Why would humans try to establish a colony on Mars rather than on the moon?
Seems to me that both bodies are similarly inhospitable, yet the moon is so much easier to reach (and we should be able to transport much, much more cargo there).
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There are three big advantages to Mars over the Moon for a longer term settlement.
Let's break these down one at a time.
The thin atmosphere allows aerobraking, and can be useful in the production of oxygen. Electricity + Martian atmosphere yields oxygen, carbon, and likely water as well. It can be used to grow plants for food as well. You can make methane from the carbon, so rocket fuel, and likely plastics.
Producing carbon dioxide on the Moon is more challenging.
The days being nearly 24 hours allows for a much easier time to get used to things. A 2 week long day followed by 2 weeks of night on the Moon will make the thermal management much more difficult, for instance, having to be managed for the two sides. The solar power availability is limited as well.
We don't really know what the minimum gravity required for normal human life is, but the Martian gravity is much closer than the Moon's, and thus probably better.
There are a few disadvantages too. The distance is way further. You can't get there all of the time, only a few limited times when the planets align correctly. Still, it seems likely that long term stays on Mars will be easier than those on the Moon. But only time will really tell.
I'll defer to others on technical advantages/disadvantages. But it's kind of fun consider some possible answers in terms of underlying motivations and goals, too.
As you say, the Moon's a lot easier to reach - in fact, we've already sent people there to check it out! Mars is a lot farther away, so transit back and forth will take a lot longer and require greater resources for each trip. Depending on your reasons for wanting an off-Earth colony in the first place, the increased difficulty of physical interaction with Earth could be exactly the reason you'd favor colonizing Mars over the Moon.
For example, if you believe human colonies off Earth are imperative in order to reduce the possibility of future human extinction, then the additional distance to Mars could be a real advantage - more time to intercept incoming weapons from Earth, more time to identify and interdict disease or contagion on route from Earth, etc. (I mean, talk about social distancing!)
Or if you support off-world colonization because of an anarchist streak - say, for example, longing for a place you and others can be free from restraints increasingly imposed by intrusive governments here on Earth that you believe are unfairly run by an elite hegemony (not views I personally hold) - then, too, you might consider the extra distance an advantage in the struggle to get out from under the government thumb. It's just harder for them to push you around when they can't get to you, right?
Or maybe you really, really want to get away from ex? Definitely leaning toward Mars. Just sayin.
If you support colonization because you desire the benefits of technology a program like that would produce, I could imagine that cutting either way. You might favor working towards Mars believing the more difficult task would lead to richer technical innovation. Or you might favor the Moon believing that a more attainable goal increases the likelihood of success, thus increasing the chances of on-going public support for further projects. Or maybe you'd come out in a sort of middle ground - favoring Mars as an ultimate long-term goal rather than stopping after colonizing the Moon, but favoring the Moon as a desirable initial step along the way.
On the other hand, if your goal (or, say, the goal of commercial entities involved in working toward off-world exploration and colonization) is to make money participating in trade with established businesses and economies on Earth, then the difficulty of transit seem like a definite additional hurdle to overcome for Mars.
Fun question- thanks!
It is something NASA is already evaluating.
If we will want to exploit natural resources in asteroids, Mars would be much closer to them than Earth is. A decent place to get R&R, store and process or preprocess mined minerals would be very beneficial. And in case of Mars, you could just drop cargo from the belt on the surface without any risk it will miss and destroy a city with few millions people in it.
Higher gravity means you can leave professionals there longer without adverse effects, lowering the cost of shift changes - advantageous for both scientific and commercial missions.
A human colony has some essential needs that have to be supplied in order to flourish. Since the question asked why Mars might be superior, I'll focus on the Mars advantages and neglect the disadvantages.
Air - Mars has small amounts of Nitrogen available in the atmosphere ~ 2.7% as discussed in This related question which could fairly easily be collected and separated for breathing atmosphere. The oxygen can come from either CO2, hydrolysis of water, or driven out of oxidized soil with heating. By comparison, the moon's resources are much less certain and much less universal than Mars and polar conditions on the moon in the permanently shadowed regions are much colder than the Mars poles.
Water - Mars has some atmospheric water and exhibits water clouds during some seasons at various locations. Subterranean water is thought to be prevalent in the mid-latitudes due to evidence of recurring slope linea Space Article on RSL as well and of course on the surface at the poles. That makes it a lot more accessible than in permanently shadowed craters at cryogenic temperatures.
Shelter - siting colonies underground or at least with thick layers of soil or regolith may be essential for providing protection from radiation. Martian soil may be easier to work with as a building material since we're already seeing evidence at the soil around the Insight lander is hard packed and can be compacted Insight Link. Mars' thin atmosphere also offers protection from micrometeorites which decreases the hazard for anyone working on the surface in a suit. Another advantage of the Martian soils is that they appear to be much less abrasive than lunar soils. In the Apollo days the lunar regolith resembled volcanic dust in its sharpness and abrasive qualities, which would make keeping a door or suit seal difficult over time. Martian soil and dust with its exposure to weathering is much rounder and less abrasive and thus our rovers are handling long term exposure quite well.
Transportation - making in-situ storeable rocket fuel should be much easier on Mars than the moon given the availability of both CO2 and water as others have pointed out including Zubrin in his pioneering work Recent Award Link
Food - Martian soils may be more similar to Earth than Lunar soils and thus require less amendment to make them viable for crops. This Study showed that using simulants for Lunar and Martian soil resulted in Mars crops growing close to the same biomass as the earth soil control but the lunar crops had significantly lower biomass.
Human Factors - the comfort of a nearly 24 hour day and twice the gravity of the moon may ultimately contribute to less physiological hardship for Mars colonists vs. Lunar colonists.