Potential 'Realistic' Effects of Living on the Moon/Mars/etc.

Maybe something missing and/or extra in the diet could perhaps change something, but I don't know what. They'd surely know a lot about nutrition, so this seems unlikely.

For people with moderate to light skin tone, the wouldn't normally be expose to untraviolet (from the Sun, nor from artificial UV light unless it was determined they needed to be exposed to it, for, say, vitamin D production), their skin would be lighter than their relatives on Earth. But I don't think this would be enough to make a stark or reliable difference.

But about the gravity thing - people living in space would generally not have excess food because it may be expensive to produce/grow, and surely expensive to get from Earth, so no one would be fat. Due to low or no gravity, they would have smaller muscles, and look (and be!) weak compared to those from Earth. The Moon and Mars have significant gravity, but not nearly as much as Earth.

Astronauts who currently spend months or even weeks in space do a lot of exercise just to keep in shape enough so their bone density (among other things) doesn't deteriorate too much, otherwise they would risk breaking bones back on Earth, and have an even worse time getting back to walking again on Earth than they do. If someone is born on the Moon or in zero gee, and intends to spend the rest of their life there, may not need anywhere near the amount of exercise current astronauts do. I think such people would look like the classic "97 pound weakling" to an Earth person, even though their (lack of) bone mass and strength might be enough for the environment they live in.

You can have rotating/centrifugal living quarters on the Moon and in outer space, but perhaps only those visiting from Earth spend much time in them at anything near Earth gravity levels. I think this gravity difference would make more of a believable, noticable difference than anything else.

I'm also brewing a story set approx 200 years from now and involves a colony on Europa. But I may shift the time earlier and explore Ceres or space colonies. Fortunately my story, if proceeded, will be adult/NA and race isn't even on my radar. We will not be in conflict.

Tattoos etc as source of conflict sounds too Divergent-like to me. You can create a new race after a colony shield on Phobos is broken and some humans are exposed to some rare cosmic rays. Or pioneers exploring the water lake in Europa are infected by indigenous microorganisms (give me a holler if you do this. I won't do this but I'm still pondering what Europa's marine life will affect human destiny.)

If you are more into slightly hardcore, I think the conflict you need will largely depend on life on Earth. Will there be a united Federation or will Africa Union dominate the world? Why are humans colonizing into the space (is the Earth livable?) Will there be substantial cultural differences between colony immigrants comparing to those staying on Earth? Will the immigrants be satisfied with new life or will they want to come back to Earth once they have saved enough money? Will first generation immigrants have different status to newcomers? Basically cultural, economical, or political differences should have bigger impacts than race, IMO.

As benbradley suggested, without gravity, humans will suffer muscle and bone loss and maybe other health deteriorating symptoms. But it seems creating artificial gravity on huge space stations isn't that hard (in theory.) On smaller planets or moons, that may be a potential issue (or economic conflict if colony governments charge too much for fusion AG ^_^.)

1) Read up on the NASA reports and proposals for off-world living.
2) Think class differences instead of race. I own a space ship capable of traveling to Mars, you don't. These are universal and will always exist.

Jeff

Some people (including many scientists) believe that growing up under lower gravity conditions may cause humans to grow taller and leaner.

While adult astronauts do indeed stretch out in space gaining up to 2 inches extra in height while they're in space because the reduced gravity causes the fluid between vertebrae to expand. They lose the height within 10 days of returning to Earth. It's difficult to say how someone who grows up in space would be affected, as their bones might grow more due to the environment. Genetically we'd be the same but would be visually more lean due to the muscles stretching out more as muscle grows more slowly than bone (as far as I know).

Astronauts experience bone related problems in space but those who were born in space wouldn't. However, coming to Earth for the first time might be difficult as they would find it harder to adjust to the heavier gravity than vice versa.

http://www.livescience.com/33082-would-humans-born-on-mars-grow-taller-than-earthlings-.html

This is all theory however but it's credible enough to write a book about it. There could be more physiological conditions that could develop in lower gravity planets but it wouldn't be evolution as the DNA doesn't change.

Science Fiction is not just based on hard science, but also on the "soft" sciences, like sociology. If you are looking for schism between the colonists and those still on earth think of un-natural selection. A colony is going to be created by either a government or a large business entity, i.e. for profit. Both groups are only going to select certain types for a colony, most likely intelligent well educated people. Would it create a brain drain on Earth? Does this become hereditary? There are a lot of possibilities. Perhaps in your future it is discovered that people with certain birth defects that result in low IQs are the most qualified for particular jobs and they are in high demand. Whenever people are separated based on physical or intellectual abilities, trouble follows.