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Not sure if this question has been asked before, but here goes;

How would skin tone change with distance to the Sun?

For context to this question, I'm planning out a future Solar System, with Humanity having Terraformed, or is in the process of, Terraforming the Inner Planets of the Solar System, as well as many of the Gas Giant Moons, with various other colonies being constructed across the Solar System. Though the planets have been Terraformed, I would imagine that there would still be many differences with each planet's Humans, as opposed to an identical clone of Earth's Humans on each planet.

With this being said, how would skin tone of each planet's Humans change between, say Mercury, to Mars and even Europa? Would the tones get darker or lighter with distance, or would they stay roughly the same?

EDIT(1): This is all under a timeframe of roughly 5,000 years, given that each planet was colonized within 10 years of one another.

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    $\begingroup$ What is your timeframe? One year, ten years, one hundred years, one thousand years, one hundred thousand years? As you have done your research on the subject you know that for example, descendants of Dutch people have been living in South Africa for about four centuries, and they are as light-skinned as they were when they came there; and you know that people of sub-Saharan African descent have been living in Canada for about a quarter of a millennium, and they are still as dark skinned as they were originally. $\endgroup$
    – AlexP
    Dec 28, 2020 at 23:58
  • $\begingroup$ From the article on skin pigmentation on Wikipedia: "Research by Nina Jablonski suggests that an estimated time of about 10,000 to 20,000 years is enough for human populations to achieve optimal skin pigmentation in a particular geographic area". So even assuming evolutionary pressures existed, your timeline is in all likelihood too short. $\endgroup$
    – jdunlop
    Dec 29, 2020 at 0:17

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The variance is unlikely to be substantial.

Look at Earth's Many-Varied Splendour!

Homo sapiens sapiens has many shades of skin tone - and we all evolved on Earth. Some of us lost our melanin when we moved to different climes - but whether that is purely a matter of insolation or a by-blow from other evolutionary adaptations to new environments is still up for discussion amongst evolutionary forensic specialists.

With that in mind, assuming humans have been on the other planets for the tens of thousands of years it would take, I would expect to see just as much variance there as here. You wouldn't be able to look at the colour of someone's skin and say "That's a Martian." You'd probably be able to look at their stature and say that, though, given the major gravitational differences.

A Tough Challenge Being Overlooked

Mars could probably be terraformed. It's got enough gravity to hold an atmosphere, though it would require replenishing, and some tricks with orbiting mirrors could provide enough sunlight to warm the place up.

Venus ... could be terraformed, though with even more work. You'd need to remove 98% of its atmosphere just to start, cool the planet significantly, and either set up orbital mirrors or speed up its rotation.

I very much doubt that Mercury could ever be terraformed. Any civilization with the tech to do so would just build a habitat elsewhere. It doesn't have the gravity to hold an atmosphere, if it did, it would be too close to the sun to hang on to it, and even if it could, it would receive far too much insolation to be habitable without a planetary sun shade.

Likewise most of the gas giant moons. Venus and Mars live within the Goldilocks Zone for sol. None of the other planets (or their moons) do so, so artificial forms of sunlight would be the only way to deliver enough energy to make them habitable. Similarly, holding on to an oxygen-nitrogen atmosphere with the low gravity of even the Jovian moons is unlikely in the extreme. So anyone not on Earth, Venus, or Mars is likely only going to be living in artificial habitats, not out under the open sky.

So, as far as people from Europa or Mercury are concerned, their skin tone would probably be about the same as one would expect from people living in a skyscraper with sunlamps - ie. probably pretty close to Earth-standard.

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  • $\begingroup$ Their eyes might change within the time frame. Closer to the sun would favor darker colors and smaller irises while further would favor lighter colors and larger irises. This ignores the rods/cones becoming more/less sensitive and the brain simply learning to adapt - but one thing you're dead on about, a bazzillion years of evolution might take more than 5k years to substantially change. Especially if there's no immigration/intermarrying during that time period (which would be odd if there wasn't. $\endgroup$
    – JBH
    Dec 29, 2020 at 0:23
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    $\begingroup$ @JBH - also worth noting that if terraforming is taking place, the terraformers are going to take measures to make the sunlight received on the surface as close to earth-normal as well. So the proximity to the sun might actually be immaterial. Recent studies (that I have only just learned about) suggest that Earth is on the very inner edge of the Goldilocks Zone, so a sunshield at the Venus/Sun Lagrange point, reducing the sun to Earth-normal, might be necessary to successfully terraform it. $\endgroup$
    – jdunlop
    Dec 29, 2020 at 0:26

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