Mars used to look a lot like Earth. The planet had a vast sea under oxygen rich skies, and as far as we know, all the right conditions for life. But while Earth was beginning to see its first glimpse of life, Mars was losing its chance of becoming a habitable world. We don't know why, but roughly 4.2 billion years ago, Mars's magnetic field disappeared. That magnetic field protected the young planet from a constant barrage of solar winds ejected from the sun. But once it was gone, Mars's warmer, rich atmosphere was blown out of the solar system.
But could we make Mars habitable again?
NASA might have a new strategy to do just that. They theorize that by placing a large magnetic dipole shield between Mars and the Sun, they can protect the planet from the Sun's solar winds and perhaps establish a new atmospheric equilibrium.
New York Times: Mars' Atmosphere Stripped by Solar Winds, NASA Says
NASA: NASA Mission Reveals Speed of Solar Wind Stripping Martian Atmosphere
USRA: A Future Mars Environment For Science and Exploration