If you want humans on Mars, first you need a self-sufficient logistics chain on Mars. We've made breathable Oxygen on Mars, and we should be able to make a Von

If you want humans on Mars, first you need a self-sufficient logistics chain on Mars. We've made breathable Oxygen on Mars, and we should be able to make a Von Neuman probe to send, and set up that supply chain. Instead of having it manage itself, we simply make it remote control, and we don't have to worry about run-away issues. We should be able to go anywhere there's large enough of a fraction of the resources required for human life. For instance, if there's NO phosphorus, instead of just a smaller percentage, it's no-go, unless we're willing to live off a stockpile imported from elsewhere. So, ambient temperature and pressure, along with elemental composition, are really the only hard limits.