You've all heard this before but I'll repeat it again. Mars has all the necessary elements in its soil and atmosphere to sustain terrestrial biology. Mars has abundant near surface water in the form of ice and an atmosphere that shields from most ionizing radiation. Mars also has a 0.38 G surface gravity that is sufficient to sustain human physiology. The Moon doesn't have an atmosphere and most of its volitiles got cooked off when it was formed.

The problem with Mars is NOT how to get people there (we've known how to do that since the 1970s). The problem is figuring out the cheapest way to use Martian in-situ resources to build a self sufficient colony. For example:

1) We know there's subsurface ice on Mars. What's the cheapest way to turn that subsurface ice into drinking water and rocket fuel (hydrogen / oxygen)?
2) Mars is covered with iron. What's the cheapest way to smelt steel on Mars?
3) What's the minimum amount of hardware that must be shipped from the Earth to Mars to make a fully functioning machine shop on Mars?
4) Mars has plenty of silicon but almost no carbonates. Glass requires sodium carbonate. Fuzed silica needs only silicon dioxide but it's harder to make. Iron tends to contaminate glass and make it green. Manganese acts as a clearing agent for glass and can counter act the effect of iron. Glass, fuzed silica or clear plastic will be needed for green houses. What's the best technology?
5) Hydrocarbons are very rare on Mars. Hydrocarbons are essential for a colony. What's the best way to make hydrocarbons on Mars? Is it simply a matter of growing plants from water, carbon dioxide and fixed nitrogen?
6) What's the best radiation shielding technology? Should lava tubes or deep canyons be turned into habitats? Should the first colonies be underground?
7) What's the best pressure suit for a Mars colonist? The pressure suit has to be easy to manufacture on Mars. The pressure suit must also have sufficient dexterity that someone could swing a pick or dig a deep hole while wearing one. This is a tough problem! My pet idea is to use pressure tents. If you want to dig a hole, peg down an air tight tent with no bottom where you want to dig the hole. Have a compressor with an inter-cooler pump Martian atmosphere into the tent. The compressor would have a pump rate sufficient to overcome the massive leak rate around the tent's bead on the ground (the tent would be pegged into the regolith with big steel spikes). The compressor would maintain a pressure in the tent at about 0.2 bars at 20 deg.C. The worker would enter the tent and wear only a tee shirt and shorts. He'd breath pure oxygen through a face mask.

Ideas like these are why a manned Mars program is both interesting and sustainable.

Gary