The idea of "terraforming" Mars has hit a major snag recently in that we now believe that Mars lost what may have been an Earthlike atmosphere when its interior cooled off much earlier in its history than ours has. This caused its magnetic field to cease and that allowed the solar wind to strip its atmosphere and, even, possibly, its ocean(s). The idea that we could restore an atmosphere using engineered plants wouldn't work, the Sun would simply strip it away too. Then again, this may not be the whole story. One thing that remains unexplained (to me so far, that is) is why doesn't radioactivity keep Mars interior hot, as it does ours. If it formed the same way as Earth it should have a comparable amount of radioactive rocks, (adjusted for its smaller size of course), so why did its interior cool about 4 billion years ago, while it is estimated ours may continue to a hundred billion years more? Does anyone here know this? And there may be a way to retain an atmosphere without a magnetic field. We're pretty sure the Earth has lost its when the field reverses, as it does about every 700,000 years, yet no great extinctions accompany these phenomena, at least with the same regularity. In any case any mass migration to Mars will remain a somewhat impossible dream until we find some way to cheaply escape the Gravity well we all live at the bottom of. Using present tech it costs about a thing's weight in gold to launch it into orbit. The Space Elevator may solve that but at present we have still a long way to go before we even know if the materials it would require are actually possible, though early indications are positive. A world government would probably be a necessity for any such thing, and I do notice that Space exploration could be forming a nascent type of such. American astronauts are now being launched exclusively on Russian rockets and will be for about a year more. There are also satellites owned by England, France and even some of the less developed world. Very promising, and too little IMO, Space will be the key to Earth's future, everyone knows that.