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.