I've always wondered if Venus, which has many similarities to the Earth, would have had an Earth like habitat if/when it was further away from the sun like the Earth is. I also don't know if or how orbits and day length are affected by distance from the sun as well. On the assumption that Mars will get closer to the sun over time, I still don't know if the sun would expire and blow up before the orbit is significantly changed. Anyway... A reason for life on Earth is the magnetic field from molten iron in the core. A banal guess for this is simply because the Earth is the right size for gravity to create friction inside the core. Assuming that Mars has all the right stuff (elements) and enough time on its hands, could it be terraformed by diverting a myriad of comets and asteroids into it? Could it be terraformed by its own gravity if its mass was simply increased?