From Christian Davenport: Living on Mars is like living on top of Mount Everest. No, it’s like living inside Chernobyl on the top of Mount Everest, given the high levels of radiation. Mars represents such an unfathomable challenge that writers such as Homer Hickam, the former NASA official and author of “Rocket Boys,” once wrote an essay titled, “A Myth Known as Mars (Psst, NASA’s not going there, pass it on.)” Even Elon Musk, who founded SpaceX with the purpose of getting to Mars, doesn't sugar coat the difficulties and once told me that "probably people will die "on the first trips to Mars. Which is why you're exactly right -- NASA wants to go to the moon first and learn how to live there before going all the way to Mars. Mars is the horizon goal that could come at some point. But NASA -- and SpaceX -- are set on going to the moon first.

From Garrett Reisman:

Good question. You are right that the Moon is easier - much quicker to get to, and more importantly much quicker to get home in an emergency, and you can go pretty much anytime, you don't have to wait for a window to open up every 2 years. That means you can go for a few days, a few weeks, a few months or a few years which would let us get experience living for extended periods of time in partial gravity outside the Earth's magnetosphere in a safe and incremental manner. If you go to Mars, you pretty much have to commit for a round-trip duration of about 2.5 years.

But Mars is a much more suitable place for humans to live. It has an atmosphere, a gravity level of about twice that of the Moon, reasonable temperatures and lots of resources that can help support a human colony including oxygen for breathing, water for drinking and methane for fuel. Plus there is the promise of incredible scientific discoveries including, probably, proof of life existing outside of the Earth.

One day it might be possible for a human colony on Mars to be truly self-sustaining. That won't happen on the Moon without continual resupply from Earth.