Been googling a bit...
There is water on Mars:
On June 19, 2008, NASA announced that die-sized clumps of bright material in the "Dodo-Goldilocks" trench dug by the robotic arm had vaporized over the course of four days, strongly implying that they were composed of water ice which sublimated following exposure. While dry ice also sublimates, under the conditions present it would do so at a rate much faster than observed.[43][44][45]
On July 31, 2008, NASA announced that Phoenix confirmed the presence of water ice on Mars, as predicted on 2002 by the Mars Odyssey orbiter. During the initial heating cycle of a new sample, TEGA's mass spectrometer detected water vapor when the sample temperature reached 0 °C.[46] Liquid water cannot exist on the surface of Mars with its present low atmospheric pressure, except at the lowest elevations for short periods.[47
source:
http://en.wikipedia.org/wiki/Phoenix_Mars_Lander#Presence_of_shallow_subsurface_water_ice
Now if we colonize in the Melas Chasma we are already 12 kilometers below the martian surface, which helps a lot pressure wise. For oxygen it's Hemp to the rescue. Mars is like 98%CO2 and hemp loves CO2 and gives O2 in return, and Hemp will grow in Martian soil (according to some guys blog anyways). So toss up some tents, plant your seeds and you've started micro-terra-forming a little campsite to start on construction of the colony.
Yes I know I'm just fantasizing about being a hemp farmer on Mars, but this is an enjoyable discussion just the same.
eb
Dude with hemp you can have material for not just terra forming, but for food, buildings, vehicles and fuel! That would be fantastic