Mars is very cold right now with almost no atmosphere, and it has remained relatively unchanged for billions (yes, billions) of years. Terraforming it would en

Mars is very cold right now with almost no atmosphere, and it has remained relatively unchanged for billions (yes, billions) of years. Terraforming it would entail thickening the atmosphere and heating it up. That'd cause a lot of changes to its surface. Water frozen for eons might start flowing, gases currently frozen in the ground might start evaporating etc. Eventually it might start raining, which would start massive planet-wide erosion. There's a reason you see craters on the Moon and not on Earth: It rains.