The fourth planet in the solar system, Mars, is today being discovered by humans. Because of the iron oxide it is called as Red Planet. Today Mars has very thin atmosphere and poles (North/South) covered with the ice.
According to some scientists Mars atmosphere can be recovered by oscillating carbon dioxide. NASA until now has sent many orbiters like Mariner and Viking, also land missions ( rovers) like Curiosity and Pathfinder. Curiosity now exploring past and present habitability of Mars surface.
When we mention past habitability of Mars, according to above mentioned missions the planet had "water" in its history. Canyons on the surface describe us a river network that appeared on the surface. Also on the poles there is water in ice form. As a result of its small core Mars could not protect its atmosphere and magnetism therefore became indefensible against radiation from the Sun.