In Mars already live aliens. They won't be glad. 
Acctuly not becouse Mars is ugly planet and very dangerous.
Acctuly not becouse Mars is ugly planet and very dangerous. That could be changed through terraforming. In theory, a new atmosphere could be created by slowly introducing plants and water to Mars.
It's a quite interesting possibility: http://en.wikipedia.org/wiki/Terraforming_of_Mars
apparently scientists thinks that terraforming mars will be impossible
even if it is possible, our great-great-great grandchildren will be dead by then... not to mention us
