What If...?

I used to love when Marvel Comics would run the ocassional "What If..?" issue, well, Scientific America poses the question: "What if Earth Switched Places With Mars?"
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*Via David Brin on Facebook, who adds, "But Musser assumes that Earth would freeze. It would -- but not forever! Volcanoes would supply CO2 till a greenhouse melted the seas. Simple calculation. If Mars were bigger, it'd have oceans now, under a dense gaia-greenhouse."