to predators) and poison any native Martian life that eats them. By being prominently green, they get attacked by fewer native life forms than those poor, pale Earthlings who are actually even less edible.
  • Waste deposition: Copper is toxic to Martian life. Martians have adapted to this by depositing copper in their skin, to be sloughed off with dead skin cells. The result is green skin.
  • Selective vision: Martians are able to communicate by subtle differences in appearance of their skin in the narrow band of color near green. Because there is so little green, it's easy to spot. Martians have hundreds of words for green shades, each denoting their health, status, diet, allegiances, and current emotional states. Poor humans look at them and wonder why Martians say so little and express so few emotions. Martians turn a lovely shade of dismissive malachite and their cheeks turn a warm emerald to express their humor.
  • Reproduction: While seeing lots of green, Martians are also turned on by green mates. Chartreuse males are seen as sickly and diseased, and cyan females are rejected as being unappealing and under-oxygenated. OMG, did you see Grog? Forest green - huba huba!
  • Radiation Shielding: Some green-reflecting compound, the equivalent to melanin, absorbs radiation from the environment. Martians are protected from this radiation by the thick deposits of this compound in their skin.
  • Religion: There is little or no green on Mars, so green is special to the Martians. They paint themselves green to set themselves apart from all lower life, as an expression of their intrinsic closeness to god. They keep invading Earth because it's CLEARLY a sacred land, populated by wicked, godless human heathens. Martians keep trying to get rid of humans to free the holy land. They're green not with envy, but holy fervor. This also explains some of the apparent illogical behavior of the Martians - they aren't motivated by conquest, but religious fervor.
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    $\begingroup$ Ooh, I really love the Selective Vision one! I'd imagine that another side effect of it being so different is some predators might not be able to detect green light? $\endgroup$
    – Benjamin Hollon
    Jul 13 at 4:47
  • $\begingroup$ @BenjaminHollon It's true, the Martians would be grey to martian animals that didn't see green. You need a reason to see a color. I've had it suggested monkeys/apes can see so many colors because it was an advantage to know when colorful fruit was ripe. In this case, green's advantage is in communication/social signaling. $\endgroup$
    – DWKraus
    Jul 13 at 4:52
  • $\begingroup$ One wonders what the religious significance of abducting and probing helpless livestock is... $\endgroup$
    – Sebastian Lenartowicz
    Jul 14 at 10:43
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    $\begingroup$ @SebastianLenartowicz That's GREY aliens, not green ones. Blasphemers and heretics. $\endgroup$
    – DWKraus
    Jul 14 at 10:58
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    $\begingroup$ There's a reason tigers are orange: with red/green colorblindness (which most animals have) the tiger's primarily orange camouflage blends in incredibly well with the green foliage. Similarly green Martians would be well-camouflaged against brown-red landscape $\endgroup$
    – thegreatemu
    Jul 14 at 17:47
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Our skin pigmentation has nothing to do with mimicry, it's an adaptation to mitigate the problems arising from the interaction between the solar radiation and our skin.

Do the same for your Martians: their equivalent of melanin is a pigment providing different hues of green, from deeper to lighter. You can also top it with a blood pigment, equivalent of our hemoglobin, which gives their blood a green color, thus making their skin look green-ish.

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Bilirubin protects from mutagens and oxidants.

High levels of bilirubin in the blood is called jaundice.

baby jaundice

https://ihv.org.uk/news-and-views/voices/kelly-anne-and-siennas-story-of-baby-jaundice/

But the pigments that cause jaundice can protect against mutagens and oxidants.

The anti-mutagenic and antioxidant effects of bile pigments in the Ames Salmonella test

Bilirubin and biliverdin are potent antioxidants, and as such a possible physiological role for them has been postulated [3]. Specifically, they have been found to protect lipids [4], [5], [6], [7], [8], [9] and proteins [10], [11], [12] from oxidation.

Your martians live in a mutagenic environment. Their intrinsic skin color is very pale, like the depicted baby, and so the high levels of bilirubin and related bile pigments make them delicate shades of yellowish green.

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    $\begingroup$ Gross, but educational. In general frogs do sufficiently prove possibility of green skin pegments of different intensities and shades of green $\endgroup$
    – MolbOrg
    Jul 13 at 6:09
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For the same reason that poisonous frogs are brilliantly colored.

Being green makes them stand out, and also makes them memorable. Most animals lack the cunning to realize that something would not be so conspicuous without good reason, but most are cunning enough to remember how badly they got beaten the last time they tried to eat something green.

For a Martian, at some point, the cost of beating up a predator once and the metabolic cost of being green was less than the cost of beating up a predator every day.

If those circumstances no longer apply, runaway sexual selection may have occurred: the green is now sexually attractive, not because it means you can scare off predators, but because it means your offspring will be sexually attractive.

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  • $\begingroup$ The sexual-selection angle is a neat idea. If being green makes you more visible/vulnerable to predators, it may be an honest signal of fitness, and thus remain attractive, even if predation is no longer a concern. $\endgroup$
    – Jon Purdy
    Jul 14 at 19:28
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Symbiotic plants on the skin

One thing that is near universally green are plants. Any part that is using photosynthesis looks green to our eyes (in nearly all cases). Why green isn't used in photosynthesis is unknown.

A symbiosis can form between skin and certain single cell or multicellular plants. The advantages can be numerous. The plants are moved and can spread, can be growing in all skin tissue a bit like mitochondria do in all cells, are somewhat protected by the larger organism and get food. The Martian gets protection from possible unfiltered sunrays, allowing them to not require pigments themselves.

Besides this obvious benefit, it can also reduce the damage of any skin disorder like albinism, allowing more Martians to survive. There can be a lot of possible advantages in healing and the strength/flexibility variability of these cells as well, as you offload this (partially) to different organisms. It reduces complexity for the Martian DNA to simply have some other fully realised forms of organisms do this for you, much like the complex gut flora and fauna help in the gut.

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  • $\begingroup$ Phycobilin are photosynthesizing creatures which come in red, orange, magenta and blue. $\endgroup$
    – Marlin Pierce
    Jul 12 at 19:46
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    $\begingroup$ You don't even need symbiosis, if the Martians themselves can photosynthesize (though probably not as their primary source of energy) $\endgroup$
    – Mooing Duck
    Jul 12 at 22:25
  • $\begingroup$ that could make a good baptism ritual whene a baby martian is first exposed to those plants to develop the symbiose $\endgroup$
    – Nephanth
    Jul 13 at 9:12
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    $\begingroup$ @MooingDuck true, but I suspect the symbiosis is a better way. The energy gain from the plants is negligible compared to what we use. The increased complexity of the skin and added possible detriments makes it an unsecure method. In contrast, symbiosis offloads the complexity, while allowing different and easily replaced organisms to join the skin. If one is bad, it'll die and be replaced by a better one. $\endgroup$
    – Trioxidane
    Jul 13 at 14:38
  • $\begingroup$ @Nephanth: Unnecessary. Humans already have symbiosis with literally thousands of other plants and creatures, and we inherit most of them via birth, food, or human interaction. Demodex live in your eyelashes for instance. $\endgroup$
    – Mooing Duck
    Jul 13 at 16:11
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Have you ever seen a true color picture taken from Mars?

It looks like a Netflix show when a scene happens in a latin-american country - everything is just hues of one basic color. For Mars, it is actually something that looks orangey to yellowish to my eyes. You can judge for yourself by browsing NASA's APOD for a few minutes, looking for Mars pictures.

Martians are visual creatures just like us, so they are green - or paint themselves green - in order to stand out, so that they can actually see each other.

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    $\begingroup$ However, green only stands out from orange/red if you have color vision similar to humans. Even most Terran mammals are dichromats and cannot distinguish between green and red well at all. Including around 10% of human males. $\endgroup$
    – JohannesD
    Jul 13 at 21:28
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    $\begingroup$ @JohannesD - that's true, but it's worth pointing out: dichromatism evolved the way it did, seeing the frequencies of light that it did, because it was genetically beneficial. It's entirely possible a martian dichromat would see two different frequencies - one for the ground and one for something-other-than-ground $\endgroup$
    – Kevin
    Jul 14 at 16:44
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    $\begingroup$ @Kevin: Yeah, the spectral peaks in our colour vision are basically adapted to the absorption/transmission spectrum of our atmosphere. (Also, if you want to viscerally see how CO2 is a greenhouse gas, look at how clearly it shows up on those graphs…) There’s nothing inherently special about the “visible spectrum” except that our atmosphere is transparent to it, so it’s a useful range to see in. It would be a bit different on other planets—on Mars, with a 95% CO2 atmosphere, you’d be best off with fine colour discrimination in the green–red–IR range (about 550–1500nm iirc). $\endgroup$
    – Jon Purdy
    Jul 14 at 19:20
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The surface of Mars is a rather inhospitable place. Very low oxygen content, relatively high charged particle radiation and very low temperatures for instance.

In order to survive just those conditions alone your Martians will need a better oxygen transport mechanism, better protection from charged particle radiation and a nice, thick layer of insulation.

In low oxygen pressure environments, certain variants of hemocyanin can provide significant improvements over hemoglobin when it comes to oxygen transportation. This has been observed on Earth in arthropods and mollusks.

Since hemocyanin comes in a variety of shades of blue we're half way (in Human vision at least) to the green you're looking for. Fortunately, there are all sorts of interesting fats and waxes that have the required yellow to complete our martian's coloration. Subcutaneous fatty deposits are a mammalian staple for heat retention, and certain types of wax have may provide some protection against charged particle damage.

Alternatively, a much more yellow-colored lymphatic fluid would complete the green coloration well enough, as long as the skin itself is largely translucent.

And finally, hemocyanin contains copper atoms. In order to maintain their blood the Martians would require a source of copper, and may use it in other aspects of their biology. Some copper compounds are bright green, such as the copper carbonate that gives the Statue of Liberty (and just about any weathered bronze or copper object) that bright green color, as well as being the base for malachite.

Of course you'll need a ready supply of copper, but fortunately the volcanic regions of Tharkis and other volcanoes around Mars seem to have you covered there.

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Their alien biology uses, instead of hemoglobin, a compound similar to hemocyanin (but a slightly-different color). Where hemoglobin transports oxygen using iron and makes our blood red, Martian blood binds the oxygen as copper oxide, and is green or cyan.

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  • $\begingroup$ I’d missed Corey’s better answer along the same lines, but since this was useful to some people anyway, I’ll leave it up. $\endgroup$
    – Davislor
    Jul 14 at 13:41
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Fashion and Body Art

Have you seen how lately there are more and more people covering their body in big tattoos? The same happens in Martian culture since ages.

In reality their body is not uniformly green, but only if viewed from a distance. If you get close you would see complex tattoos made with green ink on their grayish body.

Since they have those big eyes, they are able to resolve thinner and more tightly packed lines than humans at the same distance. So what appear as a uniform green for a human without a loupe, at close inspection are amazing tattoo patterns, which they can easily discern when talking face to face.

And maybe they "encode" the history of their family in their tattoos, as sort of tribal marks. Or maybe they are just a fashion thing that will end when the next Martian fashion guru declares "violet is the new green"!

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Their skin absorbs IR in the 1000nM spectrum to make the most of available energy. Due to a non-linear optical property that IR frequency is doubled and re-emitted in the green color range. Healthy Martians have some excess energy to burn and its emitted as green light. They actually glow green. FWIW this is actually how green laser pointers work.

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  • $\begingroup$ Considering the temperature, there is not that much IR radiation on Mars, and it would make little sense to make the most of it by absorbing and then re-emitting it with frequency doubling. $\endgroup$
    – L.Dutch ♦
    Jul 14 at 13:26
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Nanobot Swarm

In place of a space-suit, each Martian is covered by a swarm of nanobots, which repair the skin, gather data on incoming radiation and chemical hazards in the surrounding breeze, and act together as a whole-body antenna for HF radio communication. The nanobots gather power from ambient UV radiation, and emit green light.

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