There's been a lot of talk about Mars lately. Tech mogul-turned-presidential advisor Elon Musk has made no bones about his ambitions to send people to Mars, eventually to establish a permanent human presence on the red planet. While the technological challenges to settling Mars are considerable, maybe prohibitive - although, I have to say, I think that if anyone can pull if off, it's Elon Musk.
I've said and written, in fiction and non-fiction, that I believe the first successful off-planet colonies will be established not by government, but by the private sector. Elon Musk seems determined to prove me right.
But let's set aside the technical challenges for a moment and postulate a human colony on Mars as a fait accompli. What happens to that human population over time? The thumbnail, speaking as a biologist, is this: People born and raised on Mars would not be able to return to Earth. It would be a one-way trip for those who became Martians, and in time, Homo martianus. Here's why.
Gravity. Mars lacks our comfortable, accustomed, steady 1G in gravity. Instead, the Red Planet has about 1/3 G; a person who weighs 180 pounds on Earth weighs 60 pounds on Mars. But imagine going the other way, from Earth to a planet with a 3 G acceleration. That would make that 180-pound Earthling weigh 540 pounds. An Earthling in such an environment would be rendered unable to walk and would have trouble breathing; bear in mind that atmospheric pressure would also be increased. That applies to a born-and-raised Martion coming to Earth. They would, on arrival, be crushed by their own weight. Any children born and raised on Mars would be taller, leaner, with denser and longer bones. They would look funny to those of us born on Earth; tall, thin, with greatly expanded chests. That brings us to:
Oxygen. Any human habitat would require an artificial supply of oxygen. Mars' atmosphere is not breathable, but even if we can provide a high enough O2 supply to sustain our Martians, the lower atmospheric pressure would require enhanced respiratory efficiency. Unless every human lived in pressurized domes, the atmospheric pressure would be the equivalent of living in the high Andes.
See Also: Elon Musk Sets Ambitious Goal for Sending Humans to Mars—Can He Actually Do It?
Radiation. Mars has no magnetic field to shield it from harmful solar radiation and cosmic rays, unlike Earth. Martians would either have to live underground - always - or would, in time, grow darker. We see this already on Earth, where populations of humans who live in equatorial regions with more exposure to the sun and its ultraviolet radiation tend to have more melanin in their skin. Any human populations on Mars, at least on the surface of Mars, would, over a span of generations, grow darker.
Immune system. Mars is, as far as we know, a sterile desert. The only pathogens any human population on Mars would be those that the settlers brought with them, and it's a safe bet those settlers would be among some of the most medically-examined people in the history of mankind. After a few generations, the unused immune system would wither away. Forget gravity; any Martian, after a few generations, who returned to Earth would very likely grow very sick very quickly. Picture the fate of the invading Martians from H.G. Wells' "The War of the Worlds," only the Martians are humans.
Environment. We are fortunate to live on Earth, with its oceans, forests, grasslands, and the great diversity of animal life, and even the various bacteria and other microscopic creatures that make up our environment. We would either have to find a way to ensure some biological diversity on Mars as well, or the colony wouldn't last. Consider, for one thing, that our digestive systems are dependent on specialized bacteria in our guts that help our systems break down foods - we would have to find a way to preserve and pass on those bacteria. And, this may be my own bias talking, but I would fear depression becoming an issue. We are attuned to Earth and its great diversity of life and landscapes; in comparison, Mars seems a very poor place indeed.
A human population on Mars may never be practical, at least with any foreseeable technology. Mars may be a destination like a hazardous-duty military posting, where people spend a year or two, then rotate home. But were there to be any permanent colony, with people born and raised on Mars, throughout the generations they would become very different from people living on Earth, and would likely be unable to interbreed with Earthlings any longer. They would be something new and different: Homo martianus.
