They had decided to have a child and they knew the consequences on Mars. They had brought enough carbon in the form of plants and soil to keep them. It was the vanity of immortality that led them to the impasse. At 12 their son was growing up and it was then that they realized that someone must die for the others to live.
News from Earth was filled with horrible consequences of warming from carbon dioxide and it was then that the first solar war began. Energy was abundant on Mars as well as raw materials. Earth was unwilling to trade energy for oil as they had become so mired in their own habit of substance abuse that they could not change.
NASA had actually created the tools of this war by a mistake they made in 2023. A spacecraft was to gain momentum from asteroids with a bungee cord harpoon. What they didn't consider was that multi-body physics is a dangerous game in space. As the asteroid belt was destabilized and orbits became erratic the Earth was stoned into understanding of what they had done.
One of the greatest achievements of Mars was the infinite precision FPU. This was the bow and the asteroids were the arrows.