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Mars is extremely hospitable to humans, compared to almost every other planet except Earth. Mercury: tidal lock to Sun, heavy solar radiation and solar flares up close, complicated orbital movement, low gravity, basically no atmosphere. May be habitable underground in particular belts with reasonable temperatures. Venus: a runaway greenhouse effect, metal-melting temperatures and colossal pressure on the surface, colossal never-clearing clouds, very strong winds, sulfur and chlorine abound in the atmosphere. Could be habitable using artificial islands floating in the upper atmosphere where the temperature and pressure are comfortable. At least the gravity is close to that on Earth. Jupiter is not habitable by itself, just due to the very strong gravity. Various moons of Jupiter are all pretty cold (though some have liquid water under the surface), and at his distance from Sun, the amount of sunlight reaching them is rather small, even when reflected by the giant disc of Jupiter in the sky. It's cold, solar power in impractical, uranium and thorium are not apparently abundant. Saturn: see Jupiter, only even farther and colder. Neptune: see Jupiter, but even without serious moons. Pluto / Charon binary: not even a planet. At this distance, Sun is but another star in the sky. Compared to these, Mars, with its conditions that resemble Earths's polar regions over large swaths of the surface, acceptable distance from Sun, serious amounts of water (as ice), acceptable gravity, acceptable amounts of sunlight, and relative closeness to Earth looks not that bad at all, and basically the only practical candidate for an autonomous, self-sufficient world. |
People would have bone density problems much sooner in life, as an example.