Mars is the the fourth planet from the sun and could become habitable in the future. Some believe life has already exsisted there or may exist there now. Mars is much smaller than the Earth (only Mercury is smaller) and has a weaker gravitational field. The Martian atmosphere is very much thinner than our atmosphere. Mars is on average 142 million miles from the sun which is further and colder than Earth. The distance varies between 129- and 160 million miles. Despite all this life can't be ruled out there and serious scientists connected with NASA are searching for life on Mars, also there may be liquid water on the surface in summer.  Exobiologists think life is more likely under the surface of Europa.
Exploration: There have been several rovers landed on Mars. AWESOME And in time colonization may be possible and probably AWESOME. 
- Colonies on Mars will flourish and we will eventually conquer the universe, says Stephen HawkingStephen Hawking thinks we will colonise Mars.