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Intriguing questions. In one way the answer is obvious, but there is much more depth as soon as one gives it some thought. I expect a Ph.D. student could write a whole dissertation on the subject, so I'll just summarize some ideas here. You get to cherry-pick the useful ones and expand on them.

Illustration of astronauts on Mars.
Image Credit: NASA/Pat Rawlings, SAIC

The obvious answer is that since humans have lived for long periods in Earth orbit, in the ocean depths, and for short periods on the Moon, it seems we can live anywhere that we can supply air, water, and food.

For long durations, we must add spare parts for life support systems to the other items. So living anywhere requires funding to pay for the materials necessary (at a minimum, air, water, and food) and their transport, as well as the infrastructure to manufacture the systems to transport the necessities to the human outpost.

If the goal is to limit the necessity for constantly supplying the outpost with the necessities, the question still comes down to how much materiel is sent to the outpost initially vs. how hospitable the planet having the outpost is. Suppose we want an outpost on a giant icy moon of Jupiter's. With sufficient available power (solar, or some other source) and an air-tight habitat, colonists could grow plants hydroponically (provides food and clears the air of carbon dioxide while producing oxygen) and melt the ice for water lost in the recycling process. Some of the water could be broken down to free additional oxygen for breathing and to replace losses. Radiation protection would have to be built into the habitat too. All this could be done.

A more hospitable planet could still be cold enough that the only water is ice (which could be melted) but some atmosphere would be good, at least as thick Earth's atmosphere at the bottom of the "death zone" recognized by Himalayan mountain climbers. People could then survive in an atmosphere that doesn't support life by wearing oxygen masks, but not pressure suits.

(I don't know at what altitude a pressure suit would be required; it is probably higher than the base of the death zone. Note that the atmosphere cannot be poisonous; that's different from not supporting life. Chlorine gas is poisonous and will hurt organs and kill people. Nitrogen, the largest fractional component of Earth's atmosphere, won't support life but doesn't damage us.)

Just how warm a planet would be live-able is not obvious to me. Venus is too hot, but a planet that had Death Valley temperatures everywhere might still be live-able, if there is accessible water.

Astronomers looking for exoplanets want to find Earth-like planets in the habitable zone, where temperatures permit liquid water to be present on the exoplanets' surfaces. This is a reasonable shorthand way of describing the requirements for a habitable planet, but it doesn't take into account that both nature and humans can create small habitats suitable for life in what would otherwise be considered very extreme conditions.

I hope this discussion helps you narrow down your question, or inspires you to consider more of the possibilities.

Stephen Edberg
NASA Jet Propulsion Laboratory
California Institute of Technology

Currently there are not any planned human missions to Mars, but there are several concept missions that will help pave the way for humans to visit Mars in the future.

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Mars does not have a sufficient (big enough) atmosphere to protect it from impacts by such objects as meteorites, asteroids and comets. Also, Mars is not as active a planet as the Earth is, for example, so there isn't enough activity (such as volcanic, water erosion, or other forms of resurfacing) to cover over craters.

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Since Mars has less gravity than Earth, you would weigh 62% less than you do here on Earth.

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Mars and the Earth are at their closest point to each other about every two years, with a distance of about 56 million km (34.8 million miles) between them at that time. The farthest that the Earth and Mars can be apart is: 401 million km (249 million miles). This is due to the fact that both Mars and the Earth have elliptical orbits and Mars' orbit is tilted in comparison with the Earth's. They also orbit the sun at different rates.

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Yes, this is due to iron in the soil. This iron rusts and causes the surface and the sky (from kicked up dust) to take on a reddish color. This is why Mars is called the Red Planet.

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No, at least none have been found. So far we have not found any evidence of life (in any form) on Mars.

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Yes, but the Martian atmosphere is too thin for liquid water to exist for long on the surface. Water on Mars is found in the form of water-ice just under the surface in the polar regions and in hypothesized briny (salty) water, which seasonally flows in other areas on Mars.

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The temperature on Mars can be as high as 70 degrees Fahrenheit (20 degrees Celsius) or as low as about -225 degrees Fahrenheit (-153 degrees Celsius).

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It takes Mars nearly two (1.9) Earth years to complete one year on Mars.

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Actually very similar to the length of an Earth day. A day on Mars only takes 24.623 hours.

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Quite a few: Missions to Mars

Mars is about half as big as the Earth: Mars is 6,779 km (4,212.28 miles) across (diameter).

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Yes, but Mars' atmosphere is thin.

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Mars has seasons similar to those of the Earth, but longer. The seasons on Mars last about twice as long as those on Earth. However, the length of each season is not exactly the same since Mars has a slightly more elliptical orbit around the sun than does Earth. Spring in the northern hemisphere (autumn in the southern) is the longest season at 194 Sols (Martian days). Autumn in the northern hemisphere (spring in the southern) is the shortest at 142 days. Northern winter/southern summer is 154 Sols, and northern summer/southern winter is 178 Sols. That brings a Martian year to 668 Sols (or about 687 Earth days, about twice as long as an Earth year). A year, of course, is the time it takes for a planet to orbit once around the sun.

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Yes, Mars has two moons. Their names are Phobos and Deimos.

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Mars does not have a global magnetic field, but NASA's Mars Global Surveyor (MGS) orbiter found that areas of the Martian crust in the southern hemisphere are highly magnetized.

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Last Updated: 28 Apr 2014