Two books

Speaking of colonizing moons, it seems that colonizing Phobos may be
easy in the short term. There is reason to believe that Phobos is a
captured asteroid and may contain a large ammount of volitals.
Without the need of a reentry heat shield, extra propellant for
decent or long periods of aerobraking makes setting up a colony much
easier on Phobos than on the surface. Your mentioning of heat shields in this context made me have a realization, though. Sometimes Mars advocates say going to Mars is "easier" than going to the moon, because you can aerobrake at Mars, but not at the moon. However, one might use the atmosphere of Mars to assist in landing on Phobos instead. This would combine the advantages of aerobraking with the advantages of being in a much shallower gravity well, and the ability to build very large yet flimsy structures. If we targeted Deimos instead, we could add the advantage of near-continuous sunlight. As to Mars. It doesn't take much more deltaV to reach Mars than to
reach Luna. Mars has two huge advantages over Luna.
One, it has an atmosphere of CO2. A few plants, some inflatable
clear domes and wham, instant colony. Yes, but does local availability of Carbon really save you all that much weight? The NASA Summer Study on orbital habitats assumed all carbon would be brought up from Earth. They assumed a structural mass of 500,000 tons (lunar derived). I'm leaving out soil and shielding since both would be readily available on Mars. On the Earth derived end, they list biomass (from which all our carbon would come) at 5,900 tons. Other items on the Earth derived end bring the total there up to 52,600 tons. So excluding shield and soil, the habitat would mass 552,600 tons, of which the biomass (mostly carbon, but with a significant amount of other elements too) would only represent a hair over 1% of the total. So does being surrounded by a carbon-bearing atmosphere really save you all that much? As for the mass of the inflatable clear domes, any which could maintain adequate pressurization against a 1% atmosphere could do the same against 0, I would think An added benefit of an
atmosphere, micro-meteor shield and a small bit of UV protection. The Martian atmosphere is a very poor UV shield. On both the moon and Mars, precautions would have to be taken regarding exposure to UV for both people and structures. As a meteor shield, it's certainly better than nothing at all, but markedly inferior the Earth's atmosphere in this regard. Another benefit of an atmosphere, temerature regulation. Temperature
extremes are less than on Luna. This is a good point. Two, Mars shows evidence of highly concentrated, near surface water
in easily accessible areas. Luna's water (if it exists) is located in
significantly sub-zero, shaded craters and may not be in
concentrations high enough to warrant mining. Plus, very specialized
equipment with thermal protection of the nearly absolute zero
temperature would be needed to extract the water. Another good point. Luna has the added difficulty of a 336 hour night. You need a lot of
power storage or you need a nuke. Mars practically has a 12 hour
night. True, but I'm not convinced we could rely on a major way on solar energy on Mars either. We certainly don't here on Earth, where the nights are no longer than on Mars. And then Mars receives less sunlight than we do at our distance from the sun. Zubrin assumes nukes for Mars, too. Luna's benefits are proximity. Luna is a 3 day trip with
1960's techniques, Mars is 6 months away. Communication delays to
Luna are only seconds long, Mars is 15 minutes. Two more good points for Luna. If telepresence advances considerably, the latter may become a highly significant one.