Our tale of two planets begins four billion years ago. One planet was Earth, and the other planet was Mars, and the two had much in common in their infancy. Rivers and lakes etched their surfaces, craters pockmarked their faces, and volcanoes rose from their plains. But something seems to have changed on one and not the other.
In Earth’s burbling warm water, fate and chemistry combined amino acids into complex molecules, and in a process we still don’t understand, these gave rise to single cells that figured out how to make copies of themselves. Tiny mistakes in those copies eventually turned them into oxygen-exhaling organisms we call algae. Endless forms flowed from these humble ancestors, and after eons, there we were: All of human culture and hope and possibility arising within a tiny slice of time.
Mars was not so lucky. Mars dried up. Mars is small, about half Earth’s diameter, so it cooled off faster than Earth did after their birth in the cloud of dust left over from the sun’s creation. Compared to its overall volume, more of Mars’ mass is exposed to the icy blackness of space. As it cooled, its iron-nickel core solidified. When this happened, we think, the Martian magnetic field shut down, robbing Mars of its protective shield, of the sort that still safeguards Earth from solar and cosmic rays. Time and the brightening sun stripped away the Martian atmosphere before the planet’s algae, if it existed, had a chance to make the air thick and warm. Mars turned to rust before any skeletons could adorn its deserts, before any creatures could look up and contemplate their place among the other dots in the night sky. While Earth is fecund and bursting with life, Mars is, and may have always been, barren.