What will it take to make Mars Habitable?
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First, let's look at Temperature. It's more than just the average daily temperature which is important, but how much it varies. For Venus and Earth, it does not vary very much. However, for Mars, it varies enormously every 24 hours. |
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A mid-summer day in Mars' Southern Hemisphere can get to 20C (68F), but you better be prepared for it to drop to -40C (-40F) at night. It won't help to plant crops during the warm Summer, since they will freeze over each night!
The main constraint on temperature is the requirement that water remain liquid. Just as on Earth plants can handle some freezing temperatures at night, as well as a long deep freeze over the winter provided they get a warm summer to grow.
Furthermore, water can stay liquid below the ice, well below freezing surface temperatures. Consider the deep lakes of Antarctica, where the average temperature is -20 C (255K), and indeed life is found there!
Our goal will be to get Mars to have at least an average annual surface temperature at or just above freezing, and minimize deep freezes during the summer nights.