Mars has everything needed to establish another branch of human civilization. Except air, water, arable land, survivable temperatures, and protection from radiation.
Mars has a thin atmosphere of carbon dioxide, and a whole polar cap full of water, and possibly liquid aquifers.
Using electrolysis, the water can be split into hydrogen and breathable oxygen, and the hydrogen can be used with 1800's Sabatier reactor technology to produce methane and more oxygen from the carbon dioxide atmosphere.
There are traces of nitrogen in the atmosphere that can be separated out to supplement the unknown levels of nitrogen in martian soil, but on the basis of what we now know from Viking and SNC meteorites, it is likely to provide an even better medium for crop growth than most land on Earth, as it is as rich, or richer, in most of the elements needed for plant growth. In addition, its diurnal cycle of 24 hours and 30 minutes is close enough to Earth's so as not to confuse plant growth.
As for temperatures - the range of temperatures on the moon goes from about -250 to +250 farenheit, while Mars has an average temperature of -63C, not much colder than the current temperature at the Admundsen-Scott station at the South Pole:
http://www.wunderground.com/global/stations/89009.html - -54C as I post this - and a range of -81C to -31C, the low end of which is higher than the lowest temperature ever recorded on Earth, -89.4C at Vostok Station in Antartica:
http://hypertextbook.com/facts/2000/YongLiLiang.shtml Protection from radiation is provided by the atmosphere, which is thick enough to block the effects of solar flares and allow domed surface agriculture at about 0.7 psi, and by the soil, if one build's one's surface shelters out of brick, or use methane-powered bulldozers to pile dirt around your shelter, or use appropriately situated landforms, such as a cliff.