Why colonise Mars?
NASA has decided that it wants to send mankind to Mars, a magnificent technical goal. The scientific payoff in terms of understanding of the red planet may be less than we would have got from the same money spent on rovers, but there will probably be a better scientific understanding of the effect of long spaceflights on humans.
However, some people assume that this is a precursor to colonising Mars as a way of spreading out into space and increasing humanity’s range. This does not make much sense at all.
As Neil deGrasse Tyson points out , no place on Mars is warmer or wetter than Antarctica (the coldest, driest place on Earth). If you want to increase the space available to humanity, before colonising Mars you might think about colonising Antarctica. Antarctica is warmer than Mars, has more water, has 1 earth gravity, has 1 earth atmosphere of pressure, and abundant oxygen and water. The cost and duration of the journey to get there is a tiny fraction, less than a millionth, of the journey to get to Mars. However, no-one is in a rush to colonise Antarctica.
The way to increase the space available to humanity is to colonise space, not planets.