The fundamental difference between the insides of Earth and Mars was decided by how much iron they had when they were still oceans of liquid lava.

Mars has a distinct, deep layer of silicate minerals that Earth completely lacks, despite both planets starting as molten balls of rock. This difference comes down to the partial melting of the magma ocean as it cooled billions of years ago. Earth's larger size and specific iron content allowed its mantle to mix thoroughly, while Mars's chemistry caused it to separate into permanent layers. This early cooling phase set the stage for why Earth has plate tectonics and life while Mars became a stagnant, red desert. It shows that a planet's entire geological destiny is written in its very first few million years. Understanding this helps us predict which alien planets might actually be habitable.